Measuring pressure performance of a large saline aquifer during industrial-scale CO 2 injection: The Utsira Sand, Norwegian North Sea

The Sleipner injection project has stored around 14Mt of CO 2 in the Utsira Sand and provides a unique opportunity to monitor the pressure response of a large saline aquifer to industrial-scale CO 2 injection. There is no downhole pressure monitoring at Sleipner, but the 4D seismic programme provides an opportunity to test whether reliable indications of pressure change can be obtained from time-lapse seismic. Velocity-stress relationships for sandstones, calibrated against measured data from the Utsira Sand, indicate that pore pressure increases of <1MPa should produce measurable travel-time increases through the reservoir. Time-lapse datasets were used to assess travel-time changes by accurately mapping the top and base of the reservoir on successive repeat surveys outside of the plume saturation footprint. Measured time-shifts are of the order of a very few milliseconds, with significant scatter about a mean value due to travel-time ‘jitter’. The ‘jitter’ is due to non-perfect repeatability of the time-lapse surveys and shows a Gaussian distribution providing a useful statistical tool for determining the mean. Observed mean time-shifts through the Utsira Sand were 0.097ms in 2001 and 0.175ms in 2006. These correspond to mean pressure increases of less than 0.1MPa. An idealised noise-free reservoir ‘impulse response’ was computed, taking into account lateral reservoir thickness variation. Convolving this with the repeatability noise distribution gives a realistic predicted reservoir response. Comparing this with the observed time-shifts again indicates a pressure increase less than 0.1MPa. Flow simulations indicate that pressure increases should range from <0.1MPa for an uncompartmentalised reservoir to >1MPa if strong flow barriers are present, so the results are consistent with the Utsira reservoir having wide lateral hydraulic connectivity.