IN-DEPTH ANALYSES OF POSTHYDRAULIC FRACTURING PROCESSES IN UNCONVENTIONAL TIGHT RESERVOIRS

Abstract

Hydraulic fracturing is the most widely used approach for unconventional tight or shale oil production. An appropriate postfracturing process usually refers to a certain shut-in period, which could be of critical importance to enhance ultimate oil recoveries. The shut-in period comprises processes of fluid filtration and spontaneous imbibition. In this study, the pressure decay characteristics during the fluid filtration process after hydraulic fracturing were studied. First, a pore-scale imbibition model with forced pressure was constructed on the basis of capillary model and fractal theory. Then, in addition to the theoretical model, coreflooding tests were performed in the core samples for optimizing the injection pressure difference with the assistance of low-field nuclear magnetic resonance (LF-NMR). Furthermore, the fluid filtration process was modeled and evaluated through another coreflooding test, which was conducted with declining injection pressure. It was found from the theoretical model and experiments that the synergistic effect of the displacement and capillary pressure was maximized for improving oil recovery with an optimized pressure difference of 5 MPa. Moreover, the pressure decay process was divided into fast-declining, transitional, and steady periods. The lab-scale shut-in time was determined to be 30 hours, which was exactly the time when the pressure decay reached the stable stage. This study will support the foundation of more general applications pertaining to hydraulic fracturing, especially fluid filtration processes in unconventional tight or shale oil reservoirs.