Impact of Aqueous Phase Trapping on Mass Transfer in Shales with Multiscale Channels

摘要

Large amounts of fracturing fluid retained in a shale reservoir could not only restrict gas production through aqueous phase trapping (APT) but also cause potential contamination of groundwater. In this work, experiments modeling in situ aqueous imbibition and flowback were conducted to quantitatively investigate the APT behavior in shales, including matrix, natural fracture, and artificial fracture. Results show that the forward imbibition process is slower than the reverse imbibition process. The aqueous phase tends to be imbibed through parallel bedding. Permeability reduction rates of matrix, natural fracture, and artificial fracture cores after imbibition could reach 100, 85, and 70%, respectively. Meanwhile, it is demonstrated that the aqueous flowback efficiency depends upon flowback timing, pressure difference, initial permeability, and bedding direction. The flowback efficiencies of matrix, natural fracture, and artificial fracture cores could be less than 7%, less than 30%, and less than 15%, respectively. Additionally, the permeability recovery rates of those could be less than 19%, less than 39%, and less than 67%, respectively. Finally, shale APT mechanisms are analyzed from both geological and engineering aspects. The quantitative investigation of shale APT is conducive to economically and environmentally developing shale gas reservoirs.