Effective medium modeling of the joint elastic-electrical properties of sandstones with partial water saturation

Abstract

Quantitatively assessing fluid saturation in reservoir rocks has been the ultimate goal of hydrocarbon geophysical investigations. Joint interpretation of acoustic and electromagnetic survey data indicates a promise to improve the fluid quantification ability. However, the saturation effects on the cross-property relationship between the elastic and electrical rock properties (i.e., the joint elastic-electrical properties) that form the key to the successful joint interpretation remain poorly understood. We have developed an effective medium modeling scheme for the joint elastic-electrical properties of sandstones with partial water saturation and studied theoretically the effects of porosity and grain and pore geometries on the saturation-dependent joint elastic-electrical properties of sandstones. Comparison between the modeling results and an available laboratory data set confirms the applicability of the approach for modeling the joint elastic-electrical properties of partially saturated sandstones. P-wave velocity is found to decrease with increasing electrical conductivity as water saturation increases in rocks with low porosity, low grain aspect ratio, and high pore aspect ratio. The water saturation effects on the joint properties are amplified in rocks with higher porosity, greater grain aspect ratio, and lower pore aspect ratio, in which P-wave velocity indicates an asymmetric V-shaped relationship with electrical conductivity as an implicit function of increasing water saturation. It also is found that the increasing porosity affects the joint properties in a gradually increasing manner, whereas the increasing grain and pore aspect ratios impose a decreasing impact on the joint elastic-electrical correlations. The results have strengthened our understanding of the joint elastic-electrical properties of partially saturated sandstones and will facilitate the joint interpretation of acoustic and electromagnetic survey data for improved fluid quantification.