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Title: Evaluation of known-boundary and resistivity constraints for improving cross-borehole DC electrical resistivity imaging of discrete fractures

Abstract

There is a need to better characterize discrete fractures in contaminated hard rock aquifers to determine the fate of remediation injections away from boreholes and also to evaluate hydraulic fracturing performance. A synthetic cross borehole electrical resistivity study was conducted assuming a discrete fracture model of an existing contaminated site with known fracture locations. Four boreholes and two discrete fracture zones, assumed to be the dominant electrical and hydraulically conductive pathways, were explicitly modeled within an unstructured tetrahedral mesh. We first evaluated different regularization constraints starting with an uninformed smoothness-constrained inversion, to which a priori information was incrementally added. We found major improvements when (1) smoothness regularization constraints were relaxed (or disconnected) along boreholes and fractures, (2) a homogeneous conductivity was assumed along boreholes, and (3) borehole conductivity constraints that could be determined from a specific conductance log were applied. We also evaluated the effect of including borehole packers on fracture zone model recovery. We found the fracture zone conductivities with the inclusion of packers were comparable to similar trials excluding the use of packers regardless of electrical potential changes. The misplacement of fracture regularization disconnects (FRDs) can easily be misinterpreted as actual fracture locations. Conductivities within these misplaced disconnectsmore » were near the starting model value and removing smoothing between boreholes and assumed fracture locations helped in identifying incorrectly located FRDs. Analysis confirmed improved model sensitivity when regularization disconnects were (1) applied along the boreholes and fracture zones, and (2) FRDs were placed where actual fractures existed. Our findings demonstrate that structural constraints used after careful evaluation of a priori information are critical to improve imaging of fracture electrical conductivities, locations and orientations.« less

Authors:
 [1]; ORCiD logo [2];  [1]
  1. Rutgers University
  2. BATTELLE (PACIFIC NW LAB)
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1562516
Report Number(s):
PNNL-SA-90198
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Geophysics
Additional Journal Information:
Journal Volume: 78; Journal Issue: 3
Country of Publication:
United States
Language:
English
Subject:
geophysics, inversion, resistivity, fracture, contamination

Citation Formats

Robinson, Judith, Johnson, Timothy C., and Slater, Lee. Evaluation of known-boundary and resistivity constraints for improving cross-borehole DC electrical resistivity imaging of discrete fractures. United States: N. p., 2013. Web. doi:10.1190/geo2012-0333.1.
Robinson, Judith, Johnson, Timothy C., & Slater, Lee. Evaluation of known-boundary and resistivity constraints for improving cross-borehole DC electrical resistivity imaging of discrete fractures. United States. doi:10.1190/geo2012-0333.1.
Robinson, Judith, Johnson, Timothy C., and Slater, Lee. Wed . "Evaluation of known-boundary and resistivity constraints for improving cross-borehole DC electrical resistivity imaging of discrete fractures". United States. doi:10.1190/geo2012-0333.1.
@article{osti_1562516,
title = {Evaluation of known-boundary and resistivity constraints for improving cross-borehole DC electrical resistivity imaging of discrete fractures},
author = {Robinson, Judith and Johnson, Timothy C. and Slater, Lee},
abstractNote = {There is a need to better characterize discrete fractures in contaminated hard rock aquifers to determine the fate of remediation injections away from boreholes and also to evaluate hydraulic fracturing performance. A synthetic cross borehole electrical resistivity study was conducted assuming a discrete fracture model of an existing contaminated site with known fracture locations. Four boreholes and two discrete fracture zones, assumed to be the dominant electrical and hydraulically conductive pathways, were explicitly modeled within an unstructured tetrahedral mesh. We first evaluated different regularization constraints starting with an uninformed smoothness-constrained inversion, to which a priori information was incrementally added. We found major improvements when (1) smoothness regularization constraints were relaxed (or disconnected) along boreholes and fractures, (2) a homogeneous conductivity was assumed along boreholes, and (3) borehole conductivity constraints that could be determined from a specific conductance log were applied. We also evaluated the effect of including borehole packers on fracture zone model recovery. We found the fracture zone conductivities with the inclusion of packers were comparable to similar trials excluding the use of packers regardless of electrical potential changes. The misplacement of fracture regularization disconnects (FRDs) can easily be misinterpreted as actual fracture locations. Conductivities within these misplaced disconnects were near the starting model value and removing smoothing between boreholes and assumed fracture locations helped in identifying incorrectly located FRDs. Analysis confirmed improved model sensitivity when regularization disconnects were (1) applied along the boreholes and fracture zones, and (2) FRDs were placed where actual fractures existed. Our findings demonstrate that structural constraints used after careful evaluation of a priori information are critical to improve imaging of fracture electrical conductivities, locations and orientations.},
doi = {10.1190/geo2012-0333.1},
journal = {Geophysics},
number = 3,
volume = 78,
place = {United States},
year = {2013},
month = {5}
}

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