lost circulation solutions for severe sub-salt thief zones

Copyright 2002 AADE Technical Conference

This paper was prepared for presentation at the AADE 2002 Technology Conference “Drilling & Completion Fluids and Waste Management”, held at the Radisson Astrodome, Houston, Texas, April 2 - 3,2002 in Houston, Texas. This conference was hosted by the Houston Chapter of the American Association of Drilling Engineers. The information presented in this paper does not reflect any position,claim or endorsement made or implied by the American Association of Drilling Engineers, their officers or members. Questions concerning the content of this paper should be directed to the individualslisted as author/s of this work.

AbstractSignificant challenges exist when drilling through

and below salt formations. The thief zone at the base ofthe salt can introduce severe lost circulation and wellcontrol problems, often resulting in loss of the interval orthe entire well. In many cases, operators choose to drillthe salt section with water-based mud systems ratherthan risk costly losses of oil- or synthetic-based fluids.The slower drilling rates and poor hole quality obtainedby drilling with water-based mud are offset by thepotential losses of the invert emulsion drilling fluid. Thethief zone immediately below the salt formation istypically a thin zone of highly fractured rock, usuallyshale. Controlling losses in this zone has proved to beextremely difficult. Very few effective lost circulationremedies have been successful, especially when drillingwith invert emulsion fluids.

The producing formations of the Hassi Messaoudfield are located below a 900-meter (3000-ft) zone ofmixed salt, which introduces a severe thief zone into thewell construction process. The severe lossesencountered drilling these wells result in significant non-productive time, massive fluid loss, borehole instabilityand loss of the drillstring. A severe pressure inversionalso contributes to the lost circulation problems, withpore pressure reductions from ± 2.01 sg (16.8 lb/gal) inthe salt to ± 0.93 sg (7.8 lb/gal) in the thief zone. Over800 wells have been drilled in the Hassi Messaoud field,with more than half of these wells encountering total lossof circulation once the salt section is drilled. The degreeof severity of the losses varies between sub blocks as afunction of local stresses, which provide additional forcesfor rock destabilization.

Through the application of specialized lost-circulation materials, these wells can now be drilledwithout significant fluid losses. The drilling time for thetroublesome intervals has been reduced by 50% in mostcases. This paper reviews the methods applied to avoidlost circulation in the sub-salt thief zone and discussesthe time and cost savings obtained.

IntroductionThe drilling performance curve for the Hassi

Messaoud field had reached a point of optimized

performance leaving a very narrow window for furtherimprovement, with the exception of the recurring sub-saltlost circulation. Loss of circulation typically occurs whilepenetrating the Trias Argileux Greseux (TAG) formationin the lower section of the 8½-in. interval (Fig. 1).Vertical well profiles indicate the thief zone is below ahigh-pressure interval, which is commonly drilled usingsalt-saturated water-based fluid with a density in therange of 1.98 – 2.1 sg (16.5 to 17.5 lb/gal). The lossesare thought to be induced due to a severe pressuretransition once the Trias Salifere (the main salt section)has been drilled, exposing the TAG formation to anexcessive annular hydrostatic pressure (>9000 psi). Thisover-pressure induces fractures, causing severeformation losses.

The shape and structure of induced formationfractures is always subject to the nature of the formation,drilling and mechanical effects, as well as geologicalinfluences over time. It is believed the HMD field isdivided into 25 separate blocks, and subsequently eachblock appears to exhibit unique geologicalcharacteristics. The field can best be described asunified single units referencing specific geology andstrategraphy.

With the experience gained developing the HMDfield, the operator was able to predict the depth andseverity of the losses with a relatively high degree ofaccuracy. Typical historical offset data are given in Table1. The challenge remained to develop an effectivesolution to prevent or cure the severe lossesencountered below the salt formation. Extensiveinvestigations were carried out, involving modification ofdrilling practices and testing of a wide range of lostcirculation materials (LCM). According to operationaldata, squeezed cement slurries across the thief zonewere successful at sealing off the losses, but alwaysinvolved a negative effect on performance due to thenon-productive time associated with repetitive tripsinvolved with the squeeze job.

