It is important to predict whether or not fluid injection into or near a fault will cause slip, since this can result in seismicity of sufficient magnitude to cause damage to surface structures. An important mechanism that is usually neglected when analyzing fluid injection into faults is the leak-off into the surrounding rock mass. The objective of the study is to outline the importance of considering this leak-off mechanism. This is achieved by comparing a two-dimensional fully coupled fluid and mechanical loading extended finite element method (X-FEM) formulation (via development of a standalone code in MATLAB) with the Universal Distinct Element Code (UDEC). The X-FEM code has the capacity to consider leak-off into the surrounding rock mass from the fault, whereas UDEC does not have this capability unless extremely fine blocks are generated in the vicinity of the fault to simulate a fracture network within the fault damage zone. A case study is presented and the results indicate that using the UDEC the normal and shear displacements are comparable to an impermeable X-FEM model. However, there is additional pore pressure produced at the center of the fault in the impermeable X-FEM calculation, compared with the UDEC. When the surrounding rock mass has high permeability this pore pressure reduces in the X-FEM model, producing less slip than the UDEC result. This X-FEM provides an advancement over previous studies that do not consider leak-off from the fault into the surrounding rock mass. The proposed method may assist with providing a more accurate prediction of fault-slip. This slip mechanism is important to understand in the context of induced seismicity and its associated concerns.

1 Introduction

Seismicity large enough to cause damage on the surface can be caused from fluid injection into or near a fault, generating slip (Raleigh, Healy & Bredehoeft, 1976; Nicholson & Wesson, 1992; Cornet et al., 1997; Majer et al., 2007; Ellsworth, 2013). The complexity due to the fluid-rock mechanical interaction makes it difficult to predict seismic events from fluid injection. Therefore, to predict this seismicity the fault mechanics due to fluid injection need to be determined. To predict fault movement these coupled fluid and mechanical processes are needed to be used in the analysis. Previous studies have predominately focused on developing numerical methods (Rutqvist et al. 2002) to generate parametric results (Rutqvist et al. 2013, 2015) however, these do not compare different numerical methods. This study introduces a coupled extended finite element method (X-FEM) approach that considers the fluid-rock mechanical interaction, including the leak-off into the rock mass. The method should be sufficiently efficient to be used in industry and uses appropriate rock properties as inputs into the numerical model. This study concentrates on the comparison of this developed code with the Universal Distinct Element Code (UDEC) (Cundall 1980) and provides evidence that the leak-off into the rock mass is an important process governing the fault mechanics due to fluid injection.

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