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Overview

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Horizontal-technology development is evolving from a leading-edgehigh-risk exercise to a normal operating day-to-day practice that recov-ers additional reserves. This years selected papers are examples ofinnovative approaches that exploit and fine-tune the technology.

It is clear that we are quickly evolving the use of horizontal technolo-gy to improve economic parameters associated with the productionfrom our fields. The number of related technologies developed as aresult of the industrys ability to complete horizontal and multilateralwells successfully is amazing.

Selection of the most appropriate development strategy is a complicat-ed process requiring not only the economic consideration of the tech-

nical parameters of the reservoir and the production process, but manyother considerations as well. Today, industry professionals who devel-op these plans are in the enviable position of having an almost unlim-ited wellbore-architecture capability. As an industry, it is important thatwe continue to understand, improve, and enhance the use of this tech-nology to improve recovery.

Aspects of our business, such as technology development, requireongoing commitment. High-quality research and development is ongo-ing within several organizations. One of our industrys biggest chal-lenges is accessing technological developments to create even betterhorizontal drilling processes. Industry associations, such as SPE, play avaluable role in collecting and sharing this information. Organizationsthat embody this focus of technology development are bound to bemore successful in the long term.

Lew A. Hayes is Vice President of Operations atPetrovera Resources Ltd. He has been involved with more

than 400 horizontal wells, including several vertical andhorizontal multilateral completions; copatented a multi-lateral system; and is coauthor of several papers related tonew technology developments. Hayes holds a BS degreein petroleum engineering from Montana Tech. He hasworked extensively in Canada with east coast offshore

experience; a full range of heavy-oil developments including steam-assisted gravity drainage; and the deep sour drilling and completions inthe foothills. A member of SPE for 20 years, Hayes serves on the JPTEditorial Committee.

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Modeling Near-Wellbore

Damage and Natural Cleanup

of Water-Based Mud

Predicting formation damage in

horizontal wells, often openholecompletions, is critical to optimizefield development. The economiceffect of near-wellbore induceddrilling-damage and cleanup effi-ciency has fostered both experi-mental and numerical studies toassess wellbore flow propertiesduring oil production. A numericalapproach was developed to modelformation damage from water-based mud (WBM) and predict

well performance for natural

cleanup when the well is subject toa pressure drawdown.

IntroductionIt is recognized that near-wellbore flowproperties are altered by mud andmud-filtrate invasion during overbal-anced-drilling operations. The degreeof formation damage depends on para-meters such as nature and characteris-tic of the drilling mud, formation prop-erties, and operating conditions (e.g.,shear rate applied on the mud, overbal-ance pressure, and temperature).Formation damage caused by drilling-fluid invasion may reduce oil and gasproductivity substantially in manyreservoirs. Productivity losses are espe-cially critical for long horizontal wells,which are often completed openhole.In such a case, the near-wellbore dam-age is not bypassed by perforations andmay lead to very large skin values.Therefore, prevention of formationdamage generated by drilling mud maynot always be possible.

ModelingIn previous work, a simplified numer-ical approach for modeling naturalcleanup was developed for horizontalwells drilled with oil-based mud(OBM). This modeling is now extend-ed to wells drilled with WBM andincludes the simulation of filtrateinvasion. For WBM, the two maindamaging mechanisms are particulateinvasion during the initial spurt-lossperiod and filtrate invasion throughthe filter cake. Even without physical

or chemical interactions between fil-

trate and formation fluids (i.e., com-patible rock/fluids systems), a funda-mental difference exists between OBMand WBM displacement processes.

Permeability Damage. In an oil-bear-ing formation, displacement of oil-in-place with a WBM filtrate is an imbi-bition process, which generates a highwetting-phase saturation in the invad-ed zone, while the OBM filtrate is anearly miscible displacement process.In addition, WBM filtrate comprises

mainly polymer molecules that caninvade deep into the reservoir.Depending on the molecular weightand filtration conditions, polymerchains can be stretched by the flow, gothrough the filter cake, then adsorbon the porous media or even plugrock pores. Polymer chains associatedwith water increase the capillaryretention of water, leading to residualwetting-phase saturation after oilflowback that is higher than the initialsaturation. The result is an additionaldamaging effect (water blocking)because of the drastic reduction of oilrelative permeability. Generally, forrating the performance of variousdrill-in fluid formulations, the perme-ability damage is quantified throughoil-return permeability measurementsand flow-initiation pressure tests per-formed at relevant flow rates on coresamples damaged during dynamicmud filtration tests.

