Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

6
Generating anisotropic seismic models
based on geomechanical simulation
It’s not rocket science, it’s rock science.
Julio Friedmann
6.1 Introduction
Seismic waves provide a means of remotely sensing the subsurface over a range of length scales.
Information from time-lapse (4-D) surveys and microseismic monitoring will compliment information
from bore-hole logging, flow rate measurements and pressure tests that will allow us to locate zones
of CO 2 saturation, map out reservoir flow compartments and identify regions of high stress and
fracturing. Commonly, it is assumed that observed time-lapse variations are simply a factor of varying
fluid content. However, it is becoming increasingly clear that fluid substitution alone cannot account
for all the observed temporal variations time-lapse seismic data (e.g., Hatchell and Bourne, 2005).
Travel time-shifts away from the reservoir and the development of stress-induced SWS suggest that
seismic properties are also sensitive to geomechanical deformation. One of the main goals of the
IPEGG consortium is to quantify the sensitivity of seismic observables to geomechanical effects.
In order to relate the information given by coupled fluid-flow/geomechanical models (e.g., in situ
stresses and strains, changes in porosity, and the movement and properties of fluid within the reservoir)
to seismic observables, it is necessary to model the elastic stiffness of the reservoir and surrounding
units. These models must be based on information provided by the coupled fluid-flow/geomechanical
simulation, and must also be constrained by geologic, engineering and seismic observations. I aim
to construct these models using rock physics theories that include intrinsic rock properties and incorporate
the effects of changes to the applied stress field and fluid saturation. In this chapter I
present and discuss a workflow to generate elastic models from the MORE-ELFEN coupled fluidflow/geomechanical
simulations.
6.2 Stress-sensitive rock physics models
The effects of stress and/or strain on rock elasticity is observed empirically to be non-linear (e.g., Nur
and Simmons, 1969; Kuster and Toksoz, 1974). The stress dependence of seismic velocities is strong at
low confining stresses, but weakens as confining stresses increase. The most common explanation for
this observation is that at low pressures, seismic velocities are dominated by the opening and closing
of discontinuities or microcracks between grain boundaries. At higher pressures, these discontinuities
close and velocities increase, but become less stress dependent. A number of approaches have been
used to account for the nonlinear response of velocity to stress, including empirically determined
relationships (e.g., Minkoff et al., 2004), Hertz-Mindlin contact forces (e.g., Makse et al., 1999), strain-
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