Abstract
Hydraulic fracturing is performed in unconventional oil and gas and enhanced geothermal systems (EGS) reservoirs to enhance formation permeability and create a conductive pathway for fluid flow to one or multiple wellbores. Typically, energy production requires the existence of a densely naturally fractured medium or a complex hydraulic fracture network or, in some cases, both. When targeting a naturally fractured reservoir, stimulation is meant to provide a connection between these natural fractures to generate large regions of hydraulically connected rock for production. Although hydraulic fracturing is performed often in naturally faulted or fractured media, the geometry and effectiveness of the hydraulic fracture crossing natural discontinuities is not well-understood. To better understand hydraulic fracture interactions with discontinuities, laboratory scale hydraulic fracturing tests were performed which contained an idealized case of one large angled discontinuity, or natural fracture. Two-block sample sizes of 15×15×25 cm3were subjected to true triaxial confinement before wellbore placement and completion. Acoustic emissions (AE) were monitored during the fracture initiation, propagation, and fracture interaction process. An in depth post-test analysis was performed on AE microcrack data to characterize the hydraulic fracture and determine its effectiveness crossing the discontinuity. Individual microcrack mode of failure was determined as well as relative volumetric deformation using moment tension inversion techniques. Both sides of the natural fracture were compared in terms of source mechanism information of microcracks to determine if changes in failure characteristics occurred. Post-test visualization of the hydraulic fractures at the surface provided an image of severe fracture dimension reduction, particularly fracture width and crossing the natural fracture. AE characterization was compared with visual inspections to determine if AE source parameters correlated with observed post-fracture imaging data. Implications of drastic hydraulic fracture width reductions crossing natural fractures include under predictions of stimulated reservoir volume from field scale AE data, and reduced production.
