An Analysis of Focal Parameters with Application to West Texas Micro-Earthquakes

Date

2017-12

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Abstract

Two types of seismic events related to hydrocarbon production have been extensively studied recently: microseismic events associated with hydraulic fracturing in unconventional reservoirs and induced seismicity associated with fluid extraction and wastewater injection in oil fields. This dissertation investigates the location errors of microseismic events due to anisotropy in velocity models and micro-earthquake distribution, magnitudes and focal mechanism solutions in the Permian Basin.

Seismic anisotropy, a typical property in fractured sedimentary rocks, may bias the identification of the location of microseismic events if using isotropic models, a common practice in seismic monitoring. To quantitatively assess such biases, we applied a nonlinear location method to synthetic travel time data in seven models with different levels of anisotropy. An effective isotropic velocity model is assumed in locating the events. The average location error is 59 m for the isotropic case, about the size of the grid interval in the velocity model, and is 156 m (158 m), 237 m (244 m), and 258 m (265 m) for 5%, 10% and 15% VTI (HTI) cases, respectively. These results suggest that ignoring 5% seismic anisotropy can cause significant biases in microseismic event locations and anisotropy should be included in the velocity model to improve the location accuracy.

Hydrocarbon production activities are thought to be inducing small earthquakes in the mid U.S. and increasing earthquake hazards. In this study, we have analyzed seismograms during March 2009 to February 2010 from the USArray Transportable Array in the Permian Basin and located 126 micro-earthquakes with magnitude ranging from 1.6 to 3.2. The events distribute broadly in the region except the northwest part. Fault plane solutions of 16 events are solved using initial P-wave polarities and reflect normal and strike-slip faulting. The resolved fault strikes vary from NW-SE in the southwest of the study area to E-W in the south and southeast and NE-SW in the northeast. Although focal mechanisms agree with current tectonic stress in west Texas, the distribution and focal mechanisms of the micro