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• NOVEMBER 1997 1191 DRILLING AND COMPLETION FLUIDS As mature hyd rocarbon reservoirs become more depleted, drilling through depleted sands is becoming increasingly common. High levels of overbalance increase the risk of lost circulation and differential sticking. Recently, three Gulf of Mexico wells were successfully drilled through depleted sands with overbalances ranging from 3,500 to 4,600 psi. In each applica- tion, a glycol-enhanced water-based mud (WBM) was used. DIFFERENTIAL STICKING Differential sticking occurs when drilling through a permeable formation with a pres- sure overbalance, resulting in loss of fluid to the formation. This loss of fluid causes the buildup of a filter cake on the face of the formation made up of the solids in the drilling fluid. If the rate of fluid loss is too high, the thickness of the cake increases to the point that the drillstring becomes embedded in the cake and sticks. Differential sticking is more likely to occur in deviated wellbores because the drill- string is in contact with the filter cake more than in a vertical well. “CLOUD-POINT” GLYCOLS The use of glycols, in particular polygly- cols, as WBM additives has been common practice for several years. Their exact mechanisms and benefits have been the subject of much debate within the industry. Particular focus has been on a group of products that have become known as cloud-point glycols. Unlike most salts that become more soluble in water as tempera- tur e increases , these polygl ycol s become less soluble. At higher temperatures, an abundance of insoluble glycol is in the water, which becomes “cloudy” because light is scattered by the fine droplets. The temperature at which this occurs, the clou d-po int temperature (CPT ), is dete r- mined by the salinity of the solution, mole- cular weight, and concentration of the gly- col. An increase in any of these three para- meters results in a decrease in the CPT. Glycols have been used primarily for shale inhibition, 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 simi- lar products act as lubricants in drilling flu- ids, reducing the torque and drag experi- enced when drilling high-angle wells. Laboratory testing shows some reduction in lubricity coefficient with addition of cloud-point glycols in certain drilling flu- ids. A reduction of 15 to 25% is observed in fluids that have a high lubricity coefficient. In fluids with a low lubricity coefficient, lit- tle or no improvement is observed. DRILLING DEPLETED SANDS Cloud-point glycols are used primarily as shale inhibitors. Analysis of filtration and lubricity data shows that these glycols have improved filtration control and increased lubricity . In a number of wells drilled in the North Sea and the Gulf of Mexico, they were used as shale inhi bitors . In addition the wells also encountered depleted forma- tions, and there was no ind icatio n of dif- ferential sticking despite high overbal- anced pressures. Case Histor y 1. The first application was at Eugene Island, where a severely deplet- ed sand existed. A low-pH, partially hydrolyzed polyacrylamide/glycol system was recommended to drill the sand. Before the depleted zone was drilled, samples of the drilling fluid were sent to a laboratory to verify the recommended treatment. Fann 90 testing indicated that 2- to 3- lbm/bbl starch and 3 to 4 vol% glycol was the optimum product mix to prevent excessive cake buildup. The addition of these materials did not affect the rheologi- cal properties and reduced the fluid loss at 250°F from 27.3 to 17.0 mL/30min. Further additions of 1.0-lbm/bbl starch reduced the fluid loss to less than 14.0 mL/30 min. After drilling the sand and reaching the production interval, a short trip was made without any excessive drag or fill. The depleted sand was exposed for 42 hours while laying down the drillstring and running casing. Even with this expo- sure time, the 7 5 / 8-in. casing was landed on bottom without any problems. Case History 2. The second well was at Rabbit Island, where two depleted sands were believed to exist. A lime-based system was used to drill these zones. As with the previous well, drilling-fluid samples were sent to the laboratory for analysis before drilling of these zones. Fann 90 analysis indicated that 3-lbm/bbl starch and 3 to 4 vol% glycol was the optimum product mix to prevent excessive cake buildup. Additions of caustic soda and calcium lignosulfonate were also recommended. After resistivity for- mation testing, the sands were found to be slightly less depleted than expected. A 9 5 / 8- in. casing string was successfully run and cemented across the sands. MECHANISMS There are three possible mechanisms for the improvement in filtration control observed with cloud-point glycols. The glycols may increase the filtrate viscosity and theref ore reduce the rate of fil tration. The glycols may combine with the poly- mers typically used for filtration control and stabilize them at elevated temperatures, improving their performance. The third mechanism proposed is based on the prin- ciple that the glycol becomes insoluble as it approaches its cloud point, forming an emulsion. These droplets can plug small pore spaces in the filter cake and seal the cake, reducing filtration rate and cake deposition. To maximize the filtration and lubricity benefits from cloud-point glycols, the downhole te mperature must exceed the CPT of the glycol. This article is a synopsis of paper SPE 38571, “Drilling Severely Depleted Sands in the Gulf of Mexico: The Benefits of Cloud-Point Glycols,” by M. McGi ll, S. Seaton , SPE, and R. Valenziano, Baroid Drilling Fluids Inc., and S. Bruner, SPE, and J. Golden, SPE, Shell Offshore Inc., originally presented at the 1997 SPE Annual Technical Conference and Exhibition held in San Antonio, Texas 5–8 October. Please read the full-length paper for addition- al detail, illustrations, and references. The paper from which the synopsis has been taken has not been peer reviewed. DRILLING DEPLETED SANDS: THE BENEFITS OF CLOUD-POINT GLYCOLS
