Investigation of Injection-Induced Seismicity Using a Coupled Fluid Flow and Rate/state Friction Model

Abstract

This paper describes a numerical investigation of seismicity induced during injection into a single, isolated fracture. A model that was developed and used that couples (1) fluid flow and (2) rate and state friction. Rate and state friction theory describes how friction on a fault depends on both sliding velocity and past sliding history. Rate and state friction is used widely in earthquake modeling. We investigated the effect of various factors, both geological and practical, that impact induced seismicity. Our modeling indicated that shear-induced pore dilation may be an important process that prevents injection from triggering slip events that propagate far from the injector. Results suggested that there must be an upper limit on the amount of shear dilation that can occur because otherwise slip events would nucleate continuously near the injector well, rather than at the periphery of the stimulated region (as is commonly observed in EGS experience). The effect of injection schedule was examined. It was found that decreasing injection pressure over time was a successful strategy for reducing the maximum event magnitude. The model predicted that significant seismicity should occur after injection ceases, which is consistent with observations from EGS stimulations. The post-injection events were caused by a redistribution of fluid pressure after injection. Production of fluid from the well immediately after injection inhibited post-injection events. In prior work, we investigated the impact of injection schedule using a simpler treatment of friction that we refer to as static/dynamic. The injection schedule findings described in this paper were consistent with our results from our static/dynamic modeling. Despite the apparent success of the static/dynamic modeling, it has important drawbacks, and we discuss some of those issues in this paper. Our eventual goal is to perform coupled fluid flow and rate and state simulation on a large network of fractures. We discuss various strategies we have implemented or are implementing to improve the efficiency of the simulations and make large scale simulations possible.