Hydro-Mechanical Modelling of Fault Slip Induced by Water Injection Using TOUGH-FLAC Simulator: The Work of DECOVALEX-2019 Task B

Park, Jung-Wook (Korea Institute of Geoscience and Mineral Resources) | Kim, Taehyun (Korea Institute of Geoscience and Mineral Resources) | Park, Eui-Seob (Korea Institute of Geoscience and Mineral Resources)

OnePetro 

Abstract

As part of the DECOVALEX-2019 project Task B-Fault slip modeling, we are developing a numerical model for simulating fault activations induced by water injection. The work of Task B is scheduled to be conducted until 2019 in three research phases. The topic of the first step is developing a numerical method for a benchmark model to simulate the injection test in a single fault zone. We present a numerical model to reproduce the coupled hydro-mechanical process of fault activation using the TOUGH-FLAC simulator. The mechanical behavior of a single fault is represented by the zero-thickness interface element of FLAC3D upon which a slip and/or separation is allowed. The fluid flow along a fault is represented using finite thickness elements in TOUGH2 on the basis of Darcy’s law with the cubic law. The hydro-mechanical coupling between the fracture hydraulic transmissivity and the slip-induced displacement was established for two different fault models (FM1 and FM2). A coupling module was developed in the TOUGH-FLAC simulator to continuously update the changes in geometrical features, as well as hydrological properties induced by mechanical deformation. Then, the transient responses to stepwise pressurization of the fault and host rock were examined during the simulation. The hydro-mechanical behavior, including the injection flowrate, pressure distribution around the borehole, stress conditions, and displacements in normal and shear directions induced by water injection were monitored along the fault and/or surrounding rock. The results of benchmark calculations suggest that the developed model can reasonably represent the hydro-mechanical behavior of a fault and the surrounding rock, including the progressive evolutions of the pathway and fault slip zone. This study will be extended and enhanced through continuing collaboration and interaction with other research teams of Task B.

1. Introduction

The DECOVALEX project, which began in 1992, is an international research and model comparison collaboration for thermo-hydro-mechanical-chemical processes in geological systems. Task B of the current DECOVALEX-2019 phase, running from 2016 to 2019, addresses the potential creation of permeable flow paths for contaminant transport in low-permeability host rocks. The objective of the task is to develop numerical models for coupled hydro-mechanical processes of fault activation. The work is planned to be conducted until 2019, through the following three steps of progressively increasing complexity: 1) The benchmark calculation of a simplified single fault plane, 2) the interpretive modeling of an observed activation in a minor fault, and 3) the interpretive modeling of an observed activation in a major fault. The model developed in the benchmark calculation will be modified and verified using the field data from fault activation experiments recently performed at the Mont Terri underground research laboratory in Switzerland.