Introduction to Directional and Horizontal Drilling

Introduction to

DIRECTIONAL ANDHORIZONTALDRILLING

J. A. "JIM" SHORT

:pelUi'\Vell Books

PENNWELL PUBLISHING COMPANY

TULSA, OKLAHOMA

DISCLAIMERThis text contains statements, descriptions, procedures, and other information,

hereinafter collectively called "contents," that have been carefully considered andprepared as general information. The contents are believed to represent situationsand conditions reliably that have occurred or could occurbut are not represented orguaranteed as to their accuracy or application in any condition or situation. Thereare many variable conditions in oilwell and gaswell drilling and related situations,and the author has noknowledge or control oftheir interpretation. The contents areintended to supplement and not to replace the user's judgment in considering,investigating, and verifying actions and situations. Use of the contents is solely atthe risk of the user. In consideration of these premises, any user of the contentsagrees to indemnify and save harmless the author from all claims and actions forlosses and damages.

Copyright 1993 byPennWell Publishing Company1421 South Sheridan/P.O. Box 1260Tulsa, Oklahoma 74101

Library of Congress cataloging in publication data

Short, J. A.Introduction to directional and horizontal drilling / J.A. "Jim" Short,

p. em.Includes bibliographical references and index.ISBN 0-87814-395-51. Directional drilling. 2. Horizontal oil well drilling. I. Title. II. Title:

Directional and horizontal drilling.TN871.23.S48 1993 - ---6221,.3381--dc20

93-16840eIP

All rights reserved. No part of this book may be reproduced, stored in a retrievalsystem, or transcribed in any form or by any means, electronic or mechanical,including photocopying and recording, without the prior written permission of thepublisher.

Printed in the United States ofAmerica

1 2 3 4 5 97 96 95 94 93

\~~

rI

Thisbook Isdedicated to my wife,Catherine Leona "Campbell" Short.

She has enriched my life, continuallyreinforcing our relationship over the

years. She truly personifies the generous,loving wife and mother.

..Miss Kitty, I love you.

CONTENTSPREFACE ix

CHAPTER 1 OVERVIEW,DESIGN GUlDELlNES 1Summary 1History and Development 2Directional Status and Applications 4Horizontal Status and Applications 10Design Guidelines 16Designing/Calculating Well Patterns 24Directional Designs 34Horizontal Designs 37Bibliography 47

CHAPTER2 DRILLINGTOOLS 53Summary 53Downhole Equipment 53Drillpipe String 54Drillstring 68Directio:nal Control 72Bottomhole Assembly 76Measurement Instruments 86Wellbore Surveys 100Bibliography 101

CHAPTER 3 DEVIATIONAND SIDETRACKING 105Summary.. 105Selecting Measurement Systems 106Orientation 108Deviating on Bottom 113Sidetrack Plug 120Sidetracking 127Other Deviation Procedures 139Bibliography 142

VII

CHAPl'ER 4 DIRECTIONALDRILLING 143Summary 143Operations 144Single-Bend 153Double-Bend 162Extended-Reach 164Slant Hole 165Casing and Cementing 166Drilling Problems 168Fishing 176Bibliography 179

CHAPl'ER5 HORIZONTALDRlLLING 181Summary... 181Operations 182Short-Turn 189Medium-Turn 192Long-Turn 199Extended-Reachand CombinationPatterns 203Formation Evaluation 204Casing and Cementing 208Completions 214Bibliography 222

INDEX. 227

VIII

ThIsbook should raIseas many questIons as you mIght have had beforeyou started readIng It.. .maybe more. That's not meant asan apology,but asa challengel

-William L.Leffler(Petroleum Refiningfor the Nontechnical Person. SecondEdition. 1985. PennWell Books).

PREFACEVertical drilling is fundamental to the oil and gas industry.

Directional drilling developed from a need to vary direction fromvertical drilling and has been facilitated by advances in technology.It is a commonly used, well-established, and proven technique.Horizontal drilling developed for similar reasons. It is widely usedand is gaining acceptance in the industry. Through continued useand technological advances, additional applications of these twoinnovative drilling methods will develop, further increasing theirimportance. Both are used worldwide to prevent waste by develop-ing and producing oil and gas not recoverable by other methods andby reducing costs.

This book is an introductory text on directional and horizontaldrilling and related activities. The material is presented in non-technical language with explanations ofcommon terminology. Thetext followsthe natural sequence ofevents; new subjects build uponprior material in a building-block fashion. This serves a dualpurpose. Those less-experienced can start at the beginning, layinga foundation and building upon it. More advanced readers may godirectly to subjects ofinterest. Each chapter starts with a summaryfor a quick review and ends with a comprehensive list of referencesas sources of additional information. Specific topics can be foundeasily from the Table of Contents or in the expanded Index.

This book is for anyone interested in directional and horizontaldrilling. It should be very helpful tobeginning employees as well asto personnel in other sectors of the oil and gas industry, includingthose in related fields such as service and supply companies. Readthe book to learn general information about directional and hori-zontal drilling, scan it for special subjects, or use it as a referenceor textbook.

IX

~-

CHAPTERl

OVERVIEW, DESIGNGUIDELINES

SUMMARYBy earlier methods, all wells were drilled vertically downward.

Directional drilling evolved from the need to drill the hole in otherdirections. Special drilling tools and procedures are used to changethe direction ofthe wellbore from vertical to directional or horizon-tal in order to penetrate targets that cannot be reached by regularvertical drilling methods. Directional and vertical drilling servemainly for the drilling of exploration and development wells.Horizontal drilling creates development wells with increased,sometimes very high, production rates. There are various wellpatterns within the directional and horizontal classifications, de-pending upon the type of well.

Directional and horizontal drilling are high-risk drilling opera-tions compared to vertical drilling. Efficient drilling programsmust be designed carefully. Successful designs have a drillable wellpath, provision for casing, and minimized hole problems. The wellpath includes the kickoff depth, the angle-build and angle-droprates, the drift and direction ofthe wellbore, the target, and limits.

Directional and horizontal drilling are flexible and applicable tomany situations; these wells are drilled worldwide in most major oiland gas fields, both on land and offshore. Usage is increasing, witha potential for widespread future usage.

OVERViEW,DESIGNGUIDELINES 1

HISTORYAND DEVELOPMENT. ..they may be a witness unto me that I [Abraham] havedigged this well. Genesis 21:30

The history of drilling fades into the distant past. China hadwells before 120P AD., later followed by drilling in France, Italy,and West Virginia. The first drilling objective was to producewater. Later needs for resources led to drilling steam for geother-mal energy, saltwater for salt, and gas for heating and oil. TheDrake well, drilled in Pennsylvania in 1859, is the acknowledgedstart of the drilling industry in the United States. Drilling equip-ment began with hand-digging tools, followedby spring pole, cable-tool, and rotary rig equipment in the late 1800s. Early "churn"drilling used a cable or flexible drilling line so that holes weremainly vertical.

Rotary drilling with a rotating drillstringdeveloped into a highlyefficient process for drilling and completing oil and gas wells atdepths greater than 30,000 feet. Rotary rigs drill on land oroffshore, and some are modified forspecial drilling services. Rotarydrilling methods were later modified for directional drilling.

Directional tools and techniques evolved slowly from verticaldrilling. An early reason fordirectional drilling was due to a "fish,"unrecoverable drilling tools lost in the hole. Directional methodsallowed drilling around and bypassing the fish, a less expensiveoption than drilling another hole. Crooked holes were anotherproblem that led to directional drilling. One other potential andless publicized incentive may have been to drill into more produc-tive areas under adjacent acreage where ownership may have beenin question.

The whipstock was the first reliable directional drilling tool.Development of new tools and techniques aided first in drillingstraight and vertical holes and later aided directional drilling.Developments in measuring instruments were the final step lead-ing to moderndirectionaldrilling..

Directional drilling is conventionally defined as a procedure fordrilling a nonvertical hole through the earth. It first gained promi-nence when it was used to control a blowout well in southeast Texasin the mid-1930s. At a safe distance from the blowout, a directionalhole was drilled at an angle to a point near the bottom ofthe blowouthole. Fluid was pumped through the deviated hole into the forma-tion, stopping the blowout. This innovative procedure done on asensational and highly productive well received widespread public-ity. It focused attention on the somewhat new drilling procedure.

2 OVERVIEW,DESIGNGUIDELINES

Directional drilling had a strong start offshore and in other areaswhere it was difficult or expensive to build a surface location. Earlyoffshore wells were drilled on wide spacing from piers and laterfrom individual platforms. Directional techniques allowed drillingmultiple wells from one location, thus eliminating construction ofan expensive structure for each well (see Fig. 1-1). These andsimilar procedures firmly established directional drilling, and itdeveloped into a reliable, efficient drilling procedure with wide-spread usage. (Note that the angles of bends are exaggerated inmost illustrations to allow easy visualization.)

Ai!,the drilling industry has matured, wells have been drilledvertically to more than 30,000 ft deep. However, very deep drillinghas become less common because of the expense and indicationsthat oil and gas donot often occur at these depths. This, in part, hasled to extended-reach, drilling directional to greater distances.

