Carbonates have become important targets for rock property research in recent years because they represent many of the major oil and gas reservoirs in the world. Some are undergoing enhanced oil recovery. Most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones, but applying relations developed for sandstones to carbonates is problematic, at best. We measured in the laboratory nine carbonate samples from the same reservoir at seismic ͑3 to 3000 Hz͒ and ultrasonic ͑0.8 MHz͒ frequencies. Samples were measured dry ͑humid-ified͒, and saturated with liquid butane and brine. Our carbonate samples showed typical changes in moduli as a function of porosity and fluid saturation. However, we explored the applicability of Gassmann's theory on limestone and dolomite rocks in the context of shear and bulk modulus dispersion, and Gassmann's theory assumptions. For our carbonate set, at high differential pressures and seismic frequencies, the bulk modulus of rocks with high aspect ratio pores and dolomite mineralogy is predicted by Gassmann's relation. We also explored in detail some of the assumptions of Gassmann's relation, especially rock-frame sensitivity to fluid saturation. Our carbonate samples showed rock shear-modulus change from dry to brine saturation conditions, and we investigated several rock-fluid mechanisms responsible for this change. To our knowledge, these are the first controlled laboratory experiments on carbonates in the seismic frequency range.

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Statoil has injected CO2 into a saline aquifer for disposal. Monitoring the behaviour of the CO2 in the sand formation and the sealing capacity of the overlying shale cap rock are key elements in understanding the dynamics of the injection process. A repeated 3D seismic dataset was therefore acquired in 1999, after injection of about 2 million metric tons of CO2. The time-lapse data show a large increase in reflectivity and a large push-down of reflections caused by the injected CO2. Gas at different levels within the sand are probably trapped by thin shale layers. Only a small part has reached the top of Utsira Fm., and no signs of CO2 are observed above the top seal.

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