CO2 saturation and thickness predictions in the Tubåen Fm., Snøhvit field, from analytical solution and time-lapse seismic data

Abstract

a b s t r a c t CO 2 migration in a saline aquifer is governed by viscous, capillary and gravitational fluid forces at an early stage of injection, where the dominant flow regime is site specific and controls the fluid migration in the pore space. This study combines the CO 2 saturation inverted from time-lapse seismic methods with an analytical expression to define the CO 2 flow regime, saturation distribution and layer thickness in the Tubåen Fm. following CO 2 injection. Quantitative estimates of the CO 2 saturation from time-lapse seismic amplitude versus offset (AVO) and spectral decomposition are compared to a viscous dominated analytical expression of CO 2 injection into a saline aquifer. The spatial extent of the CO 2 plume obtained from time-lapse spectral decomposition and inverted from time-lapse AVO analysis display good agreement with the analytical expression. The CO 2 is limited to an area close to the injection well, with an elongated shape in the channel direction. Comparison between the time-lapse seismic and analytical expression shows that the fluid flow is dominated by viscous forces. CO 2 saturation within the plume is constant and close to the residual brine saturation. The influence of gravity is ignorable on the reservoir CO 2 flow. CO 2 fills the entire sandstone unit up to approximately 50 m away from the injection before the CO 2 layer thickness is reduced to a thin wedge that propagates below the overlying shale unit. Reduction in CO 2 saturation away from the injection well is a reduction in effective CO 2 saturation relative to the thickness of the horizon. The maximum radius of the CO 2 layer from the analytic expression is 750 m, of which 400 m is above the time-lapse noise level. Time-lapse seismic analysis reveals the CO 2 layer radius is 405 m in the direction of the local fluvial channel and 273 m in the perpendicular direction.

Volumetric bounds on subsurface fluid substitution using 4D seismic time shifts with an application at Sleipner, North Sea

“…Because of this, it is common practice to include supplementary information in time-shift analyses. Apart from the examples mentioned above, such information could for instance be geophysical logs and depth dependent fluid properties (White, 2013), repeated logs and 4D reflection amplitudes (Ivanova et al, 2012), or analytical fluid migration solutions (Grude et al, 2014).…”

Saturation scale effect on time‐lapse seismic signatures

“…) can be used to simulate the non‐uniqueness of the VSR (e.g., Grude et al . ). However, by doing so, the aspect of the saturation scale is entirely neglected.…”

“…The saturation scale renders a frequency-dependent saturated bulk modulus K sat (ω), hence describing the P-wave dispersion and attenuation. We note that Brie's empirical model (Brie et al 1995) can be used to simulate the nonuniqueness of the VSR (e.g., Grude et al 2014). However, by doing so, the aspect of the saturation scale is entirely neglected.…”

“…In this approach, however, the saturation scale change is neglected and the variability of the VSR is sometimes empirically modelled (Grude et al . ). Moreover, as this is an effective elastic medium approach, signatures like time‐lapse attenuation (Blanchard and Delommot ) cannot be analysed.…”

“…Landrø's approach assumes that the elastic properties associated with saturation change are given by Gassmann's equation, whereas pressure changes are governed by the effective stress behaviour typically obtained from ultrasonic core measurements. In this approach, however, the saturation scale change is neglected and the variability of the VSR is sometimes empirically modelled (Grude et al 2014). Moreover, as this is an effective elastic medium approach, signatures like time-lapse attenuation (Blanchard and Delommot 2015) cannot be analysed.…”