In this work, the full process ofnear-wellbore damage followed bynatural cleanup is modeled. The

WBM filtrate invasion is simulatedwith standard waterflooding con-cepts, which indicated a cone-typeinvasion depth along the horizontalwell. Filtrate/oil relative permeabilitycurves (imbibition curves for invasionand drainage curves for flowback) areused as input parameters. In addition,filter-cake properties (thickness andpermeability) and final oil permeabil-ities obtained from specific laboratorymeasurements are used to model thecleanup process.

Fluid InvasionDrilling-fluid invasion occurs whenthe drill bit contacts the reservoir rockand a rapid mud invasion (spurt loss)occurs because no filter cake exists toprevent entry of solid particles fromthe mud into the pay zone. Duringthis period, progressive deposition ofthese particles creates an internal filtercake. When this internal filter cake iswell established, most of the solid par-ticles are retained outside the forma-tion, generally creating a thin externalfilter cake that controls the rate of fil-trate invasion. As shown in Fig. 1, asteady-state flow regime, character-ized by a constant filtrate-invasionrate and a constant filter-cake thick-ness, is generally observed. Thisdynamic equilibrium results from theprevention of particle deposition

because of the shearing effect of thedrilling fluid. The start of the dynam-ic equilibrium corresponds to the endof the spurt period and the beginningof dynamic invasion. The fluid lossesduring the spurt and filtration periodscan be obtained from laboratory tests.In this study, a transition period is notspecified, and the spurt loss measuredin the laboratory takes into accountthe filtrate invasion during the transi-tion period. At the end of the spurtperiod, an internal filter cake is com-

This article, written by Technology

Editor Dennis Denney, contains high-

lights of paper SPE 73733, Modeling

of Both Near-Wellbore Damage and

Natural Cleanup of Horizontal Wells

Drilled With a Water-Based Mud, by

Y. Ding, SPE, D. Longeron, SPE,

G. Renard, SPE, and A. Audibert,

SPE, Inst. Franais du Ptrole, originally

presented at the 2002 SPE International

Symposium and Exhibition on

Formation Damage Control, Lafayette,

Louisiana, 2021 February.

For a limited time, the full-lengthpaper is available free to SPE mem-

bers at www.spe.org/jpt. The paper

has not been peer reviewed.

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pletely built. Duringdynamic invasion, it wasassumed that neitherabsolute permeability inthe formation nor externalfilter-cake properties, ifany, changed. Damagedpermeability in the zoneoccupied by the internalcake as well as properties

in external cake (cakethickness and its perme-ability) can be measured inlaboratory tests. Withthese data, filtrate invasioncan be simulated with atwo-phase flow simulatorrestricted by bottomholeflowing pressure.

After drilling is complet-ed and before the well isput on production, filtrateinvasion may continue under static

conditions if an overbalanced pressureis maintained. During this static inva-sion, the thickness of the external cakeincreases because of the absence ofshear stress, thus reducing the filtrateinvasion compared with other periods.

Oil FlowbackVarious techniques are used toremove filter cake, including breakerssuch as acid or an oxidizing solution.In this study, only natural cleanup(pressure difference applied betweenthe reservoir and the wellbore) is con-sidered for removing the filter cake.When this pressure difference is largeenough, external cake can be lifted offand flow initialized to remove parti-cles in the zone occupied by the inter-nal cake. Two regions can be distin-guished regarding the oil-return-per-meability variations. The first regionis close to the wellbore wall, where theregained permeability is the combinedeffect of partial removal of solids par-ticles and reduction of filtrate satura-tion. The second region is far from the

wall, where only the filtrate saturationreduction is considered because parti-cle deposits are assumed negligible.Therefore, for the cleanup process, itcan be assumed that potential damagefrom polymers contained in the fil-trate is globally represented by a hys-teresis of filtrate/oil relative perme-ability curves.

