Fluid factor index for fluid saturation estimation

Estimation of fluid saturation is of great importance to the oil/gas exploration and production. In this work, we propose the fluid factor index for the fluid saturation estimation. The fluid factor index is defined as the ratio of the value of the fluid factor for rocks with oil/gas-water saturation to the corresponding value under the full water saturation. Similar with the resistivity index in Archie's formula, this definition of fluid factor index can largely cancel out the influences of rock porosities and hence primarily reflects the effects of fluid saturation. Five fluid factor indices (IK, IΔK, IVP/VS, Iν, and IZp) are then studied, which are calculated from sample bulk modulus, bulk modulus increment due to fluid saturation, compressional to shear velocity ratio, Poisson's ratio, and compressional wave impedance, respectively. Their sensitivities to the fluid saturation and their dependency on rock porosities are investigated through the ultrasonic experiments on the water-oil and water-gas saturated sandstone samples. The results show that, for both the water-oil and water-gas saturated samples, IΔK is an ideal fluid factor index for fluid saturation estimation as it is most sensitive to the fluid saturation and is almost independent of the rock porosities; IK and Iν are not as good as IΔK, but are better than IVP/VS and IZp with respect to their sensitivity to the fluid saturation and dependency on the rock porosities. Hence, IΔK is recommended to be used for the fluid saturation estimation. These experimental observations can be validated and explained by the theoretical simulations. This research provides the basis for using the acoustic data to estimate the fluid saturation, which is very helpful to the oil/gas exploration and production.