Leak-off of oil-based drilling fluids in saturated porous media

Abstract

The static leak-off of oil-based drilling mud into liquid-saturated cores was studied experimentally. A simple model for the leak-off was first developed extending an earlier model for the static filtration into unsaturated core. CT scan aided static filtration experiments were performed in brine and oil and brine saturated cores, simulating reservoir saturation regimes. Formation of external filter cake and internal filtration of solid particles were visualized and leak-off volumes were determined as function of time. At the end of the experiments the formed external filter cake and internal particle deposition were characterised with the aid of an Electron Scanning Microscope. Using drilling fluids containing carbonate particles it was found that leak-off volumes for saturated cores are larger than for unsaturated cores. It was observed further that leak-off volumes increase with the particle size, i.e. consistently with a more permeable external filter cake and limited internal filtration. Leak-off volumes decreased when using smaller hematite particles or using larger range of (barite) particles sizes. The filtration volumes for the smaller sized drilling fluid components in brine saturated core experiments were found to be larger than filtration volumes found for dry core experiments in previous work.