UC Berkeley

Reservoir core measurements can help guide seismic monitoring of fluid-induced pressure variations in tight fractured reservoirs,including those targeted for supercritical CO2 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-fluidpressure 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 themodel 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 lowerwave 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.