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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

Abstract

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

Authors:
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:
Research Org.:
Montana State Univ., Bozeman, MT (United States)
Sponsoring Org.:
USDOE Office of Fossil Energy (FE)
Contributing Org.:
Lawrence Berkeley National Laboratory
OSTI Identifier:
1430903
Grant/Contract Number:  
FC26-05NT42587
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Geophysics
Additional Journal Information:
Journal Volume: 82; Journal Issue: 2; Journal ID: ISSN 0016-8033
Publisher:
Society of Exploration Geophysicists
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; fractures, low frequency, rock physics, shear modulus, attenuation

Citation Formats

Saltiel, Seth, Selvadurai, Paul A., Bonner, Brian P., Glaser, Steven D., and Ajo-Franklin, Jonathan B. 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. United States: N. p., 2017. Web. doi:10.1190/GEO2016-0199.1.
Saltiel, Seth, Selvadurai, Paul A., Bonner, Brian P., Glaser, Steven D., & Ajo-Franklin, Jonathan B. 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. United States. doi:10.1190/GEO2016-0199.1.
Saltiel, Seth, Selvadurai, Paul A., Bonner, Brian P., Glaser, Steven D., and Ajo-Franklin, Jonathan B. Thu . "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". United States. doi:10.1190/GEO2016-0199.1. https://www.osti.gov/servlets/purl/1430903.
@article{osti_1430903,
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},
author = {Saltiel, Seth and Selvadurai, Paul A. and Bonner, Brian P. and Glaser, Steven D. and Ajo-Franklin, Jonathan B.},
abstractNote = {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 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 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. 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.},
doi = {10.1190/GEO2016-0199.1},
journal = {Geophysics},
number = 2,
volume = 82,
place = {United States},
year = {Thu Feb 16 00:00:00 EST 2017},
month = {Thu Feb 16 00:00:00 EST 2017}
}

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