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Unconventional gas, especially shale gas, has generated strong interest recently.Much has been written on shale gas regarding stimulation and fracturing tech-nology, environmental issues, geology, water management, reservoir perme-ability, gas recovery, and more, but far less has been presented about shale-gasdrilling. I am taking this opportunity to write about shale-gas drilling becausedrilling technology and efficiency contribute to making shale-gas wells evenmore economical. Shale-gas plays are an excellent place and application where

bottomhole-assembly (BHA), drillstring, well-trajectory, and bit-design optimiza-tions play a great role in well economy. Indeed, because the number of horizontalwells drilled is numerous, a small reduction in drilling costs and nonproductivetime could result in major cost savings for operators.Whichever US shale-resource basin (e.g., Barnett, Fayetteville, Haynesville,

Marcellus, and others) is studied, most of the wells drilled today are horizon-tal, enabling increasing the production rate significantly compared with verti-cal wells. These horizontal wells usually comprise a vertical, a curved, and ahorizontal section. Depending on the shale basin, vertical depth ranges fromapproximately 500 to 3000 m, and the lateral horizontal section may be as longas 3000 m. The first challenge is designing a bit that can drill abrasive sandstoneor limestone stringers (unconfined compressive strength up to 170 MPa) and softshale efficiently, while keeping good bit steerability and durability.

Steerability is a great concern because build rate, up to 12/30 m, must bereached to maximize horizontal length and, as a consequence, reservoir exposure.These high build rates may be attained by use of steerable mud motors or by somerotary-steerable system specially designed for high-dogleg applications. In shallowhorizontal shale-gas wells, the challenge concerns the drillstring design becausebuckling becomes an issue. Indeed, to reach an acceptable rate of penetration, suf-ficient weight should be available on the bit without exceeding the critical buck-ling loads. Therefore, some inverted BHAs are proposed that contain heavyweightdrillpipe, or drill collars above drillpipe, to help transfer the weight downhole.With the numerous wells forecasted in the near future, drilling techniques in

shale-gas wells will become well established, and drilling efficiencies will con-tinue to improve.

Drilling Technology additional readingavailable at OnePetro: www.onepetro.org

SPE 128916 An Intelligent System To Detect Drilling Problems ThroughDrilled-Cuttings-Returns Analysis by A.N. Marana, So Paulo State University,et al.

SPE 135587 The Effect of Drillstring Rotation on Equivalent CirculatingDensity: Modeling and Analysis of Field Measurements by Ramadan Ahmed,University of Oklahoma, et al.

SPE 134306 Numerical Simulation on Three-Layer Dynamic Cuttings-Transport Model and Its Application to Extended-Well Drilling by ZhimingWang, SPE, China University of Petroleum, et al.

SPE 134488 Downhole-Vibration Measurement, Monitoring, andModeling Reveal Stick/Slip as a Primary Cause of PDC-Bit Damage in Todays

Applications by L.W. Ledgerwood III, SPE, Baker Hughes, et al.

Drilling Technology

TECHNOLOGY FOCUS

44 JPT FEBRUARY 2011

JPT

Stphane Menand, SPE, is a research

engineer at Mines ParisTech in the

Geosciences Center and a perma-

nent consultant with DrillScan. He

has 13 years of experience as an

R&D project manager in drilling engi-

neeringmore specifically, in direc-tional drilling, drillstring mechanics

(torque, drag, and buckling), and

drill-bit performance. Menand has

authored several SPE and other tech-

nical papers. He earned a PhD degree

in drilling engineering from Mines

ParisTech. Menand serves on the JPT

Editorial Committee, the SPE Books

Development Committee, and the SPE

Drilling and Completions Advisory

Committee. He is the faculty spon-sor of the SPE Student Chapter at

Mines ParisTech.

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JPT FEBRUARY 2011 45

In horizontal and highly deviated por-tions of an extended-reach-drilling(ERD) well, cuttings beds form onthe low side of the annulus. The cut-tings bed partially blocks the annulus,resulting in excessive pressure lossand a higher equivalent circulatingdensity (ECD). Recently, the use ofdownhole mechanical cleaning devic-

es (MCDs) has been introduced tomitigate the problem without inducingexcessive ECD. The full-length paperpresents results of an experimentalstudy that was conducted to evaluatecuttings-removal efficiency of MCDs.Results indicate that the tools signifi-cantly reduce the amount of cuttingsin the annulus.

IntroductionEfficient cuttings transport is animportant issue in drilling highlydeviated and horizontal wells. Indirectional wells, drilled cuttingstend to accumulate on the low sideof the annulus and form a thickcuttings bed when the flow velocitybecomes insufficient to suspend the

cuttings-bed particles. Particularly inhigh-angle and horizontal boreholes,the formation of a thick cuttingsbed can give rise to numerous dif-ficulties such as lost circulation, dif-ferential sticking, and high torqueand drag. Recently, hydromechanicalhole-cleaning devices (HHCDs) havebeen developed to enhance cuttings-

transport efficiency in directionalwells. These tools are introduced inthe drillstring with different spac-ing arrangements.

