Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

Abstract
Capture of CO 2 produced at fossil-fuel burning power stations, and its subsequent storage in deep
geological formations (Carbon Capture and Storage, CCS) has the potential to become an important
tool in mankind’s struggle to reduce climate-changing CO 2 emissions while ensuring that the world’s
energy needs are met. In order for CCS to become socially acceptable and economically viable, the
risks of CO 2 leakage must be quantified and minimised. We must be able to model how CO 2 will flow
through the subsurface, and we must be able to monitor this flow. We must also be able to model
the effects of CO 2 injection on the rocks in and around a reservoir, and we must develop monitoring
methods to track these effects in the field. These requirements generate a number of scientific and
engineering questions on which this thesis focuses.
In 2000, a CCS component was added to EnCana’s enhanced oil recovery operations at the Weyburn
oil field, Saskatchewan Province, Canada. This project has provided a testing ground for many
modelling and monitoring techniques, including passive seismic monitoring. The first aim of this
thesis is to analyse the microseismic data recorded at Weyburn to see what microseismic events can
tell us about geomechanical deformation induced by injection. The second part of the thesis aims to
model geomechanical deformation using finite element techniques. The final part of the thesis links
the two techniques by showing how microseismic observations can be used as a tool to help calibrate
geomechanical models, and also how stress changes predicted by geomechanical modelling can help
interpret microseismic data in terms of storage security.
A downhole microseismic monitoring array was installed at Weyburn in 2003, and CO 2 injection
was initiated in a nearby well in 2004. CO 2 injection induced approximately 100 events during the
first year, and approximately 40 in the following 3 years. Events are located in two clusters associated
with nearby production wells. Events are located at reservoir depths and in the overburden as well.
Event locations - around production wells and in the overburden - are difficult to interpret within a
conventional framework for injection-induced seismicity, where locations are expected to form a cloud
around the injection well.
Microseismic events provide excellent S-wave sources to image seismic anisotropy in reservoirs using
shear-wave splitting (SWS). As part of this thesis I have developed an approach using rock physics
to invert SWS measurements for the presence of aligned fractures sets. Using the Weyburn data, I
find two sets of aligned fractures that strike to the NW and NE, matching fracture sets previously
identified in core samples and borehole image logs from Weyburn.
The use of microseismic data at Weyburn has highlighted how little we know about the microseismic
and geomechanical response to CO 2 injection in comparison to other commonly injected fluids such
as water. To address this uncertainty I analyse microseismic data from a set of hydraulic fracture
stimulations into a reservoir that use both water and CO 2 as the injection fluids. I make a direct
comparison of microseismicity generated by the two fluids, finding that the rates and magnitudes of
microseismicity during CO 2 and water injection are very similar. SWS measurements are also unable
to distinguish between the fracturing induced by the different injection fluids suggesting that there is
no reason to expect lower rates of microseismicity or geomechanical deformation during CO 2 injection.
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