- Copyright © 2002 Society of Exploration Geophysicists
It is widely believed that gas dissolved in water or a few percent of a separate gas phase in water can make the pore fluid mixture very compressible. The fluid bulk modulus (K), the inverse of compressibility, would drop significantly and, in turn, P-wave velocity and impedance will decrease. This suggests that seismic techniques (e.g., DHI and AVO) cannot distinguish a water zone with small amounts of gas either dissolved or as a free phase in water from economic gas reservoirs.
In exploration, the tendency is to consider natural gases as extremely light fluids with negligible modulus. Figure 1 shows, under such an assumption, the effect of gas saturation on P-wave velocity of rocks (calculated using the Gassmann equation). With a low gas modulus of 0.01 Gpa (still more than 70 times higher than air modulus at room conditions), gas saturation of a few percent has an effect on P-wave velocity that is similar to that of full gas saturation. Data suggest that low gas saturation can generate similar seismic attributes but with false hydrocarbon indicators that are similar to those of economic gas reservoirs. Consequently, many dry holes drilled based on false hydrocarbon indicators have been attributed to this condition—widely known as the “fizz-water” effect.
Unfortunately, the fizz water concept has not been rigorously defined and examined although, as stated earlier, it is widely accepted among geophysicists. It has become a standard scapegoat for almost all failures of DHI or AVO applications including deepwater reservoirs. And, because the ill-defined fizz-water concept looks so logical, it may actually prevent efforts to find the real cause or develop new techniques for seismic evaluation of hydrocarbon saturation.
Deepwater reservoirs are often undercompacted and saturated with overpressured fluids, …