These tools have helical grooves orblades on their surface to assist cut-tings-bed removal. A negative anglealso is designed on each blade toimprove the scooping effect on thecuttings bed. While rotating the drill-pipe, the blades scoop the cuttingsbed and help to bring the cuttingsinto suspension. At the same time,the circulation of the drilling fluidallows the suspended cuttings par-ticles to be carried away, thus leadingto better hole cleaning. Because ofhydrodynamic and hydromechanicaleffects, the tools help to reduce thecuttings accumulation in highly devi-ated and horizontal sections of thewellbore where the buildup of a cut-tings bed cannot be avoided. Thus,several of these tools are used in atypical drilling application to reducethe in-situ cuttings concentrationand, hence, reduce the occurrence of

hole-cleaning-related problems.HHCDs improve hole cleaning by

creating more turbulence, bringingcuttings into suspension, and scoop-ing the cuttings bed. The interactionbetween the tools and the slurry thatcontains cuttings particles and drill-ing fluid is a complex fluid-mechanicsproblem for which an analytical solu-tion would be difficult to develop.Therefore, an experimental approachis the best option to provide practical

solutions, perform sensitivity analy-sis, and obtain a better understandingof the use of these tools.

To achieve the objectives of theinvestigation, extensive cuttings-transport experiments were per-formed using a water-based fluid thatcontains 1.25 ppb polyanionic cellu-lose (PAC) as a viscosifier. Table 1 in

the full-length paper presents the testmatrix that specifies test parametersfor each experiment. Two, designs/generations (Fig. 1) of these tools(G1 and G2) were considered in theinvestigation, and their performancewas compared to standard drillpipewithout MCDs. The second-genera-tion (G2) tool was developed recent-ly. The tool is expected to performbetter than the first-generation (G1)tool in terms of cleaning the wellboreand reducing drillstring mechanicalfriction and required torque at thesurface. The new design of G2 has abearing system that allows the bladesto rotate freely without touching thewellbore wall, and as a consequence,both the blades and wellbore are pro-tected. Although mechanical benefitsare apparent from its design, it isimportant to determine hole-cleaningefficiency of the tool quantitativelyand perform a qualitative comparisonbetween the two generations underdifferent operating conditions. At thesame time, it is necessary to deter-

mine the effect of drilling parameters,such as mud flow rate, tool spacing,drillpipe-rotation speed, rate of pene-tration (ROP), inclination angle (i.e.,measured from the vertical axis), andannular diameter ratio, on the effi-ciencies of these tools. This wouldhelp to optimize the performance ofthe tool through careful control ofthese parameters. Hence, this studyfocuses on determining experimen-tally the efficiencies of these down-

This article, written by Assistant Tech-nology editor Karen Bybee, containshighlights of paper SPE 134269,Experimental Studies on the Effectof Mechanical Cleaning Devices onAnnular Cuttings Concentration andApplications for Optimizing ERD Sys-

tems, by Ramadan Ahmed, SPE(currently with the University of Okla-homa), Munawar Sagheer, SPE,Nicholas Takach, SPE, Reza Majidi,SPE, Mengjiao Yu, SPE, and StefanMiska, SPE, University of Tulsa, and,Christophe Rohart, SPE, and JeanBoulet, SPE, VAM Drilling, originallyprepared for the 2010 SPE AnnualTechnical Conference and Exhibition,Florence, Italy, 1922 September. Thepaper has not been peer reviewed.

Experimental Studies on the Effect of Mechanical Cleaning

Devices on Annular-Cuttings Concentration

DRILLING TECHNOLOGY

For a limited time, the full-length paper is available free to SPE members at www.spe.org/jpt.

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hole cleaning devices (G1 and G2)and studying parameters that affecttheir performance.

Experimental StudyCuttings-transport experiments wereperformed using the low-pressureambient-temperature flow loop devel-oped by Tulsa University Drilling-Research Projects. The tests wereintended to obtain experimental datathat can provide useful information

to improve the performance of MCDsand yield valuable insights into the useof these devices for improving cuttingstransport while drilling. Therefore,experiments were performed to covera wide range of test parameters, asshown in Table 1 in the full-lengthpaper. According to the test matrix,30 experiments with 10 different loopassemblies were conducted. Eachexperiment was repeated three timesto ensure result reliability. Thus, atotal of 90 tests were performed. Theeffects of seven operating parameters(i.e., diameter ratio, flow rate, toolspacing, tool design, drillpipe-rotationspeed, ROP, and inclination angle)were studied. Three different drillpipesizes were considered in the investi-gation. The following three types oftool spacings were considered: (1)Type 1two MCDs in the test sec-tion; (2) Type 2three MCDs in thetest section; and (3) Type 36 MCDsin the test section. Experiments alsowere conducted with only a conven-tional drillstring (i.e., no MCDs). All

experiments were performed with onetest fluid (1.25-lbm/bbl PAC suspen-sion) that has acceptable rheologicalproperties and is fully transparentto allow flow visualization and bed-height measurements. The experi-ments were performed in a large-scaleflow loop that has an 8-in. transparenttest section.

