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The full-length paper describes theapproach taken to produce a “designermud” that effectively increases fractureresistance while drilling and can be usedin both shale and sandstone. The design-er mud works by forming a “stress cage”using particle bridging and an ultralow-fluid-loss mud system. The theory isdescribed, and field data are presentedthat quantify the increase in fractureresistance and demonstrate the value ofthe system.

IntroductionMud losses are a frequent problem encoun-tered during drilling. Losses occur when themud weight required for well control and tomaintain a stable wellbore exceeds the frac-ture resistance of the formation. Depletedreservoirs present a particular problem.There is a reduction in pore pressure asreserves decline, which weakens hydrocar-bon-bearing rocks. Neighboring orinterbedded low-permeability rocks(shales) may maintain their pore pressure.This can make the drilling of certain deplet-ed zones virtually impossible when the mudweight required to support the shaleexceeds the fracture resistance of the sandsand silts. The ability to strengthen the well-bore has the following applications/benefits.

• Access to additional reserves in deplet-ed zones.

• Reduced mud losses in deepwaterdrilling.

• Loss avoidance when running casing orcementing.

• Improved well control.• Elimination of casing strings.• An alternative option to expandable

casing.

Theoretical ApproachThe approach taken was to allow small frac-tures to form in the wellbore wall and holdthem open with bridging particles near thefracture opening. The bridge must have alow permeability to provide pressure isola-tion. Provided the induced fracture isbridged at or near the wellbore wall, thismethod creates an increased hoop stressaround the wellbore referred to as a stress-

cage effect. The goal is to be able to achievethis continuously during drilling by addingappropriate materials to the mud system toproduce a designer mud.

Permeable Rocks. In permeable rocks, theparticle bridge need not be perfect becausefluid that passes through the bridge will leakaway from the fracture into the rock matrix.Thus, there will be no pressure buildup inthe fracture and the fracture cannot propa-gate. Even if a mudcake forms initially onthe walls of the fracture, the fracture couldgrow by a small amount to expose new sur-face to relieve the pressure. An additionaleffect is the initial pressure decline behindthe bridge when the fracture first forms.This will raise the effective stress across thefracture and cause closure behind thebridge, which should provide a stable foun-dation for the bridge. From these argu-ments, achieving a stress-cage effect in per-meable rocks should be straightforward.

If the mud contains particles that are toosmall to bridge near the fracture mouth, thefracture could still become sealed by thebuild up of a mudcake inside. The seal-ing/bridging will be slower, and the fracturelength might extend too far to form a usefulstress-cage effect. This is borne out by themud losses observed in the field with ordi-nary muds. Interestingly, fracture gradientsobserved in sands are usually higher thanthose predicted by theoretical models. Thisseems to be related to the presence of mudsolids and the deposition of mudcake.

Low-Permeability Rocks. In low-perme-ability rocks such as shale, the bridge willneed to have an extremely low permeabilityto prevent pressure transfer into the fractureand fracture propagation. For this reason,ways to produce mudcakes with an extreme-ly low fluid loss (ultralow-fluid-loss muds)were studied. High-pressure/high-tempera-ture (HP/HT) fluid-loss values as low as 0.1mL are achievable. The idea of usingultralow-fluid-loss mud to achieve wellborestrengthening is the subject of a patent appli-cation. It should have a particular benefit instrengthening shale. The approach alsowould work in higher-permeability rocks,

and to date there has been no downside inrunning ultralow-fluid-loss mud in perme-able formations. Indeed, an advantage is thereduced risk of differential sticking.

The driving force for bridge formationacross a shale fracture needs to be consid-ered carefully. The initial rush of fluid intothe fracture when it forms will deposit thebridging solids at the fracture mouth, but apressure difference across the bridge isrequired to hold it in place. Pressure decayinto the shale matrix behind the bridge willbe minimal, especially with oil-based muds(OBMs), which have an added sealingaction caused by interfacial-tension (capil-lary pressure) effects. In water-based muds(WBMs), there may be a slow pressureleakoff into the shale, but the challengethen would be to produce WBM with anultralow fluid loss so that the bridge at thefracture mouth has a sufficiently low per-meability. Despite these concerns and chal-lenges, initial field tests in shale have beenvery encouraging.

