Mechanisms and Parameter Optimization of Pre-Fracturing Energy Enhancement in Ultra-Low-Permeability Tight Oil Reservoirs

Abstract

Ultra-low-permeability tight oil reservoirs, including the Chang 6 and Chang 8 formations in the Changqing Oilfield and the Fuyu reservoir in the peripheral Daqing Oilfield, are characterized by poor reservoir properties and limited waterflooding efficiency. Conventional areal waterflooding either fails to establish effective displacement or results in rapid local water breakthrough, leading to rapid production decline and a recovery degree of less than 10%. Large-scale refracturing combined with modification of the water injection strategy has, therefore, become an important approach for improving single-well productivity. However, long-term injection–production imbalance may cause substantial formation-energy depletion and an increase in horizontal stress contrast, which are unfavorable for the development of complex fracture networks during refracturing. To investigate the mechanism and optimize the design of pre-fracturing energy enhancement, rock-mechanics experiments were first conducted on cores from the Fuyu reservoir in the Daqing Oilfield. The resulting pore pressure and stress responses were interpreted based on poroelastic coupling and the effective-stress principle. A three-dimensional coupled reservoir–geomechanical model was subsequently established for a Chang 6 tight oil block in the Changqing Oilfield using formation-specific geological, petrophysical, geomechanical, and production data, and field performance was further used for validation. The laboratory results show that pre-fracturing water injection increases pore pressure, reduces effective confining stress, and decreases the horizontal principal-stress difference, thereby promoting a transition in rock failure from isolated shear fractures toward intersecting fracture patterns. The laboratory-derived mechanical trends were transferred to the Chang 6 model primarily at the mechanistic level, while quantitative parameters were recalibrated using reservoir-specific data. By establishing the relationship between the energy enhancement ratio, defined as the ratio of injected fluid volume to cumulative produced fluid volume, and formation pressure recovery, and further considering sensitivity, economic feasibility, and operational constraints, the optimal energy enhancement ratio was determined to be 0.8–1.0. These results clarify the geomechanical mechanism and key design parameters of pre-fracturing energy enhancement and provide practical guidance for refracturing design in ultra-low-permeability tight oil reservoirs.