| dc.description.abstract | Low-frequency distributed acoustic sensing (LF-DAS) can be used as a monitoring tool for hydraulic fracture propagation using measured values of strain. To understand subsurface conditions with shear and normal stresses, a laboratory-scale hydraulic fracture experiment was performed to simulate the LF-DAS response to fracture propagation with embedded distributed optical fiber strain sensors. The objectives of this research were to generate hydraulic fractures of known geometry with shear and normal stresses, measure the strain response along distributed fiber sensors embedded in the sample, and use the results to reveal insights of fracture propagation.
The experiment was performed on a uniaxially-compressed, transparent 8-inch cube of epoxy with a radial initial flaw angled to the applied load. Water was injected into the epoxy block to generate a fracture with shear and normal stresses along the plane of the fracture. These experiments used distributed high-definition fiber optic strain sensors with tight spatial resolutions. The measured strains were compared to experiments with a purely normal stress component to understand how the zero-strain method for fracture geometry apply to the studied case when a shear stress on the fracture plane is introduced.
The experimentally acquired strain and strain-rate waterfall plots with shear and normal stresses on the fracture plane exhibit comparable results to the strain responses of purely normal stresses with a narrowing region of extension surrounded by compression as a fracture approached and intersected a fiber optic cable. However, unlike the experiments with purely normal stresses on the fracture plane, the introduction of a shear stress created an asymmetrical strain signature over the fracture plane. This dominant strain response on one side of the fracture plane suggests the existence of a shear stress on the plane of the fracture. | |
