Estimating the Effects of Wag-EOR on Elastic Properties for Seismic Reservoir Monitoring in Abu Dhabi

Abstract

Abstract When implementing CO2 WAG EOR, fluid distribution and pore pressure in the reservoir vary. The interpretation of seismic reservoir monitoring for WAG-EOR often includes uncertainty in the changes obtained by time-lapse seismic surveys because these changes are the integration of effects by several factors. The objective of this study is to estimate the impact of each factor on elastic property and investigate a dominant factor through a rock physics study at a CO2 WAG site in Abu Dhabi. To estimate the effect of pore-pressure change on elastic velocity, an empirical relation between elastic velocity and confining pressure was used. Sonic velocity using core plug samples of the reservoir was measured under multiple confining pressures to construct empirical relations. As for the effect of fluid saturation changes on elastic velocity, we estimated it through Gassmann's equation. Realistic changes in pore pressure and saturations during the WAG operations were based on reservoir simulation results. Finally, based on the input petrophysical data and the relations from rock physics analyses, elastic property changes in WAG-EOR are estimated. Density and P-wave velocity (Vp) changes caused by the fluid substitution from oil to water are more significant than those by the substitution from oil to CO2. Pore pressure changes have less of an impact on Vp variations than does fluid substitution. Based on the estimation of elastic property changes associated with WAG operations, density changes are affected mainly by the increase in water saturation because of the density difference between water, oil and CO2. Vp changes seem to be consistent with the increase of gas (CO2) saturation. Even in the upper zone of the reservoir where oil is swept by both injected water and CO2, the effect of CO2 is prominent as well-known low gas saturation effects. Vp is significantly affected by the small amount of CO2 which was observed in the comparison of Vp with and without 10% of CO2 saturation in the rock. P-impedance changes are consistent with Vp changes because of the larger changes of Vp than the density changes. From these observations, the changes obtained by seismic reservoir monitoring would mainly represent the effect of CO2 saturation changes. We quantitatively looked into the impact that changes in pore pressure and saturation had changes on elastic properties such as density and elastic velocity. Furthermore, it was determined which elements predominately control changes in elastic properties when these changes occur simultaneously in WAG-EOR operations. These findings play an important role in the interpretation of seismic reservoir monitoring for WAG-EOR in Abu Dhabi.