Implicit Fracture Modelling in FLAC3D: Assessing the Behaviour of Fractured Shales, Carbonates and Other Fractured Rock Types

Fractured rocks play an important role in many types of petroleum and geo-energy operations. From fractured limestone reservoirs to unconventionals, understanding the geomechanical behaviour and the dynamically coupled (dual) permeability system is paramount for optimal development of these systems. Traditionally, incorporating fracture systems in Finite Element Models, Finite Difference Models or other types of mesh-based approaches has been cumbersome for a number of reasons. Here, a simple implicit method is employed to model the transient behaviour of a fractured rock mass subjected to fluid pressure changes. Using a number of simplifying assumptions, the fractured rock mass is represented by an equivalent rock mass, for which the properties are dynamically updated during the simulation based on the local stress state. This allows for modelling of the fractured rock mass without explicitly including fractures. The model was applied to a simple example application. The results indicate that the model captures dynamic behaviour of fractures as a function of effective stress changes. Failing to account for this dynamic behaviour leads to a significant error in the BHP and does not capture the transient permeability evolution.