The modes of geologic storage of CO
2 are usually categorized as structural, dissolution, residual, and mineral trapping. Here we argue that the heterogeneity
intrinsic to sedimentary rocks gives rise to a fifth category of storage, which we call local capillary trapping. Local capillary
trapping occurs during buoyancy-driven migration of bulk phase CO
2 within a saline aquifer. When the rising CO
2 plume encounters a region (10
−2 to 10
+1m) where capillary entry pressure is locally larger than average, CO
2 accumulates beneath the region. This form of storage differs from structural trapping in that much of the accumulated saturation
will not escape, should the integrity of the seal overlying the aquifer be compromised. Local capillary trapping differs from
residual trapping in that the accumulated saturation can be much larger than the residual saturation for the rock. We examine
local capillary trapping in a series of numerical simulations. The essential feature is that the drainage curves (capillary
pressure versus saturation for CO
2 displacing brine) are required to be consistent with permeabilities in a heterogeneous domain. In this work, we accomplish
this with the Leverett
J-function, so that each grid block has its own drainage curve, scaled from a reference curve to the permeability and porosity
in that block. We find that capillary heterogeneity controls the path taken by rising CO
2. The displacement front is much more ramified than in a homogeneous domain, or in a heterogeneous domain with a single drainage
curve. Consequently, residual trapping is overestimated in simulations that ignore capillary heterogeneity. In the cases studied
here, the reduction in residual trapping is compensated by local capillary trapping, which yields larger saturations held
in a smaller volume of pore space. Moreover, the amount of CO
2 phase remaining mobile after a leak develops in the caprock is smaller. Therefore, the extent of immobilization in a heterogeneous
formation exceeds that reported in previous studies of buoyancy-driven plume movement.
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