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Differences between flow of injected CO 2 and hydrocarbon migration

2010; Zoological Society of London; Volume: 7; Issue: 1 Linguagem: Inglês

10.1144/0071183

2047-9921

Christian Hermanrud, Gunn M. G. Teige, Martin Iding, Ola Eiken, Lars Rennan, S. Østmo,

Geological and Geochemical Analysis

Abstract Knowledge of fluid flow processes in the subsurface is important for CO 2 storage operations as well as for hydrocarbon exploration. Repeated seismic surveys for more than 10 years of CO 2 injection into the Utsira Formation, in the Sleipner area, offer a unique dataset. This dataset holds information on fluid migration processes that can be analysed for the benefit of hydrocarbon exploration and CO 2 storage considerations alike. Thorough analyses of these datasets reveal several features that give useful information of subsurface fluid flow processes. The CO 2 in the Utsira Formation has flowed laterally beneath thin, intra-formational shales. At the same time, CO 2 has flowed vertically through shaly horizons that would normally be considered as barriers to fluid flow. This flow has apparently taken place through vertically stacked flow conduits through the shales. These conduits may to some extent have existed prior to the start of CO 2 injection, but may also have been augmented by the CO 2 injection process. The calculated pushdown of seismic reflectors below the CO 2 plume is less than that observed, which may point to the presence of hitherto unrecognized flow paths for the CO 2 . Hydrocarbon migration pathways are in general not recognizable in seismic data. This implies that such avenues are significantly thinner than those of the CO 2 migration in the Utsira Formation. This result points to the presence of mixed-wet migration pathways, in which capillary flow resistance may not control the (sub-horizontal) flow path thickness. A circular depression at the top of the Utsira Formation that existed prior to the injection may be interpreted as a result of a deeper seated sand remobilization feature. Such features will also promote vertical hydrocarbon migration where they are present. A more widespread occurrence of such features may explain why hydrocarbons are generally found beneath thick shales, but are less likely to be found below thin intra-formational shales below the structural spillpoint of the top seal. These observations suggest that seal thickness is an important parameter, even if the capillary entry pressure of the sealing rock is sufficiently high to preserve significant hydrocarbon columns.