SPE-172388-MS

Using Robust Torque and Drag Modelling Software For Efficient WellPlanning and Operations Monitoring. Paradigm Sysdrill for OML 126Wells – A Case Study

Okoli Ugochukwu, Addax Petroleum Nigeria; Simon Verity, Paradigm Geophysical Ltd

Copyright 2014, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Nigeria Annual International Conference and Exhibition held in Lagos, Nigeria, 05–07 August 2014.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contentsof 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 reflectany position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the writtenconsent 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 maynot be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

The quest to appraise un-stacked reservoir sands and other surface location constraints has given rise tocomplex wellbore profiles due to the requirement of building angles and azimuth changes through thesetargets. This has increased the length and inclination of the wells drilled in the past decades. There are alsolarger departures and more tortuous wellbore. These trajectories can give rise to excessive torque and dragduring the well delivery.

Excessive drill string torque and drag is one of the major limitations of horizontal and extended reachwells, and when unplanned is a primary limiter. Because of the increased costs and risks in these wells,torque and drag analyses is recognized as an important part of the risk management process.

Torque and drag models have proven to be very useful in the planning, drilling and post analysis ofthese difficult wells. During the planning stage they are used to optimize the trajectory design to minimizethe torque, drag and contact forces between the drill string and the borehole wall. They are used duringoperations to monitor the hole conditions while drilling, diagnosing hole cleaning problems, watching outfor impending differential sticking, and monitoring for high torque in planned highly tortuous trajectories.In post analysis the models help to determine the true causes of the hole problems, which will further behighlighted, documented and used to optimize operations.

Accurate torque and drag analysis gives an opportunity to build reliable well trajectories, as it will takeinto account the capabilities of the rig and the geological complexity of the formation to be traversed.Ideally the model must be able to predict with minimal error the forces and along the wellbore. Generallythe discrepancy between torque and drag prediction and actual measurements in conventional wells iswithin 20% depending on the variables used. This percentage error might increase or decrease dependingon the variables used for these models.

In this paper, a general overview of the torque and drag model is presented. It is then discussed howthe Paradigm Sysdrill software has been able to accurately predict the torque and drag values of a plannedwellbore. The error margin between the predicted and actual values was very minimal. This software hasbeen able to effectively aid in the planning, monitoring and helped to give a reasonable and concise postmortem of the subject well that will be used as the case study in this paper.

IntroductionDrilling extreme and challenging wells is becoming more common, and increased knowledge of Torqueand Drag modelling and interpretation is needed. Due to the complex wellbore profiles currently drilled,it is required to have a good model for effective wellbore friction analysis. The model must be be a reliabletool to be able to effectively predict values for Torque and Drag analysis during planning, drilling andpost-operational phases.

A primary key to successfully planning, drilling and completing a well is to first use a program thatforsees possible problem areas and includes all drilling parameters. This program should aslo be able toeffectively investigate the drillability of the well designed. During the planning phase the models are usedto optimize the trajectory design to minimize the torque, drag and contact forces between the drillstringand borehole wall.

Different models classified under soft string and stiff string models have been used for torque and draganalysis, however the best way for model evaluations is comparison of the simulated results with theactual measurements of hookload and torque values. If there is a discrepancy between the model and theactual hook load or/and torque, this may then simply mean either a problem with the model, someindication of well problems or external factors having some effect on the actual values of torque and drag

BackgroundThe Alpha-28 well used for this study is a development well drilled in the OML-126 region of the NigerDelta. This well lies in a water depth of 435ft MSL and was designed and delivered as a horizontaldevelopment producer.

The design of this example is typical of others in the field and is for a recumbent or ‘fish hook’trajectory with a measured depth of approximately 10500 ft., TVD 7650 ft. and total lateral departure of4700 ft. with a horizontal section of approximately 1100 ft. The Directional Difficulty Index11 (DDI) is6.05 Fig 2.

