50 Oilfield Review

Improvements in Horizontal Gravel Packing

Brian EdmentBaku, Azerbaijan

Fraser Elliott John Gilchrist Brian Powers BPBaku, Azerbaijan

René JansenNew Orleans, Louisiana, USA

Tim McPike Shell Exploration & Production Americas Houston, Texas, USA

Henry Onwusiri Shell UK Exploration & ProductionAberdeen, Scotland

Mehmet Parlar Sugar Land, Texas, USA

Allan TwynamBPSunbury, England

Aart van Kranenburg Shell Exploration & Production Europe Stavanger, Norway

FIV (Formation Isolation Valve), MudSOLV and QUANTUMare marks of Schlumberger. Alternate Path and AllPAC are marks of ExxonMobil; this technology is licensed exclusively to Schlumberger.

Companies have been slow to adopt openhole gravel packs for inclined and horizontal

well completions. This reluctance centered on operator concerns about the detrimental

effects of gravity on gravel placement around sand-exclusion screens. Today, however,

these attitudes are changing. Improved techniques, specialized screens, numerous

successful installations and a proven record have increased confidence in the long-

term performance of this technique for controlling sand influx in high-angle wellbores.

The number of directional and extended-reachwells increased dramatically during the pastfifteen years, especially offshore. Operatorscomplete many of these wells as open holesbecause of the high cost and difficulty ofcementing casing and of achieving clean,effective perforations. Long openhole sectionswith large inflow areas have higher productivitiesthan cased-and-perforated completions and arenot as sensitive to drilling or completion damage.However, these types of completions still requirereliable sand-control measures (next page).

Gravel packing, an effective and widely usedtechnique, places a granular filter media, orgravel, around metal sand-exclusion screensinside perforated casing or open hole. The gravelis round, well-sorted, clean natural sand orsynthetic materials sized to exclude individualformation grains and smaller rock particles, orfines—commonly referred to as producedsand—while being held in place by the screens.

Completion operations involve pumpingslurries of gravel and a carrier fluid into theannulus around a screen assembly. The gravel isdeposited as carrier fluid leaks into theformation or circulates back to surface through awash pipe inside the screens and a tubingworkstring. Gravel packs, used extensively invertical wells for decades, were less common forhigh-angle and horizontal open holes in someareas, until recently.

Sand control for open holes traditionallyconsisted of stand-alone screens without gravelpacks or, more recently, expandable screens.

Stand-alone screens, however, often failprematurely. Operators attribute these failuresto incomplete borehole cleanout and partiallyplugged screens, which result in “hot spots,” orconverging flow.1

Concentrated inflow at discrete points leads toincreased pressure drops, screen erosion,increased sand production and, ultimately, to adecline in well productivity. Newer expandablescreens are run with an initial diameter that issmaller than the open hole, and then they areextruded against the borehole wall. These systemsshow promise for some sand-control applicationsif the expanded screens are strong enough toresist rock failure and sand influx.2

However, if expandable screens are notcompliant enough to seal tightly against washed-out and enlarged or irregular boreholes, theireffectiveness may be only marginally better thanstand-alone screens. Removing trapped filtercakeafter screen expansion also is a concern. Frequentscreen-only failures led many operators to attemptmore openhole gravel-pack (OHGP) completions.3

Incomplete gravel packs and ineffectivecontrol of sand caused by early job termination,or screenout, resulting from annular blockage, or bridges, represent significant economic and operational risks for operators. A singlesand-control completion offshore costs severalmillion US dollars. Remedial interventions toclean out sand and repair wellbore damage oftencost up to a million US dollars and involvesignificant production downtime.

1. Sherlock-Willis TM, Morales RH and Price P: “A GlobalPerspective on Sand Control Treatments,” paper SPE 50652, presented at the SPE European PetroleumConference, The Hague, October 20–22, 1998.Parlar M and Albino EH: “Challenges, Accomplishments,and Recent Developments in Gravel Packing,” Journal ofPetroleum Technology 52, no. 1 (January 2000): 50–58.

2. Tiffin D, Stevens B, Park E, Elliott F and Gilchrist J:“Evaluation of Filter Cake Flowback in Sand ControlCompletions,” paper SPE 68933, presented at the SPEEuropean Formation Damage Conference, The Hague,May 21–22, 2001.

3. Ali S, Dickerson R, Bennett C, Bixenman P, Parlar M,Price-Smith C, Cooper S, Desroches L, Foxenberg B,Godwin K, McPike T, Pitoni E, Ripa G, Steven B, Tiffin Dand Troncoso J: “High-Productivity Horizontal GravelPacks,” Oilfield Review 13, no. 2 (Summer 2001): 52–73.

Spring 2005 51

> Sand-control methods. In-situ chemical consolidation and selective or oriented perforating in cemented casing with positive zonal isolation avoidsweaker zones or minimizes sand influx (top left). Cased-hole gravel packs control sand in laminated formations, lower quality sands and marginallyeconomic vertical wells (middle left). Frac packs and screenless completions combine stimulation and sand control in reservoirs with layered pay zones,poorly sorted grains or low fluid transmissibility (bottom left). In open hole, stand-alone screens control sand in formations with large well-sorted grains andin wells with short producing lives (top right). Expandable sand screen (ESS) systems provide viable well completions, but long-term reliability has not beenestablished (middle right). At high inclination angles, an openhole gravel pack (OHGP) often maintains well productivity or injectivity longer than othermethods (bottom right).

In-Situ Consolidation and Selective or Oriented Perforating

Cased-Hole Gravel Pack

Cased-Hole Frac Pack

Horizontal Stand-Alone Screen in Open Hole

Expandable Sand Screen (ESS)

Horizontal Openhole Gravel Pack (OHGP)

Cement

Resin

Perforations

Perforations

Blank pipe

Blank pipe

Fracture

Gravel

Intermediatecasing

Intermediatecasing

Intermediatecasing

Productioncasing

Productioncasing

Productioncasing

Gravel

Screens

Screens

Screens

Screens

Filtercake

Filtercake

Filtercake

Gravel

Production packerExpanded screens

Screens

Screens

Open hole

Gravel

Running screens

Gravel-pack packer

Production packer

This article reviews openhole gravel-packingtechniques, including water packing and shunt-tube technology, which achieve higher sustainedproduction and more consistent sand control inhigh-angle and horizontal wells than other types ofcompletions. A case history from Nigeria describesthe water-packing technique and postinstallationevaluation of OHGP results in a mature,heterogeneous reservoir with depleted pressure.

A second case history presents the evolutionof sand-control completions in the Caspian Seaoffshore Azerbaijan. This example reviews therelative performance of different sand-controlcompletions, including measures taken toimprove OHGP performance while reducingreservoir damage.