Horizontal drilling subsequently evolvedmainly to improve wellproductivity. It involves drilling the well in a curve from vertical tohorizontal and then horizontally. The first wells had one or moreshort holes drilled horizontally into the formation from the verticalwellbore. These "drain holes" exposed more of the reservoir to thewellbore and produced larger volumes of oil and gas.

The horizontal drilling procedure had been tested in variouscountries by the 1950s. However, inadequate equipment, lack ofdemand, and the relatively high cost compared to conventionalrecovery techniques hampered development. Interest revived in

Figure 1-1Multiple wells drilled from one location

E

OVERVIEW. DESIGNGUIDELINES 3

the 1980s, focusing on drilling a single hole a longer horizontaldistance into the formation. Tools and techniques developed at anaccelerated rate, further increasing efficiency. Horizontal drillinghas many applications. It is the latest (and very significant) drillingtechnique.

DIRECTIONALSTATUSANDAPPLICATIONS

Modern directional drilling is an established, widely used drill-ing procedure. It was originally developed for sidetracking a fish,drilling kill wells, correcting crooked-hole problems, and laterpreventing the well from crossing lease lines (see Fig. 1-2). It is stillused for these purposes. They are important, but other equallyimportant applications have developed over time, such as drillingfor attic oiland gas. Directional drilling is common in both offshoreand land operations. Major areas of usage include the Texas-Louisiana Gulf Coast, the North Sea, the Mideast, and the FarEast.

Equipment and techniques permit drilling any reasonably de-signed well pattern. Regular directional patterns are more com-mon, with slant and extended-reach holes drilled where applicable.Directional patterns can be combined with horizontal patterns,and expanded usage will lead to other applications.

MULTIPLEWELLSFROM ONE SURFACELOCATION

Drilling multiple wells directionally from one surface location isa common, important application ofdirectional drilling. Multiwelldrilling sites include offshore platforms, man-made islands andpeninsulas, and platform and earthen locations in swamps, jungles,and other isolated areas. The older, highly developed EastWilmington field in California is a significant example ofa multiwellsite. It has nearly 1,200wells, including high-angle and extended-reach, from 4 man-made islands and 4 earth-filled pier locations.

Modern directional and extended-reach techniques may drillinto large areas containing oil and gas from one surface location(see Fig. 1-3). Avertical well penetrates the reservoir at one point.Directional drilling increases coverage substantially as illustratedby the following, based on about 15,000 ft of deviated hole.

Holes at 20 cover about 3 square miles. Coverage increasesabout 340% at a low inclination angle of 40. Increasing the angle


Figure 1-2Early directional applications

A - Relief'kill'we.B - Blowoutwell

C - Bypass a fishD - Straighten crooked hole

to 60 increases coverage about 200%more than that at 40. High-angle extended-reach drilling at 80 increases coverage about130%more than that at 60, 234%more than coverage at 40 and820%more than coverage at 20.Asignificant example ofthis is anoffshore well in Australian waters drilled to a measured depth ofmore than 18,000 ft. Horizontal displacement was almost 3 milesat a true vertical depth ofless than 8,000 ft. About 28 square milesof reservoir were theoretically accessible to one surface location inthis extreme case. This area is considerably larger than the averagesize of most oil and gas fields.

There are various advantages to drilling multiple directionalwells from the same surface site. The main advantage is the singlesite requirement. It is more economical to drill many directionalwells from one platform than it is to build a costly platform for eachvertical well. The same situation occurs in swamps, jungles, an,d

OVERVIEW,DESIGNGUIDELINES 5

Figure 1-3Directional wells Increase coverage

- Kickoffdepth-+- 2,640II.

40'

Based on drBling15,000 f~ measured depth, of deviatedhole below the kickoff point

. - True vertical depth below kickoff point

other isolated areas because of the costs of building access roadsand multiple surface locations. Common gathering, separation,storage, and other production facilities further reduce costs.

Many productive formations donot contain sufficient volumes ofoil and gas to justify the costs of building individual platforms orsingle-well locations in order to drill vertical wells. The more cost-effective procedure ofdrilling multiple wells from a single locationoften allows economical development and production. This allowsproduction of oil and gas that would not otherwise be produced.

INACCESSIBLESURFACELOCATIONSInaccessible surface locations inhibit development by the drill-

ing of individual vertical wells for various reasons. Some surfacelocations are inaccessible for economical, physical, or other rea-


sons. Surface drilling sites are very costly, if available, in residen-tial and industrial areas. Ordinances and statutes prevent drillingin some areas. Shipping fairways must be left open for ships to pass,so a drilling platform cannot be constructed on the fairway. Otherrestricted areas include parks, lakes, cemeteries, recreationalareas, and major thoroughfares. Related reasons for not drilling insome areas include concerns about safety, noise pollution, and thedifficulty ofmaintaining long-term production and transportationfacilities. The only reasonable method ofrecovering the underlyingoil and gas in these situations is by directional drilling. It often ispossible to obtain a few acres for a single surface drill site and thendrill multiple directional wells into the surrounding area from thesingle site.

CHANGED AND MULTIPLE TARGETSMany wells are nonproductive dry holes. Geological and reser-

voir information obtained during drilling may suggest a productive

Figure 1-4Other directional well applications

Park area

A ~ Plug back and deviate into o~ zoneB - Inaccessible surface locationC - Multiple targetsD = Plug back and driUto oil zone

- -

OVERVIEW, DESIGN GUIDELINES 7

Figure1-5Saltdome drilling

A .Attic oil 8.Dualcompletion C.SidetracksE .Originaldry holes F-Atticgas

area near the wellbore. It is common in this case to plug back,sidetrack, and drill direction ally into the productive area. Oil andgas frequently overlay water in dipping reservoirs. A vertical holedrilled into the water zone may be sidetracked for drilling di-rectionally updip into the oil and gas zone. A well may be drilleddirectionally under an inaccessible location. Wells can be drilled di-rectionally into multiple targets for dual completions (see Fig. 1-4).Similarly, an oilwell in the gas cap or a dry hole may be sidetrackedand drilled into the underlying oil zone. Basement oil, attic oil andgas, and salt dome and fault traps are common directional drillingtargets (see Fig. 1-5).

Exploration wells may be drilled directionally from a singlelocation in a similar manner. Normally, exploration wells aredrilled vertically and the field is developed with directional wells,generally from a single surface location such as a platform. Some-times the exploration prospect may require multiple explorationwells, and the cost ofindividual surface locations is very expensive.Then a single surface location isbuilt, such as an ice island in arctic


waters. Regular and long, extended-reach exploration wells may bedrilled for exploration and later developed ifjustified.

Drilling into multiple targets is another directional drillingprocedure. Oil- and gas-bearing strata may occur at differentdepths and horizontal locations in a localized area. These may betested and produced by deviating and drilling directionally intothese multiple targets with a single directional well under favor-able conditions.

SLANTHOLESSlant holes are a special application of directional drilling in

areas where strata containing oil and gas occur at shallow depths.They are similar to drilling multiple directional wells from a singlesurface location with several differences. In these cases, the verti-cal distance to the reservoir is too short to establish sufficientcurvature and drill directionally into targets a long horizontaldistance from the wellbore. The drilling starts from the surface atan angle of30-45 with a slant hole rig. The bottom ofthese holesmay be displaced over 5,000 ft horizontally at vertical depths of3,000 ft (see Fig. 1-6). This is about twice the horizontal distanceobtainable with conventional directional drilling tothe same depth.Otherwise, slant hole drilling serves the same purpose as ex-tended-reach directional drilling and has similar advantages. Some

Figure 1~Slant hole and slant/horizontal combination

.@ . . . ;';::I::,;.;i: .; ::' : I ' : ' : ; : I:;. : .; I ' : .; , : I '

A - Slant hole B - Slant/horizontal combination


areas of slant hole drilling include Canadian gas sands, Peruvianoffshore waters, the Far East, and the Athabasca heavy oil sandsin"Canada.

HORIZONTALSTATUSANDAPPLICATIONS

Horizontal drilling is a procedure for drilling and completing oiland gas wells with improved productivity compared to wells drilledby other methods. Acurved section is drilled from the bottom ofthevertical hole, followed by drilling horizontally into the formation.Horizontal drilling may be combined with other forms of direc-tional drilling, such as a horizontal section at the bottom of anextended-reach well. Horizontal drilling is well established, adapt-able to a wide range ofsituations both on land and offshore, and itsusage is growing rapidly.

Most major fields have horizontal and some combination wells.General areas of activity include Canada, Indonesia, France,M-rica, the North Sea, and Mideastern countries such as SaudiArabia. The highest level of activity is in the United States. Somestates, such as Texas, have statutes governing aspects ofhorizontaldrilling such as well spacing and production schedules.

A field or reservoir may require fewer horizontal wells forcomplete development as compared to other methods of drilling.Vertical or directional wells efficiently deplete or drain a given areaof reservoir. Horizontal wells increase the area of drainage by amultiple related to the length of the horizontal section, which isgenerally considerably more than the average vertical or direc-tional well. The net result is fewer horizontal wells for developinga given size field as compared to vertical and directional wells.Directional and extended-reach drilling increase areal coveragefrom one surface site, and combining these with horizontal drillingfurther reduces the number of wells needed.