The well was drilled through normally and subnormally pressured formation and consists of 30� to 95/8� API casings with an 8�1/2� open hole stand alone screen completions. Sea water and hi-viscositysweeps were used for the top hole sections (36� and 26�) and a pseudo oil based mud system with mudweights between 9.4 and 10 ppg was used for the 17–1/2� section to well TD. Fig 1.

Most of the directional work was performed in the 12-¼� section where the inclination builds from 38to 90 degrees with a 160 degree change in azimuth. Typical build rates were between 3 and 3.2 deg/100ft.

Figure 1—

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Prior to the drilling operations, planned torque & drag models were developed with the ParadigmSysdril software as a guide to BHA design and were subsequently used to monitor the operations duringdrilling operations for any major divergence of the curves signifying either insufficient hole cleaning orwellbore instability.

In this paper results from the models and field measurements for both drilling and running casing arecompared and discussed.

Figure 2—

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Figure 3—

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Theory

The Torque & Drag ModelA ‘soft string’ model based on one proposed by Johancsik et al1 is used to calculate the torque and tensionprofiles of the string for various user selected operating modes. These can be compared against bucklinglimits9, 10, pipe and connection yields, make-up torque and surface equipment limitations.

The predicted hookload and surface torque are calculated at various depth intervals and can becompared against measured values acquired during drilling and tripping operations.

Operating ModesThe operating mode is used to describe the motion or condition of the string and provide the correct inputsto the model. The modes supported are as follows.

● Rotary drilling with weight on bit● Slide drilling with weight on bit● Running into hole (Slack Off)● Pulling out of hole (Pick up)● Rotating off bottom● Rotating into hole● Rotating out of hole (Backreaming)

The axial and rotational velocity, additional weight on bit and circulating paramerers are used as inputsto the model.

Friction FactorsThe model allows the user to assign any number of friction factor intervals to a given wellbore. Eachinterval consists of a single friction factor value.

Attribution of friction factorsThe single friction factor is decomposed into axial and rotational components. Eq.1 & 2. Since the frictionacts against the direction of motion and the sign of the axial velocity term is changed accordingly for thechosen operating mode.

1)

2)

Calculation of Side Force LoadsThe ‘normal force’ or side force load is used to calculate the effects of hole drag whilst rotating and ormoving the string axially.

The air weight of the string is multiplied by the buoyancy factor, Eq.3 to account for buoyant effectsof the well bore fluids. This is used in the determination of side force load.

3)

Buoyed weight of a component is determined from Eq.4

4)

The well path is calculated using a minimum curvature interpolation3, 4of the well plan or directionalsurvey. The build and walk rates experienced by each component are calculated using the inclination andazimuth at the top and bottom of each component. This is to try and capture any dog legs within the rangeof a single component.

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Figure 4—

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Figure 5—

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The vertical force per component is stated as Eq.5

5)

The lateral force per component is stated as Eq.6

6)

The normal or side force load per component is then calculated using Eq.7

7)

TorqueThe incremental torque for each component is determined from Eq.8

8)

Viscous TorqueThe effect of viscous torque is included if the user chooses to include hydraulic effects although this effectis negligible. The effective viscosity is determined from the integrated hydraulics model5, 6

The external viscous torque effect is calculated using Eq.9

9)

The internal effect is calculated using Eq.10

10)

The function for torque then becomes Eq.11

11)

Accumulated torque at component n in the string is calculated using Eq.12

12)

TensionTension and torque are summed from the bit to surface. The tension is calculated using the both the pistonforce or ‘true tension’ and the ‘effective tension’ or buoyancy factor methods.8, 7

‘True Tension’At the bit and at points in the string where there is a change of cross sectional area or a pressure

discontinuity; the axial load at the given component n is determined from Eq.13

13)

Otherwise the tension is summed using Eq.14

14)

In the drilling and circulating case the internal and external pressure profiles are determined using anintegrated hydraulics model5. The user has the ability to select one of four rheological models and candefine the flow rate and rheological parameters.

