Petrophysical Insights of coupled hydro – mechanical behaviors of swelling clay using complex resistivity and distributed fiber optic sensing

Abstract:

The coupled hydrological – mechanical - chemical processes in nano-porous clays control its swelling, ion exchange, and transport behaviors. Insights into such coupled processes can be gained using novel experimental approaches linking the different aspects of the physics together. We explore the joint use of low frequency complex resistivity and high resolution distributed fiber optic sensing for studying the coupled hydro-mechanical behaviors of swelling clay, and its linkage to the microscopic inter-particle and inter-layer hydration and surface charge dynamics. The complex conductivity signals are sensitive to the behaviors of electrical double layer (EDL), and fiber optic based distributed strain sensing can quantify three dimensional strain dynamics linking to swelling rate and geometry.

Our initial experiments were focused on bentonite clay hydration and the impacts from fluid salinity changes. Synthetic brines were used as the saturating fluids and the experiments were conducted at ambient temperature utilizing a flexible sleeve design. Our results revealed significant changes of the swelling behavior under different fluid salinity and the corresponding impedance and strain signals. The accelerated swelling rate with less concentrated brine was quantified with the dynamic strain measurements, which was accompanied by a significant reduction of the EDL polarization signals and a continuous shift of the impedance spectrum toward lower frequencies. Such behaviors are linked to swelling induced pore geometry and EDL charge structure changes. These initial results demonstrate the promises of joint geophysical approaches to understand coupled hydro-mechanical behaviors of swelling clays.