Seismic Monitoring of Gas Floods in Carbonate Reservoirs—Hirsche et al
From 1987 to 1990 the Alberta Research Council and Western Atlas conducted a major research program to assess the feasibility of monitoring hydrocarbon miscible floods in carbonate reservoirs with time-lapse seismic techniques. The results of this study, based on ultrasonic velocity measurements, indicated that saturation-induced velocity contrasts are significantly larger than Gassmann predictions in the majority of carbonate reservoir types. Seismic modelling, based on these results, suggested that injected gas should be seismically detectable in many carbonate reservoirs. While these results are encouraging there are major uncertainties in relating the velocity changes observed at the high frequency laboratory scale to the low frequency seismic scale. We have recently conducted several theoretical and experimental investigations to relate the velocity changes at core scale to the seismic scale. These newer findings tend to suggest that it should be possible to track the movement of injected gas in many carbonate fields using time-lapse seismic methods. This conclusion was indirectly supported by a detailed modelling study and a 2D test line acquired over a solvent injection well during 1987. Continued injection during the past 10 years has increased the thickness of the gas bank from 10 m. to 30 m. This test line has been recently recorded again, in March 1997, to test these concepts.