Study of adsorption behavior in shale reservoirs under high pressure

Understanding adsorption behavior is of critical importance for the development of shale reservoirs. However, most extant adsorption models cannot properly describe isothermal adsorption when the pressure exceeds a specific value at which the Langmuir volume is obtained. In this study, a modified bi-adsorbent simplified local density/Peng-Robinson model (SLD-PR) was developed in which adsorption in both organic and inorganic components of shale was considered at the same time, and a new modification was imposed on the covolume parameter in equation of state. The model was demonstrated on high pressure methane isothermal adsorption tests of terrestrial shale samples and the high pressure methane adsorption data for marine shales available in the literature samples. It is found that the proposed model can fit isothermal adsorption well for both terrestrial and marine shales with different shapes of adsorption curves, especially in the high pressure range. With the application of the model, it was determined that the methane adsorption capacity of Yanchang Formation terrestrial shale exhibits a positive correlation with its TOC content. Other factors, such as specific surface area, clay content and temperature, also significantly affect adsorption capacity. Furthermore, in the calculation of in-situ gas storage in shale reservoirs, the proposed model produces more reliable results generally when compared with the original SLD-PR model and the Langmuir model.