Abstract

Seismic wave velocities in a reservoir depend on physical properties such as elastic constants of the rock, porosities, pore fluid characteristics, pore geometry, consolidations, hydrocarbon saturations and effective pressure. During a thermal EOR process some of these parameters are changed resulting in changes in seismic velocity and acoustic impedance. Timelapse seismic is based on measuring the observed changes in seismic response to locate bypassed oil, water or gas fluid fronts and heated zones. The changes in seismic response can be tremendous or subdued depending on relative predominance of one or more of these rock and fluid properties. Because of the cost-implication of 4D seismic surveys, it is prudent to estimate the changes in seismic response and then use that estimate to evaluate the feasibility of observing the changes in the reservoir that will be useful in production/development decisions. In this paper, we present a procedure for estimating the reservoir fluid and rock physical parameters that may control the seismic response of an in-situ combustion process in Balol field of Mehasana, India. We generate and analyse pre- and post-combustion scenarios on the basis of rock physical calculations and Gassmann fluid substitution model. This is followed by a forward modeling to generate synthetic pre- and post-combustion seismic sections for comparison and analysis. We show that in-situ combustion process in Balol Field indeed gives rise to an observable change in seismic response and it is possible to track the thermal front in successive repeat surveys. We finally discuss the usefulness and value addition that may accrue due to 3D time-lapse seismic surveys in Balol for reservoir management and monitoring of thermal EOR.