Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 0.
ABSTRACT
Microseismic activity is an observable manifestation of geomechanical deformation induced by
injection. As the pore pressure is increased in a reservoir, effective compressive stress is reduced,
causing it to expand. I use a recently developed coupled fluid-flow/geomechanical modelling technique
to model this injection-induced deformation. I develop a series of simple, representative models to
study how the stress path induced by injection is modulated by the geometry of the reservoir and the
material properties of both the reservoir and surrounding non-pay rocks. I find that small reservoirs
that are softer than the surrounding rocks are most prone to stress arching effects, while flat extensive
reservoirs that are stiffer than surrounding rocks tend to experience only the hydrostatic effective
stress decrease induced by pore pressure increase. By computing the changes in deviatoric stress
induced by injection, I am able to analyse which types of reservoir will be most prone to brittle failure
(fracturing) and where this failure is most likely to occur. I find that small reservoirs are most prone
to fracturing - in small soft reservoirs the overburden is most likely to fracture above the injection
well, in small stiff reservoirs fracturing is most likely to occur in the reservoir.
Changes in the applied stress field will change the seismic properties of rocks in and around the
reservoir. Therefore in theory it should be possible to image geomechanical deformation using seismic
methods. To do so, a rock physics model is needed to map modelled stress changes into changes in
seismic velocities. I have developed a model that uses a microstructural effective medium approach to
map triaxial stress changes into anisotropic changes in P and S wave velocity. I have calibrated this
model with over 200 sets of stress-dependent ultrasonic velocity data, measured on core samples, taken
from the literature. I use this model to show that the differences in stress path caused by differing
reservoir geometries and material properties may well generate resolvable variations in the 4-D seismic
response of a CO 2 injection site. These responses have the potential to be used as a diagnostic in
imaging geomechanical deformation, and thereby be used to contribute a greater understanding of the
risk of CO 2 leakage due to fracturing.
I have developed a workflow to construct geomechanical models, make seismic and microseismic
predictions, and thereby to make comparisons with geophysical observations. In the final part of
this thesis I apply this workflow to the Weyburn field. I construct a representative geomechanical
model to simulate production from the reservoir, followed by CO 2 injection for storage and EOR. The
modelled rock mechanical properties are based on measurements made on core samples from Weyburn.
I use the modelled stress changes to make microseismic and seismic anisotropy predictions, finding
a poor match between predicted microseismic zones and where microseismicity has been observed at
Weyburn, and a poor match between predicted and observed seismic anisotropy. However, I develop
a second model that takes into account the softening effect that vugs and fractures will have on the
stiffness of the reservoir that will not be included if core sample measurements are used without
modification. This new model provides a much better match for both microseismicity and seismic
anisotropy. The model indicates that microseismicity observed in the overburden at Weyburn does
not represent fluid migration through the caprock but stress transfer through the rock frame. This
approach shows how geophysical observations can be used to help calibrate and thereby improve the
accuracy of a geomechanical model, providing a greater understanding of the risks that geomechanical
deformation can pose to the secure storage of CO 2 .
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