The rheology of the test fluid bestfits the power-law model. The aver-age cuttings size (river gravel with 2.6specific gravity) used in the experi-ments was 3.35 mm in diameter.

ConclusionsAnalysis of the experimental resultsshows some interesting trends and pro-vides better understanding of the effectof mechanical hole-cleaning tools oncuttings transport at different operating

parameters. It shows the performanceof the tools compared with that of stan-dard drillpipe. The study focused oninvestigating the effect of most of thedrilling parameters considered in thefield. As a result, the test matrix wasdeveloped on the basis of the Taguchimethod. The Taguchi approach wasimplemented to reduce the number ofexperiments while covering the most-relevant parameters such as flow rate,ROP, diameter ratio, tool design, anddrillpipe-rotation speed. For each test,the amount of cuttings accumulated inthe test section was determined fromweight measurements.

Analysis of the results by the anal-ysis-of-variance (ANOVA) approachshows that the flow rate and hole-inclination angle are the most-criticalparameters that dictate hole-cleaningperformance of the tools.

In addition to the ANOVA results,a generalized correlation was devel-oped on the basis of dimensionalanalysis. An extensive sensitivityanalysis, which was conducted using

the correlation, shows that: MCDs are effective in cleaning

highly deviated and horizontal well-bores.

Hole-cleaning performance of thetwo tools (G1 and G2) investigated inthis study is comparable. Performanceof the first-generation tool is slightlybetter than that of the second-gener-

ation tool, depending on operatingparameters; however, G2 tools rotatefreely without touching the wellborewall and thus reduce the torque anddrag required at the surface and limitwear considerably.

In horizontal wellbores, use of thetools improves hole cleaning regard-less of the bed area in the annulus.

The effect of ROP on the annu-lar-bed area is minimal. Flow-loopexperiments simulate only the effect

of cuttings-generation rate at the bit,which is related directly to the ROP.The contribution of axial tool move-ment to the removal process couldbe substantial, but it was not studiedduring these experiments. A prelimi-nary model that includes axial toolmovement has been proposed; field orexperimental data now are required todetermine model parameters.

Annular-bed area is very sensitiveto differences in tool spacing when asmall number of tools per length ofthe wellbore is used.

The drillpipe-rotation speed hasa moderate effect on the performanceof the tools.

MCDs improve hole cleaning byscooping cuttings-bed particles andresuspending, them into the high-velocity flow zone, thus, achievingfurther transportation by drilling fluid.Mud-carrying capacity remains animportant factor for the overall hole-cleaning performance with MCDs.Visual observation of the bed profilesin the annulus indicates that the bed

height in the tool zone is very lowcompared to locations where thereis no tool and that the bed heightis maximized between two adjacenttools. The axial tool movement impliesa cumulative scooping effect of thesetools, providing an additional increaseof the overall cleaning performance ofthe MCD drilling system. JPT

Fig. 1G1 is the first-generation tool (a), and G2 is the second-generation tool (b).

(a) (b)

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The objective of real-time optimizationof drilling parameters is to optimizeweight on bit (WOB) and bit rotationalspeed to achieve maximum drilling rateand to minimize drilling cost. An exten-sive literature survey of drilling-opti-mization research was conducted forthe study described in the full-lengthpaper. The general equation for rate of

penetration (ROP) was optimized usingactual field data.

IntroductionThe objective of optimizing drillingparameters in real time is to arrive at amethodology that considers past drill-ing data and predicts drilling trends toadvise of optimum drilling parametersto save drilling costs and reduce theprobability of encountering problems.To achieve effective optimization, anextensive literature survey was conduct-ed and the results of previous research-ers were considered in developing adrilling-optimization methodology in areal-time environment. The linear drill-ing-ROP model introduced in 1974 thatis based on multiple regression analysiswas used in real time to:

Achieve coefficients of multipleregression specific to the formation.

Determine an ROP-vs.-depth pre-diction as a function of certain drillingparameters.

Determine optimum drillingparameters specific to the formationbeing drilled.

The following assumptions are consid-ered to be satisfied so that the equationsgiven in this study function properly:

Bottomhole cleaning is achievedeffectively.

The bit and bottomhole-assemblycombination in use is one that is selectedproperly for the formation being drilled.

The formation interval being drilledis considered to be homogeneous.

The rig and auxiliary equipmentare functioning efficiently.

Data from three directional offshorewells in the Mediterranean area wereused to test the methodology.

Literature SurveyNumerous detailed research studieshave been performed for optimizationof drilling activities with the objec-

tive of maximizing footage drilled andminimizing drilling costs. Drilling opti-mization can be achieved by preselect-ing the magnitudes of the controllabledrilling parameters.

