Rotary Drilling Components - Drill String-

Lecture 2

PET 356

Q 1 2015-2016

Abu Dhabi Polytechnic

Drill Pipe

• The bulk of the drill string ( Figure ) is made up primarily of lengths of steel drill pipe, which provides length to the drill string and transmits rotational energy to the drill bit

• Drill pipe consists of hollow steel tubes to which are welded tool joints: a threaded "pin" (usually at the bottom end) and a threaded "box" (usually on the upper end

Drill Pipe

Drill Pipe

• The American Petroleum Institute (API) establishes standards for pipe sizes, as well as for variations on the thread types and thicknesses of the box and pin "tool joints."

• Drillpipe is available in a variety of strengths and is generally supplied in "joints" that are about 30 ft (9.1 m) long and anywhere from 2 3/8 to 6 5/8 inches (6 to 17 cm) in diameter .

• Drill pipe should never be subjected to high torque or compressional forces, since it could easily "twist off". (The exception to this rule is horizontal drilling, where the pipe is run in compression in the lateral hole section. In situations like this, heavy weight pipe specifically designed for compressive service is used)

Bottomhole Assembly (BHA)

• Between the drill pipe and the drill bit is the section of the drill string kown as the Bottomhole Assembly (BHA), which consists mainly of drill collars—heavier and thicker than drill pipe, with larger outside diameters and smaller inside diameters, and with connections that are machined directly into the body of the pipe rather than welded.

Bottomhole Assembly (BHA)• The purposes of a BHA are as follows (Mitchell, 1995):

• to protect the drill pipe from excessive bending and torsional loads

• to assist in directional and wellbore deviation control

• to maintain vertical, straight well trajectories

• to reduce the severity of sudden changes in hole angle ("doglegs")

• to reduce the potential for wearing channels into the side of the wellbore ("keyseats")

• to ensure that casing can be run into the well

• to improve drill bit performance

• to reduce rig and drill string vibration

• to aid in well testing, workovers and other specialized procedures

Bottomhole Assembly (BHA)

• Drill collars are primarily designed and configured to prevent axial and torsional buckling of the drill pipe by holding it in tension ( Figure)

• Because of their greater diameter and thickness, they serve to maintain stiffness in the drill string and control wellbore deviation

Bottomhole Assembly (BHA)• In addition to drill collars, the BHA includes numerous other components, the selection and placement of which are based on:

• drilling conditions,

• formation characteristics and

• the desired wellbore size, geometry and trajectory

These include, but are not limited to the following:

• Heavy wall drill pipe is heavier, stronger and stiffer than regular drill pipe, but at the same time more flexible than drill collars. It may be run above the drill collars to provide a gradual change in stiffness between the drill collars and the drill pipe, thus reducing the possibility of fatigue failures.

• Stabilizers are short sections of pipe, or subs, designed to keep the drill collars centered at selected points in the wellbore and maintain a full-diameter ("full-gauge") hole

Bottomhole Assembly (BHA)• They can be used to ensure that the weight of the drill collars is concentrated on the bit

• reduce torque and bending stresses in the drill string;

• prevent wall-sticking or key-seating of the drill collars;

• build, drop or maintain hole angle in directional drilling;

• maintain constant bit direction in straight-hole drilling

• The number, type and placement of stabilizers in the BHA depends on local drilling conditions and well objectives. Not every tool is appropriate for every BHA

Bottomhole Assembly (BHA)• Reamers with cutting elements on their sides, can be run in hard or abrasive formations to ensure a full-diameter, in-gauge hole is maintained

• Crossover subs are designed to join drill string components that have connections of different sizes and/or thread configurations. The most universal example of a crossover sub is the bit sub, which is a very short sub with a "box" connection on both ends. This allows the pin end of the bit to be connected to the bit sub on bottom, and the pin end of a drill collar to be connected at the top

• Hydraulically or mechanically operated drilling jars may also be used

• A vibration dampener is a type

of shock absorber designed to

prevent vibrations generated by

the bit from traveling up the drill

string to the surface.