INCREASEDPRODUCTIVITYHorizontal wells have higher production rates and produce

greater quantities of oil and gas than wells drilled by othermethods, as verified by production histories and computer simula-tions. The common contact surface area between the wellbore andthe formation limits the flow of oil and gas into the wellbore.Production is roughly proportional to the reservoir area contacted.Horizontal wells have long holes drilled horizontally into the


rI

formation compared to shorter sections in vertical and directionalwells. The net result is that the wellbore and formation have alarger common open section, thus allowing larger volumes of oiland gas to be produced. The situation is analogous to drainingwater out of a water tank with a large diameter pipe compared toa small diameter pipe.

Reservoir flow mechanics define the flow of oil and gas in thereservoir. According to the radial flow theory, oil and gas flowradially inward toward vertical and directional wellbores. Thecross-sectional area available for flow decreases as oil and gasapproaches the verti~al wellbore. This increasing flow restrictionuses more reservoir energy toproduce a given amount ofoiland gas.

However, line81'-flow theory has more influence on flow intohorizontal holes, at least' near the wellbore and during the earlyproducing life. Flow mechanisms are complex and reservoir fluidshave a fixed amount ofenergy. In summary, higher energy require-ments restrict the flowrate from vertical and directional wellboresmore, compared to the lower energy usage and correspondinglylarger flowrates from horizontal wellbores. This more efficient useof energy also enhances total recovery from the well before itreaches the economic limit for production.

Horizontal drilling also improves productivity from low-perme-ability formations. Many formations contain oil and gas but pro-duce lowvolumes from vertical and directional wells because oflowpermeability. Horizontal wells have increased flowrates because ofthe -increased flow area and decreased reservoir energy require-ment as described. Therefore, many low-permeability formationsare noncommercial with vertical and horizontal drilling but pro-duce economic volumes of oil and gas from horizontal holes. Be-cause of their greater exposure to the producing zone, horizontalwells also may be more effectively hydraulically fractured (creatingmultiple fractures compared to a few fractures), which furtherincreases productivity (see Fig. 1-7).

Oil and gas often occur in thin formations. Small volumes ofoiland gas near the wellbore, sometimes combined with low-perme-ability, may further restrict flow rates. Long horizontal sectionsincrease flow rates as described for other situations.

There are many examples of increased productivity from hori-zontal holes. A horizontal well in the North Sea flowed 30,000BOPD, approximately 10 times the production rate of an averagevertical or directional well in the field. The Austin Chalk formationin southern Texas has many horizontal wells. The average for 15wells with various horizontal section lengths was 460 BOPD and


Figure1-7Horizontal wells and low permeability

A - Vertical weD,single hydraulic fractureB - Increased weDbore exposure to formationC - Multiplehydraulic fractures

260,000 cubic feet ofnatural gas per day (260 Mcfd). This is about3 to 5 times the amount of production from an average vertical ordirectional well.

VERTICALFRACTURESVertical, or highly tilted, natural fractures frequently contain oil

and gas. These may cover wide vertical areas and contain largevolumes. Sometimes oil and gas may flow slowly into the fracturesfrom adjacent low-permeability formations, effectively rechargingthe fractures. A vertical or directional well may penetrate onefracture but seldom more than two. Often several fractures must bepenetrated for the well to be economical. A horizontal well fre-quently penetrates several fractures (see Fig. 1-8). Steeply dippingproductive formations can be a comparable-situation.


A significant example of a field with high-angle or verticalfractures is the Pearsall field in south central Texas. An averagevertical well produces about 30,000 bbls during its lifetime. This isuneconomical. Somehorizontal wellshave already exceeded 100,000bbls. One well produced more than 100,000bbls in 16months, andthe projected ultimate recovery is 375,000 bbls. This suggestsrecoveries from horizontal completions will be at least 3 andpossibly 5 times that ofvertical wells. As a note ofcaution, there areolder vertical wells that would not be commercial even with theseincreases.

Analogous situations are isolated areas of high-permeabilitycontaining oil and gas. These include sand lenses and dune-typefeatures isolated within a dense or low-permeability formation (seeFig. 1-8). Vertical or directional wells commonly drill into only oneof these high-permeability areas, and the flow rate may not beeconomical. A horizontal well can drill through several of these toproduce at a higher and often economical rate. A well in the North

Figure1-8Otherhorizontal wellapplications

-

A - Multiplesand lensesB - Vertical dry hole

C - Thin zoneD - Fractured formation


Sea area drilled a 2,OOO-foothorizontal section and encounteredseveral gooddune-type features. Initial maximum production wasas much as 5 times higher than any other (vertical or directional)well in the field.

SAND PRODUCTION AND CONINGMost wells produce at a high flow rate with a resulting high

pressure drawdown. Horizontal wells have a larger section of thewellbore exposed to the formation. Therefore pressure drawdownis less fora given production rate in horizontal wells than in verticaland directional wells. This lessens production problems related topressure drawdown.

At higher drawdown pressures, sand production is a commonproblem, especially the production of unconsolidated and fine-grained sand. Sand erodes and plugs equipment and restricts theflow rate. Screens and gravel packing limit sand entry into thewellbore and in some cases reduce production rates. Less pressuredrawdown eliminates the need for screens and gravel packing andallows higher production rates.

Water coning problems can be reduced with less pressure draw-down. Water frequently underlies oil or gas in the reservoir. Wellscompleted in the oil and gas section may produce water by coning.High drawdown causes the water to flow upward, coning into theproductive section and thus being produced with the oil and gas(see Fig. 1-9). Water production often restricts the production ofoiland gas. Produced water must be disposed ofby approved methods,further increasing production cost.

Gas coning occurs in completions in which an oil zone has anoverlying cap ofnatural gas. High drawdowns cause the gas to flowdownward, coning into the oilsection and thus being produced withthe oil (see Fig. 1-9). It is preferable to leave the gas in place toconserve reservoir energy.

Horizontal wells allow higher production rates at correspond-ingly lower drawdown pressure as described. This reduces theproblem of water and gas coning. It is possible to restrict coningfurther by placing the horizontal lateral in the reservoir in theoptimum position relative to the water, oil, and gas contacts.

OTHER APPLICATIONSHorizontal drilling is highly applicable to existing cased vertical

and directional wells with larger diameter casing and under favor-able conditions. These wells are already drilled and cased, andreentering them will be a major application of horizontal drilling


\Figure 1-9011,gas, and water coning

. Oil":::::"3'-'-

'.0

...I':':::I':':':I':':':I':':~.. . 'Gas' . . . . . (;;\

: ~-_.: . . . . ~.. . 0' . . . -.- -:- :--;- -. - .-~I7/--;

. ~ . .Oi. . . .'1-":- - :...- - -' - ...:- .:..- '- -' - ..:- ...:..;... . .//\~ .. . . Water . -: ~ -.- -'0-" "" .' .: . ',,' A ~.

- - - - -11-_ - -.Water

A.Verticalwelwithconing B.Gas weD,no coningC . Oil well,no coning

due to the large number of existing wells and the lower generalcosts involved. Many of these wells are depleted, but the higherproduction from horizontal completions may justify reentry. Forexample, an abandoned producing well in the North Sea wasreentered, drilled horizontally, and completed, doubling produc-tion from the field.

New horizontal wells have been successful, so reentering anddrilling existing wells horizontally is expected to give similarresults. One such potential field is the Pearsall field in southcentral Texas, which has about 2,000 vertical wells and limitedfield development because of low productivity. A few of the manyother prospective areas include the Niobrara in the Denver Julesburgbasin in Wyoming and Colorado, the Cretaceous Mesaverde inUtah and Western Colorado, the Baken shale in the Williston basinin Montana, and the Sprayberry in West Texas.

Horizontal drilling has the potential in secondary, tertiary, andenhanced-recovery procedures to recover part ofthe remaining oil.Large sections exposed to the formation will increase gravity


drainage efficiency.Horizontal drilling should increase injectivity,improve sweep efficiencies, and reduce the number ofwells neededfor waterflooding and steam injection for recovering heavy oils. Itis especially applicable for improving flooding sweep efficiencies,which allows production of oil from isolated areas that werebypassed by flooding from vertical wells. There are very largereserves of heavy oil in the world. This process should be equallyapplicable in miscible, carbon dioxide, and inert gas floods andsome repressurization projects.

A modified form of horizontal drilling places pipelines under-neath areas where conventional methods cannot be used. Theselocations include roads, rivers, ship channels, and industrial areas(see Fig. 1-10).

Horizontal wells should be efficient at producing methane gasfrom shallow coal beds in the western United States. This alsowould serve a secondary purpose of reducing the mining cost ofdrafting to dilute the gaseous mixture in the mine to a safe workinglevel. Other industries benefit from horizontal drilling techniquesin different forms, such as the mining industry's use ofblast holes.

Combined directional and horizontal drilling may have otherapplications. These include reduced well spacing, in situ oil shaleretorting, coalgasification, in situ leaching in the mineral industry,and heating heavy oil and tar sands. The same general proceduresdiscussed here (and/or modified forms of drilling) apply.

