The Role of Pore Pressure Depletion in Propagation of New Hydraulic Fractures during Refracturing of Horizontal Wells

Abstract Refracturing of a horizontal well is a method to restore the productivity of the well in unconventional reservoirs after the expected production decline. Placement of new fractures in a system that has been already depleted poses new challenges for operators. These challenges are due to the altered stress zones resulting from the expected pore volume depletion and corresponding pressure decline. In this study, refracturing propagation issues in a horizontal well are studied. A fully coupled poroelastic displacement discontinuity model has been developed to study refrac propagation in horizontal wells. Displacement discontinuity is a powerful numerical method for studying hydraulic fracture problems. The model has been verified using available analytical solutions for undrained and drained fracture behaviors. Maximum horizontal stress criterion is used to account for the fracture propagation. Several cases are analyzed using the model. For each set, scenarios that increase the chance of successful refracturing are suggested. Results of this study show that pore pressure depletion is the key factor in defining extent and severity of stress redistribution zones in the reservoir. The effect of pore pressure is shown to be asymmetric in horizontal wells between two parallel fractures. It is observed that the stress anisotropy between two parallel fractures decreases and it increases between the tips of two fractures. After a certain time, stress anisotropy in the area between two fractures becomes negative which means that the horizontal stresses orientation is reversed in that area. Placing any new fracture in the stress-reversed area causes an unwanted change in direction of fracture propagation and eventually an intersection between new and old fractures. In addition, because of the stress redistribution, new fracture tends to propagate toward the nearest old fracture in a single well. This tendency increases as pore pressure depletion becomes greater, increasing risk of refracturing failure. A sensitivity analysis is performed on spacing and length of the old fractures to investigate the best possible treatment plan in terms of these variables. Results that are obtained by this study give better understanding about propagation of new fractures as well as old fractures in refracturing process. Moreover, it introduces a very efficient method that aids in the design of refracturing processes in depleted horizontal wells.