Effect of Supercritical Carbon Dioxide Treatment Time, Pressure, and Temperature on Shale Water Wettability

摘要

Supercritical carbon dioxide (SC-CO2) fluid can dissolve surface matter on shale and, thus, change the surface properties of shale. TO study the influence of SC-CO2 on the water wettability of shale, shale samples collected from the Longmaxi formation in Sichuan Basin were treated with SC-CO2 at various time intervals, pressures, and temperatures. The results show that, after SC-CO2 treatment, the ratio of C and Si in the shale increased, while the ratio of O, Ca, Mg, and H decreased. Water was released from the clay minerals, and the CaCO3 content decreased in the shale. After SC-CO2 treatment, the primary pores and fractures in the shale were eroded through dissolution and new pores and fracture structures were developed on the surface microstructure of the shale. As a result, the structure connectivity, which favored the seepage of shale gas, was improved. The results also show that the shale water contact angles were increased after SC-CO2 treatment. The treatment time and pressure had a significant influence on the contact angles, but the effect of the treatment temperature on contact angles was not obvious. The change in the contact angles resulted from a reduction of the CaCO3 content on the shale surface, which decreased the surface hydrophilicity. The shale surface water wettability and the shale surface tension were decreased after SC-CO2 treatment. This reduces the hindrance function of water in the shale minimal pores and fracture, which is beneficial to CO2 diffusion into the shale matrix. This promotes CO2 adsorption capacity in the shale as well as. CH4 desorption from the shale. This study provides a theoretical basis for using SC-CO2 fluid for efficient shale gas exploitation in the Longmaxi formation of the Sichuan Basin.