Focal mechanism determination of induced microearthquakes in an oil field using full waveforms from shallow and deep seismic networks

Abstract

A new, high frequency, full waveform matching method is used to study the focal mechanisms of small, local earthquakes induced in an oil field, which are monitored by a sparse near-surface network and a deep borehole network. The determined source properties are helpful for understanding the local stress regime in this field. During the waveform inversion, we maximize both the phase and amplitude matching between the observed and modeled waveforms. We also use the polarities of the first P-wave arrivals and the average S/P amplitude ratios to better constrain the matching. An objective function is constructed to include all four criteria. For different hypocenters and source types, comprehensive synthetic tests show that our method is robust to determine the focal mechanisms under the current array geometries, even when there is considerable velocity inaccuracy. The application to several tens of induced microseismic events showed satisfactory waveform matching between modeled and observed seismograms. The majority of the events have a strike direction parallel with the major NE-SW faults in the region, and some events trend parallel with the NW-SE conjugate faults. The results are consistent with the in-situ well breakout measurements and the current knowledge on the stress direction of this region. The source mechanisms of the studied events together with the hypocenter distribution indicate that the microearthquakes are caused by the reactivation of preexisting faults. We observed that the faulting mechanism varies with depth, from strike-slip dominance at shallower depth to normal faulting dominance at greater depth.

Predicting Ground Motion from Induced Earthquakes in Geothermal Areas

“…For natural earthquakes, ground motions in different Predicting Ground Motion from Induced Earthquakes in Geothermal Areas regional stress fields can be significantly different for the same magnitude and source-to-site distance (e.g., McGarr, 1984;Bommer et al 2003;Convertito and Herrero, 2004). For induced seismicity, local stress conditions are mostly driven by field operations, which can reactivate existing faults or generate new ones with mechanisms different to those expected from the regional stress field (Oppenheimer, 1986;Li et al, 2011). In Figure 5 we show the PGA residuals as a function of r hyp , M w , and depth.…”

Passive seismic imaging of subwavelength natural fractures: theory and 2-D synthetic and ultrasonic data tests

“…Using full wavefield information, with less reliance on phase picking, promises to provide a more complete and reliable monitoring. Recent studies have shown that using waveform information could yield better constraints of microseismic sources, and thus induced fractures (Duncan & Eisner 2010;Song & Toksöz 2011;Li et al 2011;Sun et al 2015). In addition, the frequency content of downhole microseismic data is often high, above 100 Hz, which implies the ability to image small-scale structures.…”

Back azimuth constrained double-difference seismic location and tomography for downhole microseismic monitoring

“…Microseismic monitoring has gradually become a common technique to characterize the development of hydraulic fracturing in tight sand and shale gas reservoirs (Rutledge & Phillips 2003;Maxwell et al 2010;Zimmer 2011b;Li et al 2011;Yang et al, 2013). By analyzing the associated microseismicity during a hydraulic fracturing operation, it can provide important insight into the fracture development pattern and fluid migration (Zhang et al 2009).…”