SR09_FGS07.qxd
25.05.2007
14:43 Uhr
Seite 9
represents the mean effective stress and exponent h, the Hertz coefficient, is equal to of 1/6 in the case of a stack of spherical grains, as stated in the Hertz-Mindlin theory (Mindlin, 1949). Laboratory measurements performed on core samples yielded lower values of h for real rocks both for P- and S-waves (Rasolofosaon et al, 2003). In the case of the underground gas storage of CĂŠrĂŠ-la-Ronde, central France, where the water-bearing sandstone reservoir lies at some 900 m depth, the measured values of the Hertz coefficients were respectively 0.13 for Swaves and 0.09 for P-waves. The time shifts between the end of the withdrawal period and the end of the injection period are of the order of one millisecond. A feasibility study based on Gassmann's formulation for the fluid substitution effect and on the measured HertzMindlin coefficients for stress effects concluded that both effects have a comparable influence on the two-way travel times of seismic reflections generated beneath the reservoir during the injection period, namely, about 0.5 ms for the stress effect for a pressure variation of 4 MPa, and 0.75 ms for the substitution effect (Vidal et al, 2001). Both effects contribute to velocity variations in the same direction within the reservoir. Indeed, the pore pressure increases with gas injection, thereby decreasing the effective stress, which in turn contributes to an additional decrease in velocity. However, it should be noted that the stress effect can extend beyond the limits of the reservoir. The time picking accuracy for reservoirs located at depths less than 1000 meters can be as low as 0.2 ms for carefully acquired and processed seismic data.
Carbon dioxide specificities From a geophysical point of view, carbon dioxide differs from methane essentially by its density, in particular when CO2 is in supercritical state at depths greater than 700 or 800
meters. In this case, the density can reach 600 kg/m3 or more, so that the density difference between gas and brine is reduced to about 400 kg/m3, that is, less than half the difference between the densities of methane (~ 100 kg/ m3 at reservoir conditions) and water. As a result, P-wave velocities are less sensitive to CO2 substitution and consequently, the estimation of the saturation will be more difficult for CO2 than for methane. Pressure effects will be comparable for storage in similar reservoirs. Methane is stored in stratigraphic traps. In depleted structures or in low permeability reservoirs, stress effects might have the largest influence on seismic parameters. However, when carbon dioxide is injected into flat or monocline aquifers of high permeability, the injected gas can freely move away from the injection point, and the pore pressure will not change very much, except close to the injection wells. In this case, no additional stress effect will be expected. The above discussion emphasizes the fact that the estimation of stored volumes of CO2 can represent a formidable challenge. One of the key requirements to infer reliable gas saturation estimates from time-lapse seismic data is the accuracy of time measurements.
Permanent source and receivers In order to monitor the time variations with sufficient accuracy, permanent data acquisition systems composed of a low-energy source and a vertical receiver array have been developed and tested. The typical layout consists of a seismic source installed in a vault or cemented in a shallow borehole, and a series of sensors deployed in a nearby well at depths ranging from of a few tens of meters to several hundred meters (Meunier et al, 2001). This configuration has been used to automatically record about ten Vertical Seismic Profiles (VSPs) per day over an underground gas storage. The VSPs were processed to measure time
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