Oil-based muds (OBMs) have been developed to combat drilling problems caused by shale hydration. This paper presents experimental data that show the factors that control the movement of oil filtrate into a shale, as described by its "entrance pressure." Although the oil filtrate of the OBM does not hydrate the shale, it can penetrate the shale, increase the pore pressure, and cause wellbore failure. It is of primary interest to understand this when troublesome shales are drilled. It is also important to understand how factors such as the emulsifier concentration in the OBM and the porosity of the shale affect this entrance pressure.

The objective of this study is to determine and quantify the factors that control the oil entry pressures of shales. For this purpose, five OBMs, containing different concentrations of emulsifiers, water, and oil, were studied.

The study was also intended to determine the effect of shale porosity on entrance pressure. Data are reported for the Arco China shale samples having porosities of 1.8 (native), 3.9, and 4.9%.

It was observed that for a given OBM water content, as the emulsifier concentration increased, the required entrance pressure and the electrical stability (ES) of the mud increased. Also, for shales that had higher porosities and, therefore, larger pore throats, the oil entrance pressure decreased.