How does water near clay mineral surfaces influence the rock physics of shales?

Abstract

Clays and clay‐bearing rocks like shale are extremely water sensitive. This is partly due to the interaction between water and mineral surfaces, strengthened by the presence of nanometer‐size pores and related large specific surface areas. Molecular‐scale numerical simulations, using a discrete‐element model, show that shear rigidity can be associated with structurally ordered (bound or adsorbed) water near charged surfaces. Building on these and other molecular dynamics simulations plus nanoscale experiments from the literature, the water monolayer adjacent to hydrophilic solid surfaces appears to be characterised by shear stiffness and/or enhanced viscosity. In both cases, elastic wave propagation will be affected by the bound or adsorbed water. Using a simple rock physics model, bound water properties were adjusted to match laboratory measured P‐ and S‐wave velocities on pure water‐saturated kaolinite and smectite. To fit the measured stress sensitivity, particularly for kaolinite, the contribution from solid‐grain contact stiffness needs to be added. The model predicts, particularly for S‐waves, that viscoelastic bound water could be a source of dispersion in clay and clay‐rich rocks. The bound‐water‐based rock physics model is found to represent a lower bound to laboratory‐measured velocities obtained with shales of different mineralogy and porosity levels.

The Elastic Properties of Clay in Shales

“…During formation of the first water layer, Ebrahimi et al () observe an increase in shear stiffness, suggesting that water bonded to the clay surface may have an ordered structure with nonzero shear modulus. Holt and Kolstø () argue that elastic wave propagation will be affected by this bound or adsorbed water, and that the bound water will have an enhanced viscosity, resulting in an effective shear modulus at finite frequency. Holt and Kolstø () use a generalized Maxwell model to relate the real part of the shear modulus of bound water to the high‐frequency limit of bound‐water shear stiffness μ BW (∞), the viscosity η BW and angular frequency ω : …”

The impact of saturation on seismic dispersion in shales — Laboratory measurements

“…However, the displacement of water molecules during stress variation (an increase in stress results in an increase of the chemical potential of the water in between the grains, making it energetically more favourable for the water to move away from the grain contact) takes time due to viscous forces. The visco-elastic properties of adsorbed (or bound) water at grain contacts is believed to be significantly different from those of free water (Holt and Kolstø, 2017). The characteristic frequency of stiffness dispersion associated with squirt flow is often written as…”

“…where K is the bulk modulus of the background material, η is fluid viscosity and γ is the aspect ratio of soft (crack-like) pores. For aspect ratio 10 -2 -10 -3 and free water viscosity, this gives a transition frequency in the kHz-MHz range, whereas if the water inside the soft pores is bound or adsorbed to the surfaces, the viscosity may be several orders of magnitude higher (Holt and Kolstø, 2017) and shift the transition into the subseismic frequency regime. Liu et al (1994) argued that the transition frequency for local flow in shale should be in the 10 0 Hz range, and also pointed out that the Biot critical frequency is of the order 10 11 Hz.…”

Influence of subsurface injection on time‐lapse seismic: laboratory studies at seismic and ultrasonic frequencies

“…Increased volume of the shale pore fluid at seismic frequencies causes strong softening of the frame and changes the shear moduli (Szewczyk et al 2017b). Water near the clay surfaces influence the rock physical properties (Holt and Kolstø 2017). Moreover, due to observed dispersion (Figures 3 and 4), the shale is often unrelaxed or only partially relaxed even at seismic frequencies.…”