Seminar: The Effect of Hydration on Elastic Properties of Shales

Mr. Alexey Yurikov, PhD Candidate, Exploration Geophysics Curtin University

Title: The Effect of Hydration on Elastic Properties of Shales

Thursday 27 September 2018, 11AM–12PM 

ABSTRACT:

Shales play an important role in many engineering applications such as nuclear waste, CO2 storage and oil or gas production. Shales are often utilised as an impermeable seal or an unconventional reservoir. For both situations, shales are often studied using seismic waves. For example, 4D seismic is well established for monitoring of conventional oil and gas reservoirs. This technique is successfully used during production to track changes in rock formations such as variation in fluid saturation.

While the seismic response on changing fluid saturation in most sedimentary rocks like sandstones and carbonates is thoroughly studied, the effect of change of saturation on elastic properties of shales is still poorly understood. It has been shown that variation in saturation in shales leads to substantial structural changes and strongly affects the elastic properties of the rock. However, there is no agreement between reported dependencies of the elastic properties on hydration in different shales. In this work, we investigate structural changes in shales with variations in hydration using laboratory measurements and X-ray micro-computed tomography.

We observe deformation and decrease of porosity in shales with drying. Additionally, we study the elastic properties of the shale at different hydration states using low frequency and ultrasonic velocities measurements. The elastic moduli of the shale show substantial changes with variations in hydration, which cannot be explained with a single driving mechanism.

We suggest that changes of the elastic moduli with variations in hydration are driven by multiple competing factors:

  1. Variations in total porosity
  2. Substitution of pore-filling fluid
  3. Change in stiffness of contacts between clay particles
  4. Chemical hardening/softening of clay particles.

We qualitatively and quantitatively analyse and discuss the influence of each of these factors on the elastic moduli. We conclude that depending on the microstructure and composition of a particular shale, some of the factors dominate over the others, resulting in different dependencies of the elastic moduli on hydration.