Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

8
Linking geomechanical modelling and
microseismic observations at Weyburn
The minimum conditions for site closure and transfer of responsibility includes [...] the conformity
of the actual behaviour of the injected CO 2 with the modelled behaviour.
Directive 2009/31/EC of the European Parliament
on the geological storage of carbon dioxide
8.1 Introduction
The above segment from the EU Parliament’s directive on CO 2 storage highlights the need to develop
models that are validated by observation. For reservoir fluid-flow models this involves history matching
with injection and production rates, downhole pressure changes and CO 2 plume geometry as imaged
by controlled source seismics. However, can we also apply this to geomechanical models Can we find
ways to validate geomechanical models of sites like Weyburn with observables from the field
In the previous chapters I have outlined an approach to model the geomechanical deformation
produced by CO 2 injection, and to predict seismic observations based on the geomechanical model.
In this chapter I will demonstrate the workflow by applying it in its entirety to Weyburn. By doing
so I hope to constrain the geomechanical model by making comparisons with the observed seismic
results, and to improve the understanding of why the microseismic events are located as they are.
The workflow that I will use is outlined in Figure 8.1. I use the approach outlined in Chapter 5
to generate a representative geomechanical model of the Weyburn reservoir, simulating both the field
depletion and the recent CO 2 injection. By considering the induced stress changes I will compute the
changes to the fracture potential for various points around the reservoir, and thereby the likelihood
of microseismic activity, and I will compare these inferences with the observations made in Chapter
2. I use the method described in Chapter 6 to invert ultrasonic measurements made on Weyburn
reservoir cores for initial crack density and aspect ratio, and use these values as input to the workflow
in Chapter 7 to compute seismic properties. I will model shear wave splitting induced by nonhydrostatic
stress changes, and compare these results with the splitting observations made in Chapter
3. Such comparisons with observation can be used to calibrate the models, and changes can be made
to various model parameters to improve the fit between model predictions and observations. This will
improve the confidence in the models when they are used to assess the security of CO 2 storage in the
reservoir.
8.2 Model description
A general description of the Weyburn field is provided in Chapter 2. The reservoir has a thickness of
30-40m, and is laterally extensive over many kilometres with little vertical relief. The top and base of
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