A saturation law for quantitative 4D seismic analysis of a turbidite reservoir undergoing waterflood

Reservoir management can be very much improved by increasing our knowledge of fluid changes within the field. Time-lapse seismic can offer this information provided an accurate relationship between the fluid-bulk modulus estimated from seismic and the true reservoir saturation condition is known. This relationship is generally simplified by ignoring the interaction of fluid, heterogeneity and seismic at various scales, thus significantly increasing errors and uncertainty in saturation estimates. To improve the relationship, a reservoir-based saturation law is developed with specific application to a turbidite reservoir. This geological architecture allows us to construct suitable conceptual models such that analytical solutions can be derived. The resulting law takes into account both saturation and geological heterogeneity when estimating saturation from seismic. Numerical tests based on flow simulation and seismic calculation show that this law can accurately predict the true reservoir saturation to within 1 to 2 percent, whilst laws not incorporating this reservoir information can achieve 3 to 8 percent and are strongly dependent on porosity and saturation fluctuations. Whilst the new approach requires data gathered from fine scale simulations and analogue outcrops, the improvement in accuracy and reduction in uncertainty may well be advantageous.