Simultaneous Measurement of Elastic and Electrical Anisotropy of Shales under Elevated Pressure: A Preliminary Study

Abstract:

Low-porosity shales are generally considered as transverse isotropic (TI) material, both elastically and electrically. We explore the correlation of elastic and electrical anisotropy of shales to mineralogy and organic richness. Both, elastic and electrical anisotropies in rocks are known to be functions of clay and kerogen contents. The elastic anisotropy in clay- and organic-rich shales is related to their laminated and lenticular texture that is enhanced with compaction and diagenesis leading. The correlation of complex resistivity and anisotropy in shales with clay content has theoretically modeled using cation exchange capacity of the clay minerals. Although, it is appear likely that elastic and electrical anisotropy might be controlled by similar petrophyiscal properties, very little research exists that explores the relationship between these two anisotropies and how petrophysical properties might influence them.

We present an experimental data of elastic and electrical anisotropy in shales acquired with a new system that allows simultaneous measurements of acoustic velocities and complex resistivities under hydrostatic pressure in three directions: parallel, 45° and perpendicular to any orientation, such as bedding planes. Our results of five shale samples show:

• Elastic and electrical anisotropy are inversely related to pressure
• Electrical anisotropy is generally higher than elastic anisotropy
• Electrical anisotropy is more sensitive to pressure change
• Complex resistivity anisotropy for in-phase resistivity and phase is highly frequency dependent

In future, we plan to measure shale samples with varying clay and organic contents and correlate anisotropy to petrophysical properties.