Dynamic Rupture Modeling of Injection-Induced Seismicity: Influence of Pressure Diffusion Below Porous Aquifers

Abstract

Seismicity induced by pore pressure increases poses a major hazard for many fluid injection operations, such as waste water injection, secondary recovery of hydrocarbons and CO₂ storage. Diffusion of fluids outside of the target injection formation through relatively permeable faults may especially induce large seismic events, often below the reservoir in the crystalline basement. Understanding the mechanisms of seismic events induced under these circumstances is the key for hazard predictions and mitigation measures. In this study we model a simplified pressure diffusion scenario and use dynamic rupture modeling to study the nucleation and propagation of seismic events due to injection-induced pressure changes. In particular, we study the influence of the shape of the pressure diffusion profile on the dynamic characteristics of rupture. We find that high pressure gradients are more likely to cause smaller events, whereas low pressure gradients cause slip over a much larger portion of the fault, with a slightly higher slip rate. In addition, we analyze the influence of the background stress and the friction drop on the fault on the fault rupture.

1. INTRODUCTION

Human-induced pore pressure increases in subsurface aquifers/reservoirs can cause fault reactivation, which is undesirable because (i) it may damage the sealing capacity of the caprock faults, which is of particular importance for the integrity of CO₂ storage sites in the case of large-scale CO₂ injection [1,2] and (ii) it may cause felt seismicity, which has posed an especially large problem in the US over the past decade in relationship to waste water injection [e.g. 3]. In this study we aim to gain better understanding on when to expect fault reactivation due to injection into porous aquifers/reservoirs, and in particular what controls the magnitude and dynamic characteristics of induced seismic events.

There are multiple mechanisms that can induce fault reactivation during fluid injection into porous aquifers: (i) fluid pressure changes in faults lower the normal effective stress, and bring faults closer to failure; (ii) fluid pressure induced volume changes cause poroelastic stressing of the aquifer and its surroundings; (iii) cooling and contraction of the rock which also stresses the surrounding; and (iv) mineral reactions that change fault shear strength. Here we focus on the pressure related effects, i.e. poro-elastic stress changes and direct pressure effects in faults. Seismicity observed in relationship to secondary recovery (i.e. a low pressure injection of water in a reservoir to stimulate oil production after primary production has declined) and in particular large scale waste water injection into permeable aquifers/reservoirs often takes place outside/underneath the target injection layer [4]. In case of basal aquifers (aquifers overlying the basement) seismicity often takes place in the crystalline basement. The inferred mechanism for this seismicity is fluid diffusion into the basement through relatively permeable faults [5]. If the basement is critically stressed, only small pore pressure perturbations may be enough to induce seismicity [6]. Understanding what controls the magnitude of these diffusion related events is the key for assessment of seismic hazard and the development of mitigation measures.