A new chemically activated crosslinking pill (CACP)1

was proposed to the operator to shut-off losses, shouldthey occur, below the salt zone. Strict operationalprocedures were also developed, giving clear directionsas to the timing and method of applying the CACP.

AADE-02-DFWM-HO-30

Lost Circulation Solutions for Severe Sub-Salt Thief ZonesM. Ferras, Sonatrach; M. Galal and D. Power, M-I L.L.C.

2 M. FERRAS, M. GALAL, and D. POWER AADE-02-DFWM-HO-30

Performance improvement was the target of thisproposal to the operator, with the performanceimprovements being directed not at the drilling fluid, butthe large gap existing between conventional LCM andcement squeezes for severe loss of circulation. TheCACP proposed to the operator had the followingproperties.

1. A simple fluid that can be pumped through thebit and bottomhole assembly (BHA).2. Adjustable particle-size distribution to fitformation specific requirements (squeezable intomicro-size fractures).3. Forms a semi-ridged gel structure as a fillingmaterial for the loss zone.A time breakdown was conducted on a number of

offset wells in the HMD field. The results of this analysissuggested that if the sub-salt lost circulation problemscould be eliminated, a drastic enhancement of thedrilling performance was possible. The followingexample illustrates the time and cost benefits availablethrough eliminating the non-productive time (NPT)associated with the sub-salt losses.

If HMD field developed with an average of 25 wellsper year, the following savings would be realized.

• Average 49 days per well• Average 3 cement plugs per well• 2.0 days lost per plug• Total NPT = 150 days• 3.06 additional wells could be drilled• Over all performance could be improved by,

3.06 / 25 = 12.2 %• 12.2 % possible additional production.The CACP technique was proposed as a lost

circulation solution in the TAG formation with the specificrequirements to cure losses with maximum possiblesealing efficiency and increase formation integrity acrossthe weak sections. Additionally, the process of applyingthe CACP was to involve less time than that currentlyassociated with applying the cement squeezes. The newtechnique was applied successfully, sealing the TAGformation without any lost tripping time, and allowingdrilling to continue with no further losses.

Problem OverviewThe following section describes the typical formation

properties and drilling conditions for a HMD well. Thesub-salt sections are typically drilled with 1.98 – 2.1 sg(16.5 to 17.5 lb/gal) salt-saturated mud system. Thehigher mud density is required for the Jurassic Dogger,LIAS, and Trias-Salifere, which require typical pressuresof 10,000 psi to maintain over-balance (Fig. 1). Once theTrias Argileux has been penetrated, a pressurereduction is encountered. This interval contains weakformations, highlighted in Fig. 1 and Fig. 2, which arereadily broken down by the high hydrostatic pressure inthe annulus. The mechanically induced fractures

subsequently lead to total loss of returns. Severity of thelosses varies from block to block due to the variation information integrity common to the HMD field and themechanical forces applied while drilling each section.

The above scenario was encountered numeroustimes in the HMD field while drilling the troublesomesections. It must be stated that changing the casingdesign is not an option. If the 7-in. casing is set higher,the situation becomes worse due to the resultingreservoir depletion which leads to a greater pressuretransition problem. Establishing a link between the TAGand Cambrian (reservoir section) formations is not anappropriate completion strategy for the wells.

Numerous attempts had been made to attack thesub-salt thief zone using different solutions suggested bycement and drilling fluids companies. Results variedfrom total failure to partial success The most commonpractice to cure the massive losses on the offset wellswas to spot and squeeze cement into the inducedfractures. The aim of the cement squeeze was toreconsolidate the formation to allow drilling to continuewithout further losses. Depending on the severity of thelosses, this process often had to be repeated severaltimes on some wells. Prior to application of the CACP,no other remedy was able to satisfactorily solve the fluidloss problem.

