SPE 37612

OPTIMIZING HOLE CLEANING BY APPLICATION OF A PRESSURE WHILEDRILLING TOOLM.D.J. Easton, SPE, Sperry Sun Drilling Services; J. Nichols, SPE, KCA Drilling Ltd.; G.J. Riley, KCA Drilling Ltd.

Copyright 1997, SPE/IADC Drilling Conference

This paper was prepared for presentation at the 1997 SPE/IADC Drilling Conference held mAmsterdam, The Netherlands, 4-8 March 1997.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s) Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s) The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers IS

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Abstract

This paper introduces the use of monitoring annulus pressurewhile drilling (PWD), both real Time and recorded as animportant tool for improving operational efficiency andreducing risk on Extended Reach Drilling (ERD) projects.Hole Cleaning and cuttings transport effectiveness can berelated to recognizable variations in the downhole measuredequivalent mud weight (EMW). Pipe rotation is shown to beessential in removing cuttings in high angle wells, and alsoincreases the equivalent circulating density. The effects ofsurface rheology changes are evident in the EMW, thefrequency of pack-offs and hole cleaning efficiency.Operating practices can be optimised once the informationavailable from PWD is understood.

Introduction

The need to optimise field recovery and to exploit reservoirseconomically has lead to the drilling of extended reach wells.Additional reserves have the potential of increasing theamount of recoverable hydrocarbons and hence field life.

Challenges typical of extended reach drilling programsincluding hole cleaning, mud weight management for wellborestability and lost circulation hazards have been encountered.Capability of the rig and ancillary equipment impacted someaspects of the well design, drill string design and mud systemmanagement. KCA Drilling limited and Sperry Sun Drilling

Services have been involved as the drilling contractor and thedirectional drilling and MWD contractor on an extended reachdrilling program for a North Sea operator.

At the planning stage, the major hazards identified in the 12¼"hole section included potential serious lost circulation, holeinstability in reactive clays, and drilling rate control to avoidinsufficient hole cleaning with the attainable circulating rates.Successful 9 5/8” casing string running and cementing tomeasured depths of more than 18000 ft. was required. In orderto understand the downhole pressures which were beingexerted, annular pressure monitoring was specified, using thePressure While Drilling (PWD) tool as part of theMeasurement While Drilling (MWD) system in the directionalbottom hole assembly.

Tools and Techniques

Annulus pressure measurement was developed initially inresponse to requirements to monitor underbalanced drillingoperations. Recorded and real-time tools were adapted fromdownhole pressure recording tools utilised in productiontesting and monitoring. The measurement of annular pressurehas proven to be useful in many well types. Importantapplications for the tools have been in situations whereEquivalent Circulating Density (ECD) and mud weight controlis critical. These include deepwater wells with low fracturegradients, high pressure high temperature wells with smallmargins between formation pressure and fracture pressuresand in high angle and horizontal wells with high ECD’S andnarrow hole stability and fracture gradient limits.

The rigours of the drilling process required repackaging of theinternal components of the pressure gauges to MWDstandards. Strain gauge or quartz transducers can be rundepending on the pressure and temperature limits expected. Amaximum temperature limit of 175°C and pressure limit of20,000 psi can currently be covered. Dead weight testercalibration is performed on recording gauges before and afteruse in a well. The recording sonde gauge is located in a non-

SPE 37612 OPTIMIZING HOLE CLEANING BY APPLICATION OF A PRESSURE WHILE DRILLING TOOL 2

magnetic sub with annular ports. This can be run as astandalone sub, recording annular and, if required, internalpipe pressures. For real time monitoring, the sub is interfacedwith an MWD system for real time pulsing of downholeannular pressure data approximately every 30 seconds.Recording gauges can be run with the real time system toprovide information on tripping pressures, downholemeasurement of leak off data, and other information whenlow or no flow is passing through the tool. The recordinggauges are programmable and can store data at any requiredintervals from 2 seconds upwards. Usual settings are for 10 to20 second data intervals.

The tool needs to be placed as close to the bit as possible forthe best results, as significant pressure losses occur in theannulus across the drill collars. Downhole pack-off occursaround the drill collars and string stabilisers. It is preferable toplace the tool where these can be clearly seen as pressureincreases.

Surface software matches the downhole pressure to the toolTVD and provides downhole pressure readings and equivalentmud weight readings for display and data storage against timeand depth. It is desirable to capture drilling parameters tomatch the downhole data and to provide full information onthe drilling operation for real time and for post run analysis.Time based data from either mud logging units or commercialcomputerised rig monitoring systems has been usedsuccessfully. The parameters found to be useful include pumprates, pump pressures, drilling rate, hookload, weight on bit,RPM, torque, surface mud properties and pit levels.

