Entrance Pressure of Oil Based Mud into Shale: Effect of Shale, Water Activity, and Mud Properties

摘要

Oil-based muds (OBMs) have been developed to combat drilling problems often caused by shale hydration. Therefore, it is of utmost importance to understand the interaction of OBMs as they contact shales. Past research (Chenevert 1970) has shown how the movement of water from OBMs can be controlled by the addition of salt. This paper deals with the movement of the oil phase of the OBM, as described by its hydraulic "Entrance Pressure." Although the oil filtrate of the OBM does not hydrate the shale, it can penetrate and flow into the shale at a certain entrance pressure. Such flow into shale can be as damaging as water flow, because it increases the pore pressure of the shale, which can cause wellbore failure. It is of primary interest to understand this when using OBMs in shales. 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.