Heterogeneity in mineral composition and its impact on the sealing capacity of caprock for a CO2 geological storage site

It is ubiquitous that the rock of porous media is composed of multiple minerals expressing distinct geochemical properties and spatial distributions, which may influence the geochemical processes, the flow field by changing the size and connectivity of the pores. Numerical modeling is an effective means to investigate the impact of heterogeneous mineral composition. However, it is challenging to represent a successful and reliable spatial distribution of minerals as the prerequisites for numerical simulations. In this study, a novel approach for realizing a reasonable spatial distribution of mineral composition was presented, and the effect of mineral heterogeneity on the geochemical reactions was evaluated. Based on the available data from a CO2 geological storage site, the random field with lognormally distributed permeabilities was firstly generated by employing the geostatistical theory, and then porosities obeying normal distribution were obtained with a regressed log-linear model. Finally, the two-dimensional joint normal distribution model was adopted to realize the mineralogical heterogeneity. By using "3 sigma law", the generated permeabilities, porosities and mineral contents were limited to the corresponding ranges of site-specific available data with probability of as high as 99.74%. Aforementioned processes have been coupled into the existing software TOUGHREACT. The proposed approach was applied to a CO2 geological storage site to estimate the effects of spatial variation of minerals on the evolution of caprock sealing capacity. The generated spatial distribution of minerals is reasonable compared with the measured data from the CO2 geological storage site in the Ordos Basin, China The 500-year simulated results imply that the spatial variation in mineralogical composition slightly enhanced the sealing capacity of caprock.