Modeling and Prediction

Pressure losses in the drill string and the annulus arecalculated by onsite personnel using theological models incommon use: Bingham, Modified Power Law (Herschel-Bulkley), and Power Law. Predictive hydraulics wereperformed on surface mud theologies measured understandard API conditions Actual downhole ECD’s and EMWresults were compared at the rigsite with predicted ECD’s.Corrections were not applied for temperature and pressureeffects on the low toxic and synthetic oil based fluids used.

Fit of downhole measured pressures to the surface predictionswas highly variable and depended on the well and pipegeometry, hole section size, mud type and downhole pressureand temperature conditions. No generalisations can easily bemade. The commonly used “simple” models do not predict theeffect of pipe rotation, or pipe eccentricity in the hole. As aresult, actual pressures were matched with the model that bestfitted the data; predictions of surge-swab, ECD’S andcirculating rates were made on an empirical basis.

Guidelines for overall penetration rates were set using thecharts developed by Luo, Bern et.al. relating mud properties,flow rates and penetration rates for acceptable hole cleaning.

Operational Program

The system has been run on three extended reach12¼"sections for an operator in the Northern UK sector of theNorth Sea. Examples are given from the first two wells.

On well 1, a kick-off assembly, comprising a 9-5/8" mud motorwith a 1.5 degree bent housing, and a 3 stabiliser BHA wasrun from 6578 ft md to achieve kick off from a cement plug.The 12%” hole section built from 55° to 80° with an azimuthturn from 1810 to 209°, dogleg severity was 2.50/100’. The 9-5/8” casing was planned to be set below 18,000 ft rkbmd, andactually set at 20,000 ft rkbmd. The 8½" hole section wasdrilled to 23650 ft rkbmd

The drill string used 6 5/8" and 5½” drill pipe to maximise theattainable flow rate,. The string was designed so the length ofthe BHA and 6 5/8" pipe could be pumped out of the hole and400ft back inside the casing without having to stop and changeout the top-drive saver sub.

On well 1, one real time and three recorded PWD runs weremade in the 12¼” section to the section total measured depthof 20020 ft. Recorded data was also obtained on the first bitrun of the 8½" hole section.

On well 2, the PWD tool was run real time on the first two bitruns in the 12¼” section, in the build from 50° to the tangentangle of 76°. A motor build assembly was run initiallyfollowed by a tangent assembly before the BHA was trippedfor full service Formation Evaluation MWD for the final partof the open hole to a casing point at 18492 ft rkbmd. Similardrill string configurations were used as in well 1 to maximiseattainable flow rates.

Based on the experiences on previous wells, the primaryobjectives were to:. Maintain casing shoe integrity and avoid losses;● Maintain good hole cleaning to avoid cuttings build up,

pack-off and stuck pipe problems;● Maintain hole stability and integrity through the section;● Run and cement 9 5/8” casing with minimal problems.

The real time and recorded PWD tools were used to assist inidentifying potential hazards, modify and improve operationalprocedures, and to understand better actual downholehydraulics. The following observations outline the lessonslearned from the application of the tools.

SPE 37612 OPTIMIZING HOLE CLEANING BY APPLICATION OF A PRESSURE WHILE DRILLING TOOL 3

appeared to create an immobile sludge of very fine cuttings onthe hole low side.

Use and Effects of Pipe Rotation

Example 1, 12¼” Hole, 78° inclination, 03:00-09:00 hr.

Example 1 (see Figure 3) shows recorded information fromrun 3 on well 1, at 16500 ft measured depth. It clearly showsthe difference in downhole equivalent mud weights measuredby the PWD tool. Lower EMW’S are seen oriented drilling,and higher EM W’s when circulating with pipe rotation andwhen rotary drilling. After two hours drilling oriented, mudmotor stalls, poor sliding and transmission of weight werebecoming a problem. Small pressure surges of 0.05-0.1 ppgafter 5:30 am occurred while drilling, and by 6:00 am it wasnot possible to drill oriented, despite working the pipe.Circulation with rotation was necessary to clear cuttingsaround the BHA and drill string in the open hole beforecontinuing to rotary drill the remainder of the stand to the nextconnection.

Observations:

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A marked difference in ECD between rotary and orienteddrilling of approximately 100 psi. in 12%” hole. In 8%”hole this effect was doubled with 150-200 psi observedbetween orientation and rotation.

