Application of Efficiently Coupled Fluid Flow and Geomechanics Model for Refracturing in Highly Fractured Reservoirs
Authors: Sangnimnuan Anusarn , Li Jiawei , Wu Kan , Holditch Stephen A. 
Published: January 2018

Abstract
Refracturing has been an appealing technique to mitigate flow rate decline. To optimize refracturing performance, it is crucial to understand stress redistribution due to poroelastic effect, which determines candidate selection, timing, and effectiveness of refracturing. In highly fractured reservoirs, stress redistribution can be complicated and yet only considered by a few literatures. The objective of this work is to predict stress redistribution due to depletion and optimize 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 fractured reservoir are predicted using our coupled fluid flow and geomechanics model with Embedded 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 capability of simulating complex fracture geometries and high-density fracture system using structured grids. In addition, 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 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 fractured reservoir with a reservoir without natural fractures. This suggests refracturing locations whether it should be between or exactly at parent fractures such that the child fractures can propagate towards un-depleted areas of the reservoir. 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 size and shape of depleted areas resulting in alteration of stress redistribution both inside and outside SRV region. Other parameters, i.e. 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.
To the best of our knowledge, this paper, at the first time, predicts 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 suggest optimum refracturing locations as well as critical timings to perform refracturing, which provides critical insights for refracturing in highly fractured reservoirs.