An experimental investigation into hydraulic fracture propagation under different applied stresses in tight sands using acoustic emissions

摘要

Hydraulic fracturing is crucial in unlocking tight gas and shale gas and oil resources. The success of any hydraulic fracture depends on the fracture dimensions and proppant placement. Microseismicity (MS) is now a common mapping hydraulic fracture technique. In this paper, we report on the acoustic emission (AE) monitoring during laboratory hydraulic fracture studies conducted on Lyons sandstone samples under different applied external stress. We compute AE hypocenter locations, analyze event frequency content and compute focal mechanisms (FMS). Shear failure reflected in the focal mechanism is more common than tensile failure. AE locations agree well with visual expression of fractures intersection on the sample surface. Fracture orientation and development is controlled by the direction and magnitude of applied stresses. Below a critical stress magnitude, the sample inhomogeneities control the hydraulic fracture development. At lower stresses, the hypocenters indicate a greater stimulated reservoir volume, suggesting stage spacing should consider the magnitudes of in-situ stresses. The sequential acoustic emission activity is found to be episodic and discretized implying fracture propagation is not a simple continuum. SEM fracture morphology studies document a complex and non-planar development of the hydraulic fractures, affirming shearing consistent with the FMS. Furthermore, SEM imaging suggests a surface area creation far more than simple planar models would imply.