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. A 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, CO2sequestration, and remediation of groundwater in fractured bedrock.
Hydraulic fracturing enables oil and gas extraction from low‐permeability reservoirs, but there remains a need to reduce the environmental footprint. Resource use, contaminant‐bearing flowback water, and potential for induced seismicity are all scaled by the volume of injected fluid. Furthermore, the greenhouse gas emissions associated with each extracted unit of energy can be decreased by improving resource recovery. To minimize fluid use while maximizing recovery, a rapidly computing model is developed and validated to enable the thousands of simulations needed to identify opportunities for optimization. Lower pumping pressure approaches that minimize pressure loss through the wellbore perforations combined with nonuniform spacing are shown to be capable of substantially reducing fluid consumption and/or increasing created fracture surface area when the stress variation is mainly from fracture interaction instead of in situ stress. When in situ stress variation is dominant, “limited entry” methods promote more uniform growth but with higher pumping pressures and energy consumption.
more » « less- Award ID(s):
- 1645246
- NSF-PAR ID:
- 10448061
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 22
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 12870-12880
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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