Leak-off of oil-based drilling mud into liquid-saturated cores was studied both theoretically and experimentally. First a simple model for the leak-off process was developed extending an earlier analysis of static filtration into unsaturated cores. Then CT scan aided static filtration experiments were performed in dry, brine-saturated and oil-bearing cores, simulating possible reservoir saturation regimes. Formation of external filter cake and internal filtration of solid particles were visualized and leak-off volumes were measured as function of time. At the end of the experiments the formed external filter cake and internal particle deposition were characterized 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 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 barite particles having wider particle distribution size.

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