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 fractures that grow in close proximity to one an other interact and compete for fluid that is injected to the wellbore, leading to dominance of some fractures and suppression of others. This phenomenon is ubiquitously encountered in stimulation of horizontal wells in the petroleum industry and it also bears possible relevance to emplacement of multiple laterally propagating swarms of magma‐driven dykes. Motivated by a need to validate mechanical models, this paper focuses on laboratory experiments and their comparison to simulation results for the behavior of multiple, simultaneously growing hydraulic fractures. The experiments entail the propagation of both uniformly and nonuniformly spaced hydraulic fractures by injection of glucose or glycerin‐based solutions into transparent (polymethyl methacrylate) blocks. Observed fracture growth is then compared to predictions of a fully coupled, parallel‐planar 3D hydraulic fracturing simulator. Results from experiments and simulations confirm the suppression of inner fractures when the spacing between the fractures is uniform. For certain non‐uniform spacing, both experiments and simulations show mitigated suppression of the central fractures. Specifically, the middle fracture in a 5‐fracture array grows nearly equally to the outer fractures from the beginning of injection. Furthermore, with some delay, the other two fractures that are suppressed with uniformly spaced configurations grow, and eventually achieve a velocity exceeding the other three fractures in the array. Hence, these experiments give the first laboratory evidence of a model‐predicted behavior wherein certain nonuniform fracture spacings result in drastic increases in the growth of all fractures within the array.
more » « less- Award ID(s):
- 1645246
- NSF-PAR ID:
- 10450821
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 126
- Issue:
- 1
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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