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Abstract
Precise source locations can be obtained from microseismic data using arrival time picks that are based on grouping similar events and correlating waveforms. The correlations allow much more consistent estimates of phase arrival times and greatly reduces the random error associated with estimation of onset times introduced by variations of signal-to-noise ratios. Relative misfits can be improved 4- to 5-fold, producing much sharper images of the microseismic cloud. Hydraulic fracture microseismicity is often characterized by repetitive source mechanisms, resulting in similar waveforms for event populations that gradually vary with angular position from a given receiver. The similarity readily allows the application of waveform correlation and precise mapping.
I show several microseismic cases where precise mapping and source mechanism analysis are obtained. The picture revealed is different from typical dispersed clouds of microseismicity, showing more planar geometry and event generation tightly controlled by the stratigraphy. The observations are inconsistent with the microseismicity being a secondary effect of leak-off and natural fracture activation. Instead, I interpret the repetition of the microseismic signal to be due to vertical hydraulic fractures growing through the regular mechanical discontinuities of flat-lying stratigraphy, where shearing is generated on bedding surfaces or on the edge of the hydraulic fracture itself in response to a bedding-controlled rotation of local stress. In this sense, the microseismicity provides a direct picture of extension fracture growth through a reservoir, highlighting the deformation at and near bedding boundaries as the hydraulic fracture shears through or along layer interfaces.
Biography
Jim Rutledge has been a member of Schlumberger’s Microseismic Services since October 2012. He spent most of his career, from 1984 to 2012, as a staff seismologist at Los Alamos National Lab. From 2004 to 2012 he also worked as a consultant for Schlumberger Cambridge Research and MEQ Geo, Inc. He received a BS in Geology from Pennsylvania State University and an MS in Geophysics from the University of Arizona. Starting in 1989, Jim has led and participated in several studies that demonstrated the uses of microseismic monitoring in oil, gas and geothermal fields.
