SPE 37065

Successful Drilling of an Underbalanced Horizontal Well in the Rigel Halfway Pool -Laboratory Screening and Field ResultsRD. Doane, D.B. Bennion, Hycal Energy Research Laboratories Ltd. and D.M. Eilers, E.R. New, Samedan Oil of Canada Inc.

of the offshore Doig formation and above by continentalhypersaline anhydrites, dolomi~ silts and shales of theCharlie Lake formation. The fonnation exhibits a regionalsoutheasterly dip with an average depth of 1266 meters.

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The pronotmcecl north-south orientation of the JX)O1 is thoughtto be influenced by rejuvenation of deep-seated faults.Although not visible 00 seismic traces, small scale reactivationmay have controlled the axis of deposition and subsequentlyaided in the preservation of reservoir facies.

Abstract The Rigel Halfway C pool is fme-grained. well-sorted~ sublid1arenite widl patchy calcite cementation. It isinterpreted as a c~-cutting tidal channel fill deposit,juXta~ against tight sands and shales of dle Halfwaybarrier-bar system. Using a permeability cutoff to oil of 1.0mD, a porosity cutoff of 8% was detennined for two wellsincluding C-20-L (from which experimental sample selectionswere taken). Based on these cutoffs, net pay of the reservoirranges from approximately 1.1 meters to 7.8 meters. Rockquality of the formation is characterized by an averagepermeability to air of approximately 100 mO. with core samplepermeabilities increasing from 25 to 85 mD in the upperinterval (1263 to 1266 meters) to 100 to 900 mD in the lowerinterval (1266 to 1270 meters). Porosity over the entireinterval increases with depth from 15% to 25%. and originaloil in place for the pool is estimated to be 1.6 x 10' m3.

The Middle Triassic Halfway fonnation in northeastern BritishColumbia is a high quality, well sorted qu~sublitharenite. Past drilling operations using conventionaloverbalanced technology in the reservoir had indicatedsignificant fonnation damage occuned due to a combination ofmechanical and chemical fonnation damage effects associatedwidt the loss of drilling fluid to the fonnation. A series ofcompatibility and special core analysis tests were conducted torefine an underbalanced drilling procedure widt natural gas fora 300 m long horizontal well in the fonnation using a naturalgas-mist hydrocarbon system at a 1400 kPa un~rbalancepressure level. The well was drilled successfully and flowedat peak rates of up to 1100 m3/day (almost 7000 bb)Jday) of oiland 340,000 m3/day (12,000,000 scf/day) of gas during theunderbalanced drilling operation. Post test pressure transientand interfermce tests indicated an in-situ permeability of 150mD widt zero skin factor indicating that the well had beendrilled successfully in an undamaged fashion. The well hascurrently produced close to 13,000 m3 of oil (SO,OOO bbl) andover 11,000,000 m3 (400 MMscf) of gas with initial AOFpotential of approximately 520 m3/day (3300 bb)Jday).

Drilling Strategy

Utilizing drilling and production experience gained throughother wells in the area, Samedan Oil of Canada set out to drilla horizontal well (3-33). The objective of drilling the 3-33well was to target the recoverable oil by drilling a non-damaged horizontal well with high deliverability for the leastdrawdoWD and GOR perfonnance. Experience with the C Poolindicated that fonnation damage had occurred when drillingprevious wens. and that damage was severe enough to preventwater injection for a waterflood, even after perforating theinjector. A fracture job established excellent injectivity leading

Introduction

The Middle T~ic Halfway fom1ation of the Rigel fieldlocated in northeastern British Columbia is part of a regressivesequence, bounded stratigraphically below by silts and shales

SUCCFSSFUL DRn.uNO OF AN UNDERBALANCFD HORIZONTAL WaL IN mE RIGEL HALFWAY P()()L SPE 3706S

to the conclusion that to achieve a s~ well. detenniningthe best method of preventing formation damage caused by thedrilling p~ was necessary.

experiments with whole mud systems. The Rigel study wassomewhat unique in that prior selection of only two mudsystems had been made, and it was known that the well was tobe drilled 1DlderbaJanced. The pupose of the study, therefore,was to detennine which of the two mud systems wouldperform more favourably in an un~rbalanced fashion, thusnegating the need for preliminary formation damagemechanism identification through various intermediarycoreflow tests. Determination of rock wettability and fluidcompatibility was cooducted to ensure that obvious problemswould not be encountered through the use of the oil-basedproduct.

