Assessment of carbon dioxide plume behaviour within the storage reservoir and the lower caprock around the KB-502 injection well at In Salah

Abstract

Surface uplift has been detected over all three of the In Salah CO2 injection wells with corresponding subsidence also observed over the gas production area. The distinctive two-lobed uplift pattern over KB-502 has attracted much research interest, as it suggests the tensile opening of a structural discontinuity at depth. The latest reservoir simulation effort at Imperial College involved the implementation of a non-sealing fault (zone) with dynamic transmissibility in the revised reservoir/overburden model and adopted a holistic approach to matching of the estimated flowing bottomhole pressure (FBHP) at KB-502. Based upon the current knowledge of the Krechba stress field and time-elapse InSAR surface uplift images, tensile opening of a pre-existing fault (zone) aligned with the maximum horizontal stress direction and its subsequent growth was analysed with reference to CO2 injection FBHP at KB-502. The history matching results, in terms of the dynamic behaviour of the fault (zone) transmissibility, are found to be consistent with the stress analysis and the field observations.A new interpretation is proposed for the two-lobed surface uplift pattern observed over KB-502. It is postulated that, prior to March 2006 when the injection pressure peaked, the two-lobed surface response was caused primarily by CO2 injection-induced tensile opening of a non-sealing fault zone and its subsequent confined growth (lateral propagation and widening) within the C10 formation. The increasingly pronounced surface uplift pattern observed after March 2006 (against the backdrop of a steady decline in the FBHP), is predominantly a result of localised CO2 migration into and pressurisation of a fracture (or fault damage) zone in the lower caprock by elevated injection pressure, at the top of the tight sandstone formation (C10.3), which is overlying the main storage reservoir (C10.2).