Numerical ModelThe full-length paper details themodel and presents examples of vari-

ous water-based formulations of for-

mation-damage and filter-cakeremoval procedures by use of differentdrawdown pressures applied to a hor-izontal well drilled in a heterogeneousformation. Cylindrical gridblocks areused for the simulation with smallgridblock sizes near the well. Filtercakes are discretized along their thick-ness to obtain a better description ofthe initial damage and the removalphenomena. Sensitivity to variousparameters, such as fluid and filter-cake properties or drilling conditions,on the damage and removal processeswas studied.

Numerical modeling involves twosteps. First, filtrate invasion duringdrilling is modeled. Second, the natur-al cleanup of the filtrate cake duringthe flowback period is modeled.

A two-phase-flow reservoir-simula-tion model was used to evaluate fil-trate invasion during the drillingstage as well as filter-cake removalduring production. A two-phase-flowmodel had already been used to sim-ulate filtrate invasion to improve log

interpretation; however, well perfor-mance was not studied. In the past,productivity change was related toformation damage by use of nonuni-form skin along the well. However,no laboratory data were integrated toevaluate filtrate invasion. The objec-tive of this approach was to evaluatewell performance by including labo-ratory data. Therefore, permeabilityin the zone occupied by filter cakewas assumed known from laboratorytesting for each geological facies.

The mathematical modeland numerical methodsare detailed in the full-length paper.

ConclusionsA numerical model wasdeveloped that simulatesboth near-well formationdamage and natural

cleanup during the drillingand flowback production ofa horizontal well drilledwith WBM. The modelrequires knowledge of per-meability reduction in theparticle-invaded zone andthe final return permeabili-ty after flowback. It alsorequires knowledge of thereservoir oil-/mud-filtraterelative permeability curves

for both the drilling phase and the

flowback production.The model can be used to investi-gate the influence of many parameterson the flow efficiency of a horizontalwell after drilling and cleanup.Together with laboratory data neces-sary to provide the main input data forspecific applications, this model canbe used as a predictive tool to evaluateand limit the risks of formation dam-age and maintain the maximum pro-ductivity of a well.

In examples run with laboratorydata, formation damage and consecu-tive productivity reduction are muchgreater with WBM than with OBM.Serious loss of production can occurwith the use of damaging and nonop-timized drilling fluid. Investigatedparameters include the most influen-tial for a well drilled with WBM: thevalue of the endpoint of oil relativepermeability during drainage.

The model remains simple, al-though it incorporates most of theknowledge acquired on formationdamage from up-to-date laboratory

data. However, it must be improvedby taking into account laboratorywork (such as the effect of localvelocity), which may affect theamount of filter-cake removal andwater blocking on cleanup efficiency.This model also must be improved byintegrating more general reservoir-boundary capabilities, effect ofanisotropy, and, if possible, couplingwith a geomechanical model to inves-tigate sand production in poorly con-solidated reservoirs. JPTJPT

Fig. 1Drilling-fluid invasion and filter-cake formation.

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Influence of the Horizontal-

Well Length on Production

Rate and Water Cut

Numerical analysis is the main

method for analyzing productivityand water-cut performance of hori-zontal wells in various field-devel-opment patterns. In sheet depositsof Western Siberia, which havenearly unit mobility ratios of oil andinjected water, a nearly frontal driveis observed. A precise analyticalsolution of steady-state fluid filtra-tion was applied to six developmentpatterns having horizontal wells.

Introduction

Development systems and horizontal-well patterns can have a significanteffect on productivity and oil recovery.Analytical estimations help optimizehorizontal-well parameters and devel-opment systems on the basis of partic-ular geological conditions, thusreducing unjustified expenses byinstalling the appropriate develop-ment system.

TargetFig. 1 shows well patterns with hori-zontal and vertical wells used to studythis vertically anisotropic, heteroge-neous reservoir. Analytical transfor-mations detailed in the full-lengthpaper are discussed on the basis ofwell spacing, specifically an areal five-spot system with vertical injectors andhorizontal four-bore producers.Similar analytical transformationswere developed for the other cases.Expressions were obtained for rate,velocity field, filtration velocity poten-tial, flooding factor for water-free peri-od, and water-cut performance, as well

as positions of oil/water front atsequential moments of time for vari-ous values of the distance betweenwells and between rows.