Reviewing the performance of wells in the Rigel area and~hing the su~ of other horizontal welJs drilled inwestern Canada suggested that damage from the drillingprocess conbibutes to less than expected productionperformance. The damage was primarily due to two mainprocesses: 1) selection of inappropriate drilling fluids, and 2)physical damage caused by not being able to remove drillingsolids from around the welloore. It was determined that thebest way to prevent these processes from occurring would beto drill the well underbalanced with a gas/liquid system.Providing the right system is utilized, and underbalancedpressure is maintainedl,2, little or no rock/fluid or fluid/fluidsensitivity should occur, and solids invasion would not be aconcern. Samedan undertook a study jointly with HycalEnergy Research Laboratories to investigate these phenomenawith respect the Halfway sand at Rigel. With these parametersin mind, two hydrocarbon-based fluids were selected for testing(one viscosified and the other unviscosified). Resourcing aneconomic and sufficient supply of gas to maintainunderbalanced pressure supported utilization of sweet naturalgas already being produced by other Samedan welJs in cl~eproximity. With fluids and method of drilling determined, thenext phase was to evaluate the performance of both systems inthe laooratory to optimize well performance.

The use of water-based (Gel Chem or Polymer) fluids wasdiscounted at the outset due to greater potential for formationdamage caused by phase trapping of the water within theporous media either dlrough spontaneous imbibition or filtratel~s during overbalanced pulses that may be encountered orrequired (ex. bit trips, well control etc.), and due to poorproductivity o~erved in previous wells exposed to aqueous-based fluids in the past. It was felt that oil-based fluids in anunderbalanced m<xle would eliminate much of the potential fordamage to occur, and so this would be the most logical placeto start the experimental program. As with most underbalancedoperations, maintaining underbalance pressure while drillingthe well could only be achieved by co-injecting the drillingfluid with gas or another density-reducing product. Sincenatural gas was available for this operation, the coreflow testssimulated the same process using methane displaced into thedrilling fluid at a specified rate.Coreflow Experimentation

~uid/F1uid Compatibility. Preliminary work was conductedto determine whether solids would be precipitated duringmixture of the hydrocarbon drilling fluid and native crude oilsince appreciable precipitation of solids could cause significantformation damage from solick entrainment in the event of fluidl~ during driUing. H no incompatibility is observed betweenthe two fluids, the drilling fluid can then be used in a coreflowexperiment

Coreflow Experimental Procedure

Core Restoration. Test cores were originally in anunpreserved state. and therefore required a reconditioningprocedure to restore rock wettability and saturation. The coreswere mounted at reservoir conditions of temperahlre and netoverbwden pressure, and a two week restoration phase ensued.Although the common practice is to conduct the restorationphase over a six to seven week period, time constraints~uired aN>reviatioo of this procedure. Because of this fact,the wettability was measured to evaluate whether therestoratioo phase had been successful.

Sample Selection, Screening and Preparation. Properselection of test samples is essential for any coreflowexperiment For this study, a selection of small plugs widt amaximum diameter of 3.81 cm were drilled from unpreservedfull diameter core from well C-20-lJ94-A-9 and re-analyzedfor routine air permeability and porosity. From this analysistwo test samples were selected based on:. good lidtological ~tation of die zooe of interest

The samples were homogeneous quartz~e litharenites andexhibited no microfractures or oilier small-scaleheterogeneities;

. penneability and porosity of each sample was comparableto the overall quality of die target wne of die reservoir;and

. bodt samples were similar in quality to each other (to be~ later).