Most of the early studies describedin the literature have foreseen a staticdrilling-optimization process. In thepast, drilling parameters were requiredto be investigated off site because of thelack of ability to transfer data in real

time. Recent studies have performeddrilling optimization in real time; how-ever among the investigated references,there is no work with statistical correla-tions in a real-time environment.

Fig. 1 in the full-length paper givesthe time line of some importantachievements in drilling and optimi-zation history. In the 1950s, the sci-entific period began, with expansionin drilling research, better understand-ing of hydraulic principles, signifi-

This article, written by Assistant Tech-

nology editor Karen Bybee, containshighlights of paper SPE 129126,Real Time Optimization of DrillingParameters During Drilling Operations,by Tuna Eren, SPE, Eni E&P, andM. Evren Ozbayoglu, SPE (currentlywith the University of Tulsa), MiddleEast Technical University, originallyprepared for the 2010 SPE Oil andGas India Conference and Exhibition,Mumbai, 2022 January. The paper hasnot been peer reviewed.

Real-Time Optimization of Drilling Parameters

DRILLING TECHNOLOGY


Fig. 1Drilling-optimization data-transmission process; D/A=digital toanalog.

Sensors

Sensors

Rigsite Network

Rigsite NetworkOperation Center Network

D/AConverter D/A

Converter

Optimization

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cant improvements in bit technology,improved drilling-fluid technology, and,most importantly optimized drilling.After the 1970s, rigs with fully automat-ed systems and closed-loop computersystems with the ability to control thedrilling variables began to be used in oiland gas fields. In the mid-1980s, opera-tor companies developed drilling-opti-

mization techniques in which their fieldpersonnel could perform optimizationat the site, referring to graph templatesand equations. In the 1990s, differ-ent drilling-planning approaches wereintroduced to identify the best-possiblewell-construction performances. Later,drilling-the-limit optimization tech-niques also were introduced. Towardthe end of the millennium, real-timemonitoring techniques began to be used(e.g., drilling parameters started to bemonitored off site). A few years later,

real-time operations/support centersstarted to be constructed. Some opera-tors proposed advanced techniques inmonitoring drilling parameters at therigsite. In recent years, drilling param-eters became easily acquired, stored,and transferred in real time. Followingthe invention of the sophisticated andautomated rig and data-acquisitionmicroelectronic systems linked to com-puters, a range of drilling-optimizationand -control services began to be avail-able. With advanced smart-computersystems, drilling ROP and bit lives wereoptimized by performing drilloff tests.Currently, state-of-the-art, high-speedInternet-protocol communication sys-tems are functioning with microwavebroadband networks as useful tools foroil and gas operations, enabling deploy-ment of faster, more-efficient networksto the fields.

Drilling-ROP Model and TheoryThe drilling-ROP model adapted forthis study is a function of eight inde-pendent variables, as in the model

introduced in 1974. The reason thismodel was selected is that it is one ofthe most complete mathematical drill-ing-ROP models that has found wideacceptance within the drilling industry.

Optimization ofDrilling ParametersA drilling-ROP model is defined toaccomplish the unique objective of thisstudythat is, to conduct real-timeanalysis for drilling-ROP optimization.The objective of the study is to opti-

mize applied WOB and string rotation.Optimization is to be formation specific.A multiple-linear-regression techniqueis used for the optimization methodol-ogy, which is a statistical approach.Multiple regression is used to find thoseparameters of an equation that causethe equation to represent the data best.A computer program has been created

to find the coefficients of the model,mathematically correlating the ROPwith the controllable and uncontrol-lable drilling parameters. The task is toacquire drilling data at a rigsite network,transmit the acquired data to the opera-tion center, perform the analysis at theoperation center, and send feedback tothe rigsite. Fig. 1 shows the data-trans-mission methodology of the process.

The data-processing technique isperformed on the drilling dataset toachieve the general-linear-ROP-equa-

tion constants. The equation constantsdetermined are used in the generalequation to predict drilling ROP as afunction of input drilling parameters.

Fig. 4 in the full-length paper showsa schematic representation of the work-flow for a multiple-regression analysis.The minimum number of datasets fora multiple regression to solve an 88matrix is five. The coefficients for the firstfive datasets are solved first, and then theloop is repeated to solve for coefficients,each time including one more set of data.The loop is continued until the numberof requested datasets is processed. Acomputer program has been created toperform the necessary calculations.

The multiple-regression technique isbased on a regression model that con-tains more than one regressor variable.Multivariate data analysis is characteriza-tion of an observation unit by severalvariables. Multivariate-analysis methodsare affected by the changes in magnitudeof several properties simultaneously.Multiple regression considers all possibleinteractions within combinations of vari-

ables as well as the variables themselves.

Representation of ResultsTable 1 in the full-length paper givesthe minimum, maximum, and averagedata-input ranges of three importantdrilling parameters for the wellbore-section results presented. The well wasdrilled directionally in the target forma-tion, which was dominated mainly byshale and sand in a 12-in. hole.