DESIGNGUIDELINESIt is best to design directional and horizontal drilling programs

by preparing the optimal well path following the objectives of theprogram. Guidelines include various controls or limiting param-

Figure 1-10Pipeline river crossing

16 OVERVIEW, DESIGN GUIDELINES

eters based on equipment specifications and experience. Some-times guidelines require modification because ofhole and programrequirements. Normally this suggests a higher level of risk. It isbest to reduce the risk as much as possible by making the bestchoice of available factors to reduce risk.

DEFINITIONSVarious terms are summarized here for preliminary clarifica-

tion and are covered in more detail in the later text. The terms oiland gas are interchangeable for most purposes and drilling opera-tions for either oilor gas are similar. The words well and hole oftenare interchangeable. Hole generally refers specifically to the drilledhole or wellbore. Well refers to the hole or well after completion.Well is also a collective term referring to the entire rig, wellbore,and drilling site. The terms deviated and sidetracked often are usedinterchangeably, and the operations are similar (for differentreasons) as described in Chapter 3.

Well depth measured along the axis of the wellbore is themeasured depth (MD),equivalent to drilled depth. This is used fordrilling measurements, casing footage, and other measurements oflength along the wellbore. True vertical depth (TVD)is the verticaldistance between a point in the wellbore and the plane of thesurface (immediately above the point). Measured depth is alwaysequal to or greater than true vertical depth (see Fig. 1-11).

Drift or inclination is the angle between the line ofthe wellboreand a vertical line, with both lying in a vertical plane. The apex ofthe angle points upward, and the drift is the angle below theintersection of the wellbore and the vertical line.

Direction or course is the compass or azimuth direction of thehorizontal component of a line along the axis of the wellbore. Toolface is the horizontal component ofthe direction toward which thebit, other drill tool, or whipstock points. Bends are changes ofanglein the vertical plane, and turns are changes of angle in thehorizontal plane.

This text refers to holes that are either vertical, straight, curved,or a combination of these. Drillholes are seldom exactly vertical,perfectly straight, or precisely curved. Variances of a few degreesare common, the amount depending upon requirements of thespecific drilling project, the manner ofdrilling, and related factors.

GENERALTER.MSThe terms low- and high-angle refer to the drift angle. They are

not standardized in industry practice, and general usage is some-what vague. There is a natural division at a drift angle ofabout 60.


Figure 1-11Depths, angles, and departures

!'\wp:1h TVD

Drift~~ I~ angle MD~

TD

Drilling and operational techniques and problems differ signifi-cantly above and below this angle. Therefore, low angles are 60 orless and placed in the directional classification; higher angles areincluded with horizontal classifications. Asimilar definition prob-lem occurs in separating extended-reach and horizontal wells.Some operators contend that the drilling degree of difficulty isabout the same after inclinations of 70-80. Others have arbi-trarily separated high-angle directional and horizontal wells at 75of inclination. Most accept 80 as equivalent to a horizontal well.

Reference information can be very helpful. It is always impor-tant to obtain operational information and data from other wells inthe area, as well as to review well histories for reference design andoperational data. These include problems in building, holding, anddropping angle; performance of various assemblies; and drillingand formation problems. Other sources of information includeequipment suppliers, trade journals, and published literature. Theimportance ofresearching records and detailed planning cannot beoveremphasized.

It is important to simplify the design as much as possible.Directional and horizontal drilling equipment and procedures arewell established, but operations are not routine. They take longer

18 OVERVIEW, DESIGN GUIDELINES

to drill than vertical wells. Reasons for this include related andnecessary operations such as deviating, making correction runs,circulating, taking surveys, and extra tripping. Also, penetrationrates may be slower. These operations frequently take longer thanplanned. Extended operating time increases risk, and verticaldrilling problems increase in directional and horizontal drilling.Problems directly related to directional and horizontal drilling alsooccur.

Hydraulics must be calculated to ensure adequate mud pressureand volume to operate the turbine or motor and remove drillcuttings. Hole cleaning is a common problem in high-angle andhorizontal holes, so it is important to have adequate mud pressureand volume. Calculations should include hydrostatic pressure ofthe mud column and other pressures based on true vertical depthfor high-angle hole~: Measured depth commonly is sufficientlyaccurate in vertical and very low-angle directional wells. Theremay be appreciable differences between true vertical and mea-sured depth in directional wells, especially with higher angles.

Excess drag and torque can be a major problem (see Chapters 4and 5). Many directional and horizontal operations such as bendsand turns cause increased drag and torque, but they are necessary.It is useful to deviate as deep as possible to minimize the amountofdirectional hole causing torque and drag problems, and to designfor minimum changes of angle and smoothly curved sections.Vertical and straight, inclined sections should be drilled straight,while providing forcasing through sections that will cause the mostdrag and torque. Drillstrings should be designed with adequateoverpull, and the design must provide for casing wear. (Formulasfor calculating torque and drag are available and may be helpful.)Drilling and tripping cause accelerated wear, especially in bendsand turns, so consideration should be given to using heavierweights and higher grades of casing. Normal casing inspectionprocedures should be followed, and additional inspections may berequired in more complex patterns, especially when casing loadsare critical.

Regular rotary assemblies limit angle build to about 4/100 ftand angle drop to about 3/100 ft. Aggressive assemblies obtainhigher rates. Rotary assemblies are most efficient at angles be-tween 25 and 45. It is crucial not to design for rotary drilling ofstraight, inclined hole sections with drift angles less than 15,except for very short sections, because of the difficulty of anglecontrol. The design should use the minimum change of angle,usually in the order of 2.5/100 ft.

Absolute dogleg is the absolute change of angle in the combined


vertical and horizontal directions measured in deg/100 ft. It shouldbe limited to about 4/100 ft when possible. Higher changes in-crease the risk of keyseats and other hole problems. Lower buildrates allow tools such as packed-hole assemblies to pass withoutreaming. Reaming should be eliminated whenever possible; it is ahigh-risk operation, requiring additional time and increasing costs.Extended-reach and horizontal holes often change angle at higherrates with a correspondingly higher risk.

Hole diameters may be determined by the pattern type and, toa lesser extent, operator preference. Optimum hole size is 8 3/4 in.to 9 7/8 in. Acceptable sizes range from 6 3/4 in. to 121/4 in. Smallholes require smaller motors that are less reliable and efficient. Itis more difficult to deviate and drill larger holes, especially in veryhard, abrasive formations. It is important to design so that mostdrilling is in optimally sized holes.

Borehole stability may be a problem in the horizontal holesection, although it is not reported as a major problem in theliterature. Special tests and calculations aid in determining this.Sometimes heavier mud is used during drilling, and heavier weightcasing later. In practice, some rock movement may be permissiblewith good designs.

Pilot holes should be designed according to target formationdepths and other information. This may save drilling a more costlyhorizontal hole. Final course adjustments should be provided forwith tangent sections as described in the section on tangents laterin this chapter. If there is any question about its exact position, thesurface location should be surveyed again. Some reasons for thismight be a survey of questionable accuracy or inadvertent move-ment of the location stake while either building the location ormoving the rig onto the location.

Casing run in deviated holes is subject to bending and bucklingstresses similar to that described for drillpipe in Chapter 5. Thesecan cause a failure under severe conditions. There is less risk offailure in the casing collars because they are stronger than the pipebody. Still, the threaded section on collared casing may be a pointofweakness. Casing failures in these instances are uncommon butshould be considered when designing the program.

Casing sizes are generally the same as in vertical holes. Commonsizes include 7 in., 7 5/8 in., 9 5/8 in., 10 3/4 in., and 13 3/8 in.Intermediate and special sizes may be used for additional casings.The design engineer should consider placing a heavier casing in thedeviated section ofhigh-angle holes for additional wear protection,as well as placing additional centralizers through the deviated


sections as needed for good centralization during cementing.The design should allow for an extra string of casing for higher

risk wells drilled in hazardous areas, particularly in earlier wellswhere less information is available or known. Drillingproblems aredifficult to predict, especially for horizontal and high-angle, ex-tended-reach wells. The casing may be omitted if it is not needed.This procedure can save completing the well at a lesser depth beforetesting all objective ho.rizonsor trying to drill at greater depths ina smaller diameter hole with the resulting problems and higherrisks.

Formation evaluation is an important part of planning and. designinga wellprogram. Theformationsshouldbe evaluated ondirectional wells in the same manner as vertical wells, withallowances made for drift angles. It is important to plan and designcarefully for evaluation in high-angle and horizontal holes wheremore problems occur. Evaluation procedures differ as explained inChapter 5. The logging features ofmeasureme nt-while-drilling aregaining acceptance. Coring should be limited because of reduceddirectional control. Open hole formation testing also should belimited because ofthe high risk ofsticking. Mud logging is common;on most wells it is used to help in drilling, to support hole guidance,and to help in evaluating formations.

Completions should be planned and designed to optimize pro-duction rates. This includes considering the type of formation,reservoir pressure, drive mechanism, reserves, stimulation, pro-duction lift, long-term economics, and future remedial work.

RISKAND DEGREEOF DIFFICULTYDrilling operations have twobasic classes ofrisk. One is the risk

encountered during drilling and completing the well. The second isthe risk that oil and gas may not occur or volumes and flow rateswill be less than originally estimated. Both are equally important.They depend upon preliminary investigation, careful planning,and prudent operations. The well must be located where oiland gasoccur in economic quantities. Otherwise, the drilling operation is awasted cost despite operating efficiency.

