184R.ARTSErAZ,Fig. 4. Result of a reservoir simulation at the time of the first time-lapse seismic survey in October 1999 (after 3 years of injection) after 2.28 Mt of CO2 injected (van der Meer et al 2000).of a porous medium saturated with a fluid to be derived from the known properties of the same medium saturated with a different fluid. As a basic assumption in this analysis homogeneous mixtures of brine and CO2 with respect to the seismic wavelength are assumed. In reality this condition is likely to be fulfilled only approximately. The densities and compressibilities of the saturating fluids, the rock matrix and the porosity of the rock are assumed to be known. In the Sleipner case the 100% water-saturated Pand S-wave velocities are known from well logs. The main constituent of the rock matrix is quartz with a known density and compressibility. The sand porosity, and the densities and compressibilities of the water and of the CO2 are also known. From this information, the Gassman relationships can be used to calculate the elastic velocities (P- and S-wave) and the density of the rock, saturated with a CO2-water mixture, for a range of saturation states. Because of the uncertainty on the bulk modulus of the CO2 under reservoir conditions (P), velocities have been calculated for three values within extreme limits (Fig. 6). For values of bulk modulus ^T co < 0.0675 GPa velocities are fairly constant for CO2-saturations over 20%. For higher bulk moduli, the elastic properties of CO2 more closely resemble those of the replaced water and the effect on seismic velocity decreases. In general, even for small CO2 saturations, the drop in P-wave velocity is considerable (about30%). This can be explained by the fact that only a few small 'bubbles' of CO2 have a dramatic effect on the overall compressibility of the saturated rock. Therefore a sharp decrease in the P-wave velocity can be observed for small CO2 concentrations. The influence of the CO2 on the S-wave velocities is minimal because shear waves are not sensitive to the saturating fluids. The minor variation that is observed is due to the variation in the bulk density (CO2 is less dense than water).Pressure effectDuring the injection process at Sleipner, until 2001 no significant increase in well head pressure has been observed at the injection well (not exceeding the unsystematic data scatter of about 0.2MPa), the CO2 flowing easily into the very high permeability reservoir. The pressure-temperature conditions of the reservoir around the CO2 plume are such that the CO2 is expected to remain in a supercritical state. Taking this into consideration, the pressure effect on the seismic velocities is expected to be marginal.Wavelet determination and synthetic modellingIn order to perform seismic modelling, a wavelet was estimated from the seismic data. The frequency
