Integrated Simulation of Stress Evolution and Hydraulic Fracturing After Long-Term Injection and Production in Low-Permeability Reservoirs

Before refracturing, due to the long-term injection and production of old wells, the distribution of regional formation pressure shows non-uniform variations. It is urgent to coupling consider this non-uniform stress evolution in the subsequent refracturing. For this purpose, taking the Chang-6 reservoir in the W block of Changqing Oilfield as an example, an in-situ stress evolution model under long-term injection and production of vertical wells is established using the Fast Lagrangian Analysis of Continua. The simulated stress field is then imported into a hydrofracture numerical model based on the discrete lattice method for modeling fracture propagation of refracturing, achieving an integrated simulation of in-situ stress evolution and hydraulic fracturing evolution. The results show that: (1) After the production of well WJ, the pore pressure around this well decreases by about 4 MPa, and the two horizontal principal stresses experience a similar synchronous reduction, but the decreasing magnitude is only about 2.5 MPa. This indicates that production will result in a decrease in the total stress but an increase in the effective stress. (2) Hydraulic fractures tend to propagate towards the depleted area preferentially. The engineering measures, such as slowly injecting fluid or shutting in before refracturing to increase the formation pressure in the depleted area, are recommended on site, thereby avoiding or reducing refracturing fractures extending into these areas. (3) As the injection time increases, the primary growth of refracturing fractures transitions from area expansion to width expansion, indicating that a short-duration, high-volume refracturing should be adopted. On the one hand, maximizing reservoir transformation can be achieved in a short time, on the other hand, increasing the injection rate can promote the even expansion of multiple fractures.