POROSITY, PERMEABILITY, AND PORE SIZE DISTRIBUTIONS FOR SNAKE RIVER PLAIN, IDAHO BASALTS: IMPLICATIONS FOR CO2 MINERALIZATION IN BASALT

CO₂ capture and storage in geologic formations is a recognized strategy for the mitigation of atmospheric greenhouse gas emissions in which CO₂ is captured and injected as a free phase (supercritical) or aqueous phase (dissolved in water) into the subsurface. Large basalt provinces such as the Snake River Plain (SRP) in southern Idaho, USA represents a formation type that has the potential to mineralize and permanently sequester gigaton quantities of CO₂ as carbonate minerals. However, the quantitative assessment of the capacity and rate of mineralization requires, among other things, an understanding of the release of divalent base cations (Ca, Mg, and Fe) which is a function of the reactive surface area of basalt. Commonly, geometric considerations (e.g., fracture) or BET gas adsorption are used to estimate surface areas resulting in values that differ from each other by orders of magnitude.

An alternative approach to directly assess surface area is the use mercury porosimetry which provides a simultaneous measurement of surface area and pore size distribution. This approach, along with the measurement of permeability to groundwater, was applied to 15 samples collected from a well core that penetrated vertically fractured, horizontally fractured, and massive sections of SRP basalt flows. Porosity for these samples ranged from 8.5-18%, with approximately two-thirds of the porosity being associated with larger pores (greater than 1 μm in diameter). The total surface area ranged from 1.3-4.0 m²/g, with an average of 0.8% of the surface area associated with larger pores and greater than 99% of the surface area associated with pore less than 1 μm in diameter. In addition, the brine permeability (range 0.013-26.1 millidarcy) is more highly correlated with the surface area of larger pores (R²=0.79) than to the total surface area (R²=0.17) suggesting the small pores with large, aggregated surface area do not significantly contribute to advective transport. These results are consistent with a two-domain conceptual model of the basalt matrix in which the long-term mineralization rate is controlled by mass transfer between a relatively non-reactive large-pore in which fluid flow (CO­₂ plus water) occurs and a stagnant highly reactive small-pore, large surface are domain.