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. SPE 165193multi-phase flow in the formation remain insufficiently explored. Recently, we reported an analysis of the leak-off of standard oil-based drilling fluid containing barite and gilsonite into dry Bentheimer sandstone cores. The study introduced an innovative method in which CT scans of the core at time intervals revealed simultaneously the build-up of the external filter cake and internal filtration (Hua et al., 2010;van Overveldt et al., 2011). This technique made it possible to investigate leak-off beyond the capabilities of standard HTHP press test with API filter paper (Aston et al., 2002;Hua et al., 2010). The experiments showed among others things that deep-bed filtration occurred in both deposited filter cake and in the porous medium. However, since cores were initially dry it did not become clear to which extend deep-bed filtration contributed to initiation of the external filter deposition. The microstructure of the external and internal filter cake resulting from the infiltration were examined by Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) (Byrne et al., 2000). The objective of this study is to examine both theoretical the leak-off of oil-based drilling fluid into cores either fully saturated with water or containing oil. The study is a detailed extension of our earlier work (van Overveldt et al., 2011) and employs similar techniques mentioned above. Calcium carbonate particles were used instead of barite and no gilsonite was added to the drilling fluid. Effect of particle size and type on the permeability of the external filter cake was investigated by using smaller sized hematite particles instead of barite or carbonate particles in the formulation of the drilling fluid. The paper proceeds with the presentation of the theory (Section 2). Next, details about the experiments (Section 3) and the results and discussed ...