Petrophysical properties of representative geological rocks encountered in carbon storage and utilization

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This study evaluates petrophysical properties (especially porosity, permeability, tortuosity, and diffusivity) of representative geological rocks in the context of injectivity, storage space, and caprock integrity for effective utilization and long-term storage of carbon dioxide. A total of 10 geological rocks were selected as representative storage media for consideration as saline aquifers & depleted oil and gas reservoirs (sandstones and carbonates), basalts, and cap rocks, as well as utilization in organic-rich shale and coal seams. An integrated suite of laboratory tests, including liquid immersion porosimetry, gas expansion porosimetry, grain size distribution, mercury intrusion porosimetry, and gas diffusion, were performed on these various rock samples. The results exhibit a disparity of petrophysical properties among two broad groups of rocks: rocks selected for possible storage of CO2 have porosities of ∼10-25%, permeabilities of ∼10−16-10−13 m2, μm-sized pore-throat size distribution, and mostly good pore connectivity; in contrast, the potential caprocks have porosities of ∼0.5-5%, permeabilities of ∼10−20-10−18 m2, pore throat sizes of <50 nm, and probably poorly connected pore networks. An understanding of the measured facets of pore structure and contribution of fractures is also critical in the context of different testing principles and data interpretation of petrophysical analyses, as well as observational scales in the laboratory and field, and therefore reliable confidence of CO2 storage and utilization performance. Our work further illustrates the controlling influence of grain size distribution and geological processes on pore size distribution and pore connectivity for a wide range of rock types and lithologies, and particularly presents the extent and behavior of CO2 gas diffusion with a custom-designed apparatus for a holistic understanding of various petrophysical attributes of widely different geological rocks.

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