Reservoir Quality and Reservoir Heterogeneity: Petrophysical Application of the Lorenz Coefficient

摘要

Carbonate reservoirs are often challenging due to intrinsic heterogeneities that may occur at all scales of observation and measurement. These heterogeneities can be attributed to a number of factors including, but not limited to, variable lithology, sedimentary facies, mineralogy, and pore types. These inherent complexities can be related to processes controlling original deposition and/or subsequent diagenesis. Carbonate heterogeneities are poorly understood, and their impact on reservoir quality at a range of length-scales is rarely defined or numerically quantified. We show how the Lorenz coefficient can be used as a measure of heterogeneity in petrophysical measurements and compare the use of the standard technique with novel versions of the Lorenz coefficient (a "dual property" and "single property" technique). We discuss how the variability in a population can influence the resultant Lorenz coefficient as a measure of heterogeneity using a synthetic dataset. We then quantify and interpret variability in wireline log-derived porosity and permeability data, enabling a comparison of heterogeneities between different measurements and between different carbonate reservoir units. In a complex carbonate reservoir we find increased heterogeneity correlates with high quality reservoir units while in a chalk reservoir increased heterogeneity correlates with lower reservoir quality. These opposing effects are controlled by the complexity of localized carbonate diagenetic processes and are explained by relating porosity and permeability to the underlying geology. Further testing and refinement of statistical techniques will enable wider application, but our analysis demonstrates that the Lorenz coefficient has great potential to significantly improve the petrophysical characterization of reservoirs. Defining a critical heterogeneity level, or threshold, which separates the two effects above, may provide a tool for relating petrophysical indicators of reservoir quality to the underlying geology.