SPE 147100

The Practice and Evolution of Torque and Drag Reduction: Theory and Field Results John E. McCormick, SPE, Weatherford International; TzuFang Chiu, SPE, The University of Texas at Austin

Copyright 2011, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Denver, Colorado, USA, 30 October–2 November 2011. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

INTRODUCTION

With the global trend moving toward drilling deep water wells, ultra-extended reach wells (u-ERW) and

complex geometry wells, we can no longer ignore the drilling limitations caused by high torque and drag

forces. Extreme torque and drag, especially unplanned, can be detrimental to drilling operations. Over

the years, engineers have developed numerous ways to challenge the drilling limitations by reducing

torque and drag forces in order to drill further and deeper. This paper attempts to describe the practices

and the evolution of torque and drag reduction methods. The different torque and drag reduction

methods that this paper focuses on are listed below.

1. Wellpath design

2. Light weight string components

3. Lubricants

4. Hole cleaning

5. Co-polymer beads

6. Mechanical friction reduction tools

7. Increased drill string and rig capability

THEORY OF T&D

In highly inclined wells, solutions to problematic torque and drag are essential in order to complete the

drilling and completion operations. Because many restrictions are imposed by the drilling rig, top drive,

wellpath and drilling components, engineers have came up with ways to reduce torque and drag while

drilling. There are weight limitations that a rig can provide, especially in non-rotating operations such as

slide drilling. If the compressive forces in the drill string are too high, bucking will occur. In an

extended reach well (ERW), two common problems with torque are the friction resistance to drill string

rotation and the make-up torque limitations. If the rotary torque is too high, torsional failure would

result. Therefore, it is essential for engineers to accurately account for the torque and drag forces and

attempt to reduce them in order to prohibit these scenarios from occurring. At the same time, it is the

engineer’s responsibility to ensure that we do not over-design to accommodate unnecessary needs.

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METHODS OF T&D REDUCTION

Engineers have developed methods that improved over the years to mitigate torque and drag forces.

Pratically, there is no way for torque and drag to be completely eliminated because there is no such thing

as a frictionless surface. The parameters included in the torque and drag equations are shown below.

Drag is directly proportional to the normal force, coefficient of friction and tubular movement

(Equation 1). Torque is directly proportional to the normal force, coefficient of friction, radius of the

object that is rotating, and tubular movement (Equation 2). Reducing any of the components in the

equation will ultimately reduce the torque or drag. This, in return, allows more efficient weight transfer

and rotational ability.

(Eq. 1)

(Eq. 2)

In this paper, we focus on the different T&D reduction categories shown below and their evolution over

time.

1. Reducing the normal forces

2. Reducing the coefficient of friction

3. Increasing dynamic vs. static conditions

4. Increasing system capabilities

The following paragraphs explain different categories of torque and drag reduction and methods that fall

in each category.

Reducing the Normal Forces

Normal force is an opposing force to the side load against the borehole in the perpendicular direction.

Theoretically, the normal force in the vertical section of the borehole should be close to zero assuming

the tubulars are centered in the hole and no contact is made between the hole and the tubular. However,

no tubular is ever truly centered in the hole, even with stabilizers applied. Sections of the string will

touch the borehole, therefore though the normal force or the side load in the vertical section can be

small, it will never be zero. Figures 1 & 2 show the different sections of the tubular string and the

associate side load.

Minimizing the normal force will result in less drag and more efficient weight transfer. Methods that

reduce the normal force include, but are not limited to, carefully designing a wellpath and using lower

weight string components, such as aluminum drill pipe (ADP) in the lateral section of the well.

SPE 147100 3

Fig 1. Tubular String Sections

Side Force/length (lbf/length)0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Me

asu

red

De

pth

(ft)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

L E G E N D

Rotate On Bottom

Fig 2. Typical Side Force Distribution Trend for Each Hole Section

Wellpath Design

Wellpath design can determine the success or failure of a well. Wellpaths are designed for avoidance, to

meet specific drilling requirements, or to reach the target formation. It is easy to change wellpath at the

planning stage, yet extremely hard to modify once the drilling operation has begun. To conduct an ERW

drilling operation without an optimized wellpath is like “building a house on a bad foundation” (K&M).

After all, the wellpath design is one of the best ways to reduce torque and drag forces.

