Wettability and Pore Connectivity and Their Potential Influence on Shale Gas Recovery: A Comparative Study of Three Leading Shale Formations in Southern China

This work systemically investigates the wettability and pore connectivity and their potential influence on gas recovery from three overmature marine shales (Niutitang, Longmaxi, and Donggangling Formations) in southern China with a range of total organic carbon (TOC) contents, mineralogical compositions, and pore geometry characteristics. To achieve these objectives, TOC, X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), contact angle, and spontaneous fluid imbibition were performed. The contact angle and imbibition were tested using both hydrophilic (deionized water, DIW) and hydrophobic (2:1 in volume of n-decane:toluene, 2DT) fluids. Experimental results show that in both the Niutitang and Longmaxi Shales, the dominant minerals are quartz and carbonate. In contrast, the Donggangling Shale has contents of higher clay minerals and lower TOC. The integrated results from SEM images, measured contact angles, and imbibition behavior indicate that the Donggangling Shale is mixed-wet, while the Niutitang and Longmaxi Shales are both strongly oil-wet and intermediately water-wet. In the clay-rich Donggangling Shale, during spontaneous imbibition, intense water-shale interaction occurs when water contacts the shale, resulting in the generation of abundant and macroscopic induced fractures. These induced fractures not only promote the migration of water into the deeper reservoir matrix and the decrease of water concentration in the main fracturing channels but also effectively improve the seepage of shale matrix, eventually making the shale gas more easily flow into the wellbore. Therefore, shutting-in the well for a period of time after hydraulic fracturing to maximize the utilization of the induced fracture networks formed by water-shale interaction could be beneficial to shale gas recovery. For Niutitang and Longmaxi Shales with high contents of brittle minerals, after the generation of extensive fracture networks by hydraulic fracturing, rapid flow-back should be considered to reduce the retention of fracturing fluid in hydraulic fractures; the residual fracturing fluid could then be used to displace the trapped gas from the shale matrix into the hydraulic fractures.