Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

8.4. A SOFTER RESERVOIR
8.4 A softer reservoir
For the updated model, I reduce the Young’s modulus of the reservoir to 0.5GPa, while keeping
all the other properties the same as for the first model. Therefore this model has a reduced reservoir:overburden
stiffness ratio. The stress evolution for this model is plotted in Figure 8.10. The
trends are the same as for the stiffer model (Figure 8.5). However, because in this case the reservoir
is softer in comparison to the overburden, more stress can be transferred from the reservoir to the
overburden (as demonstrated in Chapter 5). As a result, the changes in effective stress in the reservoir
are reduced, while the changes in stress in the overburden are increased.
2000
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17.6
Y
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17.4
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(a)
(b)
Figure 8.10: Vertical effective stress in (a) the reservoir and (b) the overburden of the softer Weyburn
model after injection. The locations of the vertical injection wells (triangles) and horizontal
producers (lines) are marked.
The fracture potentials for the softer model are computed as for the first model, using the values
in Table 8.4. The results are shown evolving through time in Figure 8.11, with snapshots across
the reservoir and overburden in Figures 8.12 and 8.13 respectively. As with the stiffer reservoir, the
fracture potential increases during production. However, the behaviour in the overburden is different
once injection begins. The fracture potential at the producing wells is relatively unchanged. However,
the onset of injection leads to a sharp increase in f p in the overburden above the producing wells. After
an initial increase in f p in the overburden, the rocks around and above the injection well experience
a decrease in fracture potential, returning to values similar to what they were before production had
begun. This provides a much better match with observations made in Chapter 2, where events occur
in the reservoir and overburden near the horizontal production wells, but few if any events are found
near the injection well. In particular, this model shows how stress transfer into the overburden, which
is promoted by a reservoir that is softer in comparison to the overburden (as noted in Chapter 5),
can generate increases in shear stress, and therefore a greater likelihood of microseismicity, above the
horizontal production wells.
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