Risks include excess drag and torque, the possibility ofstickingor keyseating, problems within the formations or with the casing,blowouts, and other drilling problems as described in Chapters 4and 5. Additional risks in directional and horizontal wells relate tothe number and radius ofbends and turns, inclination, length oftheinclined and horizontal hole section(s), wellbore stability, andoperator experience. Aswith many new procedures, mistakes have


beenmade in horizontal drilling, sometimes compoundedby therapid increase in its use and the lack of experience. Improvedequipment and techniques and additional experience will reducerisks and associated problems.

A blowout in Texas occurred when a well, while being drilledhorizontally, caught fire and destroyed the rig. Most, if not all,similar situations can be prevented with good safety equipmentand operating procedures. The severity and likelihood ofproblemsincrease with depth and higher angles. Risk is least for verticalpatterns, increases with directional patterns, and is highest forhorizontal drilling.

The risk of successfully drilling and completing the well relatesto the ''Degree of Difficulty." Higher risks are associated with ahigher degree of difficulty and result in higher costs. Table 1-1compares the degree ofdifficulty ofdrilling directional and horizon-tal wells, referenced to vertical wells.

Table 1-1Directional/Horizontal "Degree Of Difficulty."

PatternClassification

Degree ofDifficulty

Relative Cost(% greater thanvertical)

VERTICAL(reference)

DIRECTIONALSingle-bendDouble-bendComplexExtended-reachHigh-angleSlant

HORIZONTALShort RadiusMedium RadiusLong Radius

Low

LowLowto MediumMediumMedium to HighHighLowto Medium

HighMedium to HighHigh

0.0

+ 25+ 50+ 100+ 150+ 200+ 50

+ 200+ 150+ 200

The reference well is a vertical hole located in the same area asthe directional and horizontal wells. These are approximate andare listed only to give an order of :.nagnitude of risk. THESESHOULD NOT BE USED FOR ACTUAL ESTIMATES.


WELLCOSTS AND ECONOMICSWell cost and economics depend upon the specific project. Ap-

proximate costs of directional and horizontal wells relate to thedegree of difficulty as listed in Table 1-1. These are only a rule ofthumb covering a broad range. Actual costs depend upon thespecific project, pattern complexity, and various problems de-scribed in the section about risk. Experienced personnel can esti-mate reliably, but accuracy may decrease in higher risk operations.The operator should always consider drilling a vertical hole beforedrilling horizontally because of the higher costs (see Fig. 1-12).Operators experienced in horizontal drilling have cost reductionsof20%to 50% after drilling a few wells in an area, so experience inthe area is important.

Economics should be based on drilling and completion costs andwell productivity in the conventional manner. Special precautionsshould be taken when estimating productivity. Unquestionably,there have been some horizontal wells with high productivities.However, sometimes there can be a very high decline rate, so thatthe well is not economical in spite of its high initial rate. Anyproduction reports used for estimates should be verified. Carefulextrapolations of initial production for cumulative recovery calcu-

Figure 1-12Drillingrate comparisons

0-

lnIennediale \ lnIennediale~ caq '-. - caq Intenned81e+- C88ing1-

I~

*' 1 +- Vertical -7t r~ Dr~{-I I I..,.' I , ,

r- .~-.... I+-- HorIzontalI I , ,o ---+ 'line, daya --+

Rate-tine curves (Based on mea8ll'ed depths)

Con1>Ietlon

7\7!, I


lations should be made. It is important to evaluate these correctly,especially before drilling subsequent wells.

1

DESIGNING/CALCULATINGWELLPATTERNS

Well patterns are the various types and combinations of direc-tional and horizontal wells. Common directional patterns aresingle-bend, double-bend, extended-reach, and slant hole. Complexpatterns are the base pattern with one or more bends and turns andvarious changes of angle (see Fig. 1-13). Horizontal patterns areshort, medium, and long turn radius. The turn radius is the radiusofthe 90 curve (or turn) that changes the direction ofthe wellborefrom vertical to horizontal. These patterns are the most commonand considered here as standards. There are other, differenthorizontal patterns, primarily with different rates of curvature.Combination patterns merge directional and horizontal designs.Common combinations include adding a horizontal section at theend (bottom) of extended-reach and slant hole patterns.

Well patterns are illustrated on vertical and horizontal crosssections as a schematic representation of the wellbore. Compli-cated designs may use multiple sections for clarification. Theschematic illustrates the well path, an imaginary line along the

Figure 1-13Directional with horizontal and complex patterns

~wI1hhorizontal

~wI1hhorizontal

Slant hole willhorizontel

ComplexpattemewI1hbends and Iurn8


axesofthe wellbore.It includes the kickoffdepth, the courseandangle of the well path, the target and limits, boundary lines, andother relevant features. Normally all calculations are done withcomputers and schematics are printed or plotted. The well patternmust be designed carefully, paying attention to correct distancesand angles.

CLASSIFICATIONSThree basic well classifications are vertical, directional, and

horizontal. Well classifications depend upon the shape of thewellbore, the purpose for drilling the well, and the drilling proce-dure. Each well classification is subdivided into one or more typesor patterns, each serving a specific purpose. Well patterns alsoidentify the different types ofwells under the three classifications.Often the name ofthe pattern is the same as the name ofthe welltype. Vertical wells have a vertical wellbore drilled with standarddrilling tools. They represent a majority ofwells drilled and are notcovered specifically in this text.

Directional and horizontal wellbores are drilled along a plannedpath through the earth that cannot be drilled by vertical proce-dures. They are drilled progressively deeper, in any reasonabledirection, using special tools and techniques for changing thedirection of the wellbore one or more times. Horizontal holes startvertically, curve through a 90 turn, and then continue in thehorizontal direction.

Directional and horizontal wells mostly serve separate pur-poses. Like vertical wells, directional wells locate and produce oiland gas. Horizontal wells produce oil and gas at higher rates andincrease total recovery as compared to vertical and directionalwells. They also produce economical volumes of oil and gas fromsome formations that cannot be produced commercially by otherdrilling methods. The reasons are very significant and explain theacceptance and rapid advance of horizontal drilling.

DIMENSIONALREFERENCESDimensional references are the means of using various mea-

surements of distances and angles to illustrate the well pattern.They locate and define the position ofany part ofthe well includingreference depths, well paths, targets, limits, boundaries, and otherrelevant information. The same depth and point reference systemis used in both design and subsequent drilling operations. Duringthe design process, the well plan is plotted as a two-dimensionalschematic on the well plat (see Fig. 1-14). Horizontal and verticalcross-sectional views are displayed at convenient scales.


Figure 1-14Directional well plan

jo 500 \000 \500 2,000 2,500

I I" I 1 ,SIDEVIEW- plan-

Cr8od_ ~

o

\000

2,000

3,000-

4,000-

5,000-

&,000-

7,000-,o

T -+I 1 1 I I

500 \000\500 2,000 2,500

o 500 \000 \500 2,0002,500I I IlL

TOP VIf2N ..L.TF....~ 8Ioek-~

~

500 -

\000 -

\500 -

2,000-Leue Ine

///////2,500- 1

Not.. The number 01mea8Ir_tpoiIta have been recU:ed lorclarity.

The surface location and elevation must be located precisely byconventional surveying techniques, and ground level elevation isreferenced to mean sea level. This is the base reference point forlocating all other points in the wellbore. The top of the kelly drivebushing (KB), most often 1 ft above the level of the rotary, isnormally the reference point for all depth measurements. It fre-quently is necessary to convert depth measurements in the hole tosea level reference measurements. The kelly bushing elevation(KBE) is deducted from the depth measurement to obtain themeasurement relative to sea level, i.e., above sea level or sub sealevel. The measurement of the kelly drive bushing height aboveground level (usually 10-45 ft) is recorded for future reference afterthe rig moves. The top ofthe surface or first permanent casing headfrequently is set at ground level, or its elevation recorded as apermanent future depth reference.

The location of all points in the well are identified by depth andhorizontal position referenced to the KB or base reference pointunless specified otherwise. Depths are determined as measureddepth (MD) and true vertical depth (TVD) as previously defined.


Vertical section is the vertical distance in feet between two points,usually two consecutively surveys.

The horizontal position of a point is measured as rectangularcoordinates or departures referenced in horizontal distances fromthe KB. Coordinates are the shortest straight-line distances fromthe measured point to the nearest ofeither the north-south or east-west lines passing through the KB. These are referenced to truenorth, not magnetic north. For example, the horizontal identifica-tion of a point in the wellbore may be "350.25 ft N DEP and 480.62ft E DEP." The horizontal position ofthe point is 350.25 ft north and480.62 ft east of the KB. Closure is the nearest straight-linedistance from a point to the surface location measured in thehorizontal plane, or 594.70 ft in the example.

Closure and the direction of the line of closure also locate thehorizontal position ofa point. In the example, the point is identifiedby line and closure as "594.25 ft E 36 and 5' N." The point is at adistance of594.25 ft from the KB on a line that extends from the KBat an angle of 36 and 5' north of east. The same point could beidentified as "594.25 ft N 53 and 55' east" with the line firstreferenced from the north line. Bearing references are less com-mon. These are similar to the closure and line method except thatthe angle ofthe line is always measured in degrees clockwise fromtrue north.