If hydraulic effects are not included or no additional surface pressures are applied, the internal andexternal pressures are assumed to be hydrostatic. In this case the ‘true tension’ and ‘effective tension’should be the same at surface7 and can also be expressed as Eq.15

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Figure 6—

Figure 7—

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15)

HookloadHookload is calculated by adding on the weight of the travelling block assembly and other equipment suchas the top drive or kelly to the calculated ‘true tension’ at the topmost component.

Friction Factor ReductionTo enable modelling of friction factor reduction devices such a torque reduction subs or accessories; theBHA description allows for each component to be assigned either a percentage reduction in axial orrotational friction or an axial or rotational friction factor to be used in place of the friction factor derivedfrom the wellbore friction factor interval. Eq.1&2

TractionThe software also allows additional torque and tension to be applied at any given component. In this waytraction can be simulated by adding the effective motive force in place of the axial drag contribution forthe given component. See Eq.13&14

Traction Drilling Tools that provide axial force due to rotational interaction with wellbore can besimulated by replacing the axial drag contribution in Eq.13&14 with one calculated from Eq.16

(16)

The torque contribution for such a component can be calculated using an appropriate rotational frictionfactor.

Swivel or Clutch ToolsSwivel or clutch tools that do not transmit torque and allow part of the string to rotate can be modelledby applying the appropriate values to Eq.1&2 for the relevant components at a given depth.

Figure 8—

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Complex Pressure ProfilesCasing floatation or buoyancy assisted running, partially filled casing or cement filled liners can besimulated by supplying Eq.3 and Eq.13&14 with appropriate values. In the case of buoyancy assistedcasing running a pressure discontinuity exists at the internal packer and Eq.13 must be applied.

CalibrationThe friction factor for a given interval can be derived from the measured values for pick up, slack off oroff bottom torque. These operating modes are most suitable as the axial and rotational friction factorcomponents are equal to the model friction factor. See Eq.1 & 2

The solutions to the Eq.17 & 18 are found numerically by varying the axial or rotational frictionfactors.

17)

18)

The calculation requires that the friction factors for each interval are calculated sequentially usingsurface values obtained at progressively deeper depths.

Considerations in applying the modelBefore using the Torque & Drag model1 in planning and operations monitoring it is important toappreciate some of the limitations of the model and difficulties in obtaining accurate measurements at rigsite.

One of the most important considerations in Torque & Drag modelling is the friction factor and anunderstanding of its meaning and use. It is not a direct measure of a sliding coefficient of friction butrather a lumped term that encapsulates many physical processes such as contact area, adhesion, micro

Figure 9—

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doglegs, ledging, cuttings accumulation as well as the frictional effects of the wellbore and lubricity of thefluid2.

It should also be noted that it is relates to dynamic friction in a steady state system. It does not accountfor ‘stiction’; overcoming static friction and the model does not consider the inertial effects of acceleratingthe string to tripping speed. This should be taken into account when selecting hookload and torquemeasurements for comparison to the model.

The standard torque and drag model is generally regarded as being pretty good but has some knownlimitations. The main weakness in the standard model relates to the calculation of sideforce in curvedsections. It is less accurate in short radius and complex 3D wells2. Differing friction factors obtained frompickup, slack off and torque values are indicative of some shortcomings in the model2. However shorterelement lengths used in the calculation will improve the accuracy. The Sysdrill software considersindividual BHA components and calculates a result for each. Typically this would be the length of a singlejoint of drill pipe or casing.

The torque and drag model can be applied to a planned well (defined only by turn points) or adirectional survey. (Survey stations typically � 95ft/29m) Tortuosity in the form of a helix can be appliedto a planned well to simulate the effect of survey ‘noise’. However micro tortuosity below the resolutionof the directional survey is not accounted for in the model and would show as an effect in the overallfriction factor. This should be a consideration when comparing planned and actual well data.