In the modeling work, a symmetricalelliptical fracture with a wing on each sideof the wellbore was assumed. This seems areasonable starting point. If many narrowlocalized fractures formed around the well-bore to produce the stress cage, they wouldrequire only very-small bridging particles toseal them. Field evidence suggests that larg-er bridging solids are not needed.

Laboratory TestingFracture-sealing experiments were per-formed by use of specially designed testequipment. In a previous joint-industry pro-ject, fracture sealing using hollow cylinder-block samples fractured by drilling-fluid

This article, written by AssistantTechnology Editor Karen Bybee, con-tains highlights of paper SPE 87130,“Drilling Fluids for WellboreStrengthening,” by M.S. Aston, SPE,M.W. Alberty, SPE, M.R. McLean,H.J. de Jong, and K. Armagost, SPE,BP plc, prepared for the 2004 IADC/SPEDrilling Conference, Dallas, 2–4 March.

Drill ing Fluid Strengthens Wellbore

Dr i l l i ng and Comp le t i on F lu i ds

60 NOVEMBER 2004

pressure were investigated. The study pro-duced useful results and pointed towardblends of calcium carbonate (CaCO3) andgraphite as one of the best ways to reducemud losses into the fracture.

In the fixed-fracture-width device, thecell is assembled with spacers defining thefracture width, which typically is 1 mmwide at the mouth and tapers to zero at thetip for a closed fracture or to 0.5 mm for anopen fracture. Sandstone is used to form thefracture faces. The height of the fracture is38 mm, and the fracture depth (distancefrom the mouth to the tip) is 178 mm. Thecell is bolted together and placed in a reac-tion frame; there are take-off points on eachside of the cell to collect mud filtrate thatpasses through the rock faces. Pressureswithin the fracture are monitored by pres-sure transducers at the inlet, middle point,and exit of the fracture. A valve at the exitcan be closed so that the pressure buildupcan be measured. The cell can be heated.

The system is vacuum saturated, andbrine is flowed through the fracture,through all tubes, and through the leak linesto backpressure regulators and to a massbalance. The mud sample is poured into astirred injection pot and heated as required.The injection pot is pressurized using a gassupply, and the mud is injected into the cellwhen required by opening a valve. After theinitial injection of mud, the injection pres-sure can be increased stepwise or continu-ously while monitoring leakoff into the rockand pressure changes within the fracture.

In tests for a 160-md-permeability rock,the fracture tapered from 1 mm to zero andthe exit valve was closed at the start so thedriving force for bridge formation wasleakoff into the rock. A 1.16-specific-gravity(SG) water-based-polymer mud was usedwith ordinary fluid-loss control. The mudhad an American Petroleum Inst. fluid lossof 4.2 mL at ambient temperature and con-tained CaCO3 bridging solids with nographitic particles. The laboratory test wassuccessful, and a bridge was formed near thefracture mouth with no pressure buildup inthe fracture The bridge remained intact tothe 1,900-psi maximum injection pressure.There was an initial spurt and then smallsurges each time the injection pressure wasincreased. The continuous leakoff rate wasvery low but was sufficient to match the flowrate of any fluid leaking through the bridge.Tests with OBMs showed similar success on160-md-permeability (or greater) rock.Strengthening would seem to be achievablewith CaCO3 and standard muds.

In tests on lower-permeability rock, thecombination of standard OBM or WBM

and carbonate bridging particles failed toisolate pressure. This was the case even ifthe fracture tip was open initially toincrease flow into the fracture and initiatebridge formation. To achieve success withOBM, it was necessary to use an ultralow-fluid-loss mud and a combination ofCaCO3 and graphite material.

