The filtration properties of compressible mud filtercakes

The permeability and compressibility of drilling fluid filtercakes are investigated, by experiment and by modelling, as a function of the barite content of the cake and of the filtration pressure. Filtration experiments give the rate of fluid loss as a function of pressure, and the porosity of the cake is determined at the end of each experiment. These data are then combined with models for cake growth to predict the rate of fluid loss in two cases: at a constant filtration pressure, and when there is a step increase in the filtration pressure during the course of the experiment. Good agreement with experiment is obtained. The model predicts that the cake porosity varies with distance from the filter paper, as is observed experimentally. The model described here is generally applicable to filtration of other fluids, e.g. polymeric fracturing fluids.