The software does not currently compensate for sheave friction so the overall frictional effect of themechanical system of the travelling block and pulley is also lumped in with the friction factor. This wouldcause friction factors derived from torque to differ from those derived by tension.

The model allows inclusion of hydraulic forces. This option can improve the accuracy of the predictedtension but can also introduce additional uncertainty if the hydraulic model is incorrectly calibrated.

Figure 10—

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The fluid levels in the string and annulus will also influence the tension measured at surface. Althoughthis can be modelled in the software this should be accounted for when taking measurements

Of more minor consideration is the Torque & Drag model datum is at the rotary table. The un-buoyedweight of the string plus the weight of the internal fluid above the rotary table is not included. This couldaccount for a difference of 1–2 tonnes during drilling & tripping operations. This error is more significanta shallower depths.

Measurement IssuesHookload at surface is usually measured using a load cell consisting of a pressure transducer connectedto either a clamp-on sensor or pressure plate. This is placed on the dead line or deadline anchor. The outputfrom these is usually calibrated to show the entire weight acting on the hook. Since the deadlineexperiences a fraction of the true hookload and sensors are often removed for maintenance. Care shouldbe taken to ensure an accurate calibration. This can be problematic as known hookloads at the high endof the hookload range are difficult to obtain. Hookload values may need to be adjusted accordingly.

Surface torque values are usually derived from an indirect measurement of the current drawn by theelectric motor powering the top drive or rotary table. The calibration is usually quite stable. However,changing gearing of the rotating equipment will cause the effective calibration to change.

An understanding of the model and its limitations, consistency in obtaining the measured values andin inputs and options for the model are required to obtain consistent results that can be used as a predictiveand diagnostic tool.

Figure 11—

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Implementation

Planning & Modelling for A-28Initial modelling of the 12 ¼ and 8 ½ inch sections for the A-28 well revealed that the expected tensionand torque profiles during a range of drilling a tripping activities were well within the capabilities of the5� 19.5 ppf S-135 drill pipe. Fig 6 & 7.

The planned trajectory and a detailed description of the BHAs Fig 4&5. were used for the modellingof the various operations. Additional modelling options such as applied tortuosity and hydraulic effectswere not included as safe operating margins were large.

Trip charts for drilling and casing assemblies were produced for a range of friction factors between 0.1and 0.5. This range was felt to be sufficient to account for any uncertainties in the model and any expectedhole conditions. The trip charts were then incorporated into an excel spreadsheet for use at the rig site. SeeFig 8–12.

Operations Monitoring and Data Collection.A connection procedure Fig 1. was established that in addition to good drilling practise also allowed forcollection of the necessary hookload and torque data. Hookload data was acquired with pumps off tonegate the effect of hydraulic lift/weight. Hookload values were taken from the main rig floor indicatorby the drill crew accordingly

The excel spreadsheet allowed for correction to be made to the hookload values to account forcalibration error. The expected rotating weight can be used to check the hookload sensor calibration as theaxial drag effect is minimal.

Figure 12—

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Figure 13—(Full Pipe Condtion)

Figure 14—(Partially Filled to 1000ft)

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ObservationsThe excel spreadsheet was updated in accordance with the procedures. The resulting measurementscompared to the models for drilling the 12 ¼� section, running the 9 5/8� casing and drilling the 8 ½�section can be seen in Fig 8–12. The 12 ¼� & 8 ½� analysis were regenerated with updated surveys fromthe well.

No adverse conditions were indicated by the measurents and hole cleaning throughout the operationwas good. There was very good agreement between the measurements and models used. The results in Fig8–12 have no calibration correction applied. Neither the model or measured hookload values account forsheave friction.

The pickup and slackoff loads diverge somewhat from the model for running the 9 5/8� casing towardthe landing depth. This can be explained by the fact that the casing was not completely filled due toapproaching the safe operating hookload limit. This can be simulated in the software by defining theappropriate fluid depths in the inputs for the torque & drag calculation. Fig 13&14 and is explained byEq 3, 13&14

The results for the 12 ½� section show relatively low friction factors. This can be explained by theabsence of sheave friction compensation. Correcting the modelled values or the measured values wouldresult in higher apparent friction factors.