Modeling ResultsFiltration processes of fluids with closerelative mobilities in a massive,homogenous, vertically anisotropicreservoir were analyzed. In a layeredreservoir with a weak hydrodynamicconnection between layers, systemdevelopment becomes nonuniform

(i.e., each layer has its own character-

istics and short horizontal bores).Decreased sweep efficiency in a lay-

ered reservoir is the result of both adecrease in the length of the horizon-tal section and the asymmetry of thehorizontal section in filtration ele-ments. Such a process was observed inall systems having horizontal and ver-tical wells. The one exception was aline-drive system having only horizon-tal wells. In this case, deformation ofthe well pattern does not take place.

ConclusionsAs reservoir vertical permeabilityanisotropy increases, the productivityof a given development system candecrease in multiples. However, if thereservoir is thin, as in most sheet oilpools, the difference between the hor-izontal-well rate and perfect surround-ing-well rate does not exceed 5%.

Increasing the horizontal-welllength by more than 50 to 60% of thedistance between wells in vertical-injector patterns, or by more than90% in horizontal-injector patterns,does not significantly influence the

productivity of the development sys-tems. Under equal conditions, sys-tems with a staggered well patternexhibit water-free oil recovery up to40% greater than systems with a directwell pattern. For well patterns withfrontal placement of horizontal andvertical wells, the increase in horizon-tal-bore length leads to the increase inwater-free oil recovery; however, instaggered-well-pattern systems, thegrowth of the horizontal-well lengthis accompanied by a reduction inwater-free oil recovery.

In a five-spot system with four-borehorizontal producers (Case 6), therelationship between horizontal-welllength and water-free oil recovery isnonlinear. Horizontal-well length of30 to 50% of the crosswell distanceprovides the greatest water-free oilrecovery. Longer or shorter horizontal

sections reduce water-free production.Systems with frontal location wells

have a much longer water-productionperiod, compared with the staggered-well pattern, because the dynamics ofwater-cut are less favorable. In systemswith a staggered-well pattern, chang-ing the horizontal-well length does notsignificantly influence the well opera-tion in the water-production periodbecause the water-cut and flooding-factor curves vary in a much smallerrange than in frontal systems.

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Fig. 1Well patterns considered inthe study.



lights of paper SPE 77827, Influence

of the Horizontal-Well Length on

Production Rate and Water Cut

Performance in Regular Field-

Development Systems With Horizontal

Wells, by I.R. Mukminov, Yukos Oil

Co., originally presented at the 2002

SPE Asia Pacific Oil and Gas

Conference and Exhibition, Mel-

bourne, Australia, 810 October.

For a limited time, the full-length

paper is available free to SPE mem-bers at www.spe.org/jpt. The paper


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Advanced Openhole

Multi laterals

An aggressive re-entry program is

resulting in significant productionincreases from Sonatrachs HassiMessaoud oil field. Kicking off later-als in an extremely hard and abra-sive sandstone and quartzite forma-tion has required development ofnew tools. Lateral isolation andtieback to the original cased hole as

well as providing through-tubing re-entry have posed additional chal-lenges. A combination of liner hang-er and inflatable packer technology,together with large-bore comple-

tions, has led to several successfulinstallations.

IntroductionOpenhole multilaterals can be com-plex yet well engineered to providelow-risk system installations with fullisolation betwen laterals and positivere-entry capabilities. When openholemultilaterals are lined or tied back to acased or openhole main bore, theyprovide a Technology Advancement ofMultilevels Level 3 type multilateralwith mechanical integrity at the junc-tion. An engineered re-entry systemwith sealable housing hardware wasdeveloped for openhole use and wasaccurately placed across each of thetwo laterals to provide positive repeat-able re-entry into either lateral. Thelarge-bore completion systems enablepositive through-tubing selective later-al re-entry without removing the com-pletion string.