Sand-Control Methods Produced sand, or sanding, causes problemsranging from environmental concerns related tofacility cleanouts, processing interruptions andproper disposal on the surface, to erosion ofsubsurface or surface equipment and potentialloss of well control. Excessive sand influx cancause tubular or completion equipment failuresdownhole that delay production or result in lostreserves if the cost to sidetrack or redrill a well is prohibitive.4

In the past, operators often had to restrict, orchoke back, well production to levels below thecritical inflow rate that initiated sanding.Operators successfully used this strategy in someareas to delay installation of sand-controlmeasures, thus reducing upfront costs andavoiding initial completion damage, or high skin.However, restricting production adverselyimpacts profitability and may be impractical,especially for high-cost, high-rate deepwater andsubsea wells.

Companies also used in-situ chemical consoli-dation to lock formation grains in place byinjecting resins and catalysts into formations,generally through perforations in casing. However,placing chemicals across large zones and allperforations is difficult.5

Various techniques are available to excludesand from produced fluids, including chemicalconsolidation, cased-hole gravel packs, selectiveor oriented perforating, hydraulic fracturing, fracpacks that combine fracture stimulation andgravel packing, stand-alone or expandablescreens, screenless completions and openholegravel packs.

Depending on specific conditions and factors,these techniques have been applied with varyingdegrees of success. In high-angle and horizontalopenhole wells, operators use stand-alone or

expandable screens, openhole gravel packs and,in a few cases, frac packs or gravel packing abovefracture-initiation pressure to control sand.6

In the past, cased-hole gravel packing was areliable and widely used sand-control method inconventional vertical wells, but this techniqueoften results in extremely high completion skins.7

Today, most cased-hole gravel packs are per-formed as a high-rate water pack (HRWP) or afrac pack, depending on local experience oravailable materials and equipment. In wells withcemented casing, selective and oriented perfora-ting attempt to prevent sand production byavoiding weakly consolidated intervals oraligning perforations with maximum in-situstresses to increase perforation stability.8

Hydraulic fracturing for sand control involvesconventional and tip-screenout (TSO) treatmentsperformed in formations where a reduction inpressure drop, or drawdown, can minimize sandinflux or, in some cases, prevent the onset ofsanding.9 Frac packing combines TSO fracturestimulations that create short, wide hydraulicfractures with gravel packing to control sandproduction from weakly consolidated formations.10

Screenless completions combine selective ororiented perforating, in-situ consolidation andfrac packing to control sand.11 In some areas andunder certain formation conditions, stand-aloneor expandable screens may be an alternative togravel packing or frac packing.12

The initial productivity of wells with stand-alone screens is usually good, but many of thesescreen-only completions fail to adequatelyexclude sand over time. Some wells completedwith unpacked screens do not fail completely,but must be produced at significantly reducedrates because of partially plugged or erodedscreens. Openhole gravel packs, which providebetter borehole stability, tend to maintainproductivity and control sand longer than stand-alone or expandable screens.13

Economic considerations often requireoperators to choose sand-control methods thatmaximize completion reliability over the produc-tive life of a well or a field without limiting wellproductivity. The cost of future interventions toperform well workovers or recompletions isanother important consideration when selectingand designing sand-control completions. In high-cost, high-rate wells, expensive remedialoperations affect overall project profitability and economics.

Gas contracts, for example, often includepenalty clauses for defaulting on production anddelivery quotas. For cost- and risk-sensitivecompletions, especially in deepwater and subsea

developments, high failure rates and theuncertainty associated with stand-alone andexpandable screens may justify the choice of gravelpacking. However, if additional data becomeavailable to support expandable sand screen(ESS) reliability, operators might choose to useESS systems more frequently.

Today, more companies view OHGPcompletions as a viable sand-control completionfor inclined wellbores, especially in high-ratewells and wells with long horizontal sections.Because of a proven record of installationreliability, many operators consider openholegravel packing as the base case for sand controlin subsea, deepwater, or large-diameter wellboresand ultrahigh-rate gas wells.

Water Packing in Nigeria In 2002, Shell Petroleum Development Company(SPDC) evaluated openhole gravel packing as asand-control option in the Niger Delta, Nigeria(next page).14 Previously, SPDC had drilled30 wells with horizontal multilateral andsidetrack sections in shallow unconsolidatedreservoirs. SPDC completed most of these wellswith stand-alone slotted or prepacked screenliners in open hole.

In addition to the difficulty of cleaning outhorizontal sections before and after installingcompletion equipment, stand-alone screenswithout gravel packs are prone to partialplugging, screen erosion, sand influx and loss ofproductivity, which led SPDC and otheroperators to try expandable screens. Theseproblems often are related to incorrect fluidquality and practices during installation. Fluidcleanliness and conditioning, if managedproperly, can reduce the number of stand-alonescreen failures.

An ESS provides larger inflow area with lesswellbore friction than conventional screenliners, but current designs do not alwaysadequately seal against borehole irregularities.Like conventional stand-alone screens, ESSsystems also are affected by factors related tofluid quality, possibly even more so because ofpotential plugging of small screen openings.

During 2001 and 2002, noncompliant fixed-cone tools were used to expand most of the ESSsystems. Today, more compliant expansion toolsare available, but they have hole-size limitations.Expandable screens also tend to cost more,require longer delivery times and have unprovenlong-term productivity and sand-controleffectiveness. However, there have been reportsof ESS completions that have produced sand-freefor more than five years.

52 Oilfield Review

Spring 2005 53

4. Carlson J, Gurley D, King G, Price-Smith C and Walters F:“Sand Control: Why and How?” Oilfield Review 4, no. 4(October 1992): 41–53.

5. Parlar M, Ali SA, Hoss R, Wagner DJ, King L, Zeiler C andThomas R: “New Chemistry and Improved PlacementPractices Enhance Resin Consolidation: Case Historiesfrom the Gulf of Mexico,” paper SPE 39435, presented atthe SPE Formation Damage Symposium, Lafayette,Louisiana, USA, February 18–19, 1998.

6. Ali et al, reference 3.7. Waters F, Singh P, Baker C, van Wulfften P and Parlar M:

“A Novel Technique for Single-Selective Sand ControlCompletions Allows Perforating and Gravel Packing ofTwo Zones with Zonal Isolation in One Trip: A CaseHistory from Trinidad,” paper SPE 56668, presented at the SPE Annual Technical Conference and Exhibition,Houston, October 3–6, 1999.

