Experimental Investigation Using Low-Frequency Distributed Acoustic Sensing for Propagating Fractures with Shear and Normal Stresses

Abstract

Abstract Low-frequency distributed acoustic sensing (LF-DAS) at a far field observation well is a diagnostic 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 under these conditions. The objectives of this research are to generate hydraulic fractures of known geometry when shear and normal stresses are present, measure the strain response along distributed fiber sensors embedded in the sample, and use the results to enhance the interpretation of LF-DAS data measured at observation wells. The experiment has been performed on a uniaxially-compressed, transparent cube made of epoxy with a radial initial flaw angled non-perpendicular to the applied stress. The epoxy cube is 8-in in each dimension. Fluid was injected into the sample 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 sensors were embedded at four different locations, serving as observation/ monitoring locations. Pressure and fracture propagation were also recorded. The measured strains were compared to experiments with purely normal stress components to understand how the zero-strain and zero-strain rate methods for fracture geometry can be applied to the studied case when shear stress on the fracture plane is introduced. The experimentally derived strain and strain-rate waterfall plots from this study with shear and normal stresses exhibit a narrowing region of extension surrounded by compression as the fracture approached and intersected a fiber optic cable. However, unlike the experiments with purely normal stress on the fracture plane, the introduction of shear stress has created an asymmetrical strain signature over the fracture plane. The dominant strain response on one side of the fracture plane suggests the existence of a shear stress on the plane of the fracture. This information can be used in the field to reveal the stress status during fracture propagation and more realistically evaluate the fracture front.