Anatomy of pore networks in caprock relevant to geologic CO₂ sequestration

A number of factors dictate how CO2-bearing fluids migrate into and through both reservoir rocks and caprocks, wet and ultimately adsorb and react with the solid surfaces. These include the size, shape, distribution and interconnectivity of the porous matrix, the mineralogy and surface chemistry of the pore network, the chemistry of the fluids and their physical properties. While there may be a general tendency for more clay-rich lithologies to have lower porosity and permeability, the link between pore size distribution and connectivity and pore-wall mineralogy is still poorly constrained. In order to more accurately predict the reactive-transport behavior of CO2-bearing brines in both reservoir rocks and caprocks, we began testing the following key hypotheses: connected nano- to microscale pores and fractures constitute a nontrivial contribution to total rock porosity, the mineralogy of the pore and fracture network is markedly different compared to bulk mineralogy, and reactions by scCO2-brine fluids with caprocks will alter both the porosity and permeability, thus impacting caprock integrity. Efforts focused primarily on testing the first two hypotheses for caprocks. Detailed characterization studies used neutron scattering, electron microscopy and conventional petrophysics of representative examples of regionally extensive mid-continent mudstone and shale caprock to the Mt. Simon Formation (a sandstone target for CO2 injection).