Multiscale Investigation of Pore Structure Improvement in Shale by Supercritical Water Stimulation

Shale gas formation damage induced by retained fracturing fluid after hydraulic fracturing limits the well production capacity and steady production duration. Converting the retained fracturing fluid into supercritical water (SCW) via the formation heat treatment (FHT) technology is an state-of-the-art method to alleviate the formation damage of aqueous phase trapping and enhance the permeability of shale matrix. The effect of SCW stimulation (SCWS) on the multiscale pore structure of shale was quantitatively characterized by mercury intrusion porosimetry and nitrogen sorption measurements. Experimental results show that the pore volume with a pore size range of 2 nm to 5000 nm increased significantly after SCWS. The mechanisms of SCWS promoting pore structure evolution were analyzed, which include the generation of intragranular dissolved pore, pyrite dissolved pore, interparticle dissolved pore, and hydrothermal fractures. In addition, the superiority of SCWS to alter the pore strucre of a shale is systematically summarized. It is presented that SCWS can further increase the reservoir stimulation effect after hydraulic fracturing through optimally employing retained fracturing fluid, resulting in enhancing multiscale shale gas flow capacity and ultimately improving well production. This work deeply sheds light on the influence of supercritical water stimulation on the pore structure evolution in shale matrix, which is expected to be an innovative method on increasing the estimated ultimate recovery of a shale gas reservoir.