One of the most important outputsfor the data analysis is the observed-

vs.-predicted ROP comparison chart,shown in Fig. 6 in the full-length paper.The chart includes three different datas-ets for the same database. The availabledata section is composed of approxi-mately 900 data points. The first datasetis the one without any correction. Thesecond dataset is the one with onlyWOB normalization. There is a two-fold

improvement in the coefficient-of-deter-mination magnitude (R2) between thefirst and the second dataset. The thirddataset is the interpolated form of data.The number of points in the dataset hasbeen reduced to approximately 100 byinterpolating the data points at a regulardepth interval. All parameters have beensampled accordingly to have the most-representative magnitude in referenceto what the actual data readings were.Also, the bit-rotation correction causedby the mud motor has been performed

for the third data group. The coefficientof determination is within a two-foldmagnitude in reference to the latter.

ConclusionsData quality is very important for real-time drilling optimization. Data shouldbe representative and accurate whenused as an input for multiple-regressionanalysis. Results indicate that instead ofincluding all available data points, areduced number of sampled data points(i.e., representative of the existing datatrend) could give much more accurateresults. This is attributed to the spikytrend of the raw data. During real-timeoperations, the data could be sampledat certain intervals.

Another important point is consider-ation of the wellbore inclination in theanalysis. The WOB was converted intothe vertical component of the WOB, touse the normalized magnitude of theapplied WOB. It was observed that use ofthe normalized WOB resulted in greateraccuracy of the ROP prediction and,consequently, more-accurate results for

the optimum drilling parameters.This study demonstrated that drill-

ing ROP could be predicted at rela-tively accurate levels, on the basisof past drilling trends. The optimumWOB and bit-rotation speed could bedetermined to achieve minimum-costdrilling. It is believed that by meansof effective communication infrastruc-tures and through team efforts, effi-cient real-time drilling optimizationsbased on statistical syntheses are nottoo far in the future. JPT

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Vibrations are a common contribu-tor to premature drillstring failure.The need for a better understandingof the phenomenon has driven theimplementation of real-time down-hole drilling mechanics measure-ments. The full-length paper describesa numerical-modeling tool developedto enhance understanding of the tran-

sient dynamics experienced by a drill-string during drilling operations. Afinite-rigid-body approach was chosenfor modeling simplicity, computationalcost, and physical relevance of thecomputed results.

IntroductionThe drillstring is modeled as a chainof cylindrical segments. Adjacent seg-ments are held together through setsof axial, shear, torsion, and bendingsprings. The spring constants are com-puted on the basis of material proper-ties, and the segment geometry (crosssections and segment lengths) are com-puted by use of standard linear elastictheory, to capture the respective axial,shear, torsion, and bending stiffness ofthe drillstring. At any moment in time,the spring forces and moments arecomputed on the basis of their springconstants and the deviations in relativeposition and orientation of the adjacentsegments with respect to a referenceundeformed state.

Segment lengths are chosen suffi-ciently short (typically two to fourtimes the local tool diameter) so theyeffectively can be treated as rigid bod-ies. At any moment in time, the move-ment of a segment (i.e., linear andangular accelerations) follows theclassical Newtonian laws of dynamics.Forces and moments acting on each

segment include the internal forces andmoments and the forces and momentscaused by the mud and interaction withthe borehole wall.

When updating linear and angularvelocities, some external forces actingon the bottomhole assembly (BHA)segments, such as mud drag and damp-ing as a result of an assumed visco-elastic nature of the borehole wall,can influence respective velocities, andtheir effect is considered by use of animplicit iterative scheme.

The interaction of the drillstring withthe circulating mud can be complex,requiring computationally expensivefluid-dynamics models to capture thefull drilling-fluid effect. This wouldmake computational times impracti-cal without dedicated computationalresources, so a simplified lumped-parameter model of mud effects wasused at the segment level. This includeslinear approximations of axial, later-al, and torsional drag caused by fluidinside the drillstring and annulus. Thedrag is proportional to mud viscosity,

effective area of contact between toolsegment and mud, and relative speeds.The proportionality constants werechosen on the basis of comparison ofthe model to field cases.

Accurate representation of the bore-hole wall is important to capture rep-resentative accelerations and motionssuch as whirl. The borehole wall ismodeled as a viscoelastic boundary withfriction by use of a modified Hertziancontact formulation, which takes into

account the compliance resulting fromthe hollow geometry of the tool crosssection (Fig. 1). Three parametersdetermine the interaction between thedrillstring and the borehole wall:

The first parameter represents thecombined stiffness of the tool/rock two-body system. This results in a forcenormal to the borehole wall and whosemagnitude is a function of the instanta-neous interference depth between thetool and the rock. The interferencedepth is a geometric measurement ofhow far the tool would have penetrated

into the rock wall if the two bodieswere perfectly rigid. The equivalentrepresentation of the combined stiff-ness is that of two springs in series, onethe result of the linear flexing stiffnessof the tool cross section, and the sec-ond the result of the effective nonlinearstiffness of the cylinders in contact.

