A numerical experiment for geological sequestration of CO 2 in the pre-Comond? sandstones of Las Tres Virgenes geothermal area, Baja California Sur, Mexico

The most recent studies on geological sequestration of CO2 in Mexico point towards high sequestration potential associated with geologically stable zones and high-emission centres. These studies overlook however, the potencial that high heat flow areas in the country can offer to geological sequestration of CO2 by mineral precipitation. Here we present an assessment of the CO2 sequestration capacity of the Pre-Comondu sandstones, that overlie the productive granitic basement of the Las Tres Virgenes geothermal field (10 MWe). Both, the physical properties and mineralogical description of the sandstone are defined from drilling core analyses reported by the Federal Commission of Electricity. The initial formation water saturating the sandstone is determined by means of a batch simulation of equilibrium between the primary minerals in the rock and a 1 M solution of NaCl in water. Injection of supercritical CO2 is then numerically simulated. Simulations are based on the TOUGHREACT software and the equation of State Module ECO2N. Our results show Pre-Comondu sandstone favour mineralization of CO2 through the precipitation of carbonates in moderate geologic times (100-300 yr). Mineral changes occur within two main zones in the CO2-impacted reservoir, the zone lying close to the injection point is dominated by precipitation of low-albite, kaolinite, calcite, and minor siderite, whereas the distal zone responds with moderate changes in the system illite-smectite. Changes in porosity are moderate and consistent with the two mineralization zones. Furthermore, the decline in porosity occurs in the long term, which suggests moderate risk for injection operations. Although high reservoir temperature is unfavorable to CO2 solubility, high pressure can counteract this effect, so the system can benefit from higher mass diffusivity and reactivity. This manifests in an accelerated sequestration rate.

Automated summary

The latest studies on CO2 geological sequestration in Mexico suggest that areas with geological stability and high emission centers have high sequestration potential. However, these studies overlook the potential of high heat flow areas in Mexico for CO2 geological sequestration through mineral precipitation. In this study, the CO2 sequestration capacity of Pre-Comondu sandstones in the Las Tres Virgenes geothermal field was assessed. The results show that the sandstone is favorable for CO2 mineralization through carbonate precipitation in moderate geologic times. The mineral changes occur in two zones, with the zone close to the injection point dominated by the precipitation of low-albite, kaolinite, calcite, and minor siderite. The decline in porosity is moderate and occurs in the long term, suggesting a moderate risk for injection operations. The high pressure in the system counteracts the unfavorable effect of high reservoir temperature, resulting in an accelerated sequestration rate.