Sub-Salt Lost CirculationDrilling through salt formations can be troublesome

for a number of reasons. Typically, the formationsimmediately below the salt are either mechanicallyweaker or fractured, introducing a greater risk for loss ofreturns. The lost time treating severe sub-salt losses canbe up to several weeks, with obvious cost implications,especially for deepwater drilling operations. Though notthe case in the extensively developed HMD field, oftenon exploration wells, little information regarding porepressure and fracture gradient is available. Gulf ofMexico sub-salt wells often encounter higher porepressures below the salt, creating challenging wellcontrol issues. In this instance, the higher mud weightsrequired to balance the pore pressure place evengreater stress on the weakened sub-salt formations.2 Inthe case of the HMD field, the pore pressure, andsubsequent required mud weight, was significantly lowerthan that required in the actual salt formation. As such,the formations directly below the salt are drilled in a highover-balance environment.

Losses in the formations directly below the thick saltzones are typically severe, ranging from 16 m3/hr (100bbl/hr) up to total loss of returns and the inability tomaintain a full annulus. A wide variety of lost circulationmaterials (LCM) have been applied in sub-salt thiefzones, in an effort to control losses. Pills containingsized solids, gunk squeezes, conventional cementsqueezes, and foamed cement have all been proposedas solutions to sub-salt lost circulation. While the cement

AADE-02-DFWM-HO-30 LOST CIRCULATION SOLUTIONS FOR SEVERE SUB-SALT THIEF ZONES 3

squeezes were able to reduce losses in the HMD field,the associated non-productive time remained a costlyfactor.

Chemically Activated Crosslinking PillThe CACP proposed for the sub-salt lost circulation

problem in the HMD field is a blend of crosslinkingpolymers and fibrous material. The material wasdesigned to plug deep fractures, faults and vugularformations.1 The material can be blended with abiopolymer to enhance the viscosity of the material tosuspend barite while pumping. The crosslinking processis chemically activated. After setting, the material formsa firm, rubbery, ductile plug in the fractures and voidsthat it has been squeezed into, preventing further loss offluid.

The CACP can be mixed in freshwater, seawaterand saltwater and applied in wells drilled with water-based, oil-based and synthetic-based mud systems. Onemajor advantage of the CACP is that it can be pre-mixedahead of time. This allowed for significant time savingson the HMD field. The anticipated volume of CACPrequired was pre-mixed and kept on standby. Whendrilling below the salt formation, once severe losseswere encountered, the crosslinking agent was added tothe pill and the CACP was immediately pumped into thethief zone. Though not necessary, the CACP can beused to cure losses and at the same time shut off zonesproducing water or gas. This would suggest that theCACP might be an ideal material for attacking Gulf ofMexico sub-salt thief zones.

Variations of the CACP are available that can betailored to specific applications. Depending on theformation properties and the characteristics of the losszone, higher polymer loadings can be selected, higherfiber loading, or a CACP with coarse calcium carbonatecan be used. The CACP is not recommended forproducing zones as the material does not degrade and isnot acid soluble.

Risk AssessmentThe proposal developed to counter the chronic lost

circulation problems in the HMD field had not previouslybeen proven in this type of situation. As such, thepotential risks were identified and reviewed in thecontext of applying the CACP once losses occurred.

Since the pressure profile for the open-hole sectionis very complicated with a large pressure reduction andweak formations, there is always potential for wellboreinstability to develop once formation losses occur. Themassive losses that occur can reduce the annularhydrostatic pressure to levels that cannot support theupper sections of the wellbore, introducing the potentialfor borehole collapse. Once loss of returns has occurred,any delay in curing the losses introduces the risk oflosing the well. For this reason, it was imperative todevelop a lost circulation solution that could be deployed

rapidly and become effective as soon as possible. TheCACP involved minimal downtime between the onset ofthe losses and placement of the pill across the losszone.

In order to minimize the potential for stuck pipe, thesafest practice after pumping the pill required pulling thedrillstring above the high-pressure formation (TS#2)immediately after placing the pill across the loss zone. Ifthe borehole did collapse due to reduced annularpressure being insufficient to support the borehole, anypotential for stuck pipe would be reduced. Typically,9000-psi equivalent fluid density is required to stabilizethe formation. Once the string is located above the losszone, the CACP is squeezed into the induced fracturesto reconsolidate the section and allow drilling tocontinue. The operation can be repeated to sealadditional weak zones that may be encountered drillingto interval TD.