The second parameter defining thetool/wall interaction is the restitutioncoefficient, which is the ratio of thebounce speed to the approach speed

This article, written by Assistant Tech-nology Editor Karen Bybee, containshighlights of paper SPE 137754,Modeling Transient Vibrations WhileDrilling Using a Finite Rigid BodyApproach, by J. Pabon, N. Wicks,SPE, Y. Chang, SPE, B. Dow, and R.Harmer, SPE, Schlumberger, originallyprepared for the 2010 SPE DeepwaterDrilling and Completions Conference,Galveston, Texas, 56 October. Thepaper has not been peer reviewed.

Modeling Transient Vibrations While Drilling

By Use of a Finite-Rigid-Body Approach

DRILLING TECHNOLOGY


Fig. 1Viscolastic boundary; m=buoyed mass of drillstring seg-ment, =friction coefficient, B=viscoelastic damping coefficient,and K=a stiffness constant char-acterizing the elastic interactionbetween tool and borehole wall.

KB

m

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when there is a collision between thetwo bodies. The speeds are measuredalong the direction normal to the con-tacting surfaces. This parameter effec-tively captures the viscous nature ofthe boundary.

The last parameter in the lumpedtool/wall-interaction model is the fric-tion coefficient. This is used to compute

an effective friction force on the tool atthe point of contact with the wall andin a direction opposite to the instanta-neous tangent component of the veloc-ity of the tool at the point of contact.

The drill-bit interaction with the rockis modeled using an extension of anempirically based formulation in whichreaction forces and torques predomi-nantly are dependent on the depth ofcut of the bit, rock strength, and bitgeometry. The purpose of this extensionis to characterize all six degrees of free-

dom of the bit movement. For instance,natural bit imbalance, sideways cuttingaction, and rock reactions to changesin bit-axis direction are included in theformulation. Two additional parametersare used to characterize what usuallyis called the bit walking tendency.Representative models are supportedfor a wide range of drilling tools.

Model Validation

As part of the validation of the model,a range of static and dynamic cases wasanalyzed including comparisons to:

Analytical beam equations Empirical buckling equations A field-proven finite-element torque-

and-drag model A frequency-domain vibration

model High-frequency downhole data

from the field

Case Studies

Case 1: Helical Buckling DuringDrilling an Extended-Reach-Drilling(ERD) Well. A post-event root-cause

analysis was undertaken on a twistoffevent that occurred while backreamingout of the hole during the drilling ofthe 121/4-in. section of an ERD well.Analysis of the drilling-mechanics datain conjunction with torque-and-dragmodeling indicated that during drill-ing of the section, before the drillstringfailure while backreaming, significantperiods of time were spent drillingwith weight on bits (WOBs) that wasexpected to be greater than the buck-ling limit of the drillstring.

Simulations were run with the tran-sient model at 23,750 ft to visualizethe helical buckling of the drillstring,to evaluate the loading that the drill-string would have been expected to beunder in this condition in a gauge anda 2-in.-overgauge hole, and to evaluatethe torque signature that would havebeen seen at surface in response to the

drillstring transitioning from sinusoidalto helical buckling. Within a simula-tion, the surface rotational speed wasramped up to 120 rev/min, and thenafter a delay of 6 seconds, the surfaceWOB was ramped up slowly from 0 to60,000 lbf over 60 seconds. The resultsof the simulations are shown in Figs. 7athrough 7f in the full-length paper.

The model quantified, given a num-ber of assumptions, the loading on thedrillstring as it transitioned into and wasoperated in a helically buckled state. In

this case, the expected change in surfacetorque when transitioning into a heli-cally buckled state was relatively low,which highlights the risk that the drill-string easily could have been operatedin a helically buckled state without clearsurface-torque indications.

Case 2: Whirl-Related DrillstringFailures Vertically Drilling 171/2-in.-Hole Sections With Packed Mud-Motor Assemblies. A number of cata-strophic vibration-related drillstringfailures were experienced in BHAs in afield where vertical 171/2-in.-hole sec-tions were being drilled with packedmud-motor assemblies. Analysis ofthe downhole drilling-mechanicsdata, which included lateral and axialroot-mean-squared accelerations froma multivibration cartridge within ameasurement-while-drilling tool, andstick/slip measurements indicated theBHA was in backward or chaotic whirlfor sustained periods. Simulationswere run within the transient modelwith the goal of understanding the

possible motions of the BHA and thesensitivity of the system to excitationfrom the nutation of the mud motorand to downhole friction factor atcontact points.

Results from the model indicated: The downhole friction factor at

contact points between the BHA andthe borehole wall is a key driver inkicking the system into whirl, androller reamers potentially are beneficialin offsetting the point at which thedrillstring enters whirl.

At lower surface rotational speed,the system is less likely to enter whirland when in whirl is exposed to lowerbending moments and fatigue com-pared with whirl at higher surface rota-tional speed.

