Shale Oil Shut-In and Flowback Mechanism and Optimization Strategy

Abstract

Shut-in and flowback are critical stages following hydraulic fracturing in shale oil wells. Researching the distribution of reservoir pressure and fluid flow mechanism during shut-in and flowback is important for optimizing these procedures, thereby enhancing well productivity. Therefore, based on the flow mechanism of shale oil, this paper establishes a flow equation considering imbibition and seepage, using linear source superposition equivalent to the pressure distribution generated by hydraulic fracturing as the initial condition. The PEBI (Perpendicular BIsection) grid is used to divide the grid for multi-stage fractured horizontal wells. The simulation results reveal that large-volume fracturing leads to the formation of a high-pressure zone around the wellbore, significantly surpassing the original reservoir pressure, termed as the high-energy band. This high-energy band is demarcated from the original reservoir pressure by the pressure boundary line (PBL). During production, a double-pressure funnel (DPF) manifests within the reservoir, generating a region with the utmost pressure at a specific position within the high-energy band, known as the pressure peak line. Oil located beyond the pressure peak line is unable to flow towards the wellbore. According to the DPF theory of shale oil, fracturing technology should be adopted to form long straight fractures as far as possible whenever feasible to cross the high-energy band. The shale oil optimal duration for shut-in is contingent upon the movement rate of the pressure boundary and the shale imbibition curve.