8. Behrmann L, Brook JE, Farrant S, Fayard A,Venkitaraman A, Brown A, Michel C, Noordermeer A,Smith P and Underdown D: “Perforating Practices ThatOptimize Productivity,” Oilfield Review 12, no. 1 (Spring 2000): 52–74. Almaguer J, Manrique J, Wickramasuriya S, Habbtar A,López-de-Cárdenas J, May D, McNally AC andSulbarán A: “Orienting Perforations in the RightDirection,” Oilfield Review 14, no. 1 (Spring 2002): 16–31. Bersås K, Stenhaug M, Doornbosch F, Langseth B,Fimreite H and Parrott B: “Perforations on Target,”Oilfield Review 16, no. 1 (Spring 2004): 28–37.Cooper SD, Akong S, Krieger K, Twynam AJ, Waters F,Hurst G and Parlar M: “A Critical Review of CompletionTechniques for High-Rate Gas Wells Offshore Trinidad,”paper SPE 94662, presented at the SPE EuropeanFormation Damage Conference, Scheveningen, The Netherlands, May 25–27, 2005.

9. Hydraulic fracturing for reservoir stimulation usesspecialized fluids injected at pressures above theformation breakdown stress to create 180-degreeopposed cracks, or fracture wings, extending away froma wellbore. These biwing fractures propagate perpendicularto the direction of lowest rock stress in a preferredfracture plane. Held open by a proppant transported inthe treatment fluid, these conductive pathways increasethe effective wellbore radius by establishing linear flowinto the propped fracture and back to the well. Commonproppants are naturally occurring or resin-coated sandand high-strength bauxite or synthetic ceramics, sized byscreening according to standard US mesh sieves.In standard fracturing, the fracture tip is the final area tobe packed with proppant. A tip-screenout design causesproppant to pack, or bridge, near the end of a fracture inthe early stages of the treatment. As additional proppant-laden fluid is pumped, fractures can no longer propagatedeeper into a formation and begin to widen, or balloon.This technique creates a wider, more conductivepathway as proppant packs back toward the wellbore.

10. Saldungaray PM, Troncoso J, Sofyan M, Santoso BT,Parlar M, Price-Smith C, Hurst G and Bailey W: “Frac-Packing Openhole Completions: An Industry Milestone,”paper SPE 73757, presented at the SPE InternationalSymposium and Exhibition on Formation DamageControl, Lafayette, Louisiana, February 20–21, 2002. Ali S, Norman D, Wagner D, Ayoub J, Desroches J,Morales H, Price P, Shepherd D, Toffanin E, Troncoso Jand White S: “Combined Stimulation and Sand Control,”Oilfield Review 14, no. 2 (Summer 2002): 30–47.Morales RH, Profinet J, Piedras J, Gadiyar B andHarris S: “Optimization of Frac-Pack Completions Basedon Field Experience,” paper SPE 84263, presented at theSPE Annual Technical Conference and Exhibition,Denver, October 5–8, 2003. Gadiyar B, Meese C, Stimatz G, Morales H, Piedras J,Profinet J and Watson G: “Optimizing Frac Packs,”Oilfield Review 16, no. 3 (Autumn 2004): 18–29.

11. Pitoni E, Devia F, James SG and Heitmann N:“Screenless Completions: Cost-Effective Sand Control inthe Adriatic Sea,” SPE Drilling & Completion 15, no. 4(December 2000): 293–297; also paper SPE 58787,presented at the SPE International Symposium onFormation Damage Control, Lafayette, Louisiana,February 23–24, 2000. Acock A, Heitmann N, Hoover S, Malik BZ, Pitoni E,Riddles C and Solares JR: “Screenless Methods toControl Sand,” Oilfield Review 15, no. 1 (Spring 2003): 38–53.

12. Richard BM, Montagna JM and Penberthy WL Jr :“Horizontal Completions—2 Stand-Alone Screens Vary in Effectiveness,” Oil & Gas Journal 95, no. 32 (August 11, 1997): 63–69.Delattre E, Authier JF, Rodot F, Petit G and Alfenore J:“Review of Sand Control Results and Performance on aDeep Water Development—A Case Study from theGirassol Field, Angola,” paper SPE 91031, presented atthe SPE Annual Technical Conference and Exhibition,Houston, September 26–29, 2004. Al-Lamki A, Twycross J and Clarke G: “MaximizingProductivity in the Scoter Gas Condensate Field, UKCS: A Multidisciplinary Approach to Expandable Sand ScreenDesign and Subsea Installation,” paper SPE 91004,presented at the SPE Annual Technical Conference andExhibition, Houston, September 26–29, 2004.

13. Ali et al, reference 3. Wilson A, Roy A, Twynam A, Shirmboh DN and Sinclair G:“Design, Installation and Results from the World’sLongest Deep-Water Openhole Shunt-Tube Gravel-PackWest of Shetlands,” paper SPE 86458, presented at theSPE International Symposium and Exhibition onFormation Damage Control, Lafayette, Louisiana,February 18–20, 2004.

14. Onwusiri H, Onwuzurike C, McPike T and Jansen R:“Horizontal Openhole Gravel Packing in a Depleted andHeterogeneous Reservoir,” paper SPE 84159, presentedat the SPE Annual Technical Conference and Exhibition,Denver, October 5–8, 2003.

> Niger Delta, Nigeria, concession map. The Obigbo-North field is located in the OML-11 and OML-17 blocks about 18 km [11.2 miles] northeast of PortHarcourt, Nigeria. This field was discovered in October 1963. The reservoir consists of unconsolidated sands with permeabilities ranging from 900 to7,000 mD and porosities of 21 to 33%. The Obigbo-North field comprises 66 reservoir blocks: 55 oil-bearing and 11 gas-bearing. More than 50 wells havebeen drilled in the field.

Niger Delta Exploration and Production Concessions

Owerri

EnuguNiger River

Benin City

Warri

Bight of Benin

UyoObigbo-North field

1

4 38

4 5

7

3

22630

34

4

4

4 4

4

4681

36

33

3 22 1

1

777

71

2

24

326

27

22

21 16

15

13 14

17

11

79

Port Harcourt

N I G E R I AA F R I C A

Area ofdetailed map

These disadvantages restrict ESS utilizationin remote areas and mature fields with limitedincremental production or reserve potential.Gravel packing, on the other hand, has provedreliable in cased and openhole vertical wells;recent horizontal OHGP installations have beenless susceptible to partial screen plugging andsand erosion. An OHGP generally has lowercompletion damage, or skin, sustainsproductivity longer than conventional stand-alone screens, and may sometimes be more cost-effective than current ESS systems.15

In the Obigbo-North field, SPDC selected Well QWSB-53 for an OHGP. This was the firstOHGP by SPDC and the first in the Niger Delta.SPDC and Schlumberger performed a water-packing treatment in mid-2002 with the objectiveof achieving 3,000 B/D [477 m3/d] of sand-free oil production. Water packing involves lowconcentrations—0.5 to 2 pounds of proppant

per gallon [0.06 to 0.24 g/cm3]—of graveltransported by a low-viscosity carrier fluid,usually brine (above).16

The low side of a borehole packs first untilgravel reaches the far end of a section, alsocalled the toe, or until gravel forms a bridgebecause of formation collapse or high fluidleakoff. Excessive fluid loss is caused by acombination of inefficient fluids, high formationpermeability and poor or damaged filtercake,and low reservoir pressure, resulting in treat-ments that exceed formation breakdown, orfracture gradient, pressure. Gravitational forcesdominate during this “alpha” wave, so gravelsettles out in an advancing dune front until theparticle bed reaches an equilibrium height.

