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Title: 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

Reservoir core measurements can help guide seismic monitoring of fluid-induced pressure variations in tight fractured reservoirs including those targeted for supercritical CO 2 injection. We present 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 to 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 to 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 the majority of the samples shows an exponential dependence on real contact area. A simple numerical model, with one bonded circular asperity, predicts this behavior and matches the datamore » 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. Lastly, we apply our results to a Linear-Slip Interface Model to estimate reflection coefficients and calculate shear 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.« less
ORCiD logo [1] ; ORCiD logo [2] ;  [3] ;  [1] ;  [3]
  1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Univ. of California, Berkeley, CA (United States)
  2. Univ. of California, Berkeley, CA (United States)
  3. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Additional Journal Information:
Journal Volume: 82; Journal Issue: 2; Journal ID: ISSN 0016-8033
Society of Exploration Geophysicists
Research Org:
Montana State Univ., Bozeman, MT (United States)
Sponsoring Org:
USDOE Office of Fossil Energy (FE)
Contributing Orgs:
Lawrence Berkeley National Laboratory
Country of Publication:
United States
58 GEOSCIENCES; fractures, low frequency, rock physics, shear modulus, attenuation
OSTI Identifier: