Estimation of permeability anisotropy using seismic inversion for the CO₂ geological storage site of Sleipner (North Sea)

Abstract

Since 1996, more than 11 Mt of CO 2 have been injected into a deep saline aquifer, the Utsira Sand formation, at the Norwegian Sleipner field. An unexpected application of the extensive seismic monitoring program over this field leads to the estimation of the depth dependence of the permeability anisotropy (strength and direction). Time-lapse seismic monitoring is used to follow the displacement of the injected CO 2 , considered as a permeability tracer. The upper half of the Utsira sand formation exhibits large anisotropy, with a ratio f between the maximum and minimum horizontal permeabilities larger than six. In contrast, f can be as small as two in the lower half. The direction of the maximum horizontal permeability does not exhibit substantial depth dependence and lies between N18 E and N34 E. This is in agreement with previous authors who pointed out a clear CO 2 plume structure markedly elongated in the north-northeast-south-southwest direction. The small topographical trap at the top of the Utsira sand formation is a minor extrinsic cause of the measured permeability anisotropy, compared to the intrinsic effect of the formation permeability. This permeability information is crucial for reservoir simulation and for forecasting of the CO 2 plume expansion for different scenarios of injection.

Sensitivity of Seismic Attenuation and Phase Velocity to Intrinsic Background Anisotropy in Fractured Porous Rocks: A Numerical Study

“…That is, we assume that the intrinsic anisotropy of the background is produced by heterogeneities at the microscopic scale due to, for example, preferential alignment of minerals, fine layering of sedimentary materials of different stiffness, or stress‐induced anisotropy (e.g., Dürrast et al, ; Wang, ). Furthermore, as these heterogeneities can cause not only elastic anisotropy but also hydraulic anisotropy, we allow the background permeability to be anisotropic by considering a permeability tensor (e.g., Dubos‐Sallée & Rasolofosaon, ). The fractures, on the other hand, are in the mesoscopic scale range and are assumed to be composed of an isotropic and highly compliant poroelastic medium characterized by very high porosity and permeability.…”

“…The solid and relative fluid displacement fields resulting from each oscillatory relaxation test are obtained by solving the quasi‐static poroelastic equations (Biot, ) in the space‐frequency domain under corresponding boundary conditions where ω is the angular frequency, σ is the total stress tensor, p f the fluid pressure, w the average relative fluid displacement, and η the shear viscosity of the pore fluid. In order to account for the hydraulic anisotropy in the background, the permeability κ is defined as a symmetric second‐rank tensor with six independent coefficients (e.g., Biot, ; Dubos‐Sallée & Rasolofosaon, ) …”