Solubility Trapping In Formation Water As Dominant Co_2 Sink In Natural Gas Fields

摘要

Injecting CO_2 into deep geological strata is proposed as a safe and economically favourable means of storing CO_2 captured from industrial point sources1"3. It is difficult, however, to assess the long-term consequences of CO_2 flooding in the subsurface from decadal observations of existing disposal sites1'2. 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.