When fluid flowing above the bed reaches thecritical velocity for particle transport, gravelagain moves toward the far end, or toe, of ahorizontal section. This alpha wave stops after

reaching the end of a workstring internal washpipe, the toe of a horizontal section, a gravelbridge or collapsed formation.

A second deposition process, or “beta” wave,begins packing the annulus topside back towardthe beginning, or heel, of a well. This beta waverequires enough fluid velocity to maintainturbulent flow and move gravel along the top ofthe screen-borehole annulus.

In this Nigerian well, SPDC was concernedabout the low minimum in-situ stress of0.56 psi/ft [12.7 kPa/m] calculated from well-logderived values of Poisson’s ratio in the formation.SPDC drilled out cement in the bottom joint ofcasing and the float shoe, and 30 ft [9 m] of payto perform a pressure limit test. To ensure anadequate pressure margin for packing, engineersused the fracture gradient of 0.64 psi/ft[14.5 kPa/m] determined from this test as theupper limit for placing gravel.

54 Oilfield Review

>Water packing. Gravel packing with low-viscosity fluids, usually brines, relies on gravel deposition around thelow side of a screen-borehole annulus, while a slurry with low gravel concentration moves in turbulent flow alongthe top (top and bottom right). The borehole must be sealed with an efficient filtercake to minimize fluid leakoff. Ifcirculation—fluid returns to surface—is maintained, gravel moves in an “alpha” wave (1 to 5) toward the far end,or toe, of a horizontal section. If the slurry dehydrates and forward packing ceases in intervals with high fluidlosses, gravel fills the annulus and forms a bridge, resulting in an incomplete pack beyond that point. If bridgingoccurs or after gravel reaches the toe, packing proceeds back toward the beginning, or heel, of a horizontalsection in a “beta” wave (6 to 10). Surface treating pressures provide an indication of how water-packingtreatments are progressing (bottom left).

Typical Surface Treating-Pressure Response for Water Packing

Preflush stage

Slurry stage

Displacement stage

Alpha wave: slurry transportalong the screens

Slurry at toe of well Beta wave: gravelpacking from toe to heel

Annularpackoff

Surfa

ce tr

eatin

g pr

essu

re, p

si

Treatment duration, min

Blank pipe

1 2 3 4 5

678910

SlurryHeel

Toe

CasingScreens

Gravel duneWash pipe

Beta wave

Alpha wave

Borehole wall

Borehole wall

Wash pipe

Open hole Filtercake

GravelScreen

Spring 2005 55

SPDC then drilled a 1,000-ft [305-m]horizontal section to total depth using a 0.49-psi/ft [11 kPa/m] water-base drilling fluid.After reaching total depth, drillers circulated theborehole clean with no significant fluid loss,which indicated that filtercake on the sandfaceprovided a good seal. To minimize screen plugging,SPDC displaced the open hole with a solids-freefluid of the same composition as the reservoirdrilling fluid without calcium carbonate, but withadditional salt to maintain a 0.49-psi/ft pressuregradient. The casing was displaced with filteredbrine before running the sandface completionassembly, which consisted of available screensfrom warehouse stock (right).

A volumetric calculation determined that9,237 lbm [4,190 kg] of gravel would be required to pack the 6-in. openhole annulus.Schlumberger pumped gravel using a filtered12% potassium chloride [KCl] brine carrier fluid.Injection pressure and rate, and sandconcentration were monitored at surface to trackthe alpha and beta waves (below right).

Gravel was placed in four stages at differentconcentrations while reducing the injection rateto adjust for injection pressure increases. Informations with low fracture gradients, Shellcompletion engineers often reduce the slurryinjection rate once a beta wave initiates to avoidprematurely breaking down formations prior todeveloping enough dune height to completelycover the screens. This technique lays downadditional alpha waves on top of previous dune beds.

This resulted in multiple alpha waves andcomplete screen coverage. The operator pumpeda total of 13,500 lbm [6,123 kg] of gravel, butreversed out 2,670 lbm [1,211 kg] of excessgravel, leaving about 10,830 lbm [4,912 kg] ofgravel around the screens, which corresponds toan actual borehole size of 6.25 in.

After gravel packing, the running tool andwash pipe were retrieved. A 31⁄2-in. tubing stringwith a tubing-retrievable subsurface safety valveand gas-lift mandrels for future artificial lift wasinstalled. A 10% hydrogen chloride [HCl] acidtreatment energized with nitrogen [N2] wasdisplaced in the horizontal section using 11⁄2-in.coiled tubing and a downhole tool with 360°rotating jet nozzles to generate hydraulicturbulence and make closer contact withfiltercake on the borehole wall.

15. Skin is a dimensionless measure of completion damage.Positive values represent damage. Zero is equivalent tothe productivity of an undamaged formation. Negativevalues represent stimulated producing conditions.

16. Parlar and Albino, reference 1.

> Obigbo-North field wellbore and completion schematic. Shell Petroleum Development Companycompleted Well QWSB-53 with 965 ft [294 m], or 32 joints, of screen deployed in open holewithout centralizers.

Casing cross section

7-in. liner6.184 in.4.5-in. gravel-pack packer3.958 in.2.875-in. tubing2.441 in.

Openhole cross section

7-in. liner6-in. open hole4.45-in. screens4-in. base pipe3.548 in.2.875-in. tubing2.441 in.

Surface-controlledsubsurface safety valve

FIV FormationIsolation Valve

Gravel-pack packerat 8,273 ft MD

965 ft of wire-wrapped screen

Gravel7-in. liner

Heel

Toe

End of open holeat 9,410 ft MD

9 5⁄8-in. casing

> Obigbo-North field gravel-packing treatment. SPDC and Schlumberger pumped theWell QWSB-53 gravel pack while monitoring injection pressure (red), sand concentration(blue) and injection rate (green). Gravel-packing operations were performed at 3.5, 3,2.5 and 1 bbl/min [0.56, 0.48, 0.4 and 0.16 m3/min]. The pumping profile showed pressureresponses corresponding to both alpha-wave and beta-wave gravel deposition. About75% of the gravel was deposited during the alpha wave, leaving only 25% for the beta-wave deposition.

