4D rock physics and dynamic RPTs for sandstones

Abstract

We discriminate between static and dynamic rock physics templates, static RPTs relates the elastic properties of acoustic impedance and Vp-Vs ratio of the reservoir at fixed time in its history where dynamic RPTs relate the relative change in acoustic impedance and Vp-Vs ratio of the reservoir that are related to pressure and saturation changes (Andresen et al.)

In order to explain the dynamic changes in the reservoir, a good understanding of pressure dependence of the reservoir rock is important. Hertz-Mindlin theory explains pressure dependence of wave velocity in unconsolidated sand and Eberhart-Phillip derived an empirical formula to demonstrate pressure dependence in consolidated sands. In this Thesis empirical formulas are approximated for Alvheim reservoir sandstone and laboratory measured loose sand using Eberhart-Phillip relations and the laboratory measured data to characterize effective pressure dependence of velocity in these sands. These empirical formulas provide good understanding of how rock properties might change in response to pressure changes resulting from reservoir depletion and injection.

Synthetic seismic and logs are generated for well 24_6_2 in Alvheim reservoir using the information gained from the empirical relations and used this information to visualize the 4D dynamic changes in an oil reservoir consisting of such sands. it is found a small difference between the consolidated sand and this reservoir sandstone when effective pressure in the reservoir is reduced with 10 MPa. Loose sand is found to be affected most as a consequence of effective pressure reduction in the reservoir.

Fully oil saturated sand is chosen as base case to constitute dynamic RPTs in a gas drive and water drive reservoir. When porosity, saturation and effective pressure varies, interpreting dynamic property changes in the reservoir becomes complicated. Porosity is then kept constant to solve this problem.