Managing torque and drag

SFM-TD-96 1 Schlumberger Sedco Forex MANAGING TORQUE AND DRAG... SFM-TD-96 5 Schlumberger Sedco Forex Torque and Drag Torque Losses Drag Losses... Both torque and drag losses are caused

This presentation has been prepared to give you a betterunderstanding of the different parameters involved in managing torqueand drag Recommendations presented here should be applied withinoperational needs.

We want to disseminate this knowledge because great amounts ofmoney and time are spent, every year, combating torque and dragproblems One of them, stuck pipe, is an expensive, time-consuming andalways undesirable event

You will see how torque and drag levels can be monitored andmeasures implemented to effectively control them We will go throughthis process beginning with the design of the well, followed by themonitoring of torque and drag and their management

By the end of this presentation you should have a good idea about how

to effectively predict, monitor and control torque and drag

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Schlumberger

Sedco Forex

MANAGING TORQUE AND DRAG

During the first part of this presentation, we will concentrate onunderstanding torque and drag losses Let's look at what causes torqueand drag and how they can be explained with simple physics.

Friction is the resistance to motion that exists when a solid object ismoved tangentially with respect to the surface of another which ittouches, or when an attempt is made to produce such motion

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There are three (Newtonian) laws that provide the quantitative framework

This law is often expressed in terms of a constant angle of repose, or africtional angle Ø , defined by:

tan Ø = µ

Ø is the angle of an inclined plane such that any object, whatever itsweight, placed on the plane will remain stationary, but if the angle isincreased by a small amount the object will slide down

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The second law states that a material’s friction coefficient is

independent of the apparent area of contact Therefore, large and small

objects —made of the same material— have the same coefficient offriction

The third law states that the frictional drag force is independent of thesliding velocity This implies that the force required to initiate sliding will

be the same as the force required to maintain sliding at any specifiedvelocity

In practice the first two laws of friction are generally well obeyed In thecase of the third law, experiments have shown that the friction coefficientdecreases slightly with sliding velocity but for drilling applications thiseffect is negligible

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II A friction coefficient is independent of the apparent area of contact

III Frictional force is independent of sliding velocity

When drilling a well, there are two types of friction losses that cause usspecial concern:

Torque losses, which are defined as the difference between the torqueapplied at the rig floor and the torque available at the bit

Drag losses, which are measured as the difference between the staticweight of the drillstring and the tripping weight

Torque losses are referred to as rotating friction and drag losses assliding friction In theory these types of friction are supposed to beidentical, but in practice the uncertainties on surface torque and hook-load measurements do not allow a definitive conclusion Even though theconcept of sliding friction has been extended to rotating pipe, slidingfriction is always very low as vibration causes the pipe to bounce off theborehole walls, partially eliminating friction

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Torque and Drag

Torque Losses Drag Losses

Both torque and drag losses are caused by the lateral forces and thefriction between the borehole wall and the drillstring These lateral forcesdepend on:

• Drillstring Weight

• Borehole Inclination

• Directional Changes (dogleg severity)

The last two parameters are closely related to the wellbore’s path

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The diagram illustrates the effects of well profile on the distribution ofcontact forces between the drillstring and the wellbore

The path of a borehole is the result of complex interactions betweenthe behavior of the rock and different drilling parameters like:

• Bottomhole Assembly Design

• Weight on Bit and Rotary Speed

Borehole Diameter Formation Drillability Borehole Stability

Let's now look at the effects of drillstring dynamics on torque:

When the downhole friction torque changes, a torsional wave isgenerated and propagates upwards towards the kelly (the drillpipe acts

as a transmission line for torsional waves) A rotary table that has acompletely constant speed, independent of the load, represents a fixedend for such a torsional wave As a result, torsional waves are reflectedback down the drillstring with 100% efficiency

Once vibrational energy is trapped in the drillstring, severe torsionaloscillation can build up, leading to slip-stick motion and other problems4,5.The rotary table will respond to these large torque variations withsignificant speed variations

The problem of severe oscillations associated with slip/stick motion ofthe bit can be reduced, even removed, by reducing the downhole staticfriction or controlling the rotary-table speed, in a way that dampenstorsional oscillations by inducing a motion proportional to the torquevariations This concept is called torque feedback and represents a sort

of compromise between two contradictory requirements: maintaining thedesired speed and holding constant torque

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As mentioned before, dogleg severity has an important influence onthe magnitude of lateral forces For this reason, dogleg-severity valueshave to be kept below a certain critical level in order not to exceed themechanical strength of the drillpipe and other rotary equipment Thisbecomes more important with increasing inclination of the borehole.