The well path is a line along the axis ofthe wellbore. It representsa series ofpoints connected by lines. All points should be identifiedby depth and location referenced to the KB as described. Otherpoints similarly identified are the kickoff point, target, areas, andvolumes. Well path limits are the maximum allowable difference indistance between the well plan and the actual well path duringdrilling. Conventionally, a cylindrical shape along the well pathdefines well path limits. The radius ofthe cylinder is the maximumvariance (see Fig. 1-15).

The target is the drilling objective. A target in thin formations(about 15ft thick or less) is represented as a point. The target limitis a circle with the target point as the center and a radius equal tothe allowable variance. Thicker targets are delineated as lines withcylindrical shape limits similar to the well path limits. Two or moretargets are represented individually at their respective depths.

Hard lines identifY areas that cannot be drilled. Lease bound-aries and nonproductive areas such as fault blocks should beidentified as a line on the horizontal section that cannot be crossedby the drill bit. Acljacent wellbores also are identified with limitsbeyond which drilling should not occur.

OVERVIEW.DESIGNGUIDELINES 27

Figure 1-15Wellpath, target, and limits

Kelly bushing (KB)Base reference

tKickoff point

+-- Wellpath limits

Target pont

Target limits

Totaldepth(ID)

-tSingIe-bend

,.Double-bend

CALCULATIONSThe position ofthe wellbore at any point may be calculated using

formulas and measurements of angles and distances. The datapoints representing the well path, target, etc. should be set duringdesign, then the reference data calculated. Commonly, computerprograms are used to generate the well path and all other referencepoints and measurements based on guideline input data. Thecomputers also drive printers and plotters that print schematics ofthe wellbore. The basic procedure still includes calculating therequired parameters between twopoints by one ofseveral formulaslisted in Table1-2.


Table 1-2Course CalculatIon Methods.

Average AngleBalanced TangentialCallas's Helical ArcCircular ArcMercury

Minimum CurvatureRadius of CurvatureQuadraticTangential

The minimum curvature method is theoretically the most accu-rate and most commonly used. It is an involved procedure andnormally calculated with a computer.

The average angle method is easier to calculate and may be usedfor preliminary field calculations if a computer is unavailable. It isslightly less accurate by a few percentage points but is acceptablefor field work. A hand-held calculator or portable computer at thewell site can be used to make calculations that are plotted on a fieldcopy ofthe directional drilling design. This provides a comparisonofactual drilling results with the projected results, so changes canbe made immediately as required.

In the general procedure, calculations between two points aremade and recorded. The first point is the base reference point orthekelley drive bushing. The position (horizontal location and eleva-tion) ofthis point is known. Here the reference base point will havea vertical drift and a "zero," or no direction, used in the firstcalculation. Subsequent points will have both drift and direction asexplained in the following. After drilling the well deeper for somedistance, a new or final point is selected at some measured depthbelow the first point. The drift and direction of the hole at thissecond point is recorded. The drift and direction at both points andthe measured distance between them are used to calculate thechanges between the two points.

As shown in Figure 1-16, the calculated changes between thetwo points are vertical section, CB, departures, EC and DC, andclosure, AC. Each calculation gives the incremental change, eitheran increase or decrease, from the first point to the second or newpoint. The changes are added to the known depth and position datafrom the first point to give the depth and position of the secondpoint. This locates the newer point precisely in relation to the basereference point.

For example, the changes in departures give the coordinates ofthe last point. This last or new point then becomes the first point


Figure 1-16Calculating the well path

West D East,-

I

I

Down

(or temporary base reference point) for calculations after drilling toa new depth at the next "second or new point" and measuring driftand direction. The position of each succeeding point is calculatedsimilarly while drilling the well deeper.

All compass-type magnetic drift surveys or direction measure-ments reference to magnetic north. Well plats, land title schemat-ics, and other permanent records reference tothe geographical truenorth or true bearing as a universal standard. Therefore, magneticcompass measurements must be corrected from magnetic north totrue north so that the well plat will conform with surfa~e andrelated maps.

The direction and variation in degrees between true and mag-netic north depend upon the physical location of the point ofmeasurement, in this case the well site. Magnetic declinationcharts (isogonic charts) are area maps overlain with lines ofequalmagnetic declination. The correction is taken from these charts atthe measurement location (the well site). The correction is added or


subtracted from the magnetic compass reading based on magneticnorth to give the corrected direction referenced to true north.Sometimes these corrections are large, ranging from 0 to greaterthan 20%variance (=)from true north over the continental UnitedStates. Magnetic declination changes constantly. The change isvery small, but updated values must be used. Many companieshave magnetic declination values stored in their computers withprograms for correcting magnetic measurements. Note that gyro-scopic measurements may be referenced to true north, makingcorrection unnecessary.

Offshore wells in federal waters (outside ofstate waters) shouldbe corrected to Grid North. Localized areas are defined within agrid system that has specific latitude and longitude selected as thecorresponding Xand Y axes. Agrid correction is applied in order tocorrect magnetic directions. Wells in international waters mostlyuse the Universal Transverse Mercator (UTM) grid zone system,which covers broad areas referenced to meridian lines.

KICKOFF POINTThe kickoff point (KOP) is the depth or point in the hole where

deviating or sidetracking begins. Kickoffpoints should be selectedto provide an economical, drillable well path into the target.Standard criteria are used and modified subject to the well patternand any special requirements due to the drill site location.

The KOP should be selected as deep as reasonably possible.Vertical holes can be drilled faster and more economically withfewer problems compared to directional holes. The deeper KOP alsomay allow vertical clearance to sidetrack higher in the event thefirst deviated hole section is lost. Deviating at greater depths savesdrilled hole. Deeper kickoff points can alleviate other problemssuch as difficulties with hole cleaning and running logging tools,and casing and production problems after completing the well.

However, there are exceptions. It may be necessary to kickoff atshallower depths if the deeper kickoff point requires higher thannormal angles and if the section will be covered later by interme-diate casing. Kickoff at shallow depths can be accomplished byjetting or nudging (see Chapter 5) if the formations are very softand there is sufficient distance to the target.

The KOP should be at least 100 ft below the bottom of the lastcasing in the hole and preferably 200 ft or more, especially belowsurface or shallow intermediate casing. This reduces the risk ofexcess casing wear or splitting the casing shoe. The setting depthofthe casing may be adjusted ifnecessary when the KOP is critical;


the casing may be set higher, or the casing may be set in the holeafter deviating.

The KOP should be at least 50ft, preferably 150ft, above the topof a fish. Otherwise, the deviated hole may be drilled back into thefish or may reenter the original hole. Either will require a secondplugback and sidetrack. The KOP may be located closer to the fishin critical situations by using an assembly with a high-angle buildrate. This also increases the risk of a dogleg or crooked hole.

It is easier to deviate or sidetrack in some formations than inothers. Gathering information about the formations is one goodreason to review all available data on other wells in the area. Verysoft formations may increase the difficulty of deviating and build-ing angle. The deviating tool must exert a side force on theformation to cause the hole to deviate. Very soft formations may nothave sufficient strength to exert the required counterforce. There-fore, the fulcrum (orback side) ofthe directional assembly will pushinto and may partially enter the wall of the hole, providing insuf-ficient lateral thrust. This reduces efficiency, making it moredifficult to deviate or sidetrack, build angle at a satisfactory rate,and otherwise control the direction of the deviated hole in softerformations.

Very hard formations, especially abrasive formations, are diffi-cult to drill. Deviation assemblies are less rugged, sobit weight isreduced. This restricts operations, increasing the time spent devi-ating. It is important to avoid very soft, very hard, abrasive, orlaminated formations. The KOP should be selected in medium-softor medium drillability, massive formations when possible.

The horizontal position ofthe KOP must be known with reason-able accuracy. Normally, new holes have drift and direction mea-surements for calculating the KOP. Old holes with casing may nothave been surveyed, or surveyed only with a drift instrument. Agyroscopic wellbore survey should be run to determine a preciselocation. Sometimes a precise location may not be necessary,particularly with large targets. A "cone of uncertainty" often isacceptable in these cases. The horizontal displacement should becalculated for each drift survey. These should be totaled, ignoringdirection. The sum is equal to the radius ofthe cone ofuncertainty.It is the maximum possible displacement of the KOP from thesurface location, assuming accurate, representative, original mea-surements.

The exact displacement is unknown but is probably considerablyless because ofthe spiraling tendency during vertical drilling. Forexample, assume the circle ofuncertainty is 60 ft in diameter andthe allowable diameter ofthe target is 600 ft. In this situation, the

32 OVERVIEW,DESIGN GUIDELINES

direction ofthe deviated hole is controlled for drilling into a targetthat is 480 ft in diameter, a reasonable size of target in manypatterns. The diameter of the new target is equal to the allowabletarget diameter, reduced by twice the radius of the cone ofuncer-tainty. This can save the time and cost of running the wellboresurvey if the variance is acceptable. The procedure is especiallyapplicable to large targets and less difficult patterns. It also isacceptable to some state regulatory agencies.

