Rock physics modeling and 4D seismic feasibility study in one of the Iranian carbonate reservoirs
Abstract Rock Physics is one of the vital steps of any Reservoir Geophysics studies to estimate the reservoir parameters. To do this, it is necessary to quantify the effect of reservoir parameters on the velocity and density of the saturated rocks. Carbonate reservoirs contain different heterogeneities such as the presence of pore types and irregular distribution, making it difficult to estimate the reservoir properties in these reservoirs. Various rock physics models have been provided to estimate the P and S-wave velocities. However, each of these models shows some degree of limitations, especially in carbonate hydrocarbon reservoirs with different pore types. In this paper, an integrated algorithm was introduced to estimate the dry rock modulus that include the pore type, as well as the pressure effect. The effect of porosity and pore type was considered by employing Xu and White's (1995) equation. The pressure effect was added using MacBeth's (2004) methodology. Finally, the fluid effect was included utilizing Gassmann's (1951) equation. During the time-lapse seismic feasibility study for our case study, it is observed that the impact of pore type is as important as the quantity of the porosity, and even more. The variation of pore type from the fracture to the stiff pores (increasing the pore aspect ratio), can double the p-wave velocity. In addition, it is concluded that increasing the Aspect Ratio (the ratio between the minor and major axes of an ellipsoidal pore) of the pores from 0.0.07 to 0.8, could significantly decrease the pressure and saturation sensitivity of the elastic properties from 17% to 2%. Without proper treating of pore type and pressure impact , as an example, a feasible four dimensional (4D) seismic project can be concluded non-feasible and vice versa. In this paper, utilizing the introduced methodology, one dimensional (1D) feasibility study was carried out for one of Iranian Carbonate reservoirs. We could replicate the velocity and density logs with acceptable deviation from the observed logs that are better than the original Gassmann's method. Subsequently, the injection and production scenarios were generated to estimate the impedance variation in this reservoir. We observed that the reservoir pressure increase of 1500 psi and water saturation increase of 60% could change the impedance by −2% and +7%, respectively. However, the reservoir pressure drop has negligible impact on the impedance in this field. In overall, it makes this reservoir feasible for a time-lapse seismic project to monitor production and injection.