Experimental study of the stimulating mechanism of shut-in after hydraulic fracturing in unconventional oil reservoirs

Abstract

SummaryField data and research results regarding the effect of shut-in on oil and gas recovery performance are mixed. The stimulation mechanisms of shut-in after hydraulic fracturing are not clear in the literature. In this paper, two different types of experiments were conducted to investigate the effect of spontaneous imbibition (SI) during well shut-in process on matrix permeability and mass transfer between matrix and fractures. In the flooding-flowback experiments, changes in the oil production rate before and after shut-in were used to characterize the regained permeability variation. Amott cells were used in the spontaneous imbibition experiments to measure the oil recovered by the mass transfer effect. The results indicate that water blockage in the tight reservoir is very severe, and the regained permeability is reduced by more than 75% after fracturing fluid invasion. The severity increases as the core matrix become tighter, and the pressure drawdown is set to be lower. After shut-in, the initial permeability did not recover but decreased. To describe the shut-in process more accurately, we divided the well shut-in into early and late stages. In the early stage, fracturing fluid invades the rock matrix, further reducing the permeability, and oil is recovered through the counter-current flow. The results indicate that well shut-in is not favorable to oil production with higher permeability or higher formation pressure. In the later stage, with less fracturing fluid remains in the fractures, the enhancement of hydrocarbon production is mainly caused by fracturing fluid redistribution in the low-permeable matrix.