Quantification of 4D Seismic Response in Pre-salt Carbonates via Time Shift Analysis

Heterogeneity and stiffness of pre-salt carbonates challenge the detectability and interpretation of production effects to be monitored via 4D seismic, making prior modelling crucial in a more comprehensive and robust understanding of the anomalies. In this paper we computed the time shift response for selected scenarios of fluid substitution (oil by water or gas) and pressure changes (depletion or overpressure) for two types of pre-salt rocks (microbial carbonates and coquinas). High amount of CO2 dissolved in the oil has been considered. Sensitivity of elastic moduli to effective pressure were obtained by experiments on core plug samples and revealed to be a facies dependent phenomenon, with major velocity variations for coquinas. Fluid substitution using well log data also showed higher anomalies in coquinas, since this was the most porous facies and with the higher recoverability of the reservoir. Due to the thickness of the reservoir, including aquifer, time shifts showed to be susceptible to detection, and a solution for reservoir monitoring.