In a low-damping large-hole-sizeenvironment, the the mud motor is animportant source of excitation, espe-

cially because the rotor whirls coun-terclockwise within the stator. In thecase of a heavily stabilized assembly,there is a risk that the vibrationalenergy becomes trapped within theBHA, increasing the risk of the BHAentering whirl, although the increasedstabilization reduces lateral motionswhen in whirl.

The model was used to developand support a recommendation to drillwith lower surface rotational speed.Lower rates of penetration were expe-

rienced, but in this environment thelevels of vibration were reduced dra-matically with no significant change information properties, and it was clearthat the assembly no longer was enter-ing backward/chaotic whirl. JPT

RESOURCES

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Hole cleaning during directional-welldrilling is a major concern in the oilfield and must be monitored andproperly controlled during the entiredrilling operation. Inadequate drilled-cuttings removal can cause manycostly problems. Low annular-fluidvelocity, lack of drillpipe rotation, andthe wrong mud properties are pri-

mary factors in ineffective hole clean-ing. The full-length paper presents areview of previous hole-cleaning stud-ies and discusses an approach that isbetter suited for monitoring and con-trolling hole-cleaning problems.

IntroductionThe rotary-drilling process consists of arock-cutting tool (drill bit) upon whicha downward force is applied [weight onbit (WOB)] and rotation (rev/min) isimposed. The drilled cuttings generatedby the drill bit are removed by circulat-ing a drilling fluid from surface to bot-tomhole and back to surface (cuttingstransport) (Fig. 1).

During directional-well drilling, cut-tings removal becomes more difficultand if not controlled properly can resultin serious problems such as mechani-cal pipe sticking (fishing or hole loss),excessive frictional torque (increase inrotary-power requirement) and frictionaldrag (inability to reach target), difficultyin landing casing, channeling problems

during cementing, and difficulty in log-ging. The factors that are known to affecthole cleaning include annular eccentric-ity, inclination angle, drillpipe rotation,fluid flow rate (annular velocity andflow regime), rate of penetration (ROP),mud rheology and density, and cuttings(i.e., size, shape, and density). It is welldocumented that flow rate and drillpipe

rotation have the most positive effect onhole cleaning. However, an increase inflow rate causes an increase in frictionalpressure losses, which in turn causes anincrease in equivalent circulating den-sity (ECD), pump-pressure requirement,and potential hole erosion. An increasein pipe rotation can result in prema-ture pipe fatigue failures caused by theinduced cyclic stresses.

Cuttings-Transport ProblemIn contrast to other industrial process-es, the cuttings-transport problem inthe drilling industry is more complexbecause of the many parameters thatare interconnected nonlinearly. In sim-pler processes, some of the steps couldbe neglected.

Comparing this scheme to a drillingprocess results in the following cor-relations:

Processdrilling procedure ReferenceECD limitation, pump

hydraulic-horsepower limitation, holeerosion, ROP limitation, geological-tar-get limitation, rotary-power limitation

Process inputsflow rate, rev/min,hookload

Process outputspressures, ROP,torque

Internal statesWOB, eccentricity,hole cleaning

Monitoring/controllost circula-tion, drag, torque, cuttings returns,ROP, circulating standpipe pressure

For a complete and reliable automat-ed design process, the following stepsmust be taken:

Signal measurements Monitoring of critical parameters Modeling Control

Design ParametersIn any system, including drilling, thereare various design parameters to be con-sidered. There are parameters that arereferred to as effective parameters orcontrol signals, which are processinputs, and there are affected param-eters, which are the internal-processstates or the outputs. Any changein the effective parameters will cause achange in the affected parameters. Theaffected parameters are divided into two

groupsinternal states and outputs. Theoutputs are measured parameters suchas circulating standpipe pressure, whileinternal states such as annular cuttings-bed height or pipe eccentricity cannot bemeasured while drilling is taking place.

During drilling, it is essential to main-tain certain drilling parameters (e.g.,bottomhole pressure, flow rate, annu-lar cuttings concentration, and torque)within the proper range. The desiredvalue of any of the affected parameters

This article, written by Assistant Tech-nology Editor Karen Bybee, containshighlights of paper SPE 132372,Review of Cuttings Transport inDirectional-Well Drilling: SystematicApproach, by T. Nazari, SPE, and G.Hareland, SPE, University of Calgary,andJ.J. Azar, SPE, University of Tulsa,originally prepared for the 2010 SPEWestern Regional Meeting, Anaheim,California, 2729 May. The paper hasnot been peer reviewed.

A Review of Cuttings Transport in Directional-Well Drilling

DRILLING TECHNOLOGY


Fig. 1Drilling-rig schematic.