3,000

2,500

2,000

1,500

1,000

500

00 20 40 60

Time, min

Alpha wave

QWSB-53 OHGP

Beta wave

80 100 120

6

5

4

3

2

1

0

Pres

sure

, psi

Sand

con

cent

ratio

n, lb

m/g

alIn

ject

ion

rate

, bbl

/min

Injection pressureSand concentrationInjection rate

Post-treatment cleanup improved wellperformance by diverting acid across thehorizontal section and by ensuring deeperpenetration.17 The well initially produced oil at3,250 B/D [517 m3/d]. A spinner inflow profile,pressure drawdown and total production ratefrom a memory production log indicated 100%pack efficiency, with the entire horizontalsection producing into the screen assembly.

This completion demonstrated better initialinflow capability and longer sustainedproductivity at a higher drawdown pressuresthan other wells with stand-alone screens. It alsosaved the SPDC Eastern Asset Team US$ 300,000compared with previous ESS installations.

Low-rate water packing was a technical,operational and commercial success in the NigerDelta. SPDC continued to improve fluid qualityand fluid handling, reduce nonproductive rigtime and monitor OHGP performance todetermine whether this technique was suitablefor other wells.

Horizontal openhole gravel packing was laterdiscontinued because of premature screenoutsand incomplete gravel packs on some other

wells. The water-packing technique was deemedtoo complex to execute without proper onsitetechnical expertise and supervision.

Subsequently, SPDC installed severalopenhole gravel packs using Alternate Pathshunt-tube screens and Schlumberger MudSOLVfiltercake removal service, which wereextremely successful. These high-rate gas-wellcompletions were not horizontal, but had high-angle wellbore inclinations over relatively shortintervals of about 100 ft [30 m]. SPDC continuesto evaluate Alternate Path screen technology forlonger pay intervals.

Alternate Path Technology If gravel packing is required, operators mustchoose between water packing with conventionalscreens or using screens with Alternate Pathshunt tubes, two field-proven techniques forcompleting long openhole sections. Mobil OilCorporation, now ExxonMobil, developedAlternate Path technology in the late 1980s andearly 1990s to address problems associated withgravel bridging (below).18

External shunt tubes on Alternate Pathscreens allow slurry to bypass any blockage thatforms in the annulus between screens and casingor an open hole during gravel packing. Thistechnology helps ensure a complete gravel packbelow annular bridges.19 However, shunt tubesrestrict the size of screen that can be deployed,which is a limitation.

Unlike water packing, this technique does notrely on filtercake integrity. If the annulusbecomes restricted, pumping pressure increasesand slurry diverts into the shunts. These tubesprovide a conduit for slurry to bypass collapsedhole, external inflatable packers or annulargravel bridges at the top of intervals or adjacentto zones with high fluid leakoff.

Gravel packing with Alternate Path screensuses gravel pumped at higher concentrations—4 to 8 lbm/gal [0.48 to 0.96 g/cm3]—in viscouscarrier fluids. Engineers adapted Alternate Pathscreens for use in longer openhole horizontalsections. Transport shunts without ports areattached along the entire length of screenassemblies to reduce friction pressures whilegravel packing long intervals; shunts with exitports, or nozzles, serve as packing tubes.

56 Oilfield Review

> Alternate Path gravel packing. This technology ensures complete packing of gravel around sand-exclusion screen assembliesand across an entire horizontal section. Shunt tubes attached to the screens provide conduits for slurry to bypass gravel bridgesand fill annular voids (top and bottom right). Shunt packing does not depend on filtercake to prevent fluid loss. If the annulusbetween screens and openhole packs off prematurely (1 to 3), slurry diverts into the shunts, and gravel packing proceeds towardthe toe even with no fluid returns, or circulation, to surface (4 and 5). Usually, pump rates are reduced after shunt flow begins,and treating pressure increases because of the small shunt-tube diameters (bottom left).

CasingHeel

Toe

Shunt tube NozzlesSlurry

2

Gravel

Surfa

ce tr

eatin

g pr

essu

re, p

si

Treatment duration, min

Gravel formsDisplacement stage

Pressure increaseas flow divertsinto shunt tube

Slurry stage

Alpha wave: slurry transportalong the screens

Typical Surface Treating-Pressure Response for Shunt-Tube Screens

Preflush stageAnnular packoffSlurry at toe of well

Borehole wallProtective shroud

Transport tube

Screen

Packing tube

Boreholewall

Nozzle

Protective shroud

Base pipe

Wash pipe Blank pipe Open hole Filtercake

Gravel

1

3

4 5

Screen

Spring 2005 57

This configuration reduces carrier-fluid leakoffinto the annulus, limits slurry dehydration, anddelivers slurry to packing tubes at 4 to 6 bbl/min[0.6 to 0.9 m3/min]. Slurry flows from transport topacking tubes through a manifold at each screenjoint and exits through wear-resistant, carbidenozzles to pack voids behind screens at 0.5 to2 bbl/min [0.08 to 0.3 m3/min]. Shunts andnozzles are designed to reduce gravel buildupinside shunts.

Gravel does not easily make turns throughsmall exit ports, so large angled nozzles thatextend into the flowstream increase thetendency for slurry to exit the shunts. Treatmentsare performed using nondamaging fluids withgood gravel-carrying capacity and low frictioncharacteristics.

Blank pipe above screen assemblies also canbe fitted with transport tubes to provide a pathfor slurry in the event of borehole collapse orformation of a gravel bridge at the top of aninterval. In addition, a pipe shroud withpredrilled holes surrounds the entire AlternatePath screen assembly to centralize the screenswithin the shroud and to protect the shunt tubesduring installation.

Shunt Packing in Azerbaijan BP operates the Azeri, Chirag and Guneshli(ACG) fields in the Caspian Sea (above right).Since 1997, BP has installed several differenttypes of sandface completions in 29 primary andsidetrack wells, including both producers andinjectors. During this period, sand-controlmethods evolved from water packing withconventional screens and cased-and-perforatedcompletions to stand-alone screens, expandablesand screen (ESS) systems and openhole gravelpacking with Alternate Path screens.20

Water Packing with Conventional Screens—BP completed two early Chirag producing wells, A-02 and A-03, as openhole gravel packs using thewater-packing technique. Well A-02 produced oilfrom December 1997 until March 1999 withassociated sand rates of less than 10 lbm/1,000 bbl[28.5 g/m3] and no water. Since then, BPperiodically shuts this well in because of high gasproduction. Buildup tests in December 1998 andNovember 2004 indicated positive skins of 3.2 and2.1, respectively.

