Effect Of Net To Gross On Time-Lapse Seismic Response In Campos Basin, Brazil

Quantitative time-lapse studies require precise knowledge of the pressure response of rocks sampled by a seismic wave. Usually this knowledge is obtained from measurements of ultrasonic velocities as a function of pressure. These measurements are typically made on reservoir sandstone samples. However, if the reservoir is composed of sand and shale layers, the response of shales as well as sands has to be taken into account. The pressure response of shales is quite different from that in sand: since shales have very low permeability, an increase of pore pressure in the sand will cause an increase of confining pressure in the intra-reservoir shale. To estimate the effect of intra-reservoir shale on the time-lapse response to depletion or injection, we compute the combined seismic response as a function of net to gross (NTG – sand-shale proportion). This is done by applying Backus average using typical shale and sandstone stress sensitivity for an oil field located in Campos Basin, Brazil. For a typical NTG of 0.6, there is an error of approximately 35% in reflection coefficient estimation if these shales are neglected. Consequently, not considering the small shales intra-reservoir may mislead quantitative 4D studies. We suggest expanding this approach to 3D models in order to incorporate other geomechanical effects.