Abstract

Natural and hydraulic fractures in shale reservoirs are essential for providing transport conduit for the movement of fluids during fluid injection and extraction. Variations of flow conditions in the reservoir induce tractions on the fractures that potentially cause a joint to slip or micro-earthquake (MEQ) when the shear stress exceeds a critical value based on a failure criterion. Proper identification of subsurface events in a large-scale situation is of great interest for the geothermal and petroleum industries to monitor potential modification of fracture permeability during production operations as well as hydraulic fracture propagation in stimulation treatments. The Displacement Discontinuity Method (DDM) is commonly used for modeling the behavior of fractures in linear-elastic rocks. However, DDM requires the calculation of the influences among all fractures, so it is not computational efficient for large fracture systems. This work involves using the Fast Multipole Method (FMM) with DDM to simulate large-scale naturally fractured reservoirs subjected to production and injection. Synthetic case studies containing up to ten thousand fracture elements in reservoirs with and without sealing faults are carried out and the microseismic responses analyzed. The results show the approach to be effective and efficient. In addition, the results do illustrate the role of the fluid pressure on the fracture failure mechanism in large-scale situations. Potential shear slips associated with stimulated zones in the reservoir are most likely to occur near the injector wells where the pressure is higher. Finally, fractured reservoirs with faults experienced more events due to the compartmentalization that causes zones of higher pressure to develop.

1. INTRODUCTION

Natural and hydraulic fractures in shale reservoirs are essential for providing transport conduit for the movement of fluids during fluid injection and extraction. Variations of flow conditions in the reservoir induce tractions on the fractures that potentially cause a joint to slip or microearthquakes (MEQ) when the shear stress exceeds a critical value based on a failure criterion. The Mohr-coulomb failure criterion is widely used in EGS to predict shear slippage of fractures. Identification of subsurface events in large-scale situations is of great interest for the geothermal and petroleum industries to monitor potential modification of fracture permeability during production operations as well as hydraulic fracture propagation in stimulation treatments. The displacement discontinuity method (DDM) is commonly used for modeling the behavior of fractures. However, ddm requires the calculation of the influences among all fractures, so it is not computational efficient for large fracture systems. fast summation techniques such as the fast multipole method (FMM) have improved the performance of some boundary-value problems in terms of memory requirements and cpu time [1]. Recent applications combining fmm and ddm (FMDMM) have been successfully applied to accelerate the computation of reservoir geomechanical interactions of large-scale fracture networks [2, 3].

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