Well A-03, a similar completion in January1998, produced oil with sand rates of 2 to3 lbm/1,000 bbl [5.7 to 8.6 g/m3]. Buildup tests inDecember 1998 and July 2003 both indicatedpositive skins of 4.4. Despite relatively low gravel-pack efficiencies of about 25% and higher skinfactors in each well, water packing with

conventional screens achieved acceptable sandcontrol. To date, neither the A-02 nor the A-03well has produced water, and no sand hasaccumulated in either wellbore. However, as BPbegan drilling more extended-reach wells withhigh-angle and horizontal sections through payintervals, completion engineers shifted toAlternate Path screens.

Cased-and-Perforated Wells—Well A-06 andSidetrack A-06z represent two of nine oilproducers and two water injectors in the Chiragfield with cased-and-perforated completions. In

1998, the A-06 completion initially produced sandat high rates that eventually stabilized at 1 to3 lbm/1,000 bbl [2.9 to 8.6 g/m3], but withoccasional bursts that exceeded 100 lbm/1,000 bbl[285 g/m3]. A pressure buildup test indicated alow skin of negative 0.9.

However, after water breakthrough in early2000, sand production increased dramatically,and BP had to restrict outflow from this well. InNovember 2000, a coiled tubing workovercleaned out sand fill and set a cement plug in thewellbore to isolate lower sands and reestablish

17. Arangath R, Onwusiri HN and Ogoke VC: “A Cost-Effective Approach to Improve Performance of HorizontalWells Drilled in High-Permeability Formations,” paperSPE 73786, presented at the SPE InternationalSymposium and Exhibition on Formation Damage Control,Lafayette, Louisiana, February 20–21, 2002.Onwusiri H, Onwuzurike C, Adiodun A and Uchendu C:“Rotating Jetting Nozzle Adds Value in the Cleanup ofHorizontally Gravel Packed Well—Case Histories fromSandstone Environment,” paper SPE 82237, presented atthe SPE European Formation Damage Conference, The Hague, May 13–14, 2003.

18. Shunt-screen technology was developed by Mobil, nowExxonMobil, in the late 1980s and early 1990s and islicensed to Schlumberger.

19. Jones LG, Yeh CS, Yates TJ, Bryant DW, Doolittle MWand Healy JC: “Alternate Path Gravel Packing,” paperSPE 22796, presented at the SPE Annual TechnicalConference and Exhibition, Dallas, October 6–9, 1991.

Jones LG, Tibbles RJ, Myers L, Bryant D, Hardin J andHurst G: “Gravel Packing Horizontal Wellbores withLeak-Off Using Shunts,” paper SPE 38640, presented atthe SPE Annual Technical Conference and Exhibition,San Antonio, Texas, October 5–8, 1997.Karlstad S, Sherlock-Willis T, Rajan S, Samsonsen B andMonstad PA: “An Evaluation and Design Approach toGravel-Pack Treatments in the Gullfaks Field,” paper SPE 48978, presented at the SPE Annual TechnicalConference and Exhibition, New Orleans, September 27–30, 1998.

20. Powers B, Elliott F, Gilchrist J, Twynam AJ, Edment Band Parlar M: “A Critical Review of Chirag FieldCompletions Performance Offshore Azerbaijan,” paperSPE 94824, presented at the SPE European FormationDamage Conference, Scheveningen, The Netherlands,May 25–27, 2005.

> Azerbaijan, Caspian Sea offshore oil and gas field developments. BP has a 34% interest in Chiragfield, part of the Azeri-Chirag-Guneshli (ACG) megastructure development. The ACG megastructurecontains an estimated 10 billion barrels [1.6 billion m3] of oil in place, in an area that is about 30 miles[48 km] by 3 to 5 miles [4.8 to 8 km]. Chirag field early oil production was the first phase of ACGdevelopment, along with part of the Central and West Azeri field.

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water-free production from the upper sands.Water breakthrough in these zones duringNovember 2001 again caused sand production to increase.

Early in 2002, BP abandoned the A-06wellbore and sidetracked it to quickly restoreproduction. Pressure buildup tests in the newcased-and-perforated A-06z wellbore indicated alow skin of negative 1.6, but high sand ratesrequired BP to choke back production after thiswell also began producing water in March 2003.In December 2003, Well A-06z was abandonedand sidetracked again as an OHGP completion,which has since produced oil with low sand ratesof 1 to 3 lbm/1,000 bbl.

Stand-Alone Screens—Three Chirag fieldwells, including A-09 and A-18, were completedwith stand-alone screens. In April 2002, apressure buildup test on Well A-09 indicated alow skin of negative 2.8. This well produced oiland minimal sand until water breakthrough inSeptember 2003 when sand rates becameexcessive even at low water cuts of 3 to 6%.

Well A-18, completed with stand-alonescreens, initially had to be choked back becauseof excessive sand. After BP gradually increasedthe production rate over three months withcorresponding increases in sand, produced sandbegan to decrease despite increasing oil rates.Pressure buildup tests indicated an initial skin ofnegative 1.8, which gradually decreased to anextremely low negative 5. Sand influx continuedto decrease, except for intermittent bursts, butthis well never achieved maximum productivitybecause of persistent sanding.

Expandable Sand Screens—BP completedtwo Chirag field wells with ESS systems. One, theA-08z sidetrack, was drilled as a water injector.But during the cleanup flow period in December2002, this well produced oil with no water.Pressure buildup tests indicated a positive skinof 3.3. The well produced at low sand rates of 1 to5 lbm/1,000 bbl [2.9 to 14.3 g/m3] until March2004 when it was converted to injection.

BP installed the second ESS completion in asidetrack of Well A-09. The original wellbore, A-09, had been completed with stand-alonescreens. It produced high oil rates until waterproduction increased in September 2003 fromless than 0.1% to 10% with significant volumes ofsand. BP sidetracked this well as A-09z andrecompleted the new wellbore with expandablescreens in April 2004. Well A-09z produced oilwith no water and had sand rates similar to WellA-08. However, skin gradually increased fromnegative 1.5 to positive 0.3, possibly because ofincreasing gas production.

BP completed both A-08z and A-09zsidetracks with relatively low-strength ESSscreens. Well logs and absence of initial orsubsequent bursts of sand common in bothcased-and-perforated and stand-alone screencompletions helped confirm a high degree of ESSintegrity. After a period of production, however,caliper logs revealed screen deformation,particularly across shale sections. Sand controland well productivity have not been affected, butlong-term ESS performance is still uncertain.