Pill Design and PlanningThe well that was selected to apply the first CACP

lost circulation pill was located in one of the mostchallenging blocks in HMD field. This scenario providedan opportunity to aggressively test the approach againstsevere loss conditions and a high level of boreholeinstability.

The section to be drilled would require a 2.02 sg(16.9 lb/gal) salt-saturated mud system. It would benecessary for the CACP to be designed withconsideration for the following surface and downholeconditions;

• The effect of high concentrations of barite onboth the slurry rheology and thickening time

• The compatibility of the CACP with the mudsystem when drilling through the plug

• The effect of formation salt contamination whilepumping and squeezing the pill

• Mixing time and temperature effectsThe slurry design was based on 90 minutes

pumping time and a total of 5 hours setting time, whichwould be sufficient to allow for mixing, pumping,squeezing and pulling the drillstring above the pill to3000-meters (9834-ft) measured depth. Fig. 3 illustratesthe rapid increase in the viscosity of the material 90minutes after crosslinking has been initiated.

Spotting ProcedureBased on the potential risks and the pill design

criteria, a spotting and squeeze procedure was designedthat would minimize any operational risks. For the initialtrial, 8 m3 (50 bbl) of the CACP was mixed prior todrilling into the trouble zone. Once the thief zone hadbeen drilled into and only if losses occurred, thefollowing procedure was utilized. Fig. 4 provides anillustration of the stages involved with spotting andsqueezing the CACP.


1. Pull out of hole to the top of TS#2.2. Add the chemical activator and immediately

pump the CACP, displacing all of the CACPfrom the drillstring.

3. Two options available,a. If only partial returns are observed while

spotting the pill, pull up 10 stands to ensurethe drillstring is completely above the top ofthe pill.

b. If no returns are observed it is not necessaryto pull 10 stands.

4. Close annular preventer and squeeze the pill 8m3 (50 bbl) into the formation at no more than320 L/min (2 bbl/min).

5. Hold pressure on the pill until 5 hours havepassed since initial pumping.

6. Ream through plug and continue drilling ahead.If losses are again encountered, an additional pill can bemixed and the procedure repeated.

Proven AdvantagesAfter the initial and highly successful trial, the

procedure described in this paper was continued toensure the procedure was in fact a feasible solution tothe sub-salt thief zone problems. The procedure hassince been adopted as standard practice for the HDMfield. Some of the definitive advantages of the CACPprocedure include:

• Reduced non-productive time by eliminatingtripping time associated with cement squeezes,resulting in fewer days to TD with productionbrought on line faster. Average time saving of7.5 days per well.

• NPT minimized with mixing and storage ofCACP slurry prior to drilling into loss zone.

• The procedure does not require water-basedspacers ahead of and behind the pill. Thusreduced washout across the salt sections andless cement volume required for casing.

• Reduced risk of borehole instability in the TS-2and LD-2 formations due to the reducedexposure time and under-balanced staticperiods.

• CACP is more effective at increasing formationintegrity. The drilling progress after each CACPhas been better than previously recorded.

• The CACP is easy to mix and pump. A cementpump is not required (on the initial trial a cementpump was used).

• No mud treatment is required after each plug.Previously, after each cement squeeze, mudtreatment was necessary.

• The CACP is compatible with the drilling fluidstypically utilized on the HMD field.

ConclusionsBased on the highly successful application of the

CACP in the sub-salt thief zone of HMD field, severalconclusions can be drawn.

• A well-defined and engineered strategy formanaging the onset of lost circulation below thesalt formations was critical to improving thedrilling efficiency in the HMD field.

• Strict adherence to the agreed upon decisiontree ensured the successful implementation ofthe CACP methodology.

• Significant reductions in non-productive timewere possible through elimination of tripsassociated with cement squeezes and theresulting in production coming online earlier thanplanned.

• Application of the CACP technology eliminatedthe lost time associated with preparation ofalternative pills. The CACP slurry is preparedprior to the onset of lost circulation.

• A higher quality wellbore was available forcementing due to reduced washouts typicallyassociated with placing cement plugs.

• After spotting and squeezing the CACP, fluidlosses were eliminated and full circulation wasmaintained while drilling to TD.

The CACP approach has been adopted as standardpractice on the HMD field when encountering severelosses in the sub-salt thief zone.