Similar to a drillstring, the casing makes certain demands onacceptable values of inclination and direction of the borehole Thesevalues are related to:

In extended-reach wells, downward drag prediction introduces a furtherlevel of complexity as a result of potential buckling of the string underexcessive axial compression.

'Critical' load is the magnitude of the compressive force below whichthe string does not buckle

At the critical buckling load, the string begins to deform into asinusoidal buckling configuration (“snaky” shape, along the low-side ofthe wellbore)

For compression loads above the critical helical-buckling load, thedrillstring can no longer maintain its “snaky” configuration and coils upagainst the wellbore, buckling in a helical manner String lock-upimmediately follows the onset of helical buckling due to a dramaticincrease in wall forces

When rotating, axial friction is considerably reduced and string bucklingbecomes less likely, although still possible However, once bucklingoccurs, rotating the drillstring will cause accelerated fatigue of the tooljoints

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The diagram shows the two stages of drillpipe buckling, according toincreased compressive force:

Sinusoidal buckling

Helical buckling

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Frictional torque is the lowest torque associated with drillstring rotation

in a clean wellbore, with a specific mud It is generated by contact loadsbetween the drillstring and casing or open hole

In the same manner, frictional drag is the lowest drag associated withdrillstring pick-up or slack-off

In practice, several downhole effects aggravate these basic values,causing increased torque and drag levels

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What are Torque and Drag?

Frictional Torque Frictional Drag

Even though measures can be taken to minimize the causes of torqueand drag, they must first be recognized.

Total surface torque is comprised of:

• Bit Torque (influenced by bit design and formation type)

• Frictional String Torque (determined by drillstring configuration

and mud type)

• Mechanical Torque (caused by stabilizers and cuttings beds)

• Dynamic Torque (originated by downhole vibrations)

Understanding these parameters will facilitate the implementation of aprevention/control program For example, mechanical and chemicalwellbore-stability analyses can result in mud weight/chemistryrecommendations to minimize instabilities and reduce torque and drag.Identification of excessive stabilizer torque can lead to better equipmentselection such as under-gauge stabilizers, reamers, etc Also, higherflow rates, careful rheology control and drillstring rotation can improvehole cleaning, minimizing the occurrence of cuttings beds

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Downhole EffectsSurface Torque

Bit Torque Frictional String Torque Mechanical Torque Dynamic Torque

Torque and drag losses will always be present during drilling;therefore, we should learn how to best manage them, specially inapplications like extended-reach and horizontal drilling An approachcalled PMC ( Predict, Monitor, Control ) is suggested for this purpose.

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Four criteria are used in the design of a wellbore’s trajectory and thedrillstring configuration:

Rotary torque, which must be within safe working limits for the drive system and the drillstring, allowing a suitable safety margin

Similarly, up and down drag must lie within safe working limits of thedrillstring and the down drag must not place any portion of it into criticalbuckling

The contact forces between the tool joints and the borehole wall shouldlie below safe thresholds, typically 2500 lbf for 5-in drillpipe inside casingand 1500 to 2000 lbf for the same pipe in open hole Higher forces havebeen found to accelerate pipe fatigue, casing wear and mechanicalborehole problems, such as key seating and hole enlargement

The entire drillstring must remain within the stable region with regard tobuckling, for the complete range of anticipated bit weights

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Torque/drag analysis programs have gained wide acceptance as criticalplanning tools since their introduction over a decade ago Their mainfeatures are:

