Methodology for geomechanically controlled transmissibility through active natural fractures in reservoir simulation

Abstract

Forecasting water breakthrough is a difficult task to be performed for naturally fractured reservoirs. Modeling flow through natural fractures is computationally intensive for a rigorously refined model. Hence, in many cases, it is not feasible to perform a full-field simulation considering the possibility of fracture reactivation.This work aims at applying a methodology to incorporate the flow through fractures or geomechanically active fractures in a full-field model using a commercial reservoir simulator. This is accomplished by tracking which grid blocks in a model cross the activated fractures followed by the redefinition of interblock transmissibilities. These transmissibilities are calculated using the cubic law that relates fluid flow with the fracture aperture and an average distance between crossed blocks. The methodology is well known as Embedded Discrete Fracture Model (EDFM) (Moinfar, 2013; Moinfar et al., 2014; Li et al., 2009).An in-house finite element geomechanical simulator, one-way coupled with a commercial reservoir simulator, was used to predict in-situ stresses. The classical Mohr-Coulomb failure criterion was employed to calculate fracture shear strength. The natural fracture, or a segment of the fracture, is considered permeable when the applied stress is greater than its shear strength.Various case studies indicate the efficiency and accuracy of the EDFM method in a commercial simulator. In order to demonstrate the accuracy of the method, studies were performed in five spot models, comparing the results to those with a rigorously refined fracture representation. In order to highlight the applicability of the methodology at a field-scale, three reservoir simulation case studies are presented: one comparison with the dual permeability method, one simulation of a giant field, and one simulation coupled with a geomechanical simulator. The results show that the representation of natural fractures in reservoir models may drastically change the production forecast, especially in terms of water production.