Even with high flow rates (900+ gpm), minimal holecleaning in high angle section was seen when orienteddrilling. Cuttings are picked up immediately on startingrotation, adding 50-150 psi to the ECD.

rapidly became obvious, from the real time data beingtransmitted that minimal hole cleaning was occurring abovethe BHA during the oriented drilling sections. This wasdespite high annular velocities with a pump rate ofapproximately 940 gallons per minute. The annular ECDdecreased markedly as oriented drilling progressed. This effectappeared to be caused as cuttings settled around the pipe andBHA in the high angle section, and were cleaned out of thelow angle top hole. On commencing rotation, immediate

rotation was both increasing the ECD and picking up cuttings.This appeared to occur at 50-75 rpm, and confirms the generalobservation that rotation is absolutely necessary for adequatehole cleaning whether drilling or circulating in deviated wells.On the first run, the motor bend angle restricted rotary speedsto 100 rpm, subsequent runs used 120 rpm. Very high rotaryspeeds were not used. It was suspected from previous ERDexperience that PDC cuttings degradation occurred with highrotary speeds (180 rpm). The cuttings degradation had resultedin a fine solids increase in the circulating system and also

These observations were used to confirm good practices whencirculating the hole clean. The hole was always circulated withrotation at 100 rpm. The PWD readings were checked toconfirm that a steady decrease in the EMW was occurringduring the circulation, and the shakers were carrying a highcuttings load. In order to avoid washouts or accidentalIedging, the pipe was worked, and stands backed out ifextended circulating periods were anticipated.

Example 2, 12%” Hole, 78° inclination, 09:30-17:00 hr.

Example 2 (see Figure 4) shows data from later in the sameday. After another oriented drilling set from 09:00-11:30, theremainder of the stand was drilled with rotation. At 12:30, thePWD shows a peak in downhole equivalent mud weight.Shortly after, drilling was suspended by heavy cuttingsloading at the shakers and the hole was again circulated withrotation. The increasing EMW trend indicates that the mud isstill carrying cuttings, and a second peak at 15:30 was seenbefore the EMW dropped down before the connection at16:30.

Observations:

. With controlled drilling rates and rotation, excess cuttingsin the annulus were seen as a pressure peak approx. 10-15minutes before heavy cuttings loadings were observed atthe shakers.

. Fast rotary drilling directly after oriented drilling produceda slug of cuttings up the hole. Back reaming can space outthe slug and avoided overloading the shakers.

A pressure peak of approximately 100 psi was consistentlyobserved ten to fifteen minutes before excess cuttings loadingswere observed at the shale shaker. This was subsequentlycorrelated to performing fast rotary drilling after an orienteddrilling set. The cuttings were only mobilised from around thedrill collars and pipe when rotation was started. Thiseffectively caused a slug of cuttings from the entire standmoving up the annulus into the low angle upper hole section(and thus exerted a larger effect on the bottom hole pressure).

More than half a stand of oriented drilling lead to slugs ofcuttings in the annulus when fast rotary drilling wasrecommenced at instantaneous drill rates of 150-500+ ft/hr.with 940 gpm flow rates. Cuttings overloading occurred at the

SPE 37612 OPTIMIZING HOLE CLEANING BY APPLICATION OF A PRESSURE WHILE DRILLING TOOL 4

shakers several times, resulting in additional circulating timeto unload the shakers and clean the hole.

After drilling a rotary set, circulate with rotation for a fewminutes before starting the oriented set. This removes cuttingsfrom behind the BHA, resulting in better oriented drillingperformance. After the oriented set a brief backream beforethe connection removes cuttings from behind the BHAreducing the chance of pack-offs and stuck pipe. The cuttingsare spaced along the hole to avoid surface system overload.Downreaming pipe speeds must be moderated in suchcircumstances, as the cumulative effects of pipe surge,rotation, circulating ECD’S and cuttings loadings can giveexcessive surges and induce lost circulation.

Indications of Poor Hole Cleaning

The annulus pressure and thus the downhole equivalent mudweight and ECD increase when cuttings are suspended in themud. The formation of inert cuttings beds or accumulations inwashed out hole is thus generally indicated by a lower thanexpected equivalent mud weight recorded by the tool. Excesscuttings then start to cause annular blockage, seen as pack-offsand annular pressure increases. Other indications, such asanomalous torque and drag, and abnormal cuttings loadings atthe shakers also give warning of hole cleaning problems.