The ability to model three-dimensional wellbores, accurately accountfor the drillstring geometry and support multiple friction factors Thisprovides a greater degree of accuracy than any hand-calculation method For the most part, torque and drag programs are based upon thesimplifying assumption that either there is no drag or that drag is workingeverywhere along the drillstring in the same direction Since the direction

of drag is opposite to that of motion, these programs pre-define thedirection of friction as either up-string (trip in), down-string (trip out), ortangential (rotate), depending upon the operating condition beinganalyzed

Torque/drag programs subdivide the entire drillstring into a series ofshort lengths for which calculations are performed Calculations beginwith either known or assumed forces at the bit and proceed up the string

in sequential fashion, solving for equilibrium of each subdivision, with thefinal result being the measured weight for the given operating condition

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Programs used for planning are not designed to predict drillstringbehavior during the transition from one operating mode to another, theystrictly address what the final conditions will be once a given mode isachieved While solving for the primary operational modes of tripping out,tripping in, rotating off bottom and drilling, many of today’scomprehensive programs also include such effects as buckling anddrillstring bending stiffness.

It is possible to perform extensive torque and drag modeling to find thebuild rates, inclinations and drop rates that would connect the upper holesection (surface location) of a well to the lower hole section (target) withthe least torque, drag and casing wear

This modeling can be performed using offset friction-coefficientmeasurements and directional surveys Values of downhole weight-on-bit and downhole torque, from area wells drilled with steerable systems,can provide reliable information about the area’s friction coefficient

In general, torque and drag models are useful for planning if doglegsare included in the model Simulated dogleg values should be updatedfrequently, with actual surveys, to increase accuracy of predictions

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Computer Software (cont.)

Operating Modes Planning

Offset Data Updates

Predominantly based on lab measurements, mathematical modelshave been developed for predicting bit torque using various bit types.However, actual bit torque varies substantially during drilling and isinfluenced by many factors, including:

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Bit Torque from Mathematical Models

Bit Design Weight on Bit Bit Wear Rotary Speed Bit Hydraulics Formation Hardness

Simulated curves for weight and torque are helpful to the driller whenrunning and cementing casing and liners, because deviations from thesimulations may give early warnings of hole problems.

The ability to run and cement casings and liners depends heavily ontorque and drag levels in a well Casing is sometimes not reciprocated orrotated for fear of sticking it off bottom, leaving a casing collar in the wellhead, or exceeding makeup-torque levels1 Simulations of up-, down-weight and torque, caused by casing rotation during cementing, should

be performed in the planning phase of a well to determine the feasibility

of casing movement during the primary cement job

Finally, when extended-reach wells are drilled, the same torque- anddrag-simulation curves may also be used to monitor hole cleaning, wheredeviations from properly modeled torque and drag simulations mayindicate problems like the presence of excessive cuttings beds ordeficient hole cleaning

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Casing Considerations Running Casing Reciprocation/Rotation Other Considerations

Hole Cleaning

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Example of a drag chart reflecting the increase of drag due to poor holecleaning

In order to obtain an accurate measurement, bit torque should bemonitored using a drilling-mechanics sub in the MWD tool This is ofparticular importance in extended-reach wells where torque and draglevels must be constantly optimized.

Alternately, crude bit-torque measurements can be taken by monitoringoff-bottom and on-bottom surface torque, although this is onlyapproximate since weight on bit causes variations in the drillstringtension/compression profile, affecting wall forces and string-torquemagnitude

A more empirical approach, to determine bit torque, is the use of aconservative upper limit from existing field data

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Bit Torque from Field Measurements

MWD Surface Measurement Offset Data

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In order to monitor the levels of drag in the wellbore, two fieldmeasurements are routinely taken:

Pick-up weight is the force necessary to lift the drillstring It includesthe weight of the string, as well as the force necessary to overcomeexisting drag to come out of the hole

Slack-off weight is the force necessary to lower the drillstring Itincludes the weight of the string, as well as the force necessary toovercome existing drag to go into the hole The latter manifests itself as

a 'reduction' in the wight of the string