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 sand-stone/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.
