Pore-Scale Modeling of Spontaneous Imbibition Behavior in a Complex Shale Porous Structure by Pseudopotential Lattice Boltzmann Method

Spontaneous imbibition of fracturing fluid into shale matrix is one of the primary reasons for the low flowback rate in shale gas wells after the hydraulic fracturing. This leads to concerns of impacts on both environment and shale gas production. A direct pore-scale simulation is crucial to gain a deep understanding of spontaneous imbibition behavior and its impacts. The porous structures in the shale matrix are characterized by not only a geometrical complexity but also a mixed wettability, which bring great challenges to simulation methods. An improved pseudo-potential lattice Boltzmann method is proposed to simulate the spontaneous imbibition behavior in a reproduced three-dimensional porous structure of shale. The results show that the nanoscale hydrophilic pores provide the driving force and a storage place for the residual treatment fluid. The pore size and wettability heterogeneity lead to the nonuniform menisci propagation and fracturing fluid distribution in the model. Specifically, the fracturing fluid imbibed quicker in the larger pores at the early stage and gradually migrated into the smaller pores during the process. With a limited volume of the fracturing fluid, a portion of the larger pores was finally reopened. The analysis of saturation and apparent gas permeability data during the spontaneous imbibition process showed a great recovery of the model permeability along with the reopened pores. These results provide direct evidence of the residual fracturing fluid migration pattern in the shale reservoir and its influence on shale gas production. Plain Language Summary Hydraulic fracturing is widely employed to stimulate a shale gas reservoir for a better production. After fracturing, a substantial amount of the fracturing fluid stays in the reservoir even after a period of flowback operations. It is believed that the residual treatment fluid is imbibed into the surrounding shale matrix. However, whether these trapped fracturing fluids impact the shallow aquifers and the shale gas production rates remains to be fully understood. A pore-scale simulation is performed in this work to directly investigate the migration of the fracturing fluid in a reproduced porous structure of shale. The results showed that (1) the nanoscale pores in the shale matrix provide the driving force, namely, the capillary force, and the storage space for holding the fracturing fluids. (2) The nonuniform spontaneous imbibition in the complex shale porous structure results in the fracturing fluid-filled small pores and a portion of reopened large pores. The reopening of the large pores leads to a great recovery of the model transport property of the shale gas. These results can be served as evidence for answering the engineering related problems, such as the low flowback rate of the fracturing fluid and the relatively high gas production after the shut-in operation.