TARGETThe target is the drilling objective. The size ofthe target is very

important from the viewpoint ofcost. Directional drilling technol-ogy has advanced to the point where a hole can be drilled into atarget a few feet in diameter. Drilling into the casing of a blowoutwell with a kill well is an example. However, small targets canincrease significantly both drilling time and total costs, so themaximum permissible target size is selected. A standard accept-able directional target is a circle 250 ft in diameter at 5,000 ft, 500ft diameter at 10,000 ft, etc. The maximum permissible target sizeis always used.

Targets may have elliptical or oblong shapes. When possible, asurface location or program design should be selected so that thelong dimension of the target is perpendicular to a horizontal linebetween the surface location and the target. This may reducecorrection runs with rotary assemblies, because it is easier tocontrol the angle than the direction. Directional wells on land oftenhave some flexibility in selecting the surface location. This shouldbe considered in order to improve the pattern. Geological informa-tion o1?tainedduring drilling may permit increasing the target size,or it may require decreasing the size ofthe target or moving it in amore favorable direction.

Targets for relief or kill wells range from a few feet to more thana 50 ft radius for an open hole condition. It even may be necessaryto penetrate the casing of a cased hole. A less common target is acylinder, usually oriented vertically. The standard cylindricaltarget preferably should have the same horizontal size as therecommended directional target. Horizontal hole targets are mostlyvertical, normally entered by drilling horizontally into a formation.Vertical control is critical, but there often is more latitude in thehorizontal direction.

Single targets are more common for directional and horizontalwells. It is possible for some directional wells to have multipletargets, but there are seldom more than two. These can be at


different depths and horizontal positions. Guidelines for singletargets apply to multiple targets as well.

DIRECTIONALDESIGNSDirectional well classifications are subdivided into standard

patterns includirig single-bend, double-bend, extended-reach andslant hole (see Fig. 1-17). Complex patterns have multiple bendsand turns. Each well pattern is for a specific purpose, so patternselection depends upon the reason for drilling the well. The wellpath should be designed by calculating the changes of angle andlength of the straight, inclined section required to connect thekickoff point to the target.

The process starts by selecting the minimum angle of build ordrop required to drill the hole into the target. Designs include bothdeviating and sidetracking, as described in Chapter 3. Holes withthese patterns are drilled in various sizes to measured depths ofgreater than 18,000 ft (shallower for more complex designs). Ifthere is a choice, the design for the most economical type ofassembly should be chosen. The difficulty of drilling directionalwells increases 'Yith increasing angle and depth. Complex patternswith higher angle build and drop rates and more turns and bendsare harder to drill. Directional and horizontal patterns can becombined for some drilling situations.

SINGLE-BENDSingle-bend patterns have a single bend in the vertical plane,

sometimes called bend-and-run. The pattern starts with a vertical

Figure 1-17Dlrecffonal patterns

Extended-reach Slant


hole. The next step is to deviate or sidetrack at the kickoffpoint anddrill a smooth, upward curve at an increasing angle. Normalangular build rates are 1.5-2.5/100 ft, with higher build rates inholes with higher angles. The curved section should be drilled to aninclination normally between 25 and 60. This drift angle ismaintained while drilling a straight, inclined hole into the target.The angle buildup and the drift angle of the straight, inclinedsections depend upon the vertical and horizontal distances be-tween the kickoff point and the target. Drilling this pattern issomewhat troublefree and is classified as a low level of drillingdifficulty. .

This pattern is commonly used to drill multiple wells from asingle surface location by placing the conductors close together. Itis also used for sidetracking and changing the bottomhole position,for reasons including: bypassing a deeper fish; moving the bottomof the hole updip to avoid water or downdip to avoid a gas cap; bycrossing faults; penetrating attic oil or gas or basement oil; andother similar situations. Relief (kill)wells are drilled also to controlblowouts. This pattern is used also to drill vertically throughproblem formations, followedby deviating with a higher angle at adeeper depth. The pattern also serves as a basis for extended-reachand horizontal well patterns.

DOUBLE-BENDDouble-bend (8) patterns have two bends in a vertical plane

separated by a straight, inclined section. First it is necessary todeviate from a vertical hole, and then drill the angle buildup andthe straight, inclined sections similarly to the single-bend pattern.The next step is to drop angle and drill a smooth curve in thedownward direction. The angle should be dropped at rates of 1.5-2.5 /100 ft, and then dropped to vertical. This is followedbydrillingvertically downward into the target forstandard patterns. It is bestto design for drilling with rotary assemblies when possible, espe-cially for the downward curving section.

A common variation has another change of angle in the lowersection for drilling a second straight, inclined hole section into thetarget. Changing the angle in the horizontal direction is alsocommon.

Angle-build and angle-drop rates and the drift and length ofthestraight, inclined sections should be designed based upon thehorizontal and vertical distances between the kickoff point andtarget(s). High torque and drag may limit depth in complex pat-terns with multiple bends and turns. This pattern has a moderate


to high level of drilling difficulty depending upon the number ofbends and turns.

The double-bend pattern is used for similar reasons but often inmore complex situations, usually related to the distance andrelative position of the kickoff point and target. Uses includedrilling multiple targets or long vertical targets, sidetracking ashallow fish,bypassing intervening obstacles such as otherwellboresand lease limitations, and penetrating updip or downdip reser-voirs. The double-bend is a common base pattern for more complexdesigns.

EXTENDED-REACHExtended, long-reach, patterns have one bend in the vertical

plane similar to the single-bend pattern. The main difference is alonger, straight, inclined section, often at a higher drift angle fordrilling into targets located long horizontal distances from thesurface location. The difference between single-bend and extended-reach patterns is not well defined. An arbitrary definition ofextended-reach is a horizontal separation between the surface andbottomhole location greater than 3,000-4,000 ft.

Extended-reach patterns should be designed similarly to single-bend patterns with allowances made for a longer straight, inclinedsection and higher angles. Extended-reach wells have been drilledto measured depths ofalmost 18,000 ft with horizontal, surface-to-target separations of more than 15,000 ft and at high angles(approaching 80). Torque and drag increase with depth and maylimit the total depth ofthe well, thus the pattern should be designedto alleviate the condition whenever possible. Extended-reach pat-terns are combined frequently with horizontal patterns, and inthese cases the design of the straight, inclined section often issimilar to horizontal laterals as described in the section on horizon-tal wells.

SLANTHOLESlant holes start from the surface at an angle of 30-45 by

drilling with a slant-hole rig. The surface or conductor casing is setat shallow depths, and the remainder ofthe hole is drilled straight,in an inclined direction. Alternately, it can be deviated to changethe direction from a few degrees to horizontal, sometimes a fewdegrees above horizontal. General design ofthe pattern and casingstrings is similar to other directional holes with allowances madefor the angles and tubular compression due to the pull-downsystem.

36 OVERVIEW. DESIGNGUIDELINES

Slant holes may have high drag, restrictingtubulars from fallingfreely due to gravity. Slant-hole rigs have a pull-down system (pulldown)forpushing the drillstringinto the hole during tripping whenit is needed. The pull down also helps deliver additional weight tothe bit fordrilling and is useful when running casing. The pull downcreates a downward force, so the drill tools and casing may be invarious states of compression. This must be provided for whendesigning the drill tools and casing.

Slant holes penetrate productive zones at shallow depths atrelatively long horizontal distances from surface locations. This issimilar to a specialized application ofextended-reach patterns andserves the same purpose. The shallow depth limits the horizontaldistance obtainable with conventional extended-reach patterns.Extended-reach wells require some vertical distance in order tochange the vertical direction of the hole. Slant holes start at anangle, so they drill longer horizontal distances into targets atshallow measured depths.

HORIZONTAL DESIGNSHorizontal designs are well plans with a section or lateral drilled

horizontally through the earth. Conventionally, these wells devi-ate at the kickoff point, drill through a 90 curve and then drillhorizontally into the formation. They may be drilled as new wellsor in older, cased holes, if the casing diameter is sufficiently large.Horizontal drilling is applicable in a wide range ofdepths and sandthickness. Measured depths of 10,000 ft are somewhat common,with some at depths greater than 14,000 ft. Horizontal lateralshave been drilled more than 2,500 ft into thin sands (less than 10to 15 ft thick) and nearly 2,000 ft into slightly thicker sands atdepths greater than 10,000 ft.

It is also possible to drill horizontally as an extension of adirectional pattern, including extended-reach and slant holes. Thesurface location ofthe directional well is selected and then drilledso that the bottom of the wellbore is near the desired target point.Then a curved section is drilled until the hole is horizontal, followedby drilling horizontally laterally into the formation. One high-angle extended-reach well had a total horizontal displacement ofnearly 13,000 ft, including 5,500 ft ofhorizontal hole. Another hada total horizontal displacement of more than 16,000 ft, includingmore than 1,500 ft of horizontal section. There are combinationwells in most major fields, and they are common in offshoreoperations.


The horizontal classification is subdivided into patterns basedon the length of the radius (turn) of the 900 curved angle-buildsection (see Fig. 1-18).

Table 1-3HorizontalPatternClassifications.

