Thickness Optimization of Drilling Fluid Filter Cakes for Cement Slurry Filtrate Control and Long-Term Zonal Isolation

Abstract

SPE Members Abstract In this paper, the long-term isolation characteristics of two typical filter-cake systems in a gas or water environment are investigated. The test models were designed to measure the sealing capability of a premium cement and filter-cake system used to prevent hydraulic communication at a permeable-nonpermeable boundary. The test models represented the area of a sandstone/shale layer in an actual well. In a real well, sandstone is a water- or gas-bearing formation, and sealing the annulus at the shale formation would prevent hydraulic communication to an upper productive zone. To simulate these conditions, the test models remained in a gas or water environment at either 80 or 150 F for periods of 3, 4, 30, and 90 days before the hydraulic isolation measurements were conducted. Models without filter cake, consisting of 100% cement, were tested for zonal isolation with the filter-cake models to provide reference points. Results show that at 80 F, all filter cakes tested provided adequate zonal isolation up to 90 days. However, at 150 F, the reduction in filter-cake thickness prevented communication at higher pressures. The two highest pressures recorded occurred when no filter cake was on a water-saturated core. The next three highest pressures were measured with no filter cake on a gas-saturated core. These results show how critical filter-cake removal is to the long-term sealing of the cemented annulus. Results indicate that complete removal of the filter cake provides the greatest resistance to fluid communication in most of the cases studied. Introduction When an oil well is drilled, the borehole is commonly stabilized with a drilling fluid. The drilling fluid helps remove cuttings, cool and lubricate the drillstring, control formation pressure, and line the drilled hole with an impermeable filter cake. Filter cake refers to the suspended solids that are deposited on a porous medium during filtration. Filter-cake thickness is a measure of the solids deposited on the filter paper in 1/32-in. increments during the standard 30-minute API filter test. Drilling fluid consists of a mixture of solids, liquids (water or oil) in the continuous phase, and chemicals. The solids may be active, such as bentonite and polymers, or inactive, such as barite. P. 301