Effect of Pre-Existing Fractures on Fracture Location using Chattering Dust

Abstract:

Microseismic events have been used to interpret the location of fractures during formation and deformation. Recent research has shown that acoustic emissions from chattering dust can be used in the laboratory to assess relative apertures of fractures, to track flow paths in intersecting fracture networks, and to map fluid currents in non-uniform aperture fractures. Chattering dustis a chemically-reactive source that emits acoustic signals in the 20 kHz – 500 kHz range as it is transported through a system.A key question arises about the effect of adjacent fractures on the interpretation of fracture location from an internal transportable source. In this study, chattering dust was released into different fractures within a set of parallel fractures to determine the effect of adjacent fractures on the interpretation fracture location.

Samples were fabricated from blocks of transparent acrylic (~150 mm x 150 mm) with a 78 mm thick block representing an “intact” portion and with fractures created from 24 mm thick blocks separated by a 2 mm gap. Planes with four acoustic emission (AE) sensors (Mistra F15 alpha 100 – 400 kHz) each were located on the intact and the fracture sides of a sample. 3D printed asperities were inserted into water-filled fractures to create contact between the fracture surfaces.

Accurate interpretation of fracture location was determined to depend on the symmetry in fracture distribution about the fracture plane containing the source. When the source fracture had an equal number of fractures on either side, the interpreted location from dust AE matched the location. However, a shift in location occurred when the fracture distribution was not symmetric around the source. The shift arises from the additional time delays from the presence of more fractures between the source fracture and one of the sensor planes. This suggests that accurate location of fractures from acoustic emissions or induced seismicity are affected by other fractures in the system.

Acknowledgment: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Geosciences Research Program under Award Number (DE-FG02-09ER16022).