Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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A comparison of microseismic monitoring
of fracture stimulation due to water
versus CO 2 injection
Don’t force it, get a bigger hammer.
Arthur Bloch
4.1 Introduction
I have observed a very low rate of seismicity at Weyburn. A key question to ask is whether this
is always to be expected for CCS scenarios It is not clear that CO 2 injection should induce the
same rates, patterns and magnitudes of seismicity as water injection. The lower density of CO 2 in
comparison to the oil and/or water in a formation may lead to density-driven stress changes, causing
different patterns of seismicity (e.g., Sminchak et al., 2002). Supercritical CO 2 has a bulk modulus
at least an order of magnitude smaller than water, and it has been suggested that, as it has a higher
compressibility, CO 2 represents a ‘softer hammer’ than other injection fluids, and as such will have an
inherently lower seismic deformation efficiency. Furthermore, depending on the fluid initially present
in the pore space, there may be relative permeability differences between the two fluids. This would
influence the injectivity, and therefore presumably the amount of fracturing induced. As a result, there
is still significant uncertainty as to the amount of fracturing and seismicity to expect when injecting
CO 2 .
The dataset presented in this chapter comes from a hydraulic fracture job. As such, it does not
directly represent a sequestration scenario such as Weyburn. However, in many ways it can be viewed
as representing a ‘worst-case’ scenario, where the injection of CO 2 has lead to significant fracturing
of the reservoir (albeit deliberately so in this case). I wish to make a direct empirical comparison
between water and CO 2 injection. To make the comparison as fair as possible, both fluids have been
injected at similar rates, with similar injection pressures, into similar units within the same reservoir.
Both stages have been monitored on the same downhole geophone array. I aim to use this dataset
to image the orientation and extent of the fracturing, and to use both event magnitudes and shear
wave splitting analysis to place bounds on the density of fracturing caused by fluid injection. This
will allow me to consider the seismic efficiency of CO 2 in comparison to other fluids.
4.1.1 Field background and description
In a producing North American oil-field a hydraulic fracture job was performed in order to create
and improve pathways for fluid migration and to stimulate production. Owing to confidentiality
agreements information about the geology and history of this field is limited. The reservoir is thick
and contains potential barriers to vertical flow. Therefore, in order to ensure fracturing throughout
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