Entrance of oil-base muds into shales

Oil-base muds (OBMs) have been developed to combat drilling problems often caused by shale hydration such as swelling, high pore pressures and mechanical failure. Thereby, is of extreme importance to understand the interaction of OBMs and the shale(s) in contact. Even though the oil filtrate of the OBMs does not hydrate the shale, it penetrates and flows through it. Therefore, is of primary interest to know what is the breakthrough pressure that needs to be overcome by an oil-base mud over a shale, and how the emulsifiers concentration and the water activity of the shale affects this pressure. The objective of this study is to perform a laboratory test to determine entry pressures on the Arco China shale. For this purpose, five oil-based muds with different emulsifiers, oil and water concentrations were prepared. It is also intended to understand what are the effects of the shale water activity variation on the entrance pressure. In order to perform a successful test, unaltered shale samples were immersed in epoxy brine inside a plastic tube. After the brine hardened, the tube was cut into slices of 0.26" thickness and then placed inside a metallic cell available for the experiment. The cell consists of two caps, both of them with inlet ports that allow fluids to contact the face of the sample. Through one of the ports on the top cap of the cell, oil-base mud is forced at different pressures, while the bottom cell is filled with NaCl (35000 ppm) that is pressurized at 50 psig and then isolated. The upstream pressure is increased during the test until the downstream pressure reacts to this pressure increase. At the moment some reaction on the downstream is observed, a breakthrough has occurred, consequently, the shale has been invaded by oil-base mud filtrate. The original water activity of the shale (0.72) needed to be altered to higher values (0.96 and 0.86) in order to get breakthrough. This was accomplished by placing different shale samples in desiccators containing saturated salts of known water activities. It was observed from the different tests that as the emulsifier concentration increased, the breakthrough pressure also increased. The explanation of this effect relies on the fact that as the emulsion is created, the water droplets are coated with emulsifiers. As the concentration of these emulsifiers increases, the coating becomes thicker and the droplets smaller, requiring more hydraulic gradient to overcome the capillary entry pressure of the shale. It was also observed that as the water activity of the shale is reduced, the entrance pressure for the same mud increases. This is a consequence of the reduction in diameter of the pore throats as the humidity decreases. As the breakthrough process progressed, the downstream pressure increased to a certain value, however, this pressure measurement never equalized the upstream pressure. This phenomena has not yet been fully understand, since it could be a result of osmotic effects between the oil phase and the pore fluid, in addition to the capillary effects that need to be overcome as the pore throats to be penetrated decrease in diameter.