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.

It is of great significance to accurately evaluate the in-situ stress of reservoirs and understand the propagation of multi-layers commingled fractures for the exploration and development of coal measure strata. Therefore, taking the CBM well M as an example, combined with geophysical logging and one-dimensional mechanical model to calculate the in-situ stress and rock mechanical parameters of the Benxi coal seam. Then a numerical fracturing model considering the combination of coalbed reservoir and sandstone reservoir is established on the basis of the lattice method. The influence of perforation location and injection rate on the propagation of hydraulic fractures are investigated. The results show that: (1) The in-situ stress in the coal seam of Benxi formation is normal-faulting regime. The horizontal differential stress coefficient is slightly larger than 0.25, and the horizontal difference stress is relatively large, which is beneficial to generate vertical fracture. (2) The perforation location has a significant influence on fracture morphology of multi-layer combined fracturing. The fracturing effect is optimal when the perforation location is at the top interface of the coal seam. The sum of fracture area of the coal seam and the sandstone layer is the largest, and the effective fracture area ratio is also the largest. (3) When the perforation position is in the middle of the coal seam, low injection rate reduces the expansion of hydraulic fractures in the lower mudstone layer, resulting in an increase in the effective fracture area ratio significantly. However, it is not conducive to the formation of large-area fracture in the middle coal seam and upper sandstone layer under the conditions of low injection rate. Therefore, it is necessary to optimize the appropriate injection rate based on engineering practice and geological data. The research provides theoretical guidance for parameter optimization of multi-layer fracturing in coal-measure reservoirs with typical soft and hard rock masses.