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Title: Characterization of laboratory-scale hydraulic fracturing for EGS

Abstract

To investigate the impact of reservoir stimulation by hydraulic fracturing, a series of advanced lab-scale block tests were conducted with multiple measurement methods to characterize the fracture and to assess the operation and the performance of an enhanced or engineered geothermal system (EGS). The stimulation test samples were 330-mm cubic blocks of Sierra White granite with one injection well and four producers drilled into it. Acoustic emissions generated during the fracture initiation and propagation were monitored with twenty AE sensors mounted on all sides of the block to help characterize the fracture initiation and propagation process. The recorded AE signals were analyzed to obtain the event location and the failure mechanism. The localized AE events shows that due to the low viscosity of the fracturing fluid (water), the fracture propagated rapidly and took only about 1.4 s to reach near the block surface. During the fracturing phase, about 47 percent of the events were determined to be shear/mixed mode failure events, 28 percent tensile failure events, and 25 percent compressional failure events. Just after the simulation, a step-rate (constant flowrate) injection test was carried out to investigate the reservoir injectivity enhancement. During this phase additional compressional failure events were observedmore » and the percentages of tensile failure and shear/mixed mode failure decreased. The dramatic increase of the injection rate with small increments of the injection pressure demonstrates the nonlinear response of the fracture to effective normal stress variation with much higher fracture opening increase during the low effective normal stress interval. Hydraulic fracturing tests under different stress levels show that more AE hits/events are recorded when the rock block is under higher applied stress. The reconstructed 3D fracture geometry shows good agreement with the AE monitoring results. After fracturing, a tracer test was carried out and the fracture aperture was estimated to be around 139 μm. The tracer recovery curve shows two linear portions of the tracer tail on a semi-log plot similar to that observed in some field data and predicted by numerical modeling. Laser scanning of the fracture surfaces indicates that the induced fracture has a rough surface with a joint roughness coefficient (JRC) of 12.98 and tortuosity of 1.10, both of which are slightly larger than that from the Brazilian test fracture on the same rock. The thin section and SEM observations show that the fracture follows the weak boundaries between the Quartz and Albite grains or the weak beddings in the grains.« less

Authors:
 [1];  [1];  [2];  [2]
  1. Univ. of Oklahoma, Norman, OK (United States)
  2. Leidos Inc, San Diego, CA (United States)
Publication Date:
Research Org.:
Univ. of Oklahoma, Norman, OK (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
1799242
Alternate Identifier(s):
OSTI ID: 1905150
Grant/Contract Number:  
EE0006765; EE0006765.0000
Resource Type:
Accepted Manuscript
Journal Name:
Geothermics
Additional Journal Information:
Journal Volume: 83; Journal ID: ISSN 0375-6505
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; hydraulic fracturing; enhanced geothermal system; acoustic emission; fracture observation

Citation Formats

Hu, Lianbo, Ghassemi, Ahmad, Pritchett, John, and Garg, Sabodh. Characterization of laboratory-scale hydraulic fracturing for EGS. United States: N. p., 2019. Web. doi:10.1016/j.geothermics.2019.07.004.
Hu, Lianbo, Ghassemi, Ahmad, Pritchett, John, & Garg, Sabodh. Characterization of laboratory-scale hydraulic fracturing for EGS. United States. https://doi.org/10.1016/j.geothermics.2019.07.004
Hu, Lianbo, Ghassemi, Ahmad, Pritchett, John, and Garg, Sabodh. Mon . "Characterization of laboratory-scale hydraulic fracturing for EGS". United States. https://doi.org/10.1016/j.geothermics.2019.07.004. https://www.osti.gov/servlets/purl/1799242.
@article{osti_1799242,
title = {Characterization of laboratory-scale hydraulic fracturing for EGS},
author = {Hu, Lianbo and Ghassemi, Ahmad and Pritchett, John and Garg, Sabodh},
abstractNote = {To investigate the impact of reservoir stimulation by hydraulic fracturing, a series of advanced lab-scale block tests were conducted with multiple measurement methods to characterize the fracture and to assess the operation and the performance of an enhanced or engineered geothermal system (EGS). The stimulation test samples were 330-mm cubic blocks of Sierra White granite with one injection well and four producers drilled into it. Acoustic emissions generated during the fracture initiation and propagation were monitored with twenty AE sensors mounted on all sides of the block to help characterize the fracture initiation and propagation process. The recorded AE signals were analyzed to obtain the event location and the failure mechanism. The localized AE events shows that due to the low viscosity of the fracturing fluid (water), the fracture propagated rapidly and took only about 1.4 s to reach near the block surface. During the fracturing phase, about 47 percent of the events were determined to be shear/mixed mode failure events, 28 percent tensile failure events, and 25 percent compressional failure events. Just after the simulation, a step-rate (constant flowrate) injection test was carried out to investigate the reservoir injectivity enhancement. During this phase additional compressional failure events were observed and the percentages of tensile failure and shear/mixed mode failure decreased. The dramatic increase of the injection rate with small increments of the injection pressure demonstrates the nonlinear response of the fracture to effective normal stress variation with much higher fracture opening increase during the low effective normal stress interval. Hydraulic fracturing tests under different stress levels show that more AE hits/events are recorded when the rock block is under higher applied stress. The reconstructed 3D fracture geometry shows good agreement with the AE monitoring results. After fracturing, a tracer test was carried out and the fracture aperture was estimated to be around 139 μm. The tracer recovery curve shows two linear portions of the tracer tail on a semi-log plot similar to that observed in some field data and predicted by numerical modeling. Laser scanning of the fracture surfaces indicates that the induced fracture has a rough surface with a joint roughness coefficient (JRC) of 12.98 and tortuosity of 1.10, both of which are slightly larger than that from the Brazilian test fracture on the same rock. The thin section and SEM observations show that the fracture follows the weak boundaries between the Quartz and Albite grains or the weak beddings in the grains.},
doi = {10.1016/j.geothermics.2019.07.004},
journal = {Geothermics},
number = ,
volume = 83,
place = {United States},
year = {Mon Jul 29 00:00:00 EDT 2019},
month = {Mon Jul 29 00:00:00 EDT 2019}
}

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