Laboratory Investigation of Brine Diffusion Through Oil-Based Mud Filter Cakes

Abstract

Abstract When gravel packing wells in brine after an oil-based drill-in, filter cake strength and permeability are important factors for successful operations. These relate to controlling fluid losses, firstly, by resisting mechanical wear when running production screens and, secondly, by providing a tight hydraulic seal when pumping gravel. Two oil-based mud formulations with "initial" and "depleted" bridging packages, respectively, and otherwise equal formulation, were used to deposit filter cakes on outcrop sandstone plugs in a Hassler-type core holder. Drilling mud was then displaced by gravel pack carrier brine keeping an overbalance pressure. Brine was allowed to permeate through the oil-based filter cake. The filter cakes before and after exposure to brine were compared in terms of cake strength, and volume of brine produced through. It was found that brine transport through the filter cakes at the same overbalance pressure was slower than the static mud filtration rate. Scratch tests showed a higher strength for the filter cake formed with the "initial" particle size distribution, and that both filter cakes strengthened significantly following brine diffusion. NMR T2 distributions and 1D NMR intensity profiles showed that core plugs exposed to the mud formulation with "depleted" bridging package experienced stronger alterations in NMR properties and deeper damage than the formulation with the "initial" bridging package. The methods used in this study have shown to have the potential of probing some of the mechanical and hydraulic properties of filter cakes and the extent to which core plugs are invaded by mud particles during filtration and filter cake buildup. Future work will address the effect of different base fluids, water-based or oil-based, and of different bridging packages to improve our understanding of how the fluid basis affects filter cake qualities. Other options are to look at the effect of wash pills on filter cake properties.