Vertical (<5° Inclination)

Build

Lateral (~90° Inclination)

FN Vertical

FN Build FN Lateral

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Shallow ER wellpaths are especially difficult because of the lack of string weight available in the short

vertical section to combat the large amounts of torque and drag encountered in the long deviated section.

Despite its importance, it has often been given little or no acknowledgment, and generally the goal of

designing a wellpath is to optimize the drilling speed to as fast as is reasonably possible. Many ERWs

could not have been completed if design flaws or rig limitations were present. Figure 3 shows a section

view of a typical ERW.

Fig 3. Typical ERW Wellpath Section View

There are several common wellpaths:

1. Vertical Wellpath

2. S-Shaped Wellpath

3. Complex Wellpath

4. Caternery Wellpath

Wellpaths can be designed for forces to be more evenly distributed through the wellbore rather than

concentrated in a specific section. With ERWs becoming progressively more common, wellpath design

has shown its potential to reduce frictional forces. The dogleg serverity of an engineered wellpath can be

lowed tremendously, reducing operational risk and rig time.

Vertical wellpaths are known to be the easiest to drill, however, many of the reservoirs that use vertical

wells have already been tapped. The S-shape wellpath has proved itself to be the best design option for

many challenging wells. Once the drilling operation has reached the drop section, the design of the

wellpath allows more weight to be slacked off to reach TD. Careful monitoring of torque and drag is

required for this kind of wellpath. The complex wellpath has become more common over the years

especially when drilling an ERW. The lateral horizontal section is orientated to be a specific alignment

to enhance the production in a reservoir. Some good examples of complex wellpath are the “L” shape

well that has a rapid build over a short interval. They are prevalent in the U.S shale gas plays.

The caternery wellpath is especially beneficial to reduce the torque, as its strength is to start the build

section in advance with a lower dogleg severity in the build. The drag forces may increase as the length

of the deviated section will be longer. Changing a commonly drilled “L” shape wellpath from a high

dogleg serverity well profile to a relatively lower dogleg serverity yields lower torque and drag. A series

SPE 147100 5

of torque and drag models were developed on a typical “L” shape well, as detailed below and shown in

figure 4.

High normal force and side load caused extensive wear on the casing and drill pipe, resulted in buckling,

and caused other operational problems. The dogleg serverity (DLS) for the original wellpath was

12°/100ft. A new build section was designed and modeled with the DLS lowered from 12°/100ft to

about 6°/100ft (Figures 5 & 6). The side force decreased significantly (Figures 7 & 8), and the

available weight to TD increased by up to 30,000 lbf. This is a tremendous improvement showing the

potential for a well-designed wellpath to positively impact well operations.

Fig 4. Original Wellpath for an ERW

Fig 5 & 6. Original Doglegs Serverity vs. Planned Doglegs Serverity

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Fig 7. Original Wellpath

Fig 8. Planned Wellpath

Low Weight String Components: Aluminum Drill Pipe (ADP)

The weight of the string components can dramatically alter the torque and drag forces in a well.

Selecting a lower weight pipe to use in the lateral section decreases the normal forces that are generated,

which affects both torque and drag. Light weight components are mostly beneficial in the lateral section

of the well after the build section to decrease string weight and make it easier to push to TD.

SPE 147100 7

Over the years, engineers have often considered tubular designs for ERWs using titanium, aluminum,

and thin-walled, high-strength steel. Due to cost and/or technical considerations, these options have not

often been used.

This paper will focus on ADP as an example of a low weight solution to problematic torque and drag.

Aluminum alloys have an average density of 2800 kg/m3, which is about 2.8 times less than the density

of steel --7850 kg/m3. In most cases, ADP is more than 2 times lighter than steel drill pipe (SDP) of

similar size. Therefore, the weight that is used to calculate the normal force is essentially 2.8 times less.

With less normal force, the drag force contributed by the drill string at any point in the wellpath is much

less. This allows for more weight to translate to TD.

In the 21st century, SDP has gained a more prominent role in Russia and countries of the former Soviet

Union. Still, ADP remains widely used at present time. In 2007, more than 27,000 joints of ADP were

manufactured and shipped to drilling companies. The design of ADP was first applied for drilling of the

Kola SG-3 well. In 1983, this well became the deepest well in the world, with TVD 12,262m, and still

holds this record. Drilling of such a deep well became possible only because of the application of a

lightweight drill string, which consisted of more than 90% ADP.

