Shale hydration and its influencing effect on water phase flowback: a case of Longmaxi formation in Weiyuan area, Sichuan Basin

Shales of Longmaxi formation in Weiyuan area, Sichuan Basin were taken as the research object, the shale hydration soaking and flowback experiments were carried out. Firstly, the changes of pore shape and width, pore diameter distribution and specific surface area of the samples during the hydration process were evaluated by means of nuclear magnetic resonance technology, nuclear magnetic resonance T2 spectrum and low temperature N2 adsorption. Secondly, the backflow process of water phase before and after shale hydration was simulated by high-speed centrifuge to explore the backflow characteristics of fluids in pores of different scales. Finally, the hydration mechanism of shale was discussed. The results show that the pore structure of shale is changed by hydration process. Under the action of hydration stress, the expansion and derivation of pores and micro fractures of shale are caused by clay minerals. In addition, the total pore volume of shale samples increases by 6.9%−9.0%, the pore size and specific surface area increase, and the pore opening degree and connectivity are improved. Moreover, there is a pore threshold in the process of water-phase flowback of shale and the average pore threshold of shale in the study area is 31.9 nm. The more pores with pore size greater than the threshold are, the higher water phase flowback efficiency will be. The backflow process of aqueous phase is closely related to pore size and pore type. With the increase of backflow pressure difference, the liquid overcomes the control of various micro forces, and the backflow rate continues to increase. The flowback rate is significantly improved by the hydration process. © 2022 Central South University of Technology. All rights reserved.