Rock physics research with application to CO₂ geological storage I: CO₂ behavior in capillary-dominated region and effects of multi-scale heterogeneity on CO₂ trapping

Abstract

 Deep saline aquifer formations are expected to have a large potential for carbon dioxide (CO₂) storage. Host rocks of the aquifers are mostly porous sandstones. Laboratory experiments on CO₂/brine displacement flow in porous sandstones under aquifer's temperature and pressure conditions are important for better understanding the CO₂ trapping mechanism in aquifers. A high resolution X-ray CT scanner gives in situ images of CO₂ and brine distributions in porous sandstone. The images give important clues for clarifying mechanisms of CO₂ trapping in porous sandstones. In this article, we discuss mechanisms of CO₂/brine displacements revealed by employing a high-resolution medical CT scanner, and then in the succeeding article, we discuss changes of seismic velocity during the displacements. Considering the role of capillary pressure in porous sandstone and the inhomogeneity revealed in reservoir rocks by petrological and sedimentological analyses, the following points are important for the CO₂ migration and trapping in deep saline aquifers. 1. Locally-biased CO₂ flow paths appear during CO₂ injection in brine-saturated sandstone because the fine-scaled local fluctuation (about mm sizes) in pore-size distribution. 2. In simultaneous flow of CO₂ and brine, tapping and flow behaviors of CO₂ depend on the directional relationship between anisotropy in porosity distribution and flow direction. 3. The above phenomena arise from slight differences in capillary pressure accompanied by differences in pore size distributions. 4. Flow and trapping of CO₂ are governed by sizes of CO₂ clusters in pore spaces. 5. Sizes of CO₂ clusters in pores are different between the processes of CO₂ injection and brine reinjection. 6. Heterogeneity scales from pore size to geologic structure affect CO₂ flow behavior in the reservoir at CO₂ storage sites. 7. When fractures or large pore sized channels exist in a reservoir and its surrounding formations, capillary pressure has no effect on CO₂ flow.