FLUID SUBSTITUTION MODELING FOR 4D FEASIBILITY ANALYSIS OF TARANAKI BASIN

Abstract

Hydrocarbon producing reservoirs undergo physical conditions over the life of the reservoir. Such changes are dominantly manifested in properties such as velocity, density, etc. due to the interchanges of the hydrocarbon and the replacing fluids. To ensure optimum and maximum possible recovery of hydrocarbons from these reservoirs, time-lapse 4D seismic technology is routinely used as a supportive tool to manage the reservoirs. The reservoir parameters such as lithofacies, porosity, pore fluid type, saturation and pore pressure are studied with the help of rock physics measurements and models. It is primarily because all these parameters are directly or indirectly related to the seismic velocity of the subsurface formation, and can be modeled for different reservoir conditions. Thus, for the study of 4D feasibility analysis, fluid substitution modeling is done by applying the rock-physics models to analyze the seismic attributes namely AVO in terms of amplitude and its phase. For this, various plots for density, P-wave velocity, S-wave velocity with fluid variation; crossplots for impedance/ reflectivity coefficient; and intercept-gradient AVO were plotted. Rock physics models for the crossplot of AVO were analyzed for multiple scenarios against the base (initial) case using different combinations of fluids under different substitution conditions to tap the sensitivity for characterizing these reservoirs. In this study, I modeled AVO plot sensitivity with respect to varying input parameters (such as mineralogy, fluid type, pore structure, fluid saturation, etc.), which could then be used to characterize different reservoir conditions important for 4D feasibility analysis. In the modeled results, I found that changing the pore fluid in the base case for a brine-filled sandstone reservoir for oil records a decrease in reflection coefficient value for near and far offsets. Gas substitution further decreases this value. Carbonate formations taken for this study consist of mud supported carbonates and have a strength comparable to sandstones, thus show fluid sensitivity. Reflection coefficient value for carbonates change corresponding to texture, which could easily be detected and analyzed for identification of a potential prospect.