Abstract
In this study distributed optic fiber has been used to measure both the Rayleigh and Brillouin frequency shift of two different sandstone core samples under controlled hydrostatic confining and pore pressure in the laboratory. The Berea sandstone core is relatively homogeneous, whereas the Tako sandstone core is visibly heterogeneous with a coarse-grain and a fine-grain region. Rayleigh frequency has been found to have a superior performance over Brillouin frequency in terms of better consistency (less scattering) in the tests carried out. The strain gauge readings reveal considerable anisotropy in the stiffness of the Berea core between perpendicular (vertical) and parallel to the bedding (hoop) directions. The strains converted from Rayleigh frequency shift measurements agree reasonably well with readings by one of the four hoop strain gauge channels under increasing confining/pore pressure. For the Tako sandstone core, the contrast in the grain-size, and thus rock elastic properties, is clearly reflected in the hoop strain measurement by both strain gauges and distributed optic fiber. The outcomes of the test have demonstrated successfully the use of a single optic fiber for measuring rock strain response at different regions of a heterogeneous core sample along a continuous trajectory.
Highlights
Interferometric Synthetic Aperture Radar (InSAR) is a geodetic technique that can identify subsidence and uplift induced by anthropogenic activities such as subsurface fluid extraction, or injection into reservoir formations
The optical frequency domain reflectometry (OFDR) is the first attempt to utilize the principle of Rayleigh backscattering in optical fibers for strain measurement [6]
When the water injection pressure is stabilized at 0.5 MPa, the strain readings are initialized, and Brillouin and Rayleigh frequency shift measurements are performed respectively
Summary
Interferometric Synthetic Aperture Radar (InSAR) is a geodetic technique that can identify subsidence and uplift induced by anthropogenic activities such as subsurface fluid extraction, or injection into reservoir formations. Rayleigh scattered light changes its time-intensity as the environment orenvironment temperature) is conspicuous. Rayleigh scattered light changeswaveform its time-intensity waveform(strain as the aroundor thetemperature) optical fiber changes. Theoptical resulting shift in the frequency of theshift incident light, which reflects The optical frequency domain reflectometry (OFDR) is the first attempt to utilize the principle of Rayleigh backscattering in optical fibers for strain measurement [6]. 20 km@20 cm Rayleigh and Brillouin backscattering techniques have been successfully applied in measuring metallic plate strains [14] In this study, they are applied to measure frequency shift along the interval of a single optical fiber which is attached to and circles around the peripheral of two different sandstone cylindrical core samples under controlled confining/pore pressure conditions in the laboratory. The strains obtained are compared with those measured by strain gauges attached to the same core samples
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