ERWs are a promising application for light weight string components, where they can show excellent

results in reduction of T&D and enable the drilling of longer deviated sections. To date, there is not

enough data on application of light weight components in ERWs. The majority of drilling with

aluminum is done in Russia, where tendency to ERD became popular only in recent years; titanium is

very expensive, and 160 ksi strength steels have only recently made thin walled steel pipe an option.

Reasonable price, easy handling, less expensive transportation and corrosion resistance to H2S and CO2

are advantages of ADP which make it an excellent torque and drag reducing option for complicated

applications.

A well drilled in the South China Sea is presented in figure 9; high torque and drag was expected from

the well as well as the rate of penetration slowed due to issues with transmitting power to the bit. One of

the solutions tested was placing ADP in the lateral section. ADP reduced the torque and gave the well

40% time reduction as compared to the actual estimated time required for drilling the same well with

SDP.

Fig 9. Tubular Placement for an ERW in South China Sea

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Reducing the Coefficient of Friction

Out of the three categories of torque and drag reduction methods, most of the attempts in the oil industry

have focused on reducing the coefficient of friction to lower torque and drag. The coefficient of friction

is a measure of the degree of resistance to motion of two elements sliding against each other. In a

downhole operation, the coefficient of friction refers to the metal to metal or metal to rock contact

between the drill string and the casing or open hole, respectively. For a standard drilling operation, the

cased hole coefficient of friction with a water based mud may be 0.25, while the open hole coefficient of

friction may be 0.30. Depending on the condition of the well, the formation, the penetration rate, and the

mud selection, the friction factor can be changed at the discrepancy of the user. Even a 10% reduction in

friction factor could significantly increase the chance for a driller to reach TD. Below are some of the

ways the oil industry has historically lowered the coefficient of friction.

Lubricants

Lubricants are additives to the mud system that have long been used to reduce torque and drag forces. In

the oil industry, lubricants are commonly used with the drilling mud that cools the bit and lubricates the

string. Over the years, many different additives have been tried and tested to optimize the lubrication by

maximizing the coefficient of friction reduction. Lubricants can reduce the coefficient of friction up to

30% to 40%. However, lab tests that show an 80% reduction in the coefficient of friction often result in

a reduction of torque or drag of around 5% to 15% in the field.

There are three main categories of drilling lubricants additives, and those are classified as water-based

lubricant, oil-based lubricant, and synthetic based lubricant. Depending on the drilling fluid type,

specific lubricant additive should be added to achieve optimum result. Oil-based lubricant and synthetic

based lubricant increase the potential for environmental concern given their potential impact if not

properly contained. Also, they do not work as well in open hole.

Examples of lubricants include bentonite, graphite, asphalt, diesel crude oil, and fine mica. Most modern

lubricants are a type of surfactant used for water-based mud; oil-based mud contains hydrocarbon oil

that naturally plays a role as a lubricant. Most lubricants blend fatty acid esters which make them

unstable at very low or high pH values, dissociate at high temperature and saponify in the presence of

high calcium or magnesium content.

Lubricants for water-based mud are more environmentally acceptable additives. They have the

capability to reduce the coefficient of friction between steel and sandstone. The performance of

lubricants can be fairly accurately described by the change in wettability that accompanies lubricant

adsorption on the contact surface.

Water-Based Lubricant

Water-based lubricants can be mixed with clay and other chemicals to create homogeneous mud slurry.

A combination of clay and mud can suspend cuttings, which allows the cutting to be carried to the

surface easily. Many chemicals are added to a water based mud system to achieve many purposes, such

as increasing shale stability, enhancing the rate of penetration and lubricating the drill string. Some of

the most common additives to enhance lubrication in the water based mud system include bentonite and

asphalt.

Oil-Based Lubricant

Oil-based lubricants are used to apply to an oil-based mud system that is composed of liquid

hydrocarbons. Most often they contain No.2 diesel. They are usually preferred over water-based

lubricants because they have a better lubricity and can enhance cleaning with less viscosity. In terms of

friction, it can withstand greater heat without breaking down. Also, it is preferred over water-based

SPE 147100 9

lubricants in terms of drilling salt, anhydrite, carnallite, potash, or shale formations as well as corrosion

and drilling direction or slim hole. However, these tend to be more costly and less environmentally

friendly than their water-based counterparts.

Synthetic-Based Lubricant

One of the most well known synthetic based lubricants composes ester based organic compounds. Only

the neutral, metal free, phosphate esters, or orthophosphoric acid are considered synthetic based

lubricants. It is mainly added to the drilling muds on offshore oil drilling operations and has many

advantages over the oil based lubricants such as producing a faster drilling operation, being more

environmental friendly, and being more cost effective.