Alternate Path Screens—Since November2000, BP has installed 13 Alternate Pathcompletions in five producers and three waterinjectors in the Chirag field and in five producersof the Azeri field. BP originally planned toconvert Well A-15-T1, the first Chirag field well tobe completed with Alternate Path AllPACscreens, to water injection after a briefproduction period. The sandface completionincluded 380 m [1,247 ft] of AllPAC screens withtwo transport shunts, two packing shunts and aprotective shroud.

Slurry diverted into the shunts midwaythrough the gravel-packing treatment for thiswell. BP estimated a pack efficiency of 94% basedon mass-balance calculations. A well log revealedonly minor voids in a few locations. Buildup testsin July 2001 indicated a positive skin of 3.2. Oneyear later, skin had decreased to positive 1.8. Aproduction log revealed uniform inflow acrossthe openhole section.

BP kept Well A-15-T1 as a producer, and inDecember 2000, this OHGP well produced oil at15,400 B/D [2,448 m3/d] with no water. The wellbegan producing water in January 2003. Watercut increased to about 7% at the end of 2003, andthen to 14% by mid-2004. Sand rates, however,remained at 0.3 to 5 lbm/1,000 bbl [0.86 to14.3 g/m3] in contrast to significantly greatersand production from wells having stand-alonescreens and cased-and-perforated completionswith water cuts as low as 3 to 5%.

Flow from the well recently ceased because ofliquid loading, but the OHGP maintained adequatesand control during two years of water production.This performance established openhole gravelpacking with Alternate Path screens as the designbasis for subsequent well completions.

58 Oilfield Review

> Chirag field wellbore and completion schematic. BP completed Well A-19 by running and gravelpacking 504 m [1,653 ft] of AllPAC screens with two packing shunts, two transport shunts and aprotective shroud in open hole.

Oil-bearing layer

84-degree wellbore inclination End of open hole at7,481 m MD

9 5⁄8-in. casing at 6,969 m MD

QUANTUM gravel-packpacker at 6,875 m MD

Transport tube Nozzles

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Spring 2005 59

Well A-19, the longest extended-reach well todate in the Chirag field, was drilled andcompleted in December 2004 to tap anundeveloped area of the field, 6 km [3.7 miles]northwest of the Chirag-1 platform. The sandfacecompletion included 504 m [1,653 ft] of AllPACscreens with two packing shunts, two transportshunts and a protective shroud (previous page).BP calculated a gravel-pack efficiency of 91% in the A-19 well. Pressure data indicated thatslurry diverted into the shunts during gravelpacking (right).

Pressure buildup tests in January andFebruary 2005 indicated near-zero positive skinsof 0.5 and 0.1, respectively. The well produced oilrates that exceeded the 20,000-B/D [3,180-m3/d]test-separator capacity. Sand influx averaged lessthan 1 lbm/1,000 bbl [3 g/m3] with no producedwater. BP expects the output of Well A-19 toreach 29,500 B/D [4,690 m3/d], the highest ratein this field. This is a significant achievementafter eight years of production, with reservoirpressure depleted 1,000 psi [6.89 MPa] belowinitial conditions.

Four other wells were completed usingsimilar procedures, two in the Chirag field andtwo in the Azeri field. Both Chirag wells hadpositive skins of 2. Well A-12x produced oil at9,000 to 12,000 B/D [1,431 to 1,908 m3/d] and hada sand rate of 0.4 to 1 lbm/1,000 bbl [1.2 to 3g/m3]. Well A-06y produced oil at 14,000 to 15,000B/D [2,226 to 2,385 m3/d] and had a sand rate of1 to 2 lbm/1,000 bbl [3 to 6 g/m3].

One of the two Azeri field completions, WellB-05, produced oil at 42,000 B/D [6,677 m3/d]with extremely low sand rates of 0.2 to0.3 lbm/1,000 bbl [0.6 to 0.9 g/m3]. The secondAzeri well, B-09, produced oil at 35,000 B/D[5,565 m3/d] and had similar low sand rates, butis still choked back. This level of productivity isnotable because a borehole problem preventedscreens from reaching total depth, so only onezone contributes to production.

These completions resulted in high-productivity wells. Installation of sandfaceequipment was successful in all the wells exceptone, Azeri B-09, where the screens got stuckmidway down the openhole section. This problemhas occurred in other areas, when screens wererun after displacing oil-base mud with water-base fluids.21 Difficulties running screens to totaldepth in water-base fluids motivated BP tomodify completion procedures in the next twoAzeri wells.

The Azeri Well C-04, completed in December2004, was the first completion in the ACGdevelopment to have sandface equipment run

with oil-base fluids in the wellbore. Thecompletion equipment included 600 m [1,969 ft]of AllPAC screens with two transport shunts, twopacking shunts and a protective shroud.Estimated gravel-pack efficiency was 66%. Abuildup test indicated zero total skin.

Well C-01Az, a similar completion, had apositive skin of 2.5. A higher estimated gravel-pack efficiency of 93% resulted from increaseddrilling-fluid density and correspondingreduction in inward movement of the hole beforegravel packing. A large-scale surface testidentified no significant problems withdisplacing oil-base fluids from screens, so BPplans to use these modified techniques on futureopenhole gravel packs.

Fluids and displacement procedures used inthis development program have evolved since thefirst Alternate Path completion in November2000. Initially, BP drilled cased-and-perforatedwells and completions with stand-alone screensusing oil-base fluids. Early wells with openholegravel packs were drilled using water-base fluidsthat were essentially reduced viscosity, orthinned, mud from previously drilled holesections. The wells also were gravel packed withwater-base fluids.

BP anticipated that wells would become morechallenging and that high-angle and longerextended-reach wellbores would be difficult todrill and complete using water-base fluids. BP,ChevronTexaco, Petrobras and Total initiated ajoint venture to develop oil-base fluids for drillingand completing OHGP wells.22

Reservoir sections of the first two wellscompleted with Alternate Path screens in theChirag field and one well in the Azeri field weredrilled with water-base fluids. A synthetic oil-base mud was used for reservoir drilling in thenext three Chirag field producers and the nextfour Azeri field producers.

In the three Chirag field wells and the firsttwo Azeri field wells, the entire wellbore wasdisplaced with water-base fluid before runningsandface completion equipment. On the twomost recent Azeri wells, screens were run in oil-base mud.

To date, BP has gained significant experiencethat can be applied in other wells in the Chiragand Azeri fields and the deepwater Guneshlifield. Fluid systems currently available provideBP with the option of implementing oil-basegravel packing if necessary.

