A Discrete Fracture Model Coupled with Geomechanics for Low-Permeability Waterflooding Reservoirs
Authors: Lei Zhengdong , Luo Kai , Liu Shuangshuang , Li JunChao , Yu Tao , Yuan Jiangru , Li Jiahong , Li Lei , Zhao Hang 
Published: October 2018

Abstract
Low-permeability tight reservoirs have been undergoing extensive development in petroleum industry in recent years. Waterflood has been used as oil recovery process of these reservoirs. Different types of fractures such as hydraulic fractures, natural fractures, and water injection-induced fractures exist in the tight oil reservoirs, which shows a significant impact on the waterflood performance and oil recovery. We often observe strong coupling between porous media flow and reservoir geomechanics at different scales when modeling these recovery processes. Therefore, it is great challenge for us to conduct accurate modeling of these problems. In this work, we have developed a novel coupled porous media flow and geomechanics simulation model based on the discrete fracture network concept, with the aim to explore the phenomenon of water injection-induced fracture propagation and the outcome of their interactions with other natural fractures.
A dynamic simulation modeling for modeling fracture propagation in both lateral and vertical direction is presented in this work considering the poro-and thermo-elastic stresses on the fracture. In each time step, the multiphase flow is solved with finite volume method (FVM). With the pore pressure calculated from flow simulation, we have used the Displacement Discontinuity Method (DDM) to model the stress redistribution. And a special joint element is implemented to capture the behaviors of natural fractures. Based on the relative energy release rate, the propagation of hydraulic fractures is coupled with the existence of natural fractures. Then, the fracture geometry is updated by our unstructured dynamic meshing module which can adaptively re-mesh according to the solution at the end of the time step.
Several synthetic cases and a real field case are designed and simulated in this work. According to the results, the newly created fracture connecting with pre-existing fractures can result in a complex fracture network. Meanwhile, the pore pressure changes in pre-existing fractures will cause sliding and opening before intersection with hydraulically induced fractures. The role of natural fracture properties on the interaction between hydraulic and natural fractures is also analyzed and their importance is proved by case results. We also observe contrary effects of generated fracture network on production well performance. In certain situations, the fractures can enhance the oil production by increasing swept volume, hence, the efficiency of waterflood. While in some other cases early water breakthrough is observed when fractures build a connection between injection and production wells. The significantly different influence implies the importance of accurately capturing the dynamic propagation of water injection induced fractures.
This simulation model provides a useful tool for modeling the growth of water injection induced fractures and the impact of fractures on waterflood performance. With coupled fluid flow and geomechanics full physics considered, it can be used for waterflood process optimizations for maximizing oil recovery.