Experimental study of the fracture initiation through the synergy of spontaneous imbibition and hydration of residual fracturing fluids in shale gas reservoirs

Abstract

Shale gas wells often experience shut-in. However, the positive and negative effects of shut-in are not yet clear. The success or failure of shut-in is focused on the effects of fracturing fluid and its function in the subsurface. The current studies on the hydration of shale and fracturing fluid and the imbibition of residual fracturing fluids have ignored the fact that hydration and imbibition of fracturing fluids can occur simultaneously and affect each other. With full consideration of the interaction between the spontaneous imbibition and hydration of fracturing fluid, this paper innovatively reveals the fracture initiation through the synergy of spontaneous imbibition and hydration of residual fracturing fluids in shale gas reservoirs and puts forward suggestions for improving the recovery of shale gas. The results show that the imbibition of shale can weaken the mechanical strength of shale and produce micro-fractures and defects in shale. The energy required to destroy the micro-fractures is reduced, which will promote the initiation and propagation of micro-fractures, change the spontaneous imbibition path, expand the volume of spontaneous imbibition, and carry out continuous micro-stimulation of shale gas reservoir. The fracturing fluid can dissolve soluble salts filled in the pores and fractures, resulting in more induced fractures along the direction of beddings and natural fractures. The induced fracture reduces the water saturation of the hydraulic fractures and natural fractures. The fluid in pores can disperse under the action of capillary force, which can further reduce the water saturation of the imbibition intrusion zone, thus greatly restoring the permeability of the shale gas reservoir.