Nanometer to Centimeter Scale Porosity in Geologic CO₂ Storage Formations and Caprocks

Porosity and permeability are the key variables that link the thermal, hydrological, geochemical and geomechanical processes that redistribute mass and energy in response to injection of CO₂ into the subsurface. The size, shape, distribution and connectivity of rock pores dictate how fluids migrate into and through these micro- and nanoenvironments, wet and react with the solid. The overarching goal of this effort is to characterize the nano- to macropore features, quantify sorption of CO₂ and CO₂-brine, and determine how pores evolve in reacted systems at temperature-pressure-composition conditions relevant to CO₂ injection. Representative caprocks and reservoir rocks associated with CO₂ injection activities (e.g. shallow buried quartz arenites from the St. Peter Sandstone and the deeper Mt. Simon quartz arenite in Ohio as well as the Eau Claire shale, Ohio and mudrocks from the Cranfield MS CO₂ injection test) are being interrogated with an array of complementary methods - e.g. SEM, TEM, neutron scattering, X-ray CT, neutron tomography as well as conventional petrophysics.
(Ultra)small-angle neutron scattering and autocorrelations derived from BSE imaging provide a powerful method of quantifying pore structures in a statistically significant manner from the nanometer to the centimeter scale. Results will be described comparing shale and mudrocks that indicate there are significant variations not only in terms of total nano- to micro-porosity and pore interconnectivity, but also in terms of pore surface fractal (roughness) and mass fractal (pore distributions) dimensions as well as size distributions. New data on sandstones suggest that nano- and microporosity are more prevalent in nominally coarse-grained lithologies and may play a more important role than previously thought in fluid/rock interactions. Information from imaging and scattering are being used to constrain computer-generated, random, three-dimensional porous structures. The results integrate various sources of experimental information and are statistically compatible with the real rock. These computerized porous matrices will then be used in CO₂ sorption MD simulations.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California