UltraLube II

A recent lubricant addition is the development of UltraLube II, which shows a lab result of 90%

reduction in the coefficient of friction. Rather than having to saturate the entire system, the lubricant has

a strong affinity for metal surfaces and adheres to the string, enabling lower concentrations to have a

significant effect. The lubricant forms a coating on the tool joints, lowering the coefficient of friction at

the contact surfaces, until it has been mechanically removed. It works in both water based drilling fluid

and oil based drilling fluid. It is insoluble with water and it experiences no reduction of permeability

when applied.

Hole Cleaning

Hole cleaning is one of the main concerns that engineers have encountered when drilling an ERW.

Unwanted cuttings in the hole reduce the potential for a drill string to rotate and to move. It can also

increase the rate of wear of downhole tools. In an ERW, cuttings are most likely to accumulate up in the

high angle build section. This makes the removal of the cuttings from the hole extremely difficult.

Therefore, to ensure good hole cleaning, pipe rotation is a very important parameter to consider.

Increasing the pipe rotation often increases the ability to bring the cuttings out of the hole. Increasing the

mud flow rate allows the cutting to be suspended and carried upward. Sweeps, also known as high

density pills, are also used to attempt to reduce cuttings beds.

The oil industry has developed mechanical tools that can maximize hole cleaning in the deviated section

of the well while drilling by scooping cuttings up into the high flow area of highly deviated hole

sections. Proper hole cleaning can eliminate problems with cutting build up and remediate excessive

torque and drag in complex wells. The primary benefits of the mechanical hole cleaning tools are the

increase in cleaning efficiency, time saving, operational safety and the increase in wellbore quality and

stability. The tool has specially designed grooves in the tubular that throw the cuttings upward when

rotating the drill string. The grooved upset scoops the cuttings and lifts them upward. It increases the

recirculation of cuttings, resulting in more efficient cuttings removal. The tools claim up to a 60%

improvement in hole cleaning efficiency and 30% torque and drag reduction, as well as a reduction of

wiper trips.

Co-Polymer Beads

Using beads is a mechanical way to reduce the coefficient of friction. Co-polymer beads are inserted

between the drill string and bore hole to reduce the coefficient of friction. It does not affect the chemical

characteristics of the mud system. The size of the beads ranges from fine grade to coarse grade classes.

Instead of having metal to metal contact during the drilling operation, the drill string slides down the

bore hole, rolling on the beads.

Co-polymer beads are mainly applied to the build section of the bore hole where most of the forces

accumulate. The method is to apply beads from the bit to the build section while tripping in the pipe.

10 SPE 147100

The beads slowly fall into the build section as the pipe is tripping in and ultimately reduce the torque

and drag forces. The friction reducing impact can be maximized by transporting the beads right down to

the build section of the drill string. As the beads travel down the annular section between the drill string

and the bore hole, they will settle on the low side of the annulus and work as a wall-cake for the drill

string and eventually the beads will position themselves to optimize the rolling ability.

One major concern using beads is the severe solid build up if the beads are not removed properly. Thus,

beads must be recovered properly and recycled. This can be done by circulating drilling fluid to the

surface.

Mechanical Friction Reduction Tool

Another method to lower the coefficient of friction is to apply mechanical friction reduction tools

(MFRTs). When drilling an ERW, engineers tend to prefer lubricants over MFRT due to a preference

not to have additional tools in the hole unless it is necessary. Studies have shown that MFRTs

consistently provide better torque and drag reduction than lubricants, but they are generally also more

expensive. Three mechanical friction reduction tools: sub-based roller tools, pressure pulse tools and

swivels will be discussed in this and the next section.

Sub-Based Roller Tools

The roller tools are sub-based with a mandrel that rotates inside a non-rotating casting. By raising the

tool joints of the string off the wall of the hole, the internal low friction surfaces reduce torque and drag

in the areas of high normal force within the wellbore. In addition, the MFRT sub also reduces the chance

of differential sticking when used in open hole because it has a smaller contact area against the borehole

compared to tool joints. These subs are optimal when placed in high drag and high torque locations,

often in the build section, though they can be placed in both open or cased hole.

