Experimental Investigation Using Low-Frequency Distributed Acoustic Sensing for Two Parallel Propagating Fractures

Abstract

Abstract Low-frequency distributed acoustic sensing (LF-DAS) is a diagnostic tool for hydraulic fracture propagation in the far-field using measured values of strain. To understand subsurface conditions with multiple propagating fractures, 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 two hydraulic fractures of known geometry, measure the strain response along distributed fiber sensors embedded in the sample, and use the results to improve interpretations of field LF-DAS data when multiple fractures are approaching an observation well. The experiment was performed using a transparent 8-inch cube of epoxy with two-parallel radial initial flaws centered in the cube 2.6-inches apart. Fluid was injected into the sample to generate fractures along the initial flaws. The experiment used distributed high-definition fiber optic strain sensors with tight spatial resolutions. The sensors were embedded at two different locations on opposite sides of the initial flaws, serving as observation/monitoring locations. Pressure and fracture propagation were also recorded. This paper presents a workflow to model fracture geometries, and simulate the resulting strain along a fiber optic sensor. We employed finite element modeling to numerically solve the linear elastic equations of equilibrium continuity and stress-strain relationships. The simulation domain includes one-half of the 8-inch epoxy cube with two radial fractures. The measured strains from the experiment were compared to simulation results from the finite element model. The experimentally derived strain and strain-rate waterfall plots from this experiment show responses to both fractures propagating, while the fracture below took most of the fluid during the experiment. Interestingly, a fracture first began propagating from the upper of the two flaws, but once the lower fracture was initiated, it grew much more than the upper fracture. Both fibers were intercepted by the lower fracture, further verifying the strain signature as a fracture is approaching and intersecting an offset fiber. The zero-strain-rate method was applied to both fibers to dynamically estimate the propagation of the fracture fronts as they approached the fibers. The fracture growth behavior interpreted with the zero-strain-rate method compared well to the evolving fracture dimensions obtained from video-recording of the fracture geometries. The results from this work can be used in the field to reveal stress shadowing effects of two fractures and further increase our understanding of how LF-DAS can be used in the field to diagnose fracture propagation when multiple fractures are approaching an observation well.