Water retention effects on elastic properties of Opalinus shale

Abstract

Shales play an important role in many engineering applications such as nuclear waste, CO2 storage and oil or gas production. Shales are often utilized as an impermeable seal or an unconventional reservoir. For both situations, shales are often studied using seismic waves. Elastic properties of shales strongly depend on their hydration, which can lead to substantial structural changes. Thus, in order to explore shaly formations with seismic methods, it is necessary to understand the dependency of shale elastic properties on variations in hydration. In this work, we investigate structural changes in Opalinus shale at different hydration states using laboratory measurements and X‐ray micro‐computed tomography. We show that the shale swells with hydration and shrinks with drying with no visible damage. The pore space of the shale deforms, exhibiting a reduction in the total porosity with drying and an increase in the total porosity with hydration. We study the elastic properties of the shale at different hydration states using 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 and (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.

A Seismic‐Frequency Laboratory Study of Solid Substitution in Bentheim Sandstone

“…Careful examination of both data sets shows that values of the bulk modulus of the solid‐filled rock at seismic and ultrasonic frequencies are very close, but the Ciz‐Shapiro predictions are different due to substantial difference between the seismic and ultrasonic drained moduli. We believe that the drained bulk modulus measured at low frequencies is more reliable, since it is the true drained modulus of a fluid‐saturated rock, whereas the ultrasonic modulus was measured on the dry rock, and hence could be altered by the drying process and depends on relative humidity (Rasolofosaon & Zinszner, ; Yurikov et al, ). Conversely, the shear moduli of the dry and drained rock are very close, but for solid‐filled rock the ultrasonic shear modulus is higher than low‐frequency modulus (by about 4 GPa).…”

Modeling of Compaction Trends of Anisotropic Elastic Properties of Shales

“… Comparison of compliances of contacts in artificial samples A0100 and A2080, Opalinus shale from Yurikov et al. (2018) and other natural shales from Pervukhina et al. (2011).…”

“…The shear compliance of the contacts s n B T decreases from ∼70 to ∼5 GPa −1 during compaction of the artificial samples. The values of the shear compliance of contacts in natural shale samples vary between 0.01 and 10 GPa −1 for shales from the Officer Basin, Bass Basin, Carnarvon Basin (offshore Australia), Africa, and the North Sea (Pervukhina et al, 2011) and between 0.4 and 1.1 GPa −1 for Opalinus Clay depending on saturation (Yurikov et al, 2018). In the case of artificially compacted shales, the ratio of normal to shear compliance B is within the range from 0.08 to 0.2, which is in a good agreement with the values obtained on natural shales (from 0.01 to 2 as reported by Pervukhina et al (2011).…”

“…An answer to this question can be probably found in some experimental studies, for example, Yurikov et al. (2018), who showed that the dynamic elastic moduli of fully saturated shales are often lower than those of dried out (desiccated) shales. The authors explained this behavior of the moduli by the presence of water layers (or bound water) that reduce the moduli of the wet clay packs.…”

“…The inevitable question is what effects or parameters are not taken into account by the first principles calculations based on density functional theory and molecular dynamic simulations? An answer to this question can be probably found in some experimental studies, for example, Yurikov et al (2018), who showed that the dynamic elastic moduli of fully saturated shales are often lower than those of dried out (desiccated) shales. The authors explained this behavior of the moduli by the presence of water layers (or bound water) that reduce the moduli of the wet clay packs.…”