CO 2 Reaction-Diffusion Experiments in Shales and Carbonates.

The evaluation of caprock integrity and reservoir efficiency is critical for safe CO₂ geological storage management. It is therefore important to investigate geochemical reactions between CO₂-rich fluids and host rocks and their contribution in retaining CO₂ at depth. This study deals with diffusive reaction experiments on shales and carbonate samples cored from an offshore structure in the Malaysian basin, a potential target for CO₂-enhanced gas recovery. The aim is to evaluate the CO₂ reaction front velocity in a typical shaly caprock and the mineral response of the reservoir. Rock samples were characterized in terms of texture, chemistry, and mineralogy by X-ray diffraction, electron microscopy (SEM), microanalysis (EDS), infrared spectroscopy (FT-IR), rock geochemistry (XRF), and mercury injection capillary permeability (MICP). Performed analyses show mineralogical alteration induced by CO₂ as it penetrated into the samples. Carbonate dissolution and weathering of pyrite to form secondary carbonates belonging to siderite-ankerite series were observed along two reaction fronts. Estimated diffusion coefficients of CO₂ are two orders of magnitude lower than CO_2(aq) molecular diffusion in pure water and from half to an order of magnitude lower than diffusivity computed on unaltered sample, highlighting the important effect of gas–water–rock reactions on the CO_2(aq) diffusivities in shales and carbonates. Results obtained in this study provide an insight regarding the effect of geochemical reactions on CO₂ transport and represent a further discussion point on the diffusion coefficients. [ABSTRACT FROM AUTHOR]