Experimental development of low-frequency shear modulus and attenuation measurements in mated rock fractures: Shear mechanics due to asperity contact area changes with normal stress

Abstract

Reservoir core measurements can help guide seismic monitoring of fluid-induced pressure variations in tight fractured reservoirs, including those targeted for supercritical [Formula: see text] injection. We have developed the first seismic-frequency “room-dry” measurements of fracture-specific shear stiffness, using artificially fractured standard granite samples with different degrees of mating, a well-mated tensile fracture from a dolomite reservoir core, as well as simple roughened polymethyl methacrylate (PMMA) surfaces. We have adapted a low-frequency (0.01–100 Hz) shear modulus and attenuation apparatus to explore the seismic signature of fractures and understand the mechanics of asperity contacts under a range of normal stress conditions. Our instrument is unique in its ability to measure at low-normal stresses (0.5–20 MPa), simulating “open” fractures in shallow or high-fluid-pressure reservoirs. The accuracy of our instrument is demonstrated by calibration and comparison with ultrasonic measurements and low-frequency direct shear measurements of intact samples from the literature. Pressure-sensitive film was used to measure real contact area of the fracture surfaces. The fractured shear modulus for most of the samples shows an exponential dependence on the real contact area. A simple numerical model, with one bonded circular asperity, predicts this behavior and matches the data for the simple PMMA surfaces. The rock surfaces reach their intact moduli at lower contact area than the model predicts, likely due to more complex geometry. Finally, we apply our results to a linear-slip interface model to estimate reflection coefficients and calculate S-wave time delays due to the lower-wave velocities through the fractured zone. We find that cross-well surveys could detect even well-mated hard-rock fractures, assuming the availability of high-repeatability acquisition systems.

Experimental evidence for dynamic friction on rock fractures from frequency‐dependent nonlinear hysteresis and harmonic generation

“…In Saltiel et al [], we measured the modulus and attenuation before and after fracturing under a range of uniaxial loads using 1000 measurements at the driving, fundamental, frequency (8 Hz). The error bars for each modulus and attenuation measurement are estimated from the error in linear fit of all 1000 stress‐strain measurements and the standard deviation of all 1000 phase measurements.…”

“…(13) Load cell measures applied uniaxial stress with a calibrated strain gauge. Figure and description from Saltiel et al [].…”

“…Local normal stress distribution from the calibrated pressure‐sensitive film under normal stresses of (b) 18, (c) 15, (d) 0.5, (e) 3.75, and (f) 7.5 MPa. Figure modified from Saltiel et al [].…”

“…This results in a well‐mated tensile fracture, perpendicular to the diameter of the cylindrical sample, with relatively smooth and low topography (Figure ). As discussed in Saltiel et al [], we expect minimal process zone‐related microfracturing. Optical profilometry and pressure sensitive film [ Selvadurai and Glaser , ] were used to characterize the fracture surfaces; these techniques and analysis are described in Saltiel et al [].…”

“…As discussed in Saltiel et al [], we expect minimal process zone‐related microfracturing. Optical profilometry and pressure sensitive film [ Selvadurai and Glaser , ] were used to characterize the fracture surfaces; these techniques and analysis are described in Saltiel et al []. The imaged contacts transmit shear stress as well as the measured normal stress across the fracture.…”