> Chirag field Well A-19 gravel-packing treatment. BP and Schlumberger pumped the record-settingWell A-19 gravel pack at 6 lbm/1,000 gal [17 g/m3] and about 10 bbl/min [1.6 m3/min] while monitoringinjection pressure (red), fluid injection rate in (blue), fluid return rate out (yellow) and sand concentration(green). The pressure profile during pumping operations indicated responses corresponding to bothannular gravel deposition and shunt-tube diversion.

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21. Brady ME, Bradbury AJ, Sehgal G, Brand F, Ali SA,Bennett CL, Gilchrist JM, Troncoso J, Price-Smith C,Foxenberg WE and Parlar M: “Filtercake Cleanup inOpen-Hole Gravel-Packed Completions: A Necessity orA Myth?” paper SPE 63232, presented at the SPE AnnualTechnical Conference and Exhibition, Dallas, October 1–4, 2000. Parlar M, Twynam AJ, Newberry P, Bennett C, Elliott F, Powers B, Hall K, Svoboda C, Rezende J, Rodet V and Edment B: “Gravel Packing Wells Drilledwith Oil-Based Fluids: A Critical Review of CurrentPractices and Recommendations for FutureApplications,” paper SPE 89815, presented at the SPEAnnual Technical Conference and Exhibition, Houston,September 26–29, 2004.

22. Wagner M, Webb T, Maharaj M, Twynam A, Green T,Salamat G and Parlar M: “Open-Hole Horizontal Drillingand Gravel-Packing with Oil-Based Fluids—An IndustryMilestone,” paper SPE 87648, presented at the SPE International Symposium and Exhibition onFormation Damage Control, Lafayette, Louisiana,February 18–20, 2004.

Looking Ahead Many recently discovered fields require sandmanagement.23 For example, BP estimates thatwithin five years 50% of its oil and gas productionwill be from weak and unconsolidated sandstonereservoirs. In the Niger Delta region of Nigeriawhere Shell Production Development Companyoperates, 70% of hydrocarbon reserves lie inshallow reservoirs prone to producing sand.

Selection of suitable sand-preventiontechniques is a challenge that requires asubstantial amount of data, acquired atsignificant cost. Even then, sand-controlmeasures that appear viable based on initial dataoften fail. This makes experience in a particulararea an important factor in planning and designof future well completions.

Based on experience, cased-and-perforatedcompletions yield low skins, but produce largevolumes of sand, even before waterbreakthrough. These completions requirerestricted production rates and longer flowperiods to achieve postcompletion cleanup. Theyalso involve repeated cleanout of surfaceseparators, and the transport and properdisposal of sand at surface with associatedhealth, safety and environmental risks. Sand fillin wells requires frequent remedial wellinterventions, and flow from these wellseventually must be choked back significantly.

Completions with stand-alone screens exhibitlow skins, but often produce large volumes ofsand initially. Subsequently, sand influx ratesdecrease, but there still can be occasional burstsof higher sand production. However, both cased-and-perforated and screen-only completionsproduce large amounts of sand as waterproduction increases, even at low water cuts,necessitating expensive premature sidetracks insome fields.

ESS completions have low skins and currentlyappear to provide sand control equivalent to anOHGP, but the long-term impact of screendeformation, and ESS performance andreliability after water breakthrough remainunknown. Many initial ESS completions havebeen converted to injection after short periods ofproduction; others were installed too recently forconclusive evaluation.

An OHGP with high pack efficiencies controlssand, even when intentionally designed withlarger gravel to allow limited volumes of smallerfines to be produced. In addition, openhole gravelpacks tend to control sand more effectively thanother methods after water breakthrough.

Unless formations have extremely clean, well-sorted grains, subsea production and injectionwells that may produce sand and mostcompletions in deep water—greater than 1,000to 2,000 ft [305 to 610 m]—should be gravelpacked to avoid costly remedial interventions,especially when large reserve volumes areinvolved. Many operators now prefer openholegravel packing in wells with long horizontalsections to reduce sand-related failures andminimize associated productivity decline.

Ongoing improvements in fluid designs anddisplacement procedures are helping operatorsincrease the productivity of openhole gravel-pack completions, while reducing overall fielddevelopment and operational costs. Theseimprovements include reservoir drilling usingoil-base fluids, running completion equipmentand screens in oil-base fluids, displacing theentire wellbore with water-base fluids andgravel packing with water-base viscoelasticsurfactant fluids.24

For example, improved BP completionspractices in the Chirag field have resulted in acontinuous reduction in completion skins to nearzero, with incremental oil productivity benefitsof 600 to 800 B/D [95.4 to 127 m3/d] per well(above). Switching to oil-base drilling fluidsincreased drilling efficiency, improved holeconditions, and reduced drag when runningscreen assemblies in high-angle and horizontalwells. In addition to environmental benefits dueto eliminating discharge of water-base fluids, BPalso realized savings exceeding US$ 130,000 perwell by recycling synthetic oil-base fluids andreducing gravel-packing fluid costs throughelimination of enzymes in the carrier fluid.

Openhole gravel packing has evolved asoperators and service companies have gainedexperience and a better understanding offormation damage and gravel placement inhorizontal wells. It remains the sand-controlmethod of choice to protect screens and provideimproved sandface completions.

Successes like those achieved by Shell inNigeria and BP in Baku have establishedoperator confidence in the effective applicationand long-term performance of openhole gravelpacks. The reliability of OHGP completions iscontributing to a shift in the well-constructionand sand-control philosophies of many drillingand completions teams. —MET

60 Oilfield Review

> Completion skin data for the Chirag field. Cased-and-perforated completionsin the Chirag field yielded low skins. Stand-alone screens also exhibit lowskins. However, both types of completions produce excessive sand as waterproduction increases. ESS completions have low skins and control sandproduction, but their performance after water breakthrough is currentlyunproven. BP has reduced openhole gravel-pack completion skins to almostzero by continuously improving fluid designs and practices. Openhole gravelpacking is a proven completion technique that controls sand even after wellsbegin producing water. For these reasons, openhole gravel packing is currentlythe basis of design in Azeri, Chirag and Guneshli field well completions.

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23. Acock A, ORourke T, Shirmboh D, Alexander J, Andersen G, Kaneko T, Venkitaraman A, López-de-Cárdenas J, Nishi M, Numasawa M, Yoshioka K, Roy A, Wilson A and Twynam A: “PracticalApproaches to Sand Management,” Oilfield Review 16,no. 1 (Spring 2004): 10–27.

24. Kefi S, Lee J, Pope TL, Sullivan P, Nelson E,Hernandez AN, Olsen T, Parlar M, Powers B, Roy A,Wilson A and Twynam A: “Expanding Applications forViscoelastic Surfactants,” Oilfield Review 16, no. 4(Winter 2004/2005): 10–23.