Microcrack Damage Observations near Coalesced Fractures Using Acoustic Emission

Abstract

Hydraulic fracturing is a common practice in several industries and environments, including energy production using enhanced geothermal systems, hydrocarbon extraction from unconventional oil and/or gas reservoirs, and mining and civil engineering excavation methods. Understanding damage related to the coalesced fractures induced by hydraulic fracturing and the surrounding material is fundamental when efforts to predict material and system behavior are sought. Inducing fracture networks in rock can create large amounts of microcracking in surrounding regions that are not connected to the wellbore. The degree of microcracking can vary depending on fluid, rock type, stress/temperature boundary conditions, as well as inherent material properties and heterogeneities. Regions of rock containing microcracks near coalesced macro-scale fractures can behave differently than the original matrix material due to the permanent structural changes. Understanding of how the coalesced fractures can interact with the surrounding rock containing microcracks requires the characterization of damage in terms of physical property evolution. In this study, a laboratory hydraulic fracture test was performed on a two-block specimen separated by a discontinuity as an analogue to a large natural fracture. The induced hydraulic fracture was monitored with acoustic emissions (AE) throughout the initiation and propagation stages. Individual AE event, source characterization was performed to record the failure mechanism and relative volumetric deformation induced by microcracking. Source characteristics were used in conjunction with cloud-based event density techniques to determine regions of differing damage within the cloud of microcracks. Quantitative three-dimensional event density imaging results were compared with permeability measurements on sub-cores taken from the specimen post-test.