Time-lapse well log analysis, fluid substitution, and AVO

Abstract

Time-lapse seismic monitoring of reservoirs is based on changing seismic response due to fluid saturation, temperature, and pressure changes. The observable changes in seismic response can help locate bypassed oil, water- or gas-flood fronts, and heated zones. However, there are risks associated with a 4D seismic project including false anomalies caused by acquisition, processing artifacts, and the ambiguity of seismic interpretation in relating seismic changes to reservoir changes. Lumley and Behrens (1998) proposed a four-step feasibility and risk assessment study before undertaking a 4D seismic project. The proposed third step is seismic modeling from sonic and density logs. Our study demonstrates a procedure for the modeling of time-lapse seismic response changes due to simulated changes to well logs. We also simulate AVO changes at a gas-oil contact (GOC) and an oil-water contact (OWC). The objective of time-lapse AVO is to distinguish between reflections from a gas or oil sand that has been flooded by gas or water. As a model, we use a well log from White Rose Field, offshore Newfoundland, Canada. Three production scenarios are investigated. The method for calculating the seismic response changes is based on the Gassmann equation and a modification of the procedure described by Bentley et al. (2000). Gassmann's equation relates the bulk modulus of a saturated rock Ku), to the dry rock bulk modulus Kd), the solid grain modulus Ks), the fluid bulk modulus Kf), and the porosity ϕ. Densities are calculated from \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[{\rho}\_{f} = S\_{g}{\rho}\_{g} {+} S\_{o}{\rho}\_{o} {+} S\_{w}{\rho}_{w}\] \end{document}(1) \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[{\rho}\_{o} = \frac{{\rho}^{std}\_{o} {+} R\_{s}{\rho}^{std}\_{g}}{B_{o}}\] \end{document}(2) \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[{\rho}\_{u} = {\rho}\_{s} (1 {-} {\phi}) {+} {\rho}_{f}{\phi}\] \end{document}(3) where ρ o , ρ g , ρ w , ρ s , ρ u , ρ f , are the densities of oil, gas, water, solid grains, saturated reservoir rock, and fluid mixture at reservoir conditions. Sg, So, and Sw are gas saturation, oil saturation, and water saturation, …