Evaluation of Heterogeneity Effect from Distribution of Clast-Matrix on CO2 Transport in Conglomerate Cores

Geologic heterogeneity is known to significantly affect fluid flow in rocks. In this study, we performed numerical simulations to investigate the heterogeneity effect from the spatial distribution of clast-matrix in conglomerates on CO2 transport under the reservoir conditions. Natural conglomerate cores drilled from candidate formation for geological CO2 storage in South Korea were numerically analyzed as well as regularly distributed clast-matrix cores. Throughout the study, CO2 distribution, velocity of the CO2 front, breakthrough curves (BTCs), and differential pressure (∆P) were assessed to highlight the heterogeneity effect.

In the regularly distributed clast-matrix cores, the CO2 front with corrugate shape migrated faster than the matrix-only core. Even though the clast ratio was changed more than 10 %, there was not much difference in the velocity of the CO2 front (~2.0×10-4 m/min). Nevertheless, there was a correlation between the clast ratio and the velocity, which was clearly shown in the L-5 core having alternating layers of matrix and clasts distributed in square pattern.

At natural conglomerate cores (JC-1 and JC-2), the migration pattern of the CO2 significantly differed over time due to irregular distribution of clasts and matrix. However, the clast-matrix ratio and the velocity of the CO2 front were strongly correlated with a large variation in the velocity (~2.6×10-3 m/min) despite the small change of the clast-matrix ratio. The variance of the cumulative velocity of CO2 front 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 became greater where the clast ratio increased more.

Finally, differential pressure (∆P) was significantly affected by the distribution of clasts and matrix; ∆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 ∆P was shown in the cores having the lower clast ratio, implied that the heterogeneous distribution of clast-matrix could influence a degree of overpressure generation.