Causes of underpressure in natural CO2 reservoirs and implications for geological storage

Abstract

Geological carbon storage has the potential to reduce anthropogenic carbon dioxide emissions, if large volumes can be injected and securely retained. Storage capacity is limited by regional pressure buildup in the subsurface. However, natural CO2 reservoirs in the United States are commonly underpressured, suggesting that natural processes reduce the pressure buildup over time and increase storage security. To identify these processes, we studied Bravo Dome natural CO2 reservoir (New Mexico, USA), where the gas pressure is up to 6.4 MPa below the hydrostatic pressure, i.e., less than 30% of the expected pressure. Here, we show that the dissolution of CO2 into the brine reduces the pressure by 1.02 ± 0.08 MPa, because Bravo Dome is isolated from the ambient hydrologic system. This challenges the assumption that the successful long-term storage of CO2 is limited to open geological formations. We also show that the formation containing the reservoir was already 2.85 ± 2.02 MPa underpressured before CO2 emplacement. This is likely due to the overlying evaporite layer, which prevents recharge. Similar underpressured formations below regional evaporites are widespread in the midcontinent of the United States. This suggests the existence of significant storage capacities with properties similar to Bravo Dome, which has contained large volumes of CO2 over millennial time scales.

On Moderate-Rayleigh-Number Convection in an Inclined Porous Layer

“…In deep geological formations the layer may not be strictly horizontal; for example, in carbon sequestration the saline aquifers are generally inclined at an angle to the horizontal [18][19][20][21].…”

Convective carbon dioxide dissolution in a closed porous medium at low pressure

“…Closed sites are typically fault bounded and do not allow compensation of pressure changes by brine migration. Therefore, the volume of CO 2 vapour in a closed system remains essentially constant over time and consequently CO 2 dissolution reduces the pressure in the vapour phase (Akhbari & Hesse 2017). This leads to a decline of the aqueous CO 2 concentration beneath the gas-water contact and therefore reduces the density difference driving convective dissolution.…”

“…Here we show that the pressure drop in the gas can limit CO 2 dissolution long before saturation of the brine becomes a limiting factor. These negative feedbacks in closed systems are common in experiments on CO 2 dissolution (Farajzadeh et al 2009;Moghaddam et al 2012;Mojtaba et al 2014;Shi et al 2017) and in some natural CO 2 reservoirs that serve as analogs for geological CO 2 storage (Akhbari & Hesse 2017). Engineered geological storage sites are typically selected such that CO 2 is supercritical to maximise the storage capacity (Orr 2009).…”

Multiphase flow and underpressured shale at the Bruce nuclear site, Ontario, Canada

“…Likewise, the simulations included only pure water and methane gas, but the salinity of brine at the site (Nuclear Waste Management Organization 2011) could have significant effects on gas solubility (Duan & Mao 2006). Further analyses of the effects of brine-relevant gas solubility on multiphase methane and pressure at the site would therefore also be beneficial (Akhbari & Hesse 2017). Due to the common co-occurrence of gas phase and underpressures in other sedimentary basins around the world (Vinard 1988;Corbet & Bethke 1992;Law & Spencer 1998), examples of which can be found in both glaciated (Masters 1979) and nonglaciated (Davis 1984) regions, improved understanding of the Bruce site may inform efforts to understand those systems as well.…”