BackgroundThe Hassi Messaoud field in central

Algeria has 1,056 vertical wells,approximately 450 of which are pro-ducers. The nonproducers are candi-dates for a re-entry and horizontalrework program. Some nonproducingwells were drilled in the wrong part ofthe field, and others experienceddrilling problems. Sonatrach hasbegun a program to re-enter thesewells that have been shut in sincebeing drilled and rework them.

The ability to place multiple drainholes into reservoirs with challenging

geology and other constraints can

make multilaterals more viable thanhorizontal or multiple single wells. Asingle, horizontal trajectory may notbe able to intersect all the desired tar-gets and marginal reserves.

These nonproducing wells have7-in. 32-lbm/ft production casing setat approximately 3350 m and a41/2-in. slotted liner through the pro-ducing formation. In the re-entry pro-gram, the 41/2-in. liner is removed bymilling. Then, two intermediate-radius 57/8- or 6-in. closely spaced

openhole laterals are drilled to reachnarrow targets in the Cambrian for-mation. High build rates are requiredto reach the horizontal pay from theexisting 7-in shoe.

The target formation is the 75- to80-m thick Cambrian, a three-layersandstone and quartzite formationwith lenticular sandbars in an erodedanticline. The Upper Cambrian layeris the target layer, while the two lowerlayers are plagued with water/oil con-tact or water/gas contact problems.The upper zone is broken down fur-ther into sublayers or reservoir beds,and the target zone is in one of these6- to 8-m-thick sections. Water andgas encroachment from the lowerzones can shut off production and areto be avoided. The relatively thinreservoir beds require 1- to 6-m spac-ing between laterals. Proximity of the7-in. shoe to the reservoir and areageology determine lateral spacing andfinal completion systems.

Installation Scenarios

Depending on the thickness and geol-ogy of the beds, three installation sce-narios were considered for thethrough-tubing completion of theseopenhole systems.

If laterals are too close together toisolate with an inflatable packer, bothlaterals will be drilled and both multi-lateral re-entry (MLR) nipple systemswill be installed in tandem, orientedaccordingly with a conventionalthrough-tubing completion tied backto the casing string.

If there is more than 2.5 mbetween laterals, the lower lateral willbe drilled followed by installation ofone MLR nipple, an inflatable packerfor isolation, and a second orientationriser. The second or uppermost lateralwill then be drilled. The second MLRnipple will be oriented to the secondupper lateral and run with a conven-tional through-tubing completion tiedback to the casing string.

If existing geology permits thelower-lateral build rates not to exceed40/30 m, a 41/2-in. slotted liner will berun and tied back to the vertical well-bore with an inflatable packer and ori-entation riser. The upper lateral will bedrilled and one MLR nipple installedfor the upper lateral and run with aconventional through-tubing comple-tion tied back to the casing string.

Later Installations. In later installa-tions, the lower lateral was drilled suc-cesfully off a cement plug, reducing

overall costs of the complete system. A41/2-in. liner was tied back to the open-hole main bore with an inflatable pack-er to provide the drilling and comple-tion anchor for the upper lateral. Thetieback liner provides formation sup-port in the lower lateral. The upper lat-eral is drilled and completed in thesame manner as before, with an MLRwindow placed acrosss the upper exitand a liner dropped off in the lateral justoutside the window. Either a polishedbore recepticle (PBR) or an inflatable

This article, written by Assistant

Technology Editor Karen Bybee, con-

tains highlights of paper SPE 77199,

Advanced Openhole Multilaterals,

by D.G. Hurst, SPE, S.P. Hart,SPE,

and W.H. Brown, SPE, Weatherford

Intl., originally presented at the 2002

IADC/SPE Asia Pacific Drilling

Technology, Jakarta, 911 September.


paper is available free to SPE mem-

bers at www.spe.org/jpt. The paper


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packer and PBR are on the end of theliner just outside the window to enableeasier re-entry and isolation, if needed.

A more advanced solution is toallow the upper leg also to have a linertied back to the main bore to provideformation support on the upper junc-tion. This tieback liner has a premilledwindow in the liner that is orientedand aligned to allow tool or produc-

tion access to the main bore below thetop junction. This is typical of a Level3 system that is applied to a hard-for-mation open hole. Both legs providerisk-free re-entry, mechanical stabilityin the junction, and, with appropriatenipple profiles installed, provide themeans to shut off or choke back eitherof the production legs.

