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D R I L L I N G A N D C O M P L E T I O N F L U I D SAs mature hydrocarbon reservoirs become

more depleted, drilling through depletedsands is becoming increasingly common.High levels of overbalance increase therisk of lost circulation and differential

sticking. Recently, three Gulf of Mexicowells were successfully drilled throughdepleted sands with overbalances rangingfrom 3,500 to 4,600 psi. In each application,

a glycol-enhanced water-based mud(WBM) was used.DIFFERENTIAL STICKING

Differential sticking occurs when drilling

through a permeable formation with a pressureoverbalance, resulting in loss of fluid

to the formation. This loss of fluid causesthe buildup of a filter cake on the face of the

formation made up of the solids in thedrilling fluid. If the rate of fluid loss is toohigh, the thickness of the cake increases to

the point that the drillstring becomesembedded in the cake and sticks.Differential sticking is more likely to occurin deviated wellbores because the drillstringis in contact with the filter cake more

than in a vertical well.CLOUD-POINT GLYCOLS

The use of glycols, in particular polyglycols,as WBM additives has been common

practice for several years. Their exactmechanisms and benefits have been thesubject of much debate within the industry.

Particular focus has been on a group ofproducts that have become known ascloud-point glycols. Unlike most salts that

become more soluble in water as temperatureincreases, these polyglycols become

less soluble. At higher temperatures, anabundance of insoluble glycol is in thewater, which becomes cloudy becauselight is scattered by the fine droplets. The

temperature at which this occurs, thecloud-point temperature (CPT), is determined

by the salinity of the solution, molecularweight, and concentration of the glycol.

An increase in any of these three parametersresults in a decrease in the CPT.Glycols have been used primarily for shaleinhibition, lubrication of the mud filter

cake, and as a carrier for deformable asphalt

to help prevent differential sticking.IMPROVED LUBRICITY

Some evidence exists that glycols and similarproducts act as lubricants in drilling fluids,reducing the torque and drag experiencedwhen drilling high-angle wells.

Laboratory testing shows some reductionin lubricity coefficient with addition ofcloud-point glycols in certain drilling fluids.A reduction of 15 to 25% is observed in

fluids that have a high lubricity coefficient.

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In fluids with a low lubricity coefficient, littleor no improvement is observed.DRILLING DEPLETED SANDS

Cloud-point glycols are used primarily asshale inhibitors. Analysis of filtration andlubricity data shows that these glycols have

improved filtration control and increased

lubricity. In a number of wells drilled in theNorth Sea and the Gulf of Mexico, theywere used as shale inhibitors. In addition

the wells also encountered depleted formations,and there was no indication of differentialsticking despite high overbalanced

pressures.Case History 1. The first application wasat Eugene Island, where a severely depletedsand existed. A low-pH, partiallyhydrolyzed polyacrylamide/glycol system

was recommended to drill the sand. Beforethe depleted zone was drilled, samples of

the drilling fluid were sent to a laboratoryto verify the recommended treatment.

Fann 90 testing indicated that 2- to 3-lbm/bbl starch and 3 to 4 vol% glycol wasthe optimum product mix to preventexcessive cake buildup. The addition of

these materials did not affect the rheologicalproperties and reduced the fluid loss at250F from 27.3 to 17.0 mL/30min.Further additions of 1.0-lbm/bbl starchreduced the fluid loss to less than 14.0

mL/30 min. After drilling the sand andreaching the production interval, a shorttrip was made without any excessive dragor fill. The depleted sand was exposed for

42 hours while laying down the drillstringand running casing. Even with this exposure

time, the 75/8-in. casing was landedon bottom without any problems.Case History 2. The second well was atRabbit Island, where two depleted sandswere believed to exist. A lime-based systemwas used to drill these zones. As with the

previous well, drilling-fluid samples weresent to the laboratory for analysis beforedrilling of these zones. Fann 90 analysisindicated that 3-lbm/bbl starch and 3 to 4

vol% glycol was the optimum product mixto prevent excessive cake buildup. Additionsof caustic soda and calcium lignosulfonate

were also recommended. After resistivity formation

testing, the sands were found to beslightly less depleted than expected. A 95/8-in. casing string was successfully run andcemented across the sands.MECHANISMS

There are three possible mechanisms forthe improvement in filtration controlobserved with cloud-point glycols. The

glycols may increase the filtrate viscosityand therefore reduce the rate of filtration.The glycols may combine with the polymers

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typically used for filtration controland stabilize them at elevated temperatures,improving their performance. The third

mechanism proposed is based on the principlethat the glycol becomes insoluble as itapproaches its cloud point, forming anemulsion. These droplets can plug small

pore spaces in the filter cake and seal thecake, reducing filtration rate and cake

deposition. To maximize the filtration andlubricity benefits from cloud-point glycols,the downhole temperature must exceed the

CPT of the glycol.This article is a synopsis of paper SPE38571, Drilling Severely DepletedSands in the Gulf of Mexico: TheBenefits of Cloud-Point Glycols, byM.McGill, S. Seaton, SPE, andR.Valenziano, Baroid Drilling Fluids Inc.,andS. Bruner, SPE, andJ . Golden, SPE,Shell Offshore Inc., originally presentedat the 1997 SPE Annual Technical

Conference and Exhibition held in SanAntonio, Texas 58 October. Pleaseread the full-length paper for additionaldetail, illustrations, and references.The paper from which the synopsis hasbeen taken has not been peer reviewed.

cloud point

1. n. [Drilling Fluids] ID: 1817

The temperature at which a solution of a surfactant or glycol starts to form micelles (molecularagglomerates), thus becoming cloudy. This behavior is characteristic of nonionic surfactants, which areoften soluble at low temperatures but "cloud out" at some point as the temperature is raised. Glycolsdemonstrating this behavior are known as "cloud-point glycols" and are used asshaleinhibitors. The cloudpoint is affected by salinity, being generally lower in more saline fluids.

See: glycol, inhibitor, micelle,polyalkalene glycol, surfactant, thermally activated mud emulsion

2. n. [Heavy Oil] ID: 11499

The temperature at which wax crystals first start to form in acrude oil. Wax appearance temperature (WAT)and waxprecipitationtemperature (WPT) are other synonyms.

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