Evaluation of heterogeneity effect on CO2 transport in regularly distributed and natural clast-matrix conglomerate cores

Abstract

A series of numerical simulations were performed to investigate the heterogeneity effect resulting from spatial distribution of clast-matrix in conglomerates on CO2 migration under the reservoir conditions. Natural conglomerate cores drilled from geological CO2 storage site in South Korea as well as constructed cores comprising regularly distributed clast-matrix were numerically analyzed. Throughout the study, CO2 distribution, velocity of the CO2 front, breakthrough curves (BTCs), differential pressure (Delta P), gravity number (Ngv), and capillary number (Nc) were assessed to highlight the heterogeneity effect. In the regularly distributed clastmatrix cores, the CO2 plume with the corrugated shape of the front migrated faster than the matrix-only core. Even though the clast ratio was changed more than 10 %, there was not much difference in the arrival velocity of the CO2 plume (-2.0 x 10-4 m/min). Nevertheless, there was a correlation between the clast ratio and the arrival velocity, clearly shown in the L-5 core having alternating layers of matrix and clasts. At natural conglomerate cores (JC-1 and JC-2), the migration pattern of the CO2 plume significantly differed. However, the clast-matrix ratio and the arrival velocity were strongly correlated with a large variation in the arrival velocity (-2.6 x 10-3 m/min) despite the small change of the clast-matrix ratio. The variance of the cumulative arrival velocity was correlated with the clast-matrix ratio and the irregularity of the BTCs, which implied that the instability in the frontal displacement of the CO2 plume became greater where the clast ratio increased more. The Delta P was significantly affected by the distribution of clasts and matrix; Delta P increased as the CO2 passed narrow or heterogeneous pathways enclosed by the clasts and decreased as CO2 passed wide or diverging pathways. Interestingly, the highest Delta P was shown in the cores having the smallest clast ratio, implied that the heterogeneous distribution of clast-matrix could influence a degree of overpressure generation. The lowest Ngv below 1 and highest Ngv appear in JC-2 where the total clast ratio is more than 50 % and in H-1 consisting of matrix-only, respectively. The obtained ranges of Nc show that all the models are under capillary-dominated flow conditions relevant for CO2 sequestration. The magnitude of both Ngv and Nc correlates to the total clast ratio.