ConclusionsThe Sysdrill Torque & Drag software has been proved as an accurate and reliable tool for both design andoperations monitoring puposes.

Measured hookload and torque values have shown good agreement with the model in all sections ofthe well including in the 3D build section.

Some factors to ensure success are as follows.

● Appropriate inputs to the model● Consistent measurement procedures that take account of the modelling inputs.● Accurate callibration of rig sensors.● Understanding of the measurement errors and sources.● Interpretation of the measurements as compared to the model and a willingness to take corrective

action● Use of “actual” inputs(trajectories, mud properties, BHA dimensions, etc) if available for the

re-calibration of the torque and drag model.

RecommendationsThe procedures followed in the design and execution of the Alpha-28 well are typical of those used inothers in the field. To achieve accurate output for preliminary design and operations monitoring. It isrecommended that all required information (surveys, tensile strength of tubulars and connections,planned/actual WOB and RPM, connection torque values) should be entered correctly.

The software has since been further developed for operations monitoring including built in linearcalibration tools, sheave friction correction, automated back calculation of friction factors and improveduser interface.

Realtime monitoring using WITSML data acquired at the rig could be used in future to automate theprocess.

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AcknowledgementsThe authors would like to thank Addax Petroleum & Paradigm for permission to publish this paper. HopeOkwa, Tayo Ajimoko, Bob (C.R.) Anderson and Horace Awi who offered valuable technical and editorialfeedback on content and Gary Lowson at Paradigm for his help in understanding the model implemen-tation in the software.

Nomenclature

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API CONVERSION FACTORS

References1. Johancsik, C.A., Friesen, D.B., Dawson, R. “Torque and Drag in Directional Wells–Prediction

and Measurement.” (SPE/IADC 11380-PA) June 1984.2. Mitchell, R.F., Samuel R., “How Good Is the Torque/Drag Model?” (SPE-105068-PA) 20093. Howard L. Taylor (Sun Oil Co.) | Mack C. Mason (Sun Oil Co.) “A Systematic Approach to Well

Surveying Calculations” (SPE-3362-PA) 19724. S. Sawaryn, J. Thorogood. “A Compendium of Directional Calculations Based on the Minimum

Curvature Method.” (SPE-84246-PA) 2005.5. A. Merlo, R. Maglione, C. Piatti. “An Innovative Model For Drilling Fluid Hydraulics.”

(SPE-29259-MS) March 1995.6. R. Robertson, H. Stiff, “An Improved Mathematical Model for Relating Shear Stress to Shear

Rate in Drilling Fluids and Cement Slurries” (SPE-5333-PA) February 1976.7. Aadnoy, B.S., Kaarstad, E. “Theory and Application of Buoyancy in Wells.” (SPE-101795-MS)

November 2006.8. R. Samuel, A. Kumar,. “Effective Force and True Force: What are They?” (SPE-151407-MS)

March 2012.9. X, He, A, Kyllingstad,. “Helical Buckling and Lock-Up Conditions for Coiled Tubing in Curved

Wells,.” (SPE-25370-PA) February 1993.10. Mitchell, R.F. “Tubing Buckling – State of the Art.” (SPE-104267-PA) September 200911. Alistair Oag, Mike Williams; “The Directional Difficulty Index – A New Approach to Perfor-

mance Benchmarking” (SPE/IADC 59196), 2000

m � 0.3048* � ft

m � 0.0254* � in

m2 � 0.00064516 � in2

m/s � 0.00508 � ft/min

rad � �/180* � deg

rad/m � 0.057261 � deg/ft

N � 4.448222 � lbf

N.m � 1.355818 � ft.lbf

Pa � 6894.757 � psi

Pa.s � 0.001* � cP

* Conversion factor is exact.

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