Time‐lapse critical reflection: Sensitivity and uncertainty analysis

Abstract

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 offset xc (a dynamic parameter). Two static parameters of the overburden are considered in the analysis: its thickness and its velocity (vRMS). Stochastic uncertainty analysis by means of Monte Carlo simulations was carried out to determine the sensitivity of xc to velocity contrast. Results show that the effect of the velocity values in each medium, overburden and reservoir, 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 on overburden velocity is just the opposite. From the inversion viewpoint, that is, predicting changes of the reservoir P-wave velocity from changes in critical offset is a strongly dependent of the uncertainty in the initial estimate of the reservoir Pwaver velocity, followed by its degree 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.