Development of efficiently coupled fluid flow and geomechanics model for refracturing optimization in highly fractured reservoirs

Refracturing can be an appealing technique to mitigate flow rate decline in wells where the original treatment failed to adequately stimulate the formation. To optimize refracturing performance, it is crucial to understand the stress redistribution due to the poroelastic effect, which determines candidate selection, timing, and effectiveness of refracturing. In reservoirs containing natural fractures, stress redistribution can be complicated. Very few papers in the literature discuss refracturing in naturally fractured reservoirs. The objective of this work is to predict stress redistribution due to depletion and optimize the timing and locations for refracturing in reservoirs with complex hydraulic and natural fractures. In this study, pressure and stress distribution due to depletion in a highly naturally fractured reservoir are predicted using our coupled fluid flow and geomechanics model with Embedding Discrete Fracture Model (EDFM). The model was developed based on a well-known fixed-stress split, which is unconditionally stable. EDFM was coupled to the model to gain the capability of simulating complex fracture geometries and high-density natural fracture system using structured grids. The model was validated against classical Mandel's problems to ensure accuracy. In addition, the effects of natural fractures density, hydraulic fracture spacing, differential in-situ stress, and reservoir permeability have been studied. Synthetic cases with multiple natural fractures were created to study the effects of natural fractures on stress evolution. The results show that there is a significant difference in stress redistribution due to production when comparing a highly naturally fractured reservoir with a reservoir without natural fractures.. The stress distribution has to be considered when determining where to initiate the new hydraulic fractures. The critical time to perform refracturing is also recommended at different scenarios as orientation and magnitude of principal stresses change as reservoir pressure declines over the time. Beyond the critical timing, the child fractures may not be able to propagate towards intact areas at all and may damage parent fractures as a result of the reversal of maximum horizontal stress. This difference indicates that effect of natural fractures cannot be neglected in highly fractured reservoirs when performing refracturing. A change in density of natural fractures directly affects the size and shape of depleted areas resulting in alteration of stress redistribution both inside and outside SRV region. Other parameters, such as. hydraulic fracture spacing, differential in-situ stress, and reservoir permeability should also be taken into consideration when studying refracturing as they affect magnitude and redistribution of principal stresses and yield different optimum locations and critical timing in highly fractured reservoirs. This paper predicts the stress evolution induced by depletion in highly fractured reservoirs and considers the effects of heterogeneous natural fracture distribution and density on stress redistribution. The results can be use to determine the optimum refracturing locations as well as critical timings to perform refracturing, which provides critical insights for refracturing optimization in highly fractured reservoirs.