The laboratory studies show an 80% reduction in the coefficient of friction, which translates to a 50%

reduction in the local friction factor downhole. Field results are primarily wellpath dependent, averaging

between 10-35%, and can be accurately predicted using T&D modeling software.

Increasing Dynamic vs. Static Friction Conditions

Dynamic friction is lower than static friction. A number of methods for increasing dynamic friction

conditions vs. static friction conditions are used, including rotating pipe while tripping in.

Pressure Pulse Friction Reducing Tool (PPFRT)

A PPFRT is a type of MFRT that introduces vibration into the string to break static friction. It oscillates,

creating axial (lateral) movement in the string. An agitator can significantly improve the weight transfer

to TD and reduce the friction during a downhole operation, as well as improve ROP and assist with anti-

stalling for rotary steerable systems and motors. PPFRTs are especially beneficial in non-rotating

scenarios, such as slide drilling. PPFRTs vibrate and excite the drill string to decrease friction. This in

turn improves the rate of penetration by providing more efficient power transfer to the bit.

PPFRTs employ one of the earliest T&D reduction methods, which dynamically excites and vibrates to

reduce friction. There are three components that make up an agitator: the power section, the pulsing

system and the oscillating system. When fluid is pumped into the string through the power section, the

rotor rotates in the stator and creates a flow path that allows the toator valve assembly to move in a

linear motion. Rather than just rotating the tubular, a PPFRT allows for axial motion as well in order to

break the friction caused by the metal to metal contact area.

SPE 147100 11

Swivel

Swivels are used in highly deviated wells to run completions or liners. The swivel allows the pipe above

the liner or screen to rotate independently when tripping into the hole. The free-to-rotate upper mandrel

allows the tool to reduce the torque and drag forces which ultimately enables more weight to translate

downhole. The swivel can be activated by applying differential pressure to lock the two mandrels of the

tool together and allows torque to transmit through the tool. It can significantly reduce drag associated

with running strings.

Increasing System Capabilities

Another significant impact on the extension of the drilling envelope has been an increase in the size of

rigs and the strength of string components. Rather than attempting to reduce the friction factor, rigs and

string components have increased in size and strength to withstand more demanding conditions. With

more challenging wells, increased torque and drag have often been met by increasing the system

capabilities rather than attempting to reduce friction during well operations.

Tubular Grade

S-135 strength pipe has been a standard for use in the oilfield for many years, yet recent innovations

include the development of 150 ksi and higher strength drill pipe. X and G pipe with 75 ksi to 105 ksi

yields have been used for sour service operations to the present day, but may soon be replaced with sour

service drill pipe that has a minimum yield above 120 ksi.

Rotary Shoulder Connections

Different kinds of rotary shoulder connection advancements have been made to ensure that the tubular

connections can withstand high torque and drag scenarios. As more ERWs are being drilled, it is

necessary to improve upon the conventional connections that the API verified in the 1960s. One large

innovation was the development of double-shouldered connections, which have enabled drill strings to

handle much larger amounts of torque. Other innovations include increasing the taper length, thread

revolution, and thickening the double shoulder connections so the connections can allow for higher

torsional limitation.

Rig Capabilities

Larger, more powerful rigs are becoming more common in the oilfield. Top drives have moved in many

areas from strictly offshore rigs to include land rigs. The most challenging wells in today’s oilfield

utilize the largest, most powerful rigs in existence, with hookload and surface torque capacities that far

exceed those of ten years earlier.

CONCLUSION

Advancements in friction reduction methods are being made constantly in an effort to achieve the

industry’s consistently rising expectations on expanding the drilling envelope. Planning torque and drag

allows engineers to find ways to reduce torque and drag before spudding of their wells. With the rise of

ERW and other complex projects, engineers strive to find ways to drill deeper and longer. Many T&D

reduction methods have been discussed in the paper, and are listed again below. Depending on various

drilling parameters, the effectiveness of each of these methods can vary.

Wellpath design

Lubricants

Light weight string components

Hole cleaning

Co-polymer beads

12 SPE 147100

Mechanical friction reduction tools

Increased drill string and rig capability

The future of the oil and gas business resides in drilling and completing wells in challenging

environments. Finding ways to reduce torque and drag is a significant effort in solving the challenges in

sustaining the global energy demand.

ACKNOWLEDGEMENT

The authors would like thank Weatherford International for their support and encouragement to publish

this paper. We would also like to thank VAM Drilling and Aquatic for the technical expertise provided

in regards to hole cleaning and ADP respectively.

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