Evaluation of membrane quality and entrance pressure of oil-based mud at elevated temperatures

This study fully explored the impact of temperature on the membrane quality of oil-based mud (OBM) using static gravimetric measurements, membrane efficiency and entrance pressure tests. More than 30 shale samples were used to assess the membrane behavior of several OBMs under elevated temperatures. Results suggested that OBM acted as a semi-permeable membrane at low temperatures (25-55C) since ion flow out of OBM and into shale was restricted despite the existence of a large ionic concentration gradient, while water flowed freely out of shale and into OBM because of the existence of a chemical potential difference. For temperatures higher than 55C, relatively higher ion flux into shale and slightly lower water flow out of shale was observed. Excessive heat may have possibly destabilized or even destroyed the emulsifier's structure that serves as a mechanism through which OBM performs as a semi-permeable membrane. The instability of OBM's emulsifier was further confirmed through entrance pressure testing where an appreciable reduction in entrance pressure of OBM was measured at high temperatures.