Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

5.4. RESULTS
Overburden
Extension in the overburden
Side
burden
Reservoir is contracting
Compression
in the side
burden
(a)
Overburden
Compression in the overburden
Side
burden
Reservoir is inflating
Extension
in the side
burden
(b)
Figure 5.11: Cartoon illustrating stress arching during (a) production and (b) injection. During
production, the shrinkage of the reservoir induces stretching in the overburden, and compression
of the sideburden as it supports the load. The reverse happens during injection, with compression
in the overburden and extension in the sideburden.
compacts with decreasing pore pressure, the overburden should subside. However, the weight of the
overburden is supported by the sideburden, and so it does not subside. Instead there is extension in
the overburden, while the sideburden is compacted by the extra weight that it is required to support.
This process is illustrated in Figure 5.11a. With these simple injection models I have demonstrated
the inverse process occurring during inflation. The increase in pressure inside the reservoir pushes the
top of the reservoir upwards. However, the overburden is not lifted as it is connected mechanically to
the sideburden. As a result, there is extension in the sideburden, while the overburden is compressed.
This is illustrated in Figure 5.11b.
In Figure 5.12 I plot the change in vertical effective stress for the small, soft reservoir and for the
stiff, extensive reservoir. For the stiff, extensive case, σ ′ 3 decreases due to the pore pressure increase,
but there is no stress change outside the reservoir. There is no stress arching. In contrast, for the
soft, short case, although the effective stress inside the reservoir decreases, it does not decrease by as
much. This is because part of the load is supported by the overburden, which compacts, and by the
sideburden, which extends. Stress arching has occurred.
The γ 3 parameter describes the extent to which this process is occurring. I have found that
arching is likely to occur when the reservoir is soft in comparison to the over- and sideburdens. This
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