• Other specialized tools (e.g.,

downhole mud motors for

directional drilling, measurement-

while-drilling (MWD) and logging-

while-drilling (LWD) devices) may

be included as part of the BHA

depending on well requirements

Drill Bits• Drill bits used in the oil and gas industry range in diameter from 3 7/8 to 36 inches [98.4 to 914.4 mm], and come in a wide range of designs for various formation types and areas of application

• The earliest type of rotary drill bit was the drag bit or "fishtail" bit (Figure), which utilized flat cutter blades to scrape away at subsurface rock

• Most rotary drill bits used today fall into one of two major categories: rolling cutter bits and fixed cutter bits (some of which include variations on the old drag bit design)

Drill Bits: Rolling Cutter Bits

• Also known as a roller cone bits or rock bits, rolling cutter bits come in a variety of designs (Figure)

• The cones on this type of bit (usually three cones, but sometimes two) are designed to individually rotate on bearings about their own axis as the drill string turns the body of the bit

• The toothed cones "mesh" together to provide a self-cleaning action, which is enhanced by the directed high-velocity flow of drilling mud through the bit nozzles

Drill Bits: Rolling Cutter Bits

• Rolling cutter bits vary according to size and geometry, the type and configuration of their cutting elements, and the types of bearings used to join the bit body and cones

• Bit geometry is constrained by the size of the borehole and the type of cutting action desired. Cones may be aligned at an angle, for example, or offset from the centerline of the bit in order to attain the best possible cutting action for a given formation type

Drill Bits: Rolling Cutter Bits

• Rolling cutter bits may be defined in terms of two basic types of cutting elements:

• Milled-tooth bits (Figure) employ steel tooth cutters that are fabricated as integral parts of the bit cones

• In general, long, widely spaced teeth are characteristic of soft formation bits, while shorter, more closely spaced teeth used in harder formations

Drill Bits: Rolling Cutter Bits • Insert bits (Figure) employ cutting elements that are pressed into specially machined holes in the bit cones

• The tungstencarbide inserts come in different shapes, and the same general rule applies as for milled teeth: long-extension, chisel-shaped inserts for soft formations, and shortextension, rounded "button" inserts for hard formations

• Bearings may be non-sealed, where the drilling fluid lubricates the rollers, or sealed, where a rubber seal isolates a high performance lubricant within the cones

• In the case of journal bearings, a precision machined hard metal alloy surface replaces the cylindrical roller bearings. Journal bearings are designed to be stronger and more wear-resistant, for maximum service life

Drill Bits: Fixed Cutter Bits • Fixed cutter bits (Figure) are similar to drag bits in that they have no moving parts such as cones or bearings. Rather, they consist of stationary cutting elements that are integral with the body of the bit and are rotated directly by the turning of the drill string.

• The bit body is fabricated from steel or from a tungsten-carbide alloy, and the cutting elements consist of industrial grade polycrystalline diamond cutters (PDC), which are inserted into the bit, or natural or synthetic diamonds which are impregnated in the bit (Smith International Incorporated, 2005)

• The cutter patterns and spacing are “optimally designed” for the expected drilling conditions

Drill Bits: Fixed Cutter Bits • Along with their body materials and the types and geometries of their cutting elements, fixed cutter bits vary in their shape or profile

• In general, bits designed for soft formations have longer, more parabolic, sharper-nosed profiles than harder-formation bits, which tend to have more compressed, wider profiles

Drill Bits: Related Tools • Other bit-type tools have been designed for special purposes, notably hole openers and underreamers

• These tools are run above a bit to maintain or enlarge the hole size. Under reamers have collapsible arms that are held open by the pressure of mud circulating through the drillstem. These arms enable them to enlarge the bottom of the hole

• Whatever type is used, all bits perform their job with the help of the drilling fluid, which cools the bit and circulates rock chips from underneath

• Drilling fluid technology has become increasingly sophisticated and deserves treatment as a separate element of the drilling system