Geomechanical modeling of induced microseismicity

Geomechanical modelling can provide insights enabling us to successfully answer questions such as: (1) Given a known stress field, geology, rock mass fabric and injection strategy, what are the most likely resulting microseismic characteristics (e.g., hypocentres, source mechanisms and magnitudes)? (2) What does measured microseismicity reveal about the existing stress field and local geomechanical properties of the rockmass? We use two different modelling approaches to investigate various aspects such as event locations, source mechanisms and energy balances.

The first method uses bonded-particle modelling. This tool simulates rock deformation using an assemblage of rigid, round particles that are bonded together (Potyondy and Cundall, IJRMMS, 2004). This grid of particles can deform freely and bonds can be broken to represent local failure. Bonds are characterized by normal and shear strengths as well as friction coefficients to model respectively tensile and shear failure. One advantage of this approach is that it reveals microseismic event locations, including their moment tensors. This is achieved by integrating local bond failure in both space and time (Hazzard and Young, IJRMMS, 2004).

In the second approach we use the distinct element method to investigate how the heterogeneity of the rock affects the development of fractures. This method computes the stresses and strains inside discontinuous media such as fractured layered rock masses. We examine the likelihood of failure throughout a stratigraphic column, thus revealing the most likely positions for microseismic events.

Geomechanical modelling is a powerful tool to help bridge the gap between geophysical data analysis and engineering applications of microseismic data by providing a framework for advanced interpretation strategies (Van der Baan, Eaton, Dusseault, Intech, 2013). In particular, it helps answering questions such as: Why is failure occurring in specific locations and not others? What are the failure mechanisms? Where does the input energy go?

Acknowledgments

The authors thank the sponsors of the Microseismic Industry Consortium for financial support, and Itasca for providing licenses to their 3DEC and PFC software packages.