Theoretical investigation of surface chemistry on geologic CO² capillary confinement

Abstract

It can be concluded from this work that substrates where isolated minerals are wetted in laboratory settings provide an incomplete picture of CO² wetting behavior in GCS systems. Compositions of surfaces in reservoirs should be treated as a continuum when looking at the effect on wetting behavior, just as other parameters (i.e. pressure and temperature) are in other studies. Furthermore, site screening should take into account both brine ionic strength as well as reservoir rock composition together to give the most accurate prediction of CO² interfacial behavior. Future experiments on wetting is recommended and proposed herein on realistic rock core samples. This testing will not only yield more accurate wetting results, but will also tie them to residual trapping capability of that rock as well.

The concept of storing anthropogenic carbon dioxide in geologic formations is being examined as an option for reducing greenhouse gas accumulations in the atmosphere. Securing CO² safely and predictably requires a fundamental understanding of the processes that govern trapping and storage mechanisms. Sites that have the most potential and capacity are unfortunately those where relatively little is known in terms of their geologic carbon sequestration (GCS) capabilities. The present study attempts to characterize these sites by quantifying the relative impact that their reservoir rock surfaces have on CO² wetting - an essential and often overlooked factor in trapping analysis.

To accomplish this, the study employs simulations coupled with established analytic relationships to predict theoretically the influence that altering the surface chemistry and microstructure can have on supercritical CO² wetting. Intermolecular forces and their relative influences were the foundation for the analysis, along with an in-depth geologic characterization of expected reservoir composition. Computer programs were developed and utilized to find combinations of rock and structure that lead to optimum combinations of contact angle and interfacial surface energy.

The results show that bounds on contact angle variation in intermediate composition rocks exceed the sums of the individual rock types. Ranges of contact angle and interfacial tensions for all sedimentary rocks were established and compared to other ranges for different parameter (pressure, salinity) variation. It was found that the impact of surface chemistry on wetting is relatively low as compared to of the effect of pressure variations. Among individual rock types where no lithic combination is present, sandstones exhibit the highest relative contact angles whereas limestones have the lowest. The relative impact of electrolytic bulk fluids was also born from the study, and was determined to act in conjunction with surface chemistry to effectively alter wetting behavior based on the presence of ions in the medium. Surfaces with bulk properties closest to that of the brine, coupled with relatively high ionic concentration in the system will lead to greater barriers to adsorption of CO² and larger equilibrium contact angles.