To simulate the case of a shale, sandstonethat had been sealed with resin to give vir-tually zero permeability was used. In thiscase, it was essential for the fracture tip to beopen at the beginning to allow fluid to flowinto the fracture. Pressure isolation wasachieved at 300-psi injection pressure withthe ultralow-fluid-loss mud and carbon-ate/graphite blend. The bridge was dis-turbed and there was some pressure transferat 900-psi injection pressure, with full pres-sure transfer at 1,900-psi injection pressure.This may have been slight leakage ratherthan bridge total failure.

Additional observations from the experi-mental studies include the following.

• The fluid should contain a smooth/con-tinuous range of particle sizes, from claysize (approximately 1 µm) to the requiredbridging width.

• Ideal packing theory is useful for select-ing the optimum size distribution in low-weight muds.

• High particle concentrations are best foran efficient seal.

• Fracture sealing has been successful to300°F and 4,000-psi overbalance pressurein some tests.

• Mud weight is not a critical factor informing a successful bridge.

Field ExperienceExample 1—Extended Leakoff Test. Theobject of this test was to see if the designermud could increase fracture resistance in ashale formation. The well was a vertical wellin the Arkoma basin in the U.S. After settingthe 95/8-in. casing at 3,012 ft and perform-ing a casing-integrity test, 10 ft of 81/2-in.hole was drilled with a regular OBM toexpose the shale formation. After circulatingclean, an extended leakoff test was per-formed using the regular mud. The mudhad a relatively high HP/HT fluid loss, a 9-lbm/gal mud weight, and was free of bridg-ing solids. The formation fractured atapproximately 1,200 psi, at which point thepump was stopped to minimize fracturegrowth. Pressure stabilized at 800 psi,which is the propagation pressure of thefracture determined by the far-field stressstate. After bleeding back the pressure tohydrostatic, the test was repeated and thepressure plateaued at 800 psi with no indi-

cation of a breakdown pressure; the fracturewas reopening. After pressure bleedoff, theopen hole was displaced to a pill of thedesigner mud with a graphite/CaCO3 blend.During this leakoff test, the earlier fracturestayed sealed and pressure was increased tomore than 2,000 psi before the seal broke.

Example 2—Schiehallion North Sea Well204/20-C21z. In this application, a 360-ftsection of sand/shale formation was drilledwith designer mud while exceeding thesand fracture gradient. The casing was an81/2-in. sidetrack between the 95/8-in. cas-ing and the 7-in. liner. In the original well,two well-control incidents occurred whiledrilling thin hydrocarbon-sand stringersabove the targeted reservoir sands. The sec-ond well-control incident resulted in adownhole loss/gain situation, and the wellwas temporarily suspended to evaluate thefindings and plan a sidetrack.

To prevent an influx, a 1.54-SG minimummud weight was required to drill the high-pressure sand stringers. An OBM containingequal parts CaCO3 and graphitic materialwas used. The carbonate size ranged from50 to 400 µm, and graphite-particle sizeranged from 160 to 600 µm. The sectionwas started with 1.51-SG mud weight usingthe designer mud. The planned leakoff testjust below the 95/8-in. shoe was stopped ata 2.15-SG equivalent mud weight, with noleakoff observed. The formation type at thispoint was shale. Test pressure was greaterthan the sand/silt and shale fracture gradi-ents and was even greater than the overbur-den. On drilling ahead, the mud weight wasincreased to 1.54 SG before entering thehigh-pressure sand stringers. There were nomud losses in adjacent formations despitethis mud weight being greater than thesand/silt fracture gradient.

Major contributors to this success weredrilling the section with controlled drillingparameters to allow stress-cage building andusing HP/HT drilling parameters because ofthe small trip margin and no riser margin.The large particles were kept in the mudsystem by maintenance additions. Mud den-sity and rheology were carefully monitored.There was no mud damage to rig equipmentor mud pumps from the bridging particles.A 7-in. liner was run and cemented with nolosses or gains. JPT

For a limited time, the full-length paperis available free to SPE members atwww.spe.org/jpt. The paper has notbeen peer reviewed.