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. 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, CO 2 sequestration, and remediation of groundwater in fractured bedrock. Plain Language Summary A technology developed for measuring vibrations on fiber optic cable, distributed acoustic sensing, is used to measure strain in bedrock in response to pumping and injection. This technology is extremely sensitive to dynamic strain, measuring displacements approaching the size of a water molecule. Field tests showed that fluid-induced strain in rock exhibits complex geometry. The technology can be used to better understand flow in bedrock, optimize geothermal energy extraction, and identify leakage from subsurface injection systems.

From Fluid Flow to Coupled Processes in Fractured Rock: Recent Advances and New Frontiers

“…Becker et al. (2020) showed that distributed sensing with fiber optics can probe nano strain oscillations across flowing fractures along a borehole drilled through a fracture network, thus opening avenues to identify such flowing fractures in a fractured reservoir. Z. Zhang et al.…”

“…Thermo-Hydro-Mechanical Interactions Within a Fracture Network Becker et al (2020) showed that distributed sensing with fiber optics can probe nano strain oscillations across flowing fractures along a borehole drilled through a fracture network, thus opening avenues to identify such flowing fractures in a fractured reservoir. Z. used coupled forward hydro-mechanical modeling to show how low-frequency DAS and DSS signals can inform interactions between flowing and non-flowing fractures within a fracture network.…”

Dynamic motion monitoring of a 3.6 km long steel rod in a borehole during cold-water injection with distributed fiber-optic sensing

“…Also, it is well known that for FIMT type of installations, the gel filling allows for a creep and differential movement of fibers with respect to its surroundings which makes strain sensing unreliable for greater deformations and longer periods (e.g. Lipus et al, 2018, Becker et al, 2020. However, a creep over many meters or even kilometers is most likely improbable.…”

On the Comparison of Records from Standard and Engineered Fiber Optic Cables at Etna Volcano (Italy)

“…Optical fibers are able to carry light and propagate it over long distances with minimum attenuation, and so is a preferred medium for broad-band transmission systems around the world. When using the scattering properties of the glass fibers, the fiber can be turned into a distributed sensing system that provides substantial advantages over conventional electronic sensors [ 1 , 2 , 3 , 4 ]. Optical fiber sensors are able to measure external environment variables (e.g., temperature and strain; [ 5 ]) along their length with fine spatial intervals.…”