Experimental observation of moduli dispersion and attenuation at seismic frequencies in saturated tight rock: Effect of microstructure and fluid viscosity

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Summary The frequency-dependent elastic properties of fully saturated rocks are notably influenced by fluid pressure diffusion at the microscopic scale. Our experimental evaluation, utilizing forced oscillation and ultrasonic transmission methods under varying effective pressures (${P}_{{\rm{eff}}} = [ {1 - 20} ]$MPa) and temperatures ($T = [ {{3}^{\rm{o}}{\rm{C}},{\rm{\ }}{{10}}^{\rm{o}}{\rm{C}},{\rm{\ }}{{23}}^{\rm{o}}{\rm{C}}} ]$), provides critical insights into how rock microheterogeneity and pore fluid viscosity affect elastic dispersion and attenuation at frequencies of $f = [ {1 - 300,{\rm{\ }}{{10}}^6} ]$ Hz. We employed a sandstone rock sample with $8.2\% $ porosity and conducted measurements using three different fluids: N2, brine, and glycerine. In its dry state, our chosen rock exhibits frequency independent elastic moduli at measured effective pressures due to the absence of fluid flow, resulting in negligible deviations in local measurements at different locations. However, this uniform response changes markedly when the rock is saturated with fluids. Gassmann's predictions agree with the measured undrained elastic moduli. Under fluid-saturated conditions, rock's elastic moduli increase with frequency, revealing significant differences depending on measurement positions. This variation suggests that differentiation in elastic properties is amplified during wet measurements, particularly at seismic frequencies. Our modelling indicates that the dominant mechanism is squirt flow, arising from microscopic compressibility heterogeneities within the rock frame and saturating fluid. As the viscosity of the saturating fluid decreases with rising temperature, the magnitude of attenuation peaks diminishes, while their frequency spread widens. This behavior aligns with predictions from the squirt flow model, which considers the microstructure and varied pore types within the rock. Consequently, the observed frequency dependence in elastic moduli is primarily attributed to fluid flow processes driven by microheterogeneity, which are highly sensitive to small variations in the rock's microstructures. In microstructurally complex regions, it is evident that assuming isotropic and homogeneous conditions for forced axial oscillation measurements can introduce errors. The inherent heterogeneity of the rock must be taken into account to accurately interpret the frequency dependence of elastic moduli. This is especially relevant for applications in geophysical hydrocarbon exploration and seismic monitoring of reservoir geomechanical integrity during CO2 geo-sequestration.