AcknowledgementsThe authors wish to thank the management at Sonatrachand M-I L.L.C. for support and permission to publish thispaper. In addition, we wish to thank Jim Redden andMary Dimataris for assistance in preparing and revisingthis manuscript.

References

1. Bruton, J.R., Ivan, C.D., and Heinz, T.J., “LostCirculation Control: Evolving Techniques andStrategies to Reduce Downhole Mud Losses,”SPE/IADC 67735, SPE/IADC Drilling Con-ference, Amsterdam, 27 Feb - 1 Mar 2001.

2. Sweatman, R., Faul, R., and Ballew, C., “NewSolutions for Subsalt-Well Lost Circulation andOptimized Primary Cementing,” SPE 56499,1999 SPE Annual Technical Conference,Houston, Oct 3 - 6, 1999.


Table 1 - Hassi Messaoud Historical Offset Well Data (Examples)Well, date Offset 1

1969Offset 2

1978Offset 3

1987Offset 4

1988Offset 5

1999Depth of Losses (ft) 11000-11056 10997 10991 11171 11037-11043Fluid density (lb/gal) 17.5 17.2 16.9 17.0 16.9

Top of G 35 10932 10974 10968 10984 10925Top of Camberien Ri 11142 11207 11158 11280 11211Top of Cambrien Ra 11260 11289 11230 11414 11293

Cement Plugs pumped 6 + 2 LCM 1 1 2 97-in. Casing Depth 11155 11217 11165 11273 11224

ArgileuxDogger

LagunaireL.D.1L.D.2 Dolomite, Marl Pressurized formation.

High volume water influx.

JUR

ASSI

QU

E

LIAS

S#1S#2 Salt,

AnhydrideShale

intercalation

Very high-pressure zone.Potential risk of stuck pipeif mud level (pressure)drops in the annulus.Time is a major factor.

Salifere

S3 SaltArgileux Dolomitic

Shale gradingTo Silt

Argileux Greseux Loose Sand

TRIA

S

EruptiveQuartzite de Hamra fine to very

fine sandGres d’EL AtchaneO

RD

Argile D’El Gassi

LOSSZONE

RIRaR2R3

CAM

BRIA

N

Set 7-in. casing shoe 3-4to meters into Cambrian

Figure 1: Risk Assessment Chart for HMD wells.


L L LL

L L LL

L L LL

S/ET Ref LITHOLOGY DESCRIPTION

TRIA

SSA

LIFE

RE

TRIA

S A

RG

ILEU

XTR

IAS

GR

ESEU

X

Salt

Upp

erM

ediu

m

G10G20

G30

G35

G40

G50

G60

G70

CAMBRO

Reddish BowenShale

SALIFERE

MassiveSalt

Shale (colored)SALIFERE

Anhydride,Dolomitic

Shale

SandstoneDolomitic

Shale, Marl

BRUN ShaleVF SandStone

B

G10 = Top of Trias ArgileuxG20 = Seismic markerG35 = Last salt markerG40 = first sand marker

Low

er

Black ShaleLime Stone(Colored)A - CambrianB - Andesite

A

Figure 2: Lithology Chart for Hassi Messaoud Field, AlgeriaT.A.G.I - Detailed


0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

0 1 2 3

Tim e sin ce Cr oss lin ke r a dde d (hr )

Fann

35

Dia

l Rea

ding

(lb

/100

ft

3 rpm

6 rpm

10 se c G el

10 min G el

Fig. 3 - Comparative rheology profile for CACP after crosslinker has been added and while hotrolling at 86°C. After 3 hr, the samples were too thick to be measured.

Mud CACP TAGSalt

Fig 4 - Spotting and Squeezing Procedure for CACP in TAG formation. When the bit is below the salt and inthe TAG formation, if and only if there is a loss of returns, the bit is picked up to a position above the thief zone(above the TAG and TS2 formations), the CACP is displaced from the drill string and squeezed into the loss zonewith the annular preventers closed. At this point, pressure is held on the pill for several hours. After the pill has set,the plug can be drilled/reamed through and drilling can recommence.

lost circulation solutions for severe sub-salt thief zones

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