Solubility trapping in formation water as dominant CO2 sink in natural gas fields

Abstract

Injecting CO(2) into deep geological strata is proposed as a safe and economically favourable means of storing CO(2) captured from industrial point sources. It is difficult, however, to assess the long-term consequences of CO(2) flooding in the subsurface from decadal observations of existing disposal sites. Both the site design and long-term safety modelling critically depend on how and where CO(2) will be stored in the site over its lifetime. Within a geological storage site, the injected CO(2) can dissolve in solution or precipitate as carbonate minerals. Here we identify and quantify the principal mechanism of CO(2) fluid phase removal in nine natural gas fields in North America, China and Europe, using noble gas and carbon isotope tracers. The natural gas fields investigated in our study are dominated by a CO(2) phase and provide a natural analogue for assessing the geological storage of anthropogenic CO(2) over millennial timescales. We find that in seven gas fields with siliciclastic or carbonate-dominated reservoir lithologies, dissolution in formation water at a pH of 5-5.8 is the sole major sink for CO(2). In two fields with siliciclastic reservoir lithologies, some CO(2) loss through precipitation as carbonate minerals cannot be ruled out, but can account for a maximum of 18 per cent of the loss of emplaced CO(2). In view of our findings that geological mineral fixation is a minor CO(2) trapping mechanism in natural gas fields, we suggest that long-term anthropogenic CO(2) storage models in similar geological systems should focus on the potential mobility of CO(2) dissolved in water.

Geologic carbon storage is unlikely to trigger large earthquakes and reactivate faults through which CO 2 could leak

“…The high solubility of CO 2 makes dissolution one of the main trapping mechanisms in the long term. For instance, it has been observed in carbonate-dominated reservoirs containing naturally occurring CO 2 that up to 90% of this CO 2 can dissolve at the millennial timescale (the remaining 10% would be trapped in precipitated minerals) (60). CO 2 dissolution also operates over relatively short timescales and provides a significant storage capacity (61,62).…”

Convective shutdown in a porous medium at high Rayleigh number

“…One of the primary mechanisms for longer-term storage is the dissolution of CO 2 into the underlying ambient brine (Orr 2009), which forms a dense solute that can lead to downward convection. Geochemical field observations in natural CO 2 reservoirs suggest that convective dissolution provides a very significant and persistent mechanism for the transport of CO 2 (Gilfillan et al 2009). …”

Modeling mass transfer of CO2 in brine at high pressures by chemical potential gradient

“…The sequestration capacity, long-term CO 2 behavior in receptor formations, and the quantification of possible CO 2 leaks are the main concerns [2,4,9,10], and there remains a need to study the potential mobility of CO 2 dissolved in brines over a wide range of spatial and temporal scales [4,9,11], the CO 2 concentration distribution in saline aquifers, as well as the density distribution in geological media [8]. This highlights the significance of research on CO 2 dissolution and the mass transfer of CO 2 dissolved in brines.…”