The use of seismic attenuation for monitoring saturation in hydrocarbon reservoirs
We used a theoretical rock physics forward modelling and synthetic time lapse seismic data to show the possible use of attenuation to monitor hydrocarbon saturation in reservoirs. Given the time variant changes in saturation and the properties of the reservoir rock, the rock physics anelastic model (Mavko et al. 1998) is used to formulate a theoretical curve to observe the dynamic effects of saturation on attenuation. The partial saturation part of the curve is used to formulate a relation between attenuation and saturation. Attenuation is measured in time lapse synthetic seismic records and the curve-based relation is used to invert the approximate corresponding saturation that gave rise to the attenuation measured in seismic data (AMSD). Results from the study show that: AMSD can be used to monitor hydrocarbon saturation in reservoirs, if a relationship between AMSD and saturation is known; the saturation values predicted by the theoretical model are consistent with the saturation values inverted from the seismic derived attenuation. Apart from saturation, attenuation depends on porosity and mineral composition of the rock, and the property of the saturating fluid(s). For the case studied, at a saturation of 0.7, a 10% reduction in porosity caused 5.9% rise in attenuation, while a 10% reduction in the bulk modulus of the saturating fluids caused 11% reduction in attenuation.