122326) A comparison between different rock physics models in carbonate reservoirs for fluid substitution

The accuracy of estimation of saturation changes in carbonate reservoirs, using time-lapse seismic or amplitude-versus-offset (AVO) studies, depends on several factors such as repeatability, rockphysics models, fluid and rock properties. The precision of these methods is highly dependant on the accuracy of the rock-physics models, which vary from carbonate to sandstone reservoirs. There are several theories that compare the elastic properties of dry and saturated rocks. These theories are divided in two main groups: poroelastic theories and effective medium. Poroelastic theories include Gassmann’s theory and Biot’s theory; the former being the most commonly used in rock physics. Models based on Gassmann’s theory use special assumptions that are based on the properties of loose sandstone at low frequencies. These properties, in some cases, cannot be used for other lithologies (e.g. carbonates). Effective-medium theories include the differential effective medium (DEM) theory and self-consistent (SC) theory. Generally, if a rock’s microstructures are compatible with the model’s assumption, then the model can be effectively used. In the rocks with spherical and connected pores (i.e. loose sandstone) Gassmann’s equation is suitable at low-frequency. For granu­ lar rock (i.e. sandstone), SC models provide the best match. In most of carbonate rocks, there are isolated pores and fractures, and these microstructures are compatible with the DEM assumption. This presentation will review previous works, and provide a comparison between the mentioned models using laboratory tests on carbonate and sandstone core samples from a field in southwest Iran. The results will show that the DEM model is most compatible with the dense and low-porosity carbonate samples.