A fully-coupled highly-parallelized geomechanical simulator for assessing induced seismicity

Induced seismicity caused by anthropogenic modification of the subsurface effective stress state has been observed in a number of industrial applications -- from hydrocarbon extraction (e.g., Groningen [1]), drilling waste-water disposal (e.g., [2]) to the hydraulic stimulation of deep geothermal systems (e.g., [3]) – leading to the termination of the project in some cases. These activities change the stress state of a rock formation at kilometres depth and perturb the delicate balance on existing faults, possibly causing them to slip. It is crucial to evaluate possible fault reactivation already at the design stage. In this contribution, we propose a numerical model capable of predicting behaviour of rock formation intersected by multiple faults and subject to fluid injection or extraction. A poroelastic formulation is used to describe the behaviour of the porous rock, while lower-dimension lubrication flow is employed to model the fluid flow in the faults and cracks. The finite-element method is used to solve both the mechanical deformations of the rock and the fluid flow in the matrix. Faults and cracks are treated via interface elements, which allow for contact and friction. A computationally efficient block conjugate gradient approach [4] is used to solve the resulting hydro-mechanical system of equations in a robust and staggered way – without the recourse to any numerical parameter. Proper convergence is shown even in the case of strong non-linearities associated with permeability changes associated with mechanical deformation. High efficiency and scalability of the proposed simulator is achieved by using MPI routines [5], which makes it capable of large-scale simulations. We present several benchmarking tests to verify the fidelity of the model and demonstrate its efficiency. Further, we use a case with multiple faults to analyse the combined role of pore pressure evolution and poroelastic static stress transfer on fault slip. [1] R. Van Eijs, F. Mulders, M. Nepveu, C. Kenter, B. Scheffers, Correlation between hydrocarbon reservoir properties and induced seismicity in the Netherlands, Engineering Geology, 84 (3–4) (2006) 99–111. [2] Ellsworth, William L. “Injection-Induced Earthquakes.” Science 341, no. 6142 (July 12, 2013): 1225942. [3] Kim, Kwang-Hee, Jin-Han Ree, YoungHee Kim, Sungshil Kim, Su Young Kang, and Wooseok Seo. “Assessing Whether the 2017 Mw 5.4 Pohang Earthquake in South Korea Was an Induced Event.” Science 360, no. 6392 (June 2018): 1007–9. [4] Prevost, Jean H. “Partitioned Solution Procedure for Simultaneous Integration of Coupled-Field Problems.” Communications in Numerical Methods in Engineering 13, no. 4 (April 1997): 239–47. [5] Richart, N., and J.F. Molinari. “Implementation of a Parallel Finite-Element Library: Test Case on a Non-Local Continuum Damage Model.” Finite Elements in Analysis and Design 100 (August 2015): 41–46.