Fluid substitution and seismic forward modeling in one of the Iranian sandstone oil reservoirs

Changes in seismic signal properties related to the production of hydrocarbon, substituted fluid and reservoir conditions, suggest the seismic reacquisition for monitoring of the hydrocarbon fields during the injection scenarios. Due to the fact that many Iranian hydrocarbon reservoirs are in their second half of the production life, this possibility must be investigated at certain time intervals (in order to carry out 4D seismic survey). Therefore, the rock physics model is constructed using the petrophysical data to calculate the elastic properties of the rock and then, using seismic forward modeling, the calculated elastic properties are related to seismic properties of the reservoir. Finally, the possibility of the seismic reacquisition will be examined based on the seismic properties changes in the reservoir. In this paper which investigates one of the Iranian sandstone oil reservoirs, two rock physics models of Gassmann and the Hertz-Mindlin using the lower limit of Hashin-Shtrikman were used to calculate the elastic properties of the dry rock. Regarding the most significant changes in the seismic amplitude, replacing 30% of oil with gas is considered as a selected scenario for fluid substitution. Compared to the in-situ conditions, after replacing the fluid, the Gassmann model with homogeneous saturation and the Hertz-Mindlin using the lower limit of Hashin-Shtrikman show compressive wave velocity reduction of 17% and 13%, respectively, while the reduction of compression wave velocity in heterogeneous saturation is about 3%. The results of seismic forward modeling show that if the distribution of fluid in the reservoir is homogeneous and about 30% gas is substituted by oil, the change of seismic signal in the reservoir and the time delay created in the underlying reservoir layers will be observable and seismic monitoring will be applicable. However, if the distribution of the fluid is heterogeneous, this will be possible by replacing more oil with gas.