Laboratory Tests. In certain drilling operations where it isnot known whether to use an oil-based or water-based fluid, ordifferent methods of drilling are under consideration (ex.overbalanced versus underbalanced etc.), a suite of preliminarycoreflow tests is useful for narrowing down the selection oftest flui~ without conducting extensive and expensive Initial displacement with forDlation brine established a water-

wet condition which is a likely possibility for this type of rock,

SPE 37065 R. DOANE, DB. B~ION. DX mERS. E.R. NEW

Experimental Resul~followed by approximately ten days of discootinuousdisplacement with native crude oil. During the oildisplacement, if the rock has a tendency to be more neutral oroil-wet. the transition can occur.

Contact Angles. Results of the contact angles measured onrestored rock and restored rock soaked in an oil/surfactantmixture clearly showed that the oil/core interface angle on therestored sample was 90" or neutrally wet The contact angleon the second sample soaked in drilling fluid with surfactantindicated that as expected, the surfactant caused the wettabilityof the rock to change to strongly oil-wet

Contact Angle MeasuremenL Previous analysis on theHalfway rock indicated that wettability of the formation wasneutral (neither oil- or water-wet), and it was essential that thiswettability be restored in the test samples. The ~bility ofutilizing a surfactant to prevent solids build-up in downholetubulars also required investigation of how much the ~oirwettability would be altered by the surfactant. Two contactangles were measured on a post-restoration cores (one soakedin drilling fluid with surfactant added).

Therefore, it was concluded dlat it would be unwise to utilizethe surfactant as it may increase propensity for prematurewater breakthrough and reduce oil pem1eability in the nearwellbore region due to an adverse alteration in relativeperDleability .

Underbalance Drilling Fluid Tests. Two underbalanceddrilling fluid tests were conducted on separate core samplesusing the two mud systems previously described. Test #1utilized a core exhibiting an initial permeability to air of 231roD and porosity of 19.4%. and test #2 was conducted on acore with an air permeability of 293 mD and porosity of19.4%.

Compatibility Tests. Fluid-fluid compatibility tests indicatedthat the produced crude oil and oil-based mud system werecompatible in all ratios at reservoir temperature coodition.

Corenow Test II - Viscosified Hydrocarbon with Methane

Co-injection. Initial effective penneability to oil was 71 mD,and following underbalanced exposure to the drilling fluid, thepenneability decreased by 67%. Subsequent exposure atoverbalance pressure caused an additional 21 % loss inpenneability for a fmal decrease of 88%. Damage to the coreduring the underbalance phase was believed to be due toa<B1ermce of the viscosifted oil and microfines to the sandfaceof the core, and invasion of this same material during theoverbalanced pulse caused further impaimlent. P~t-testpetrographic analysis of the core revealed dlat some pores andpore throats throughout the entire length of the core wereoccluded by fmes coated in oil as well as Hmud oil" which wasbelieved to be the vjscosified hydrocarbon.

For bodt ~ts, initial permeability to native cro~ oil wasmeasured to establish a baseline permeability to undamagedrock. followed by die underbalanced drilling fluid exposure.For this procedure, methane gas was injected into hydrocarbondrilling fluid in a 5:1 ratio (gas to oil) and die mixture wascirculated past the sandface of the core. To simulateunderbalanced conditions expected in die field, native crude oilwas continuously displaced through die core toward diesandface at 700 kPa above die drilling fluid circulationpressure. Following dtis phase a regain permeability wasmeasured to determine die effect of the drilling fluid on

permeability.Coreflow Test H2 - Unvjscosified Hydrocarbon withMethane Co-injection. Initial effective permeability to oil was41.7 mD and th~ was no reduction in this value followingunderbalanced exposure to the drilling fluid. However, theoverbalance pulse with the same fluid did cause a 57%reduction from the baseline permeability. Impairment in thiscore was due to invasion of drilling microfines which becameentrained in die porous media during the overbalance pulse.Post-test petrography indicated that oil-coated fines occludedsome pores and pore dtroats, with a similar "mud oil" coatingon some pores contributing to the impairment.

As with many underbalanced drilling operations, maintenanceof a true underbalanced pressure is not always possible.Among many possibilities, this can be due to pressure spikesencountered as joints are made up or as the tubulars are beingtripped. loss of fluid can occur if zones of low ~ arepenetrated (less likely in a horizontal application), and if weDcontrol is required to prevent a kick for example, overbalancepressure may be required.

To simulate the ~bility of losing underbalance pressure atany time during the drilling or completions operation, each testsample was subjected to an overbalance pulse of the samedrilling fluid system. This was followed by regain permeabilitymeasurement to observe any damage caused by fluid invasionduring the overbalance pulse. The mechanical entrainment ofdrill solids, which were included in the mud system beingcirculated, would likely comprise the major portion of anydamage observed in this particular situation.

Study Conclusions

Results of the study clearly indicated that when the Halfwaysand was exposed to water-based and certain oil-based m\Q,irreversible reductions in permeability were effected. As well,when crushed reservoir rock was worked into the core face, theresulting mechanical damage was significant.

SUCCESSFUL DRIWNG OF AN UNDERBALANCFD HORIZONTAL WBL IN THE RIGEL HALFWAY POOL SPE 37065

Although the unviscosified drilling fluid was recommended foruse (with natural gas co-injection) over the viscosified version.it was stressed that ~ was only if underbalance pressurecould be maintained in order to prevent fonnation damage.Using a surfactant in the drilling fluid to prevent solids build-up in the wellbore was discouraged since altering thewettability of the fonnatim to a more oil-wet cmdition couldhave a significant and negative impact on the productionperfonnance of the well.

enabled drilling out of the intemlediate section andcommencing circulation to natural gas prior to drilling theHalfway. The Halfway woe was penetrated with natural gasrates of 114 *103m3/day of gas down die 101 mm drill pipewith 100% returns. Immediately upon peneb"atlng theHalfway, oil influx into the wellbore commenced at a rate of600 m3/day of sour oil. Cuttings were transported to surfaceat a high velocity such that they were virtually unworked bymechanical action.

The study concluded that not enough confidence could begained through using a conventional gas/hydrocarbon mudsystem as damage would be severe if the underbalancedcondition was compromised. It was felt that in order tomaintain production as high as possible with the lowest gas/oilratio, up to the allowable, an undamaged horizontal we1lborewould be required. The only drilling process that was left ifoil and water-based drilling fluids were eliminated was to drillthe well underbalanced with gas. Inv~gations into the use ofgas as a drilling fluid led to the conclusion that the effects ofdrilling fmes on the fonnation could be negated if the fines areremoved before they have a chance to become mechanicallycrushed in the wellbore. AJso, a conditioo of l00~underbalance would be much easier to continuously maintainusing a gas-based system.

Drilling proceeded in a westerly direction for approximately300 meters in an underbalanced coodition widt a single bit, andduring this time dte total gas injection rate increased to 170*1Q3m3/day. Total withdrawal from the well reached peakrates of up to 1100 m3/day of sour oil and 340 * lQ3m3/day of

total gas. Bottom hole pressure was monitored throughout thedrilling operation, and the well cootinued to flowunderbalanced during the entire time. This was in partattributable to reducing joint connection time by using doublejoints in a top drive rig.

As drilling proceeded it was felt that influx rates wereapproaching die maximum well potential. The well was drilledby utilizing a motor and rotating the drill pipe, and whentrajectory correcti~ were required rotation ceased. Thismethod enhanced cuttings transport which increased thepenetration rate ten-fold over conventional overbalancedoperatiom in the Halfway in the past. When die bit wasscrubbed and the angle could not be maintained. the decisionwas made to remove the drill string and cease operatiom as itwas felt that flow rates were acceptable and well productivitymay be jeopardized by a kill and tripping operation to installa new bit. The well was self-killed (with clean produced oil)and left with a series of plugs in place while rigging out.

Drilling Operation

With the laboratory work completed, and the decision made toutilize gas as the drilling fluid, it was then necessary todetermine which gas to use and flow rat~ of the gas toprovide adequate hole cleaning ability. Although nitrogen istypically the gas of choice, consultation with suppliers raisedtwo problems: 1) the proximity of Rigel centres of productionof nitrogen, and 2) the quantities of nitrogen necessary tomaintain 1400 kPa \Ulderbalance pressure at all tim~. Thelogistics and c~ts strongly supported utilization of sweetnatural gas readily available to Samedan through facilitieswithin 200 meters of the target well 3-33, thus natural gas wasselected as the drilling fluid of choice with a very smallamO\Ult of the compatible hydrocarbon fluid, used in theprevious lab tests, added in mist fonD for lubrication ~.

Well Test Results

The horizontal well was brought on stream flowing 100% oiland gas (no water) through a separator. Two tests wereconducted simultaneously 00 the horizontal well:

T5 #1 was a conventional flow and build-up t5 on thehorizontal well where quartz gauges were set in thevertical section of the hole just at the point of kick-off.The in-situ penneability to oil. as determined by ananalytical horizontal model suggests dIat the horizootalpenneability parallel to the well axis is ISO OlD,perpendicular is 120 mD and the vertical penneability is40 mD (these penneabilities are reduced from virginconditions as this reservoir was already partially depleted).The skin factor determined from this analysis was +/-0.25, or essentially zero skin, indicating that the drillingprocess had not damaged the wellbore and hence theobjective of minimal damage was achieved.

1

Approval was received from the B.C. Ministry to use naturalgas, and a pressurized surface control and flare system was~taUed on location since sour gas could Dot be vented directlyto atmosphere. With personnel safety as a prime concern, astate-of-the-art Williams rotary head directed the well effluentto the pressurized and contained separation equipment fromwhich gas returns from the wellbore were flared in a 30 meterstack.

A medium -kick-()ff" radius was selected for the build-upsection of the horizontal well and 178 rom pipe was set in ahorizootal mode 100 meters west of well centre. This offset

SPE 3706S R. DOANE, DB. BENNION, DM EILERS, E.R. NEW s

The secood test was an interference test nIB between thehorizontal well and a vertical weD approximately 200meters away. Results of this test confltDled the magnitudeof horizontal penneability in die reservoir validating theinterpretation of the horizontal well test. The 3-33horizontal well has since produced approximately 80,000bbl of oil and 400 MMscf of gas. The initial wellproductivity determined from the build-up test wasapproximately S30 m'lday under AOF conditions. It isbelieved that with the wellbore in its undamaged state, andwith the penneability ~ed with this reservoir, thatSamedan achieved very good separation of oil and gas andhence a relatively low producing GOR at high oil rates.

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Conclusions

A suc~ underbalanced well was drilled in the HalfwayFormation in the Rigel .C. pool using a natural gas-hydrocarbon mist system. Lab test studies indicated theimportance of underbalanced condition maintenance to obtaina zero skin condition. The well flowed at ra~ of up to 11 ()()mJ/day (7000 bb1fday) during the drilling process with post-~tPT A work showing zero skin. indicating that the objective ofzero invasive formation damage appeared to have beenachieved.

Acknowledgement

The authors wish to express appreciation to Vivian Whiting forassistance in the preparation of the manuscript and to themanagement of Hycal Energy Research Laboratories andSamedan Oil of Canada for the funding of this work andpe~ion to preSmlt the data.

References

Bennion, O.B., et al, "Underbalanced Drilling - Praisesand Perils", SPE 35242 presented at die SPE PermianBasin Oil and Gas Recovery Conferences.

Bennion, 0.8., et at. -UnderbalaIx:ed Drilling andFonnation Damage - Is It a Total Solution?-, JCPT,November 199.5.

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