Sub-meter scale lithological heterogeneity and its influence on the CO2 trapping capacity in CO2 storage reservoirs

Abstract

Lithological heterogeneity at mm- to cm-scale exists in the form of sedimentary structures such as cross bedding and planar bedding in heterogeneous siliciclastic reservoirs. At this scale, sedimentary structures typically consist of two lithologies with significant differences in their porosity, permeability, capillary entry pressure and mineral composition. The lithology with high porosity, high permeability, low capillary entry pressure and a high abundance of quartz and feldspar characterizes the reservoir rock and is conducive to fluid flow. On the other hand, the lithology with low porosity, low permeability, high capillary entry pressure and a high abundance of clay and carbonate minerals characterizes the intraformational baffle and acts as flow barrier. The presence of both lithologies at mm- to cm-scale govern local fluid flow and geochemical reactions. This is especially important in CO2 storage reservoirs where CO2 flow and mineralization might be significantly affected. Hence, it is necessary to account for lithological heterogeneity at sub-meter scale in regional scale dynamic simulations. However, intraformational baffles are typically neglected in static reservoir models primarily because small scale intraformational baffles are not commonly resolved by wireline logs. Therefore, their lithological properties are not accounted for in reservoir models. This study addresses the characterization and incorporation of intraformational baffles in reservoir models so that the reservoir scale estimation of carbon mineral trapping capacity can be improved. Firstly, rocks from the CO2CRC’s Otway Research Facility were characterised using a range of sample and data analysis methods. The variability in porosity, permeability, capillary entry pressure, grain size and mineral composition was used to classify five homogeneous rock type classes: coarse sandstone, fine sandstone, siltstone, mudstone and carbonate-cemented sandstone. A new workflow was developed where homogeneous rock types and composite rock types consisting of two lithologies characteristic of intraformational baffles were represented. Rock properties for composite rock types were derived so that mm-scale heterogeneity can be upscaled for integration in coarser discretised reservoir models. Lithotype logs of composite rock types were derived and used to build a high resolution 2D static reservoir model of the site. Multiphase fluid flow and reactive transport simulations were run on 120 and 8 realizations, respectively, to develop a detailed understanding of CO2 flow and geochemical reactions at mm-scale. It was found that the capillary entry pressure characteristic of the reservoir rock significantly governed the range of turning point saturations while the amount of clay and carbonate content of the baffle governed the amount of carbon mineralized. Reactive transport simulations were also run on two 2D reservoir models to quantify the error associated with the estimation of carbon mineral trapping at reservoir scales if intraformational baffles are not accounted for in dynamic simulations. The results show that the neglection of intraformational baffles in reservoir rocks might lead to an underestimation of mineral trapping capacity by nearly 2.5 times.