Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 410

Question 9.5.2 Wetting angles, critical pore throat radii,
and maximum amount of CO 2 that can
be immobilized under a seal

Calculate the CO 2 -water-mineral wetting angle ( θ ) and the critical pore throat
radii of the caprock samples in Table 9.5.2 , assuming that θ = 0 in experiments
where the invading fl uid was CH 4 or N 2 . Then, calculate the range of critical
pore throat radii of reservoir rocks and caprocks from the p c,b values in
Table 9.5.1 using γ gw ∼ 22 mN/m and the θ value calculated in the fi rst part of
this question. What is the maximum amount of CO 2 that can be immobilized
under a seal that has p c,b = 1 MPa? Express the results as: (a) the thickness of
the CO 2 plume, (b) the mass of CO 2 per area, or (c) the area necessary to store
the CO 2 captured annually from a 1 GW gas-fi red power plant. Assume that
φ = 0.25, ρ H 2 O = 1,000 kg/m 3 , ρ CO 2 = 600 kg/m 3 . A modern gas-fi red power sta-
tion with 1 GW net electrical capacity and 90% capture produces about 2.5 Mt
CO 2 /year for storage.

Fractures and faults

In the previous section, we have analyzed a small sample of a caprock,
which we hope is representative of the entire caprock. However, on a
distance of several kilometers one could expect the caprock to have
fractures and faults, which could lead to potential leakage of CO 2 .
Large faults are likely to be detected through remote sensing studies
and accounted for in the planning of a sequestration operation. Small
faults and fractures, however, may be ubiquitous and essentially impos-
sible to detect. These potential preferential fl ow paths include healed
fractures in the seal rock that could re-open as a consequence of the
pressure changes caused by CO 2 injection and m wide “micro-

annuli” that may exist in wells at the boundaries between the well cas-
ings, the well cement, and the caprock formations ( Figure 9.5.3 ).
Small cracks and fractures in caprocks are poorly understood. They
may become less important with depth if lithostatic pressure, the pres-
sure exerted by the other rocks, helps to re-seal fractures (see
Figure 9.5.4 ) [9.19]. Their properties may depend on the mechanical
properties of the rock. For example, shale gas production data suggest
that wells drilled in shale formations that have high clay and/or organic
contents have shorter gas production lifetimes, perhaps because these

∼ 10 µ