Evaluation of representative elementary volume for a vuggy carbonate rock—Part: Porosity, permeability, and dispersivity

Abstract

Abstract This work highlights variation in petrophysical properties of a macroporous vugular carbonate rock. The macroporosity consists of vugs and moldic pores with sizes ranging from mm to cm scale. Carbonate rocks exhibit multi-scale heterogeneities that make the understanding of fluid flow and recovery a challenge. Properties such as porosity and absolute permeability often vary within a volume and sampling at the representative elementary volume (REV) is needed to determine the appropriate continuum properties for reservoir characterization and prediction. A classical approach to examine the variation with rock sample size and the REV is through analysis at different scales. Measuring petrophysical properties at various scales has therefore been a part of the REV investigation for the studied material. To study the variation with sample size and the effect of heterogeneities on fluid flow, laboratory tests were conducted at different scales, ranging from cm to m scale. Totally 37 sub-samples of a large outcrop block with different sizes and shapes have been included in this work. The main objective has been to determine porosity, effective permeability and dispersivity as function of rock sample size. Porosity–permeability correlations for heterogeneous material can be inadequate and strongly scale dependant. Thus, we also investigate the variation of the porosity–permeability ratio with sample size. The experimental results show that the variance and coefficient of variation decreases with increase in sample size for all investigated properties. Based on the reduction of variation with increased rock sample size a representative volume for measurement is proposed for the studied material. The results also indicate less variation for the porosity–permeability relationship with increased sample volume. It is shown that a single measurement at traditional laboratory scale can lead to erroneous interpretation of a measured property for the vuggy carbonate material. However, multiple measurements at laboratory scale can be used to determine the representative properties.