Title: Distributed Acoustic Sensing as a Distributed Hydraulic Sensor in Fractured Bedrock

Abstract

Distributed acoustic sensing (DAS) was originally intended to measure oscillatory strain at frequencies of 1 Hz or more on a fiber optic cable. Recently, measurements at much lower frequencies have opened the possibility of using DAS as a dynamic strain sensor in boreholes. A fiber optic cable mechanically coupled to a geologic formation will strain in response to hydraulic stresses in pores and fractures. A DAS interrogator can measure dynamic strain in the borehole, which can be related to fluid pressure through the mechanical compliance properties of the formation. Because DAS makes distributed measurements, it is capable of both locating hydraulically active features and quantifying the fluid pressure in the formation. We present field experiments in which a fiber optic cable was mechanically coupled to two crystalline rock boreholes. The formation was stressed hydraulically at another well using alternating injection and pumping. The DAS instrument measured oscillating strain at the location of a fracture zone known to be hydraulically active. Rock displacements of less than 1 nm were measured. Laboratory experiments confirm that displacement is measured correctly. These results suggest that fiber optic cable embedded in geologic formations may be used to map hydraulic connections in three-dimensional fracture networks. Amore » great advantage of this approach is that strain, an indirect measure of hydraulic stress, can be measured without beforehand knowledge of flowing fractures that intersect boreholes. The technology has obvious applications in water resources, geothermal energy, CO₂ sequestration, and remediation of groundwater in fractured bedrock.« less

Authors:

Becker, M. W.  ^[1];

• Search DOE PAGES for author "Becker, M. W."
• Search DOE PAGES for ORCID "0000-0001-8242-0591"

Coleman, T. I.  ^[2];

• Search DOE PAGES for author "Coleman, T. I."
• Search DOE PAGES for ORCID "0000-0002-2082-2314"

Ciervo, C. C. ^[1]

---

+ Show Author Affiliations

1. California State Univ. (CalState), Long Beach, CA (United States)
2. Silixa, LLC, Missoula, MT (United States)

Publication Date:

Fri Aug 14 00:00:00 EDT 2020

Research Org.:

California State Univ. (CalState), Long Beach, CA (United States)

Sponsoring Org.:

USDOE Office of Energy Efficiency and Renewable Energy (EERE)

OSTI Identifier:

1799240

Grant/Contract Number:  

EE0006763

Resource Type:

Accepted Manuscript

Journal Name:

Water Resources Research

Additional Journal Information:

Journal Volume: 56; Journal Issue: 9; Journal ID: ISSN 0043-1397

Publisher:

American Geophysical Union (AGU)

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; Environmental Sciences & Ecology; Marine & Freshwater Biology; Water Resources