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We present an evaluation of the time lapse critical reflection method from the point of view of sensitivity and uncertainty analysis. The purpose of this analysis is to establish the link between changes in the p-wave velocity in the reservoir, due to fluid substitution, and the critical distance (XC), a dynamic parameter. Two static parameters of the overburden are considered in the analysis: its thickness and its effective or equivalent P-wave velocity. Stochastic uncertainty analysis by means of Monte Carlo simulations was carried out to determine the sensitivity of XC to velocity contrast between that of the reservoir and the corresponding one to the overburden (incident medium). Results show that the effect of the velocity values in each medium, overburden and reservoir (reflecting medium), on estimates of XC depends on the velocity contrast between the two media. It turns out that the greater the velocity difference between the two media, the greater the effects associated with the reservoir P-wave velocity. On the other hand, the dependence of XC on overburden velocity (Vrms of the incident medium) is just the opposite. From the inversion viewpoint, predicting changes of the reservoir P-wave velocity from changes in critical distance is strongly dependent of the uncertainty in the initial estimate of the reservoir P-wave velocity, followed by degree of importance by the uncertainty in the overburden velocity. Other sources of uncertainty in this analysis turned out to be negligible. Geomechanical effects have not been accounted for in this analysis.

Time-lapse critical reflection: does it really work in seismic monitoring of low porosity and high effective stress conditions?