Influence of Pore Geometry, Pressure and Partial Water Saturation to Electrical Properties of Reservoir Rock: Measurement and Model Development

Pressure and saturation are of two important parameters to be considered to evaluate the electrical properties of reservoir rock. As confining pressure can cause pore space of rock to collapse as well as rock properties to change, it for some reason is necessary to examine the degree of the pressure and saturation changes in affecting the electrical properties in detail. This work mainly focuses on the investigations of electrical properties of sandstones and carbonates. The effects of pore geometries, confining pressure, and partial water saturation on electrical properties are investigated. The new electrical dispersion models from 0.01 Hz until 0.2 MHz for shaly sandstone are also developed.

Petrographic image analysis is carried out to asses pore geometry of media. Circularity, gamma, pore aspect ratio, pore size distribution and pore angle distribution are calculated to evaluate their effect on electrical dispersion. An observation on confining pressure and water saturation is then evaluated to obtain their impact on pore structure and complex resistivity.

The result of this work, indicated by electrical resistivity dispersion in low water saturation, shows that surface conductivity plays as a dominant factor particularly in shaly media. In evaluating the surface conductivity, it is necessary to consider the pore geometry in that a large structure is easier to saturate rather than a small one. The high water salinity moreover can significantly reduce diffusive layer thickness and if the pore radii are very small, the diffusive layer thickness will touch each other and block an anion movement.

The results further show that pore radius distribution provides more contribution to the electrical resistivity dispersion and becomes a basis to extract porosity from resistivity. The confining pressure only contributes small changes in pore geometry of media and the use of imaginary resistivity offers a better detection on it. Modified D-model and Archie’s model are developed to calculate the dielectric permittivity and effective conductivity in high water salinity of saturated-rock as a function of water saturation degree.