1 Mud pumps

2 Swivel

3 Kelly

4 Drill bit

5 Drillpipe

6 Annular space

7 Cuttings

8 Shakers

9 Mud pits

10 Rotary table

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will be designated as the reference.Comparing the measured signals withthe references, the controller will adjustthe effective parameters automaticallyso that the reference values will be sat-isfied. In most cases, it is essential forthe controller to optimize the effectiveparameters so that they are in somespecific operational range. For example,

flow rate as an input cannot be greaterthan some specific value determinedby pump characteristics. The controllerand some of the monitoring methods arebased on real time or an offline model. Averified general model can be presentedon the basis of the physical relations ofthe process parameters or measureddatasets of process parameters. Hybridmethods for modeling of different drill-ing processes also have been presented.

If there were no limitations on theinternal states, the monitoring process

would be less complicated. Becausesome important drilling parameters thatcannot be measured have to be keptin specific working regions, the moni-toring process becomes important forthe drilling process. Monitoring also isimportant in setting up alarms to alert

when undesirable conditions occur.Measurement and monitoring duringdrilling operations have been practicedfor a long time. However, to ensure asystematic control of the process, anautomated monitoring by use of inter-nal-states estimation must be carriedout first. In the full-length paper, thefocus is on hole cleaning as an internal

state that should be monitored.

Cuttings RemovalDrilled-cuttings removal (cuttingstransport) from the annulus duringdrilling has been the subject of researchfor several decades. Different investiga-tors have taken different approaches indealing with the hole-cleaning problem.To set the stage for the present work, itis important to summarize the drilling-process parameters, which leads to adetailed review of past studies.

Past studies investigated the effectsof some or most of the process inputson hole-cleaning parameters such ascuttings concentration. A summary oftheir results includes:

Mud flow rate has a significantpositive effect.

Mud rheology has a moderate posi-tive or negative effect, depending oncuttings size, pipe rotation, hole incli-nation, and annular eccentricity.

Hole angle has a significant nega-tive effect with increase in inclination.

Mud weight has a small positiveeffect.

Mud type has a small-to-moderate

positive effect. Hole size has a small-to-negligible

effect for the same annular fluid veloc-ity.

Rotation speed has a significantpositive effect.

Eccentricity has a significant nega-tive effect.

ROP has a moderate negative effect. Drill-bit type has an unknown

influence as a result of the regrinding ofcuttings after they have been generated.

Cuttings size has a small negative

or positive effect, depending on severalconditions.

Parameter CombinationsInvestigation of the sensitivities of com-binations of parameters results in thefollowing points:

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Hole inclination, annular mudvelocity, and drillpipe rotation are themost important parameters.

Consideration of combinations offluid velocity and mud rheology revealsthat low viscosity in turbulent flowhas the same result as high viscosity inlaminar flow in vertical wells. A turbu-lent flat flow profile is more effectivein inclined wellbores. Increasing the

viscosity causes a peak in the minimumtransporting velocity.

Low flow rate and high rotaryspeed in horizontal wells provide anenhancement in cuttings transport.

In an attempt to find a globalcondition for sufficient hole cleaning,water-based, low-gel-strength, low-viscosity mud with pipe rotation wasintroduced.

Conclusions and

Recommendations

Some of the weak and strong points ofresearch in addition to some recom-mendations for future investigationsare listed below.

There currently is no generalizedsystematic model developed for holecleaning; therefore, the development

of a universal model can be verybeneficial.

Some powerful investigations ofsensitivity analysis have been con-ducted, with confirmed conclusionsreleased on controllability of holecleaning by input vectors.

Previously, misinterpretations ofinputs have occurred. Adding andclearly defining more parameters as

inputs, internal states, or outputs willgeneralize the structure of drilling hole-cleaning processes.

Some datasets in previous experi-mental investigations caused uncertain-ties that cannot be removed using sta-tistical analysis. Gathering of completedatasets with field and experimentaldata will be highly useful in sensitiv-ity analysis or verification of new sys-tematic hole-cleaning models. Differentdata-mining methods such as principal-components analysis can be used to

find a confirmed mathematics-basedsensitivity-analysis measurement. Previously presented models of

hole cleaning were confirmed on thebasis of some specific condition of drill-ing or by a limited number of experi-ments. Because the drilling process is

highly nonlinearly dependent, a gener-alized model of this process, or at leastpart of it, can be obtained by a hybriddata-/physics-based method, such asfuzzy modeling.

For design of an automatic moni-toring system for hole cleaning, com-plete datasets including different failurepoints or a complete set of modelsthat covers different failures and their

results on outputs is required. A reliable monitoring system is a

must for an automatic and online hole-cleaning control package.

Steps toward automation of a hole-cleaning process are initiated by dis-covering all the effective and criticalaffected parameters. More research onpresenting a drilling process (or part ofit) in a systematic view will increase thegenerality of the results.

The relations between effective andaffected parameters can be reached by

both theoretical and data-mining meth-ods, or even a combination of the two.Any efforts toward global modeling ofthe different drilling processes will helpthe other steps of hole-cleaning-mon-itoring automation (such as real-timetorque-and-drag modeling). JPT

www.shale-gas-water-management-2011.comcall: 1-800-721-3915 mail: [email protected]

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