Migration of CO2 through Carbonate Cores: Effect of Salinity, Pressure, and Cyclic Brine-CO2 Injection

Abstract

Geo-sequestration of carbon dioxide (CO2) in saline formations is one of the feasible options for reducing the concentration of carbon in the atmosphere. This process involves the capturing of CO2 from emission sources followed by its compression and then injecting the compressed CO2 in deep geological formations for its long-term storage. The suitable geological formations for the sequestration of CO2 are normally comprised of sandstones, shales, coal beds, and carbonates. Amongst these, carbonates are the highly reactive formations of a hydrophobic nature and have complex pore structures, and hence, CO2 sequestration and its subsequent evolution in carbonates require a thorough investigation. Further, planning CO2 geo-sequestration in carbonate formations requires assessment of CO2 behavior under high salinity and pressure conditions as these factors play a major role in retaining injected CO2 safely for a long geological time period. Thus, exploring multiphase CO2-brine migration processes in carbonates using practical experiments is of great significance for estimating the storage capacity of potential geo-sequestration sites and for ensuring their storage security. The multiphase characteristic of saline carbonate formations can also get affected during their flooding with supercritical CO2. Hence, the aim of this study was to investigate the movement of CO2/brine through different saline carbonate cores under representative reservoir conditions. The laboratory-scale CO2 core flooding experiments were conducted on two different cores of carbonate formations to estimate the effect of salinity and injection pressure on CO2 migration along with the influence of cyclic brine-CO2 flooding on characteristics of the considered cores. A series of practical experiments were performed considering 3% and 7.5% levels of salinity under two different injection pressures of 8 and 10 MPa for evaluating the effects of different hydrogeological parameters on the multiphase flow behavior and sequestration capacity of the carbonate formations. For this, brine and supercritical CO2 were injected through Edward white and Edward yellow carbonate cores to obtain the changes in pressure drop across the cores with time. The results show that under high salinity conditions, pressure drop in Edward white and yellow carbonate cores are 2 and 0.3 MPa, respectively, while in the case of low salinity, 1.5 and 0.2 MPa of pressure drop was observed in the selected cores. A high differential pressure (DP) trend was observed using a 10 MPa injection pressure, while a low DP trend was recorded for the 8 MPa injection pressure. An increment in pressure drop across the cores with consecutive injection cycles of brine and CO2 clearly indicates some pores clogging in cores due to the reactive nature of the selected carbonate samples. Thus, the results of this study provide a better understanding of changes that occur at CO2-brine-carbonate interfaces under reservoir like conditions of a typical site, which can help in planning the geo-sequestration of CO2 in carbonate saline formations.