PatternName

TurnRadius,ft

BuildRate0/100ft

HorizontalExtension, ft

ShortMediumLong

2-60 1,000+-95300-800 19.1-7.21.000-3,000 5.7-1.2

100-8001,500-3,0002,000-5,000

Angular build rates are in degrees per 100ft ofmeasured depth.Horizontal classifications are not standardized in the industry.Table 1-3 contains a summarized average ofclassifications used byvarious operators and service companies. These are guidelineswithin a wide variation of angle-build rates. There are gapsbetween the pattern ranges in Table 1-3. It is more difficult to drillin the gap areas because of equipment limitations, and it isnaturally easier to drill within the pattern ranges. A few wells aredrilled outside ofthe pattern ranges, but most are drilled within theranges listed in Table 1-3.

The turn radius of about 300 ft is a natural division betweenshort- and medium-turn patterns for several reasons. It is aboutthe minimum turn radius that most standard tubulars can passthrough safely with careful handling. Most shorter-turn curvesrequire special articulated or smaller diameter tubulars. Standarddeviation tools cannot build angle at higher turn rates in a con-trolled manner. The ability to use standard tubulars and deviationequipment is important .for conducting efficient operations andcontrolling costs. The difference between medium- and long-turnpatterns is less well defined.

Design procedures for all horizontal hole classifications aresimilar. First it is necessary to evaluate the oil- and gas-bearingstrata carefully. The next step is to select the correct length for thehorizontal section and find the best position for the horizontalsection in the reservoir, including areal location, direction, anddepth relative to formation boundaries. The horizontal section isoften placed parallel to fluid interfaces and perpendicular to frac-tures. Sometimes the horizontal section is oriented based on


Figure 1-18Horizontal patterns

Longraciua Mediumradius

t~~ t~ depth --+

Short raciua

t~~-g~11 ~~ (b~~, ..,'~~ ~-_ft1~1,500-3, ,000It, -+\

--+ I

1+-2,000 - 5,000It~

+ I

analysis of fracture propagation. This ensures the most efficientfracturing at completion. The reasons for drilling the horizontalwell as described earlier in this chapter often determine theposition. These factors determine the true vertical depth to thehorizontal lateral and its length and position in the reservoirrelative to the surface location.

It is important to evaluate the advantages and disadvantages ofthe range ofturn radii and to select the one most applicable to thewell under consideration. The kickoff point is equal to the truevertical depth to the horizontal section less the length of the turnradius. The surface location should be positioned a distance equalto the turn radius from the point where the hole becomes horizon-tal. The hole size ofthe curved and horizontal sections is chosen foroptimum operations. The vertical hole normally is a standard sizelarger.

The design engineer should also provide for a tangent section(two in areas with less information and possibly with thin reser-voirs). The measured depth is calculated, and a cross-sectionaldiagram is drawn to scale. It is important to verify that drillingassemblies can drill the pattern efficiently. Plans should call for


drilling the curved section with a drilling assembly that buildsangle at the rate required by the pattern design. Preference shouldbe given to drilling withsteerable assemblies as often as possible.The design is completed by selecting the path and target limits,casing points, and completion procedures.

The procedure is modified slightly for drilling a horizontal holewhen reentering an old well with casing. The smallest casing sizedetermines the maximum size of the horizontal drill tools. Thismay limit the turn radius and the resulting length of the lateralhole section. A turn radius should be selected based on the size oftools available and the procedure for deviating through the casing.It is important to provide for plugging back and removing a sectionof casing (see Chapter 3). Then the design is completed as de-scribed.

The applicable turn radius is selected by evaluating variousfactors. A longer vertical section is easier to drill but requires ashorter turn radius for a given depth to the position ofthe horizon-tal section in the formation. It is more difficult to drill a shorter turnradius because of the higher angle-build rate as compared to alarger radius turn (seeFig. 1-19). Problems with hole cleaning andhigh drag and torque increase with increasing measured depth,such as for a longer turn radius. It is helpful to have a goodunderstanding of the design and use of bottomhole assemblies.

SHORT-TURNShort-turn patterns, sometimes called drainholes, are drilled in

existing, cased wellbores. They have a short turn radius of a fewfeet to about 60 ft and build angle at very high rates. Severalhorizontal holes may be drilled from the same wellbore. Theaverage maximum length ofthe laterals is about 300-700 ft in theoptimum case, but generally is considerably shorter.

Short turn radius patterns are less common, partially due toinherent disadvantages. The procedure requires milling a sectionof casing. Special pipe is required to drill the short turn radius.'Horizontallaterals are somewhat short and may be drilled withoutdirectional control. The short turn radius and the small hole sizelimit completion procedures. Special drilling equipment and proce-dures can be complicated.

The pattern is not applicable in all situations. Successful short-turn projects cost less than those with a larger turn radius but givesmaller increases in production. Smaller targets can be penetratedmore accurately because ofthe equipment used and the short turn


Figure1-19Buildupangle vs. turnradius and measured length of curvedsection

radius. The curve turns in a minimum horizontal distance, so thepattern is applicable in areas such as a small lease with limitedspace. The short curvature allows placement ofartificial lift pumpscloser to the reservoir. This increases production efficiency inreservoirs with lower pressure. It also may serve as a pilot programfor determining the applicability of drilling horizontally withlonger horizontal sections.

The design includes removal of a section of casing by milling.Articulated or small diameter drillpipe can be used to sidetrack offspecial whipstocks. Most drainhole equipment has a fixed builduprate, so the vertical depth to the horizontal lateral determines thekickoff point.


1,500- -2,4001,400- Long - 2,2001,300- radius -2,0001,200-

J-- 1,800 :=1,100 - .1,000- - 1,600==900-- 1,400 .!!ai 800- 4)- 1,200

700 - Medium -1000 1ii600 - radius Short ' >...c: -800 :::J500 -

< \radius ()

400 - -600 r::

\

...

300 - -400200 -100- -200

o - -0I I I I I I I I I I I I I I I I I01020 40 60 80 100 120 140 160

Buildup angle /100 ft

MEDIUM- TURNMedium-turn holes are the most common horizontal drilling

pattern, especially on land operations. They have a turn radius of300-800 ft, corresponding to angle-build rates of 19.1-7.2 /100 ftMD. Horizontal laterals average about 1,500-3,000 ft in lengthwith maximum penetrations of more than 4,000 ft. The pattern isvery flexible and applicable to most drilling conditions encoun-tered, including deeper holes, high pressures, and formation prob-lems. Horizontal sections have been drilled in cased wellboresbelow 14,600 ft TVDj two horizontal laterals, about 3,000 ft and2,000 ft long and about 180apart, were drilled below 7 in. casingfrom the same wellbore.

Most wells are drilled in open holes with diameters between 7in.and 9 in. Wells with a longer turn radius in the upper end of theclassification may have larger hole diameters of up to about 12 1/4 in. The shorter turn radius is used for sidetracking in cased holeswith larger casing, usually with diameters of 7 in. to 7 5/8 in. orlarger. Smaller hole sizes are selected for the shorter turn radiusand drilled with slim-hole tools and techniques. Drilling with splitdrilling assemblies reduces torque and drag, and increased bitweight is used in applicable situations. Steerable assemblies areused when possible, and measurement-while-drilling is used mostcommonly.

Sometimes information about the formation and precise depthsis unknown. A vertical hole can be drilled through the targethorizon(s) first for logging and evaluating the formations. Then, ifjustified, the vertical hole can be plugged-back, and the curved andhorizontal sections can be sidetracked and drilled. This can savethe high cost ofdrilling the horizontal section if the formations arenot productive. This is more commonly used for exploration wellsand forwells drilled along the edge ofa reservoir. Tangent sectionsmay be used as described in the section on tangents.

LONG- TURNLong-turn patterns have a turn radius of 1,000-3,000 ft, corre-

sponding to angle-build rates of 5.7-1.2 /100 ft MD. Horizontallaterals average about 2,000-5,000 feet in length with maximumpenetrations of more than 5,700 ft. This pattern is usual inhorizontal drilling, especially in offshore operations where longhorizontal displacements are common. The pattern is applicable inmost drilling conditions, including in deeper holes, under highpressures, and wherever formation problems occur. It seldom isused for reentering older cased holes because of larger hole sizes


and the possibility ofdeviation at shallower depths. The pattern iscommon offshore for drilling multiple wells from a single drillingplatform with long horizontal displacements. Designing thesepatterns may be more difficult because deeper holes increaseexposure to drilling problems. The most serious problems are highdrag and torque, and cleaning the hole efficiently.

Average wells in long turn radius patterns are generally deeperthan those of other patterns, so larger casing sizes and largediameter holes (up to 12 1/4 in.) are used. This helps minimizeproblems and improves well control. It allows the use of standardtool sizes in the deeper sections and provides for an extra string ofcasing ifunexpected drilling problems occur. Some deeper patternsrequire larger hole and casing sizes in the shallower deviated holesection. It may be preferable to drill a 12 1/4 in. deviated hole andopen it to the larger size depending upon drilling conditions anddepth. Deviating in larger size holes often is difficult, especially inharder formations. Long turn radius holes commonly have longerhorizontal displacements. Drillin

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Introduction to Directional and Horizontal Drilling