Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

calibrate. This model has been calibrated with over 200 different core measurements, and has been
found to be remarkably consistent.
I have applied this model to simple geomechanical models in order to determine the sensitivity
of seismic properties to stress path effects. The results have demonstrated the potential to diagnose
what stress path a reservoir is following using 4-D seismic techniques. By diagnosing the stress path,
geomechanical models can be calibrated, allowing the risk of fracturing to be determined. I have also
demonstrated how anisotropy could potentially be used as an indicator of reservoir compartmentalisation.
Having developed and demonstrated the workflow to move from geomechanical modelling to seismic
predictions, I apply the concept to the real example of the Weyburn field. I have generated a simple
model that represents the major features of the reservoir. The initial material parameters used to
populate the model were derived from core sample measurements. However, the predictions from this
model do not provide a good match with either microseismicity or anisotropy observations. It is a
well known (and yet often ignored) fact that mechanical tests on core samples do not, for obvious
reasons, include the effects of large scale fractures on the overall properties of a material. Upscaling
commonly finds that the rock mass is softer than determined by core tests, and this is particularly true
for heavily fractured rocks, such as the Weyburn reservoir. When the Weyburn model is recomputed
with a smaller Young’s modulus for the reservoir, a much closer match is found between model
predictions and observations.
There are many free parameters that can be varied in a geomechanical model, all of which can
influence the result. We generally do our best to constrain the information put into such a model
using many sources to aid model population, from 3-D controlled source seismics, borehole logs and
core work. However, this information does not directly correspond to the information that is needed
- i.e. the poroelastic and plastic response of the rock mass, as a whole (not a limited, core-sized
sample), to the relatively large and long-term stresses applied during CO 2 injection (as opposed to
the low-magnitude, short duration stress applied by an acoustic or seismic wave). These measurements
provide proxy information for what we really need. As such, each parameter in the model has a degree
of uncertainty to it.
The question then must be: how should we deal with this uncertainty in order to have any kind
of confidence in model predictions One option is to use a stochastic method, where a probability
function is assigned to each parameter, and the resulting probabilities for the results are computed.
However, even such a method must first assign probabilities to the input parameters, which does
not get away from the original problem, in that the probability function will be based on proxy
measurements and will not directly represent the required parameter. Therefore we must have a
method for determining which models provide the most appropriate results, a decision which must
be based on comparison with observations in the field. A number of observations might be made
with which to groundtruth geomechanical models, such as surface deformation or borehole tiltmeters.
In this thesis I have used observations of induced seismicity and anisotropy to constrain my models,
and have demonstrated that some of the information used to construct the original model, in this
case the Young’s modulus provided by core sample measurements, does not actually do a good job of
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