Operational Issues and SolutionsMilling/Drilling.Starting a pathway isdifficult in hard formations, and rate

of penetration (ROP) is slow. A seriesof special tungsten insert mills, thatcut a short pilot hole quickly andeffectively to provide direction andpathway for the diamond bit to begindrilling past the top of whipstock forthe initial sidetrack, were developedand tested. In the past, it was notuncommon to make two to three millor bit runs to create a short kickoffhole past the whipstock tip. In addi-tion, the milling/drilling action usual-ly damaged the whipstock tip so thatit had to be retrieved and replaced tocomplete the build and lateral.Different mill designs were evaluatedto determine an optimum design thatwould cut in a radial fashion. Aftersome field testing, the final milldesign would enable a 50-cm cutoutwith no damage to the whipstock top.Experimentation showed that ROPcould be improved with a bit designthat used a crushing action.

For the Upper Cambrian, the mostsuccessful bits for ROP were an Intl.Assn. of Drilling Contractors 837, a

tricone insert bit with heavy gaugeprotection. Average ROP in the hori-zontal section was 2.9 m/hr with a250-RPM motor; average bit life wasapproximately 30 hr. Sidetracks offthe whipstock used a diamond bit at1- to 1.5-m/hr ROP. A natural dia-mond bit was used to complete thesidetrack and rat hole, and a triconebit was used to drill the lateral.

Isolation/Anchors. Issues had to beaddressed to obtain a suitable anchor

to ensure low-risk milling/drillingoperations and to prevent misalign-ment or realignment of tools relativeto the lateral in later operations. Useof a combination of cased- and open-hole-completion products provided abasis for drilling the lateral and tyingthe production string back to the orig-inal cased hole. Initially, a mechanicalshear-release latch system was used to

run the lower anchor system consist-ing of a centralized tailpipe, shoe, col-lar, and inflatable packer with orienta-tion riser. Improvements led to theuse of a hybrid hydraulic-release liner-hanger setting tool and setting-sleeveorientation riser that provided alower-risk cementing operation andensured quick and positive running-tool release before cement setting.

Milling/drilling of inflatable packersin openhole hard formations has notcaused any problems. The hydraulic-

release tool has required some refine-ments to provide more clearance dur-ing running and retrieving and to iso-late internal components from cementcontamination. Other modificationshave been made to improve procedur-al processes to enable high-load latch-ing and unlatching during space outof the tubing string.

Initial installations used inflatablepackers for isolation and anchors forboth lateral legs. Lower inflatablepackers usually were cement inflated,and upper inflatable packers were liq-uid inflated. Later and current instal-lations use a liquid inflatable packerfor the top lateral, but the lower later-al now is drilled off a cement plug.Drilling off a cement plug is effectivefor the short-radius build and is morecost effective.

Re-entry. Through-tubing selectivere-entry into lateral bores associatedwith Level 4 cased multilateral wells isproven technology. The challenge wasto adapt this technology for use with

Level 1 openhole multilaterals andensure that the equipment and tech-niques used provided a cost-effectiveand low-risk solution. The thin reser-voir sections dictated closely spacedlaterals, requiring varied completionschemes and more complexity inaligning re-entry components. Forfuture re-entry of openhole comple-tions, large tubing completions withlarge-bore MLR nipples providegreater producing capabilities withadditional flexibility for workover

operations in the laterals. Just likeconventional completion engineeringand design, re-entry wellbore comple-tion tubulars should be no smallerthan the completion tubing forworkover flexibility. The MLR systemused in these wells uses selective pro-files to provide optimum completioninside diameter.

MLR System. The MLR nipple systemis a sleeve that is installed in the wellas part of the completion and is posi-tioned adjacent to the drilled lateralexit from the main bore. The MLRnipples provide selective access forintervention tools to monitor andmanage the lateral bores without theneed to pull the completion string towork over the well. To accommodatethe close tolerances required to posi-tion the MLR nipple, careful andaccurate measurments were taken

during drilling the lateral bores. Thecollet orientation latch used fordrilling provides depth and orienta-tion data for both lateral wellbores. Aquick and simple alignment methodwas devised to enable rigsite lowercompletion assembly makeup. A sim-ple acme-type thread connection thatcould be adjusted on site and lockedin position was used that also couldprovide close coupling of the MLRnipples, if required. By careful equip-ment selection, it is possible to identi-fy and selectively exit MLR nipples aslittle as 1.5 m apart.

