Rock physics modeling to monitor movement of CO2in Sleipner gas field, North Sea: An ideal CCS field
Sleipner gas field in the North Sea is the world’s first industrial scale CO2 injection project designed specifically to reduce the emission of greenhouse gas. Here CO2 separated from natural gas produced at Sleipner gas field is injected into the Utsira sand, which is a major saline aquifer in the North Sea basin. In time-lapse threedimensional seismic data (4D), CO2 plume is imaged as a number of bright sub-horizontal reflections within the reservoir. Correlation of log data with the seismic data indicates that CO2 accumulates within a series of interbedded sandstones and mudstones beneath a thick cap rock of mudstone. Nine reflective horizons have been mapped within the reservoir on the six seismic surveys from 1999 to 2008. Comparison with the baseline seismic survey of 1994 (pre-injection) provides clear impression of the migration of CO2 plume. In this paper, we attempt to model CO2 distribution quantitatively within the reservoir by applying a pressure-dependent differential effective medium (PDEM) theory using 4D seismic data. Preand post-injection acoustic impedances are calculated by inverting respectively post-stack seismic data of 1994 and 2001 using a model-based inversion technique. 3D CO2 saturation volume is estimated using PDEM theory from inverted acoustic impedance of the year 2001 taking the reference of that from the results of pre-injection data of the year 1994. Since the gas distribution type is seldom known, we estimate the saturation distribution using both a homogeneous and a patchy distribution pattern in our rock physics model. We estimate saturation for homogeneous distribution of CO2 to be 0-20% and for CO2 as patches of gas as 0-80% of the total porosity within ~200 m thick reservoir unit. 5-7% uncertainty in the predicted CO2 saturation is estimated using a Monte-Carlo simulation technique. Our results indicate that a large amount of CO2 is accumulated as patches of gas within sand layers capped by mud layers, though some amount of gas may have dissolved uniformly with water.