Detailed Joint Structure in a Geothermal
Reservoir from Studies of Induced Microearthquake Clusters
W. Scott Phillips, Nambe Geophysical, Inc.
Leigh S. House, Los Alamos National Laboratory
Michael C. Fehler, Los Alamos National Laboratory
Submitted to the Journal of Geophysical Research
Published Journal of Geophysical Research, 1997, 102, 11745-11763.
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Microearthquake clusters form distinct, planar patterns within five study regions of a
geothermal reservoir undergoing hydraulic fracturing at Fenton Hill, New Mexico. The
patterns define individual, slipping joint surfaces of dimension 40 to 120 m, containing 80
to 150 events each. Sharp, straight edges truncate the clusters; we interpret these as
marking intersections with aseismic joints. Each edge orientation is consistent with an
intersection between the active joint and a plane orientated parallel to one of the other
clusters we identify. Therefore, it appears that cluster shapes constrain the geometry of
seismic and aseismic joints; both could be important components of the fluid-flow
network. The distribution of inferred slip plane orientations is consistent with, but fails to
provide sufficient constraint to differentiate conclusively between two, very different, stress
field estimates, one measured using pressurization and wellbore breakouts, the other using
focal mechanisms of the largest microearthquakes. An impermeable joint model, requiring
pore pressure in excess of the normal stress on a joint before slip can occur, was
inconsistent with many of the inferred slip plane orientations.
The high-quality locations were possible because events from the same cluster generated
nearly similar waveforms, permitting the precise determination of relative arrival times.
Standard deviations of arrival-time residuals fall between 0.1 and 1.1 ms for these clusters.
Major axes and aspect ratios of the 90% confidence ellipsoids range from 6 to 28 m and
1.5 to 8, respectively. Small events dominate the seismic energy release and thoroughly
populate the identified, active joints, allowing the hypocenters to reflect details of the joint
To further investigate the reservoir structure, we applied a source-array, slant-stack
technique to waveforms from the well-located clusters, yielding directions that scattered
energy left each cluster. By studying paths of scattered waves, we expected to pinpoint
impedance contrasts that might have indicated concentrations of fluid-filled joints.
However, results show that scattered energy in the S-wave coda left the source region in
the same direction as the direct S wave. Direct waves may have excited borehole tube
waves that became trapped in the vicinity of the geophone tool, overwhelming any energy
scattered from the reservoir.