Influence of salt solutions on the permeability, membrane efficiency and wettability of the Lower Silurian Longmaxi shale in Xiushan, Southwest China

Abstract The transmission of pore pressure is considered as one of the primary factors of wellbore instability in shales. Environmental and economic considerations make water-based drilling fluid (WBDF) as a possible method to retard or even inhibit shale hydration. Before that, the mechanisms of salt solutions on the physical properties of shales should be clarified. The influence of various types (NaCl, KCl, CaCl2, HCOONa, and HCOOK) and concentrations (5 wt%, 10 wt%, and 20 wt%) salt solutions on the permeability, membrane efficiency, and wettability of the Lower Silurian Longmaxi shale in Xiushan, Southwest China (Longmaxi shale), was investigated with HKY-3 shale pressure transmission device, and JC2000DM contact angle measuring instrument. Results showed that salt solutions with better capability to retard pore pressure transmission in Longmaxi shale were 20 wt% HCOONa, 5 wt% HCOONa, 20 wt% KCl, 20 wt% HCOOK, and 5 wt% NaCl. Here the speed of pore pressure transmission in shale was judged by the permeability after pressure transmission test (PTT). Among the five types of salt solutions, HCOONa had the best capability to retard pore pressure transmission. 5 wt% NaCl, 10 wt% KCl, and 20 wt% HCOONa or 20 wt% NaCl presented higher membrane efficiency than other salt solutions. Meanwhile, the relationship between membrane efficiency and water activity of NaCl, KCl, and CaCl2 solutions were positively correlated while the relationship of HCOONa and HCOOK solutions were negatively correlated. After PTT, the contact angle of Longmaxi shale in contact with salt solutions increased up to 44.3%. Higher contact angle leads to the smaller contact area and then less water invasion into shales, resulting in improved wellbore stability. The order of the contact angles of Longmaxi shale in contact with the same type of salt solutions with various concentrations after PTT was 5 wt% > 10 wt% > 20 wt%. The achievements of this paper can offer a guideline to formulate high-performance WBDF for horizontal drilling in Longmaxi shale.