Title: The direct-current response of electrically conducting fractures excited by a grounded current source

Abstract

Hydraulic fracture stimulation of low permeability reservoir rocks is an established and cross–cutting technology for enhancing hydrocarbon production in sedimentary formations and increasing heat exchange in crystalline geothermal systems. Whereas the primary measure of success is the ability to keep the newly generated fractures sufficiently open, long–term reservoir management requires a knowledge of the spatial extent, morphology, and distribution of the fractures — knowledge primarily informed by microseismic and ground deformation monitoring. To minimize the uncertainty associated with interpreting such data, we investigate through numerical simulation the usefulness of direct-current (DC) resistivity data for characterizing subsurface fractures with elevated electrical conductivity by considering a geophysical experiment consisting of a grounded current source deployed in a steel cased borehole. In doing so, the casing efficiently energizes the fractures with steady current. Finite element simulations of this experiment for a horizontal well intersecting a small set of vertical fractures indicate that the fractures manifest electrically in (at least) two ways: (1) a local perturbation in electric potential proximal to the fracture set, with limited farfield expression and (2) an overall reduction in the electric potential along the borehole casing due to enhanced current flow through the fractures into the surrounding formation. Themore » change in casing potential results in a measurable effect that can be observed far from fractures themselves. Under these conditions, our results suggest that farfield, timelapse measurements of DC potentials can be interpreted by simple, linear inversion for a Coulomb charge distribution along the borehole path, including a local charge perturbation due to the fractures. As a result, this approach offers an inexpensive method for detecting and monitoring the time-evolution of electrically conducting fractures while ultimately providing an estimate of their effective conductivity — the latter providing an important measure independent of seismic methods on fracture shape, size, and hydraulic connectivity.« less

Authors:

Weiss, Chester J. ^[1]; Aldridge, David F. ^[1]; Knox, Hunter A. ^[1]; Schramm, Kimberly A. ^[1]; Bartel, Lewis C. ^[2]

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+ Show Author Affiliations

1. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
2. CARBO Ceramics, Inc., Houston, TX (United States)

Publication Date:

Sun May 01 00:00:00 EDT 2016

Research Org.:

Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

Sponsoring Org.:

Sponsor Funded; USDOE

OSTI Identifier:

1333854

Report Number(s):

SAND-2015-2297J
Journal ID: ISSN 0016-8033; 579468

Grant/Contract Number:  

AC04-94AL85000

Resource Type:

Accepted Manuscript

Journal Name:

Geophysics

Additional Journal Information:

Journal Volume: 81; Journal Issue: 3; Journal ID: ISSN 0016-8033

Publisher:

Society of Exploration Geophysicists

Country of Publication:

United States

Language:

English

Subject:

58 GEOSCIENCES; 30 DIRECT ENERGY CONVERSION; 4D; resistivity; finite element; fracture stimulation; geothermal