Advances in Borehole Seismic Fracture Diagnostics

Abstract

Summary Recent redesign of the hardware, software, and data-reduction techniques associated with a triaxial borehole seismic system (BSS) have made possible better estimates of hydraulic fracture geometry. The redesigned triaxial system now incorporates three geophones per axis and provides 30 times the downhole gain of the replaced system, resulting in improved signal-to-noise ratios. This stronger signal, with an increased digitization rate for each of the six simultaneously digitized channels from two borehole seismic tools, has made possible the acquisition and processing of data that previously were inaccessible. The new electronics makes use of a null system and an improved calibration system that includes a synthetic event generator to produce sinusoidal signals of specified amplitude, frequency, and phase. These make readjustments to maintain system balance while the tool is in place downhole. A maximum likelihood event location scheme, incorporating an algorithm based on the use of directional statistics, is used to compute the location of microseisms and error estimates for these locations. Based on the ability to locate perforation shots, a 25-ft [7.6-m] uncertainty in the location of individual microseisms is the estimated accuracy of the redesigned system. This resulted in a fairly high level of confidence in the determination of the azimuth of the Nov. 1, 1986, hydraulic fracture in the fluvial zone at DOE's Multiwell Experiment (MWX) site.