Integrated Simulations of Vertical Well Refracturing in Tight Oil Reservoirs

Refracturing is affected by the reorientation and change of the in-situ stress field caused by pre-existing hydraulic fractures and well production. However, the comprehensive influence of primary fracturing, water injection, and well production in vertical wells on the redistribution of the in-situ stress field is not well understood. To investigate the above problems, an integrated simulation procedure with a 4D geomechanical model is conducted to accurately describe pressure distribution and to update stress states, which couples the primary fracturing, well production, refracturing, and post-production. This work analyzes the changes in the in-situ stresses induced by hydraulic fracturing, reservoir depletion, water injection, and soaking. The results show that the variation of the in-situ stresses is different concerning the location of fractures. Hydraulic fracturing increases the three-dimensional principal stress in the fracture area, while the minimum horizontal principal stress increases more and the horizontal stress difference decreases. In contrast, the stress in the matrix region farther from the fracture decreases, the minimum horizontal principal stress decreases more, and the horizontal stress difference increases. The refracturing of vertical wells needs mainly considering the stress change in the fracture area. However, the increase in fluid injection volume does not necessarily improve fracturing efficiency. The horizontal principal stresses increase with liquid injection. Simultaneously, the stress contrast increases at the initial fracture location, and stress reversal occurs in the vertical fracture direction after first fracturing. The refracturing fracture may propagate to the stress redistribution area by changing the orientation of the perforation. Through the integrated simulation, the effects of different water injection rates, primary hydraulic fracturing, and well production on the change of the in-situ stress are clarified, which provides insights for the design and evaluation of refracturing for vertical wells in tight oil reservoirs.