Microseismic Activity Observed During Depressurization of an Oil Storage Cavern in Rock Salt

摘要

In November 1978, the Los Alamos National Laboratory installed a triaxial downhole geophone package at the Bryan Mound Strategic Petroleum Reserve, a salt dome near Freeport, Texas. Monitoring was for the purpose of detecting microseismic activity during the depressurization of one of the oil storage caverns. Seismic activity started soon after the start of depressurization, reached a peak 5 days later, and rapidly died off. Nineteen locatable microearthquakes with magnitudes between -1 and -2 were observed during and after depressurization. All but two of these events were located near the side of the oil storage cavern. Because the shear-wave spectra of these events were characterized by well-defined corner frequencies we were able to calculate the radius of the rupture surface, the seismic moment, and the stress drop from the source spectra. Estimates of the source radius vary from 15 to 100 m, stress drops range from 0.3 to 0.01 bar, and seismic moments range from 3 x 10 exp 14 to 3 x 10 exp 15 dyne/cm. Important conclusions from this study are that comparatively small changes in the internal pressure of oil storage caverns may be enough to cause failure in the salt near the cavern walls and that downhole microseismic surveys can locate zones near large underground excavations that are undergoing rock failure. (ERA citation 06:032987)