Coupled hydro-mechanical simulations of discrete fluid-driven fracture propagation through fractured rock masses using a lattice modeling approach

Fluid-driven fractures are critically important in a number of geoengineering application, such as to increase the permeability of an oil/gas reservoir and stimulate the productivity. On the contrary, near the underground storage sites for radioactive wastes or carbon dioxide, the propagation of fractures induced by pressurized gas should be avoided to detain the pollutants. Numerous numerical models have been developed to reproduce the physical phenomena of the fluid-driven fractures and have better understanding of the fracturing mechanism. However, it is still challenging to explicitly model the fluid-driven fracture propagation because it involves tightly coupled hydro-mechanical behavior with a singularity at the crack tip and complex interactions with pre-existing discontinuities in heterogeneous rock masses. This study investigates hydraulic fracture propagation and formation of discrete fracture networks using a coupled hydro-mechanical simulation code, TOUGH-RBSN. The modeling tool combines a multiphase fluid flow and heat transport simulator, TOUGH2, with a geomechanical and fracture-damage model, called the rigid-body-spring network (RBSN). Fractures are modeled as discrete features, and hydrological properties (e.g., permeability, porosity) of fracture elements are evaluated by fracture opening and aperture changes calculated at time steps of the simulations. Modeling capabilities for hydraulic fracturing processes are presented through simulations of a virtual fractured reservoir consisting of multiple pre-existing natural fractures. Case studies are conducted by changing the reservoir configurations, such as confining stress condition (e.g., degree of stress anisotropy), the matrix permeability, and the viscosity of injected fluid. In the preliminary results, the stress field and the fluid pressure distribution are provided to demonstrate modeling of complex hydro-mechanical interactions between propagating fractures and pre-existing fractures. The fracture patterns also can qualitatively interpret the characteristics of hydraulic fracture propagation attributed to the variables for reservoir setting.