Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

pressure, P fl
σ ′ ij = σ ij − β w I ij P fl , (5.1)
5
Geomechanical simulation of CO 2
injection
Even the self-assured will raise their perceived self-efficacy if models teach them better ways of doing
things.
Albert Bandura
5.1 Introduction
Microseismic monitoring provides information about the geomechanical deformation occurring in and
around the reservoir. As such, the interpretation of microseismic activity can be greatly improved
by geomechanical modelling. Geomechanical models commonly use finite element techniques to simulate
the deformation caused by pore pressure changes in the reservoir. Injection of CO 2 will increase
the pore pressure in the reservoir. This represents the loading for the geomechanical model, which
computes the deformation both inside and around the reservoir. By examining the stress evolution
it is possible to identify areas in and around the reservoir where fractures are likely to form or be
reactivated. In this chapter I use a relatively novel technique where a geomechanical model is coupled
to a fluid-flow simulator. This allows changes in pore pressure to be passed directly to the geomechanical
model, and changes in porosity and permeability to be returned to update the fluid model. The
modelling method has been developed by the Integrated Petroleum Engineering, Geomechanics and
Geophysics (IPEGG) consortium. In this chapter I introduce the modelling technique and demonstrate
it with several simple numerical simulations. I use these to examine how factors such as the
reservoir geometry and material properties affect the stress evolution during CO 2 injection.
5.2 Effective stress and stress path parameters
The concept of effective stress was introduced by Terzaghi (1943). When a stress is applied to a porous
material, part of the stress will be supported by the matrix material, and part will be supported by
the fluid in the pores. The part of the stress supported by the matrix is termed the effective stress,
given in tensorial form as σ ′ ij . It is the effective stress that will determine the deformation of the rock
frame. The effective stress is a function of the external stress applied to the rock, σ ij , and the pore
where I ij is a 3×3 identity matrix, and β w is the Biot-Willis parameter (e.g., Mavko et al., 1992),
assumed here to be 1. The magnitudes of the principle effective stresses are given by the eigenvalues of
σ ′ ij , and when referring to these I will use only one subscript, e.g., σ′ 1. The corresponding eigenvectors
give the orientations of the principal effective stresses. One of the principal stresses is usually orientated
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