A clear indicator of the formation of cuttings beds is whensmall pack-offs are seen by the tool, of the order of 100-300psi, especially on picking up off bottom. This often occurswhile oriented drilling, and indicates a partial blockage of theannulus. When drilling with a mud motor these smallvariations in standpipe pressure can be masked on standpipereadings by the variations in pressure drop caused by the mudmotor power and torque.

Abnormally high cuttings loadings have been related todrilling oriented, followed by rotation. Hole cleaning waseffectively nil without pipe rotation even at 940 gpm flowrates in the 12¼" section with the effects becoming morepronounced at higher angles, as demonstrated in examples 1 &2.

Drilling difficulties were also caused by cuttings settlingaround the drill pipe well above the BHA and the open holesection. In the 8½” section on well 1, after orienting 2½"

stands, the final part stand was rotated. After the connectionsevere difficulty was experienced in orienting the pipe and indrilling successfully. The recorded PWD trace showed anincreasing trend in the EMW before the connection. It appearsthe cuttings were mobilised and carried up the hole by therotation. On the connection they settled around the pipe inside

the casing, restricting pipe movement and transmission ofweight, and causing difficulties with oriented drilling for morethan one hour. The plots indicated that further rotation withcirculation was necessary to remove the cuttings from thewell.

In hole sections where washouts are suspected or are known tooccur, the immobile cuttings in the washout may not be seenby the PWD until they are dislodged by pipe movement. Theythen pack-off the bottom hole assembly, causing largepressure surges. The PWD may indicate a problem, but otherindicators such as excess drag, overpull, and anomalous MWDreadings must be observed as well. Backreaming and piperotation must be used. Hole stability is necessary for troubleavoidance. Overgauge hole results in low annular velocitiesand poor hole cleaning, leading to cuttings accumulations,pack-off’s and a severe danger of stuck pipe. The correct mudweight for the hole angle, identification of early signs of holeinstability and minimisation of out of gauge hole are essential.

Example 3 Well 2, 12%” section, 55° inclination

On well 2, example 3 (see figure 5) shows the effect of lowmud rheology. There was overloading of the shakers, pack-offs and excessive ECD’S when the cuttings were lifted intothe mud stream. As the yield point was increased from 15 to25 during the day, this effect was mitigated then disappeared.

Hole Cleaning observations from the PWD were backed up bymonitoring cuttings loading at the shakers, and pit volumedecrease as new hole was drilled.

Example 4, Well 2, 76° Inclination

Example 4 (see figure 6) shows the observed decrease inEMW measured by the PWD tool as the hole was circulatedclean. On reaching 76° inclination, it became obvious that theBHA was requiring too much orientation to hold angle for fastdrilling. After orienting 50% for 6 joints at 76° inclination, theECD had exceeded the predicted values by 10 ppft (0.2 ppg).It was obvious that the annulus was loaded with cuttings, andon circulating the hole clean, the ECD dropped back toBingham model predictions. Combined with shakerobservation, the PWD clearly indicated that the hole hadcleaned up. The trip out was uneventful.

With the new assembly in the hole, the ECD was approx. 5pptf (0.1 ppg) above model predictions, and a precautionarycirculation after 1000’ drilled dropped the ECD back to thepredicted ECD line. This justified drilling ahead withoutneeding to circulate the hole clean periodically, saving approx.

SPE 37612 OPTIMIZING HOLE CLEANING BY APPLICATION OF A PRESSURE WHILE DRILLING TOOL 5

1 - 3 hours a day over the previous practice of regularcirculation each tour.

Circulation times before tripping out were governed bycuttings returns and PWD data. As a result, no problems wereencountered on trips, with minimal overpull or evidence ofcuttings beds.

Use of Hydraulics models

PWD tool measurements were used to calibrate hydraulicsmodel predictions. The model that best matches actual datacan be used to predict ECD’S on a look-ahead basis. Usingdata from measured drilling ECD’S, casing running speedschedules and recommended circulating rates can becalculated. This technique was used when planning the 20000ft 9 5/8" casing run on well 1. Actual annular pressure loss datawas used to select the Modified Power Law as the best fitmodel. The casing was run, circulated and cemented with nomud losses, a notable achievement in the field.

Pipe running speed schedules were also used in the 8½”section to moderate high observed surge pressures whenrunning in hole, and when moving pipe with rotation and flowto reduce excess surge pressures.

On well 1, annular pressure readings were available for theentire 12%” section. Comparison of actual ECD data withmodeled data suggests that actual downhole pressure losseswere affected by temperature and pressure. Figure 1demonstrates the predicted and actual pressure lossesmeasured in the 12’/4” section of well 1, compared withsurface predictions of expected pressure losses in the annulus.It can be seen that the actual ECD increases with depth at aslower rate than predicted. Increasing surface rheology valuestowards the end of the section were not reflected in themeasured ECD values downhole.

Surface models can give a base trend from which deviationsdue to cuttings loading, pack-offs, pipe rotation, etc. can bemore clearly seen.

ROP Control - The optimisation of overall drill rates.On all wells, overall ROP was restricted to approximately 150ft/hr (including connections), in accordance with therecommendations of the hole cleaning charts of Luo, Bern etal . Actual instantaneous ROP’s while rotary drilling were upto 500 ft/hr. below the kick-off point, and instantaneous drillrates of 50-90 ft/hr oriented and 150-200 ft/hr weremaintained to 16000 ft measured depth.

On well 1, in the 12¼" section, excellent hole cleaning wasapparent when rotating at 100-120 rpm. After the first 1,000ftthe mud rheology was increased to higher than planned andhigher than previously used in other long reach wells. In thepast the 6 rpm figure has been kept around 10, throughout thesection it was maintained at 12-15 (see table 1). The PV wasmaintained on average around 42 cp and this allowed the useof 6“ mud pump liners through the section. At the section TDof 20020 ft. it was still possible to maintain a circulation rateof 860 gpm. A lot of attention was paid to the PV and at theone point it started to climb drilling was stopped and the finesstripped out by circulating the hole clean and then reducingthe circulation rate and installing 230 mesh screens on theshakers. The YP was maintained between 23 -26 which gaveeffective hole cleaning.

Hole cleaning ability was related to flow rate. Flow rates of940 gallons per minute gave effective hole cleaning at anoverall rate (including circulations and connections) of 100-150 ft/hr on run 1. On run 3, when flow rates were lowered to800 gpm (but with similar mud properties), the EMW peaksdue to cuttings slugging were higher, and more circulation wasrequired, even though drill rates were slower. On run 4, to thesection TD of 20020 ft, when flow rates were raised to 850-900 gpm, cuttings slugging was less severe.

On well 2, the mud rheology was again increased at the rigsitewith the YP maintained at 27 -33 where the mud programcalled for 20 - 25 YP. This produced higher ECD’S, so pipemovement, especially when reaming and rotating onconnections, had to be controlled to reduce surge pressures.Lower flow rates were used on well 2, 790-820 gpm, butraised low end rheology appeared to compensate for thereduction in annular velocity.

On wells 1 and 2, the main limitations on overall drill ratewere surface system shaker capability at high cuttings loads,and only two mud pumps, leading to flow rate restrictions anda lack of mud pump redundancy. Well 2 was additionallyhandicapped by kelly drilling with no top-drive system, hencean inability to backream effectively. The control of drillingand equipment risk was paramount in this section. In spite ofloss of mud pumps, and rig systems shutdowns while in openhole there were minimal hole problems on the resumption ofoperations. This was proof of trouble avoidance by followinggood operational practices.

Conclusions

The use of annular pressure measurement on extended reachwells allows a substantial improvement in the understandingof downhole conditions. Operational procedures have been

SPE 37612 OPTIMIZING HOLE CLEANING BY APPLICATION OF A PRESSURE WHILE DRILLING TOOL 6

improved, leading to a reduction in non-productive time andbetter drilling efficiency.●

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Real time measurement of annular pressure has confirmedthe importance of rotation as an essential mechanism inefficient hole cleaning at high angles.Circulating with pipe rotation has improved hole cleaningand reduced the risk of stuck pipe, and time lost inunnecessary circulation.API Hydraulics models do not include the effects ofrotation and cuttings build up on downhole annularpressure losses. However, comparisons with actual annularpressure allows the choice of the model that best fits theempirical data.Higher flow rates improved hole cleaning only whenrotation exceeded a threshold speed and low end rheologywas optimised.Daily footage rates were increased from approximately1000 feet per day to 1500 feet per day without increasingwell risks.

NomenclatureBHA = Bottom Hole AssemblyECD = Equivalent Circulating Density, psi/1000 ft or

pounds per gallonEMW = Equivalent Mud Weight, psi/1000 ft or pounds

per gallonFEMWD =Formation Evaluation MWD

PWD =Pressure While Drillingpptf =psi per thousand feet

AcknowledgmentsThe authors wish to thank the management of KCA Drillingand Sperry Sun Drilling Services for their support andpermission to submit this article.

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