Advances in Borehole Seismic Fracture Diagnostics

Abstract

ABSTRACT Recent redesign of the hardware, software, and data-reduction techniques associated with a triaxial Borehole Seismic System 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, together 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 were previously inaccessible. The new electronics makes use of a null system and an improved calibration system which includes a synthetic event generator to produce sinusoidal signals of specified amplitude, frequency, and phase. These make readjustments to maintain system balance possible while the tool is in place downhole. A maximum likelihood event location scheme, which incorporates a newly developed algorithm based on the use of directional statistics, is used to compute the location of microseisms and error estimates for these locations. The accuracy of the redesigned system, based on the ability to locate perforation shots, indicates a 25 ft (7.6 m) uncertainty in the location of individual microseisms. This results in a fairly high level of confidence in the determination of the azimuth of the November 1, 1986, hydraulic fracture in the fluvial zone at DOE's Multiwell Experiment Site.