The MLR nipple assembly, designedwith a 53/4-in. maximum diameterwith reduced relief diameters toaccommodate debris, was easilydeployed in the 57/8- or 6-in. diameteropenhole main bore. A 41/2-in. uppercompletion with 33/4-in. selective nip-ple profiles was chosen to accommo-date the intervention tools. MLR nip-ple systems have been used succesful-ly in dozens of installations.

ConclusionsProper planning and implementation ofdual openhole laterals in the ultra-abra-sive Cambrian formation in Algeria hasled to improved project timetables andlower project costs. Milling and drillingoff cemented inflatable packers inopenhole applications into extremelyhard formations, accurate repeatablealignment of lateral re-entry comple-tion components, and completiontieback to the original wellbore havebecome routine processes. JPTJPT

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Single-Operation Stimulation

of 14,000-Ft-Long

Reservoir Sections

Horizontal wells with continuous

reservoir sections between 10,000and 20,000 ft are being used for thedevelopment of the laterally exten-sive low-permeability chalk in theDan/Halfdan oil accumulation, off-shore Denmark. The patent-pend-ing Controlled Acid Jet (CAJ) con-cept was developed to stimulate

very long horizontal well sections ina single operation. The method usesan uncemented liner with a limitednumber of unevenly spaced holes(perforations) to ensure efficient

acid stimulation of the completereservoir section, provided the acidis pumped at sufficiently high rates.

Introduction and BackgroundThe Dan/Halfdan oil field is approxi-mately 149 miles off the west coast ofDenmark in the North Sea. Oil pro-duction from the Dan field began in1972. The 29,600-ft-long Well MFF-19C, with a horizontal reservoir sec-tion of 20,749 ft, followed the oil accu-mulation more than 700 ft downdipfrom the main field and below thestructural saddle point. This action ledto the discovery of a 1.5-billion-STB,nonstructurally trapped oil accumula-tion, the Halfdan field.

Reservoir DescriptionThe Dan/Halfdan field compriseschalk reservoirs of Danian andMaastrichtian age. Development fo-cuses on the Maastrichtian reservoir,which is characterized by relativelyhigh porosity (25 to 35%) and low liq-uid permeability (0.1 to 2 md). The

bottom part of the Danian, the D2unit, exhibits relatively low porositiesand very low permeabilities. A zonehaving extremely poor permeabilityseparates the Danian and Maastrich-tian formations. Properties of thechalk vary on a 3- to 7-ft scale, re-flecting depositional cycles, whichcan be recognized over long distancesin the lateral sense. Each cycle con-sists of a high and low porosity inter-val, causing the significant verticalporosity variations.

Well Concepts Used. Initially, the

Dan field was developed with deviatedwells stimulated with acid or sand-propped fractures. In 1987, the firsthorizontal wells were drilled in thefield. Encouraged by the productionperformance of the horizontal wells,all subsequent wells have been drilledas horizontal completions.

The perforate, stimulate, and isolatesystem was developed to facilitate effi-cient multiple fracture treatment of hor-izontal wells. This system is used exten-sively for completion and stimulation

of horizontal producers and injectors.Packers, run on an inner tubing, iso-late the individual zones, and slidingsleeves provide access to each perfora-tion interval in the cemented liner forselective stimulation of each zone. Thesliding sleeves are manipulated bycoiled-tubing-deployed shifting tools.However, this limits the length of thehorizontal section to the reach of thecoiled tubing within the productionstring (typically, approximately 8,000 ftof reservoir section). However, thematrix-acid-stimulated zone length istypically limited to approximately 500to 700 ft, to ensure good acid coveragealong the entire zone. Effective stimu-lation of the distant part of each zonein the cemented and perforated linermay be difficult to obtain, and aciddiversion has traditionally beenimproved by the use of high pumprates, foamed acid, viscosity-controlledacid, benzoic acid flakes, etc.

Proposed TechniqueThe new completion and stimulation

method jets acid onto the formationalong the entire uncemented liner andis controlled by a limited number ofpredrilled holes. The acid is pumped athigh rates and exits the holes at highvelocities, resulting in jetting of the for-mation. By limiting the number andsize of holes, a choke effect is obtained,and a significant pressure drop occursbetween the inside and the outside ofthe liner during stimulation. A nonuni-form geometric distribution of theholes is used to compensate for the fric-

tion pressure drop along the liner sec-tion (i.e., the average hole spacingdecreases toward the bottom of theliner). The open annulus outside theliner, in combination with the overpres-sure on the inside of the liner (causedby the choking over the holes), ensuresthat acid eventually reaches the bottomof the very long liner, and the well isthus stimulated along its full length.

The initial flow distribution alongthe uncemented liner is shown inFig. 1. Initially, both the liner andliner/wellbore annulus are filled withmud. Also, a high resistance to flowexists at the wellbore face (mud cake).When the acid first contacts the for-mation on top of the liner, the mudcake and formation break down andconsiderable volumes start leaking offto the formation. Hence, as soon as aneffective connection to the reservoirhas been established, the stimulation

pressure will fall, assuming constantstimulation rates. At this stage of thestimulation job, the fluid leakoff intothe top of the reservoir section is amixture of acid jetting out of thepredrilled liner holes in the top sec-tion and fluids flowing from the moredistant part of the liner annulus in thedirection of the heel. As the acid frontmoves along the liner, a breakdownzone is created in which acid mixeswith the mud and breaks down themud and the mudcake.

Horizonta

lWells



lights of paper SPE 78318, Controlled

Acid Jet Technique for Effective Single

Operation Stimulation of 14,000+ ft

Long Reservoir Sections, by

J.H. Hansen and N. Nederveen,

SPE, Mrsk Olie og Gas AS, originally

presented at the 2002 SPE European

Petroleum Conference, Aberdeen,

2931 October.


paper is available free to SPE mem-

bers at www.spe.org/jpt. The paperhas not been peer reviewed.

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Eventually, when all the mud is bro-ken down and residuals displaced intothe formation, the annulus will befully filled with acid. A net flowtoward the sections with the lowestskin in the well will continue, and theacid flowing toward these sectionswill wash the wellbore face. Fresh acidwill continue to be jetted at the prede-termined distribution points along the

uncemented liner, ensuring effectivestimulation along the full liner lengthby acid flow in the annulus.

Single zones as long as 14,400 ft havebeen treated effectively with the tech-nique. For comparison, experiencewith a traditional cemented and perfo-rated liner has shown a maximumzone length of 500 to 700 ft if the slid-ing sleeve is placed at the middle of theperforation interval and if the reservoirproperties are nearly constant.

ConclusionsThe CAJ completion and stimulationtechnique was developed and imple-mented for the development of the flankareas of the Dan/Halfdan low-perme-ability chalk field offshore Denmark.

The uncemented-liner completionand stimulation technique is efficientand simple to install. In addition,substantial cost and time reductionshave been achieved. The techniqueenables single-operation stimulationof reservoir intervals longer than14,000 ft.

Full acid coverage is obtainedthroughout the entire liner section.

The production performance per unitlength of well is superior to the con-ventional acid fracture and/or matrix-acid-stimulated cemented liners.

The stimulation effect is confined tothe near-wellbore area. No additionalwater production from the deeperwater-saturated reservoir units hasbeen observed.

No significant additional skin isintroduced by the completion,despite the very limited number ofpredrilled holes in the liner and the

bull heading of the drilling mud intothe formation.No significant solids production or

indications of collapse of the openhole have been observed in sectionscompleted with the CAJ liner. JPTJPT

Fig. 1Conceptual outline of fluidflow during stimulation through aCAJ liner. AInitial large leakoff at

the heel of the well. BThe mud isdisplaced, and the breakdown zonemoves toward the toe of the well.CThe mud and mudcake havebeen removed completely, and stim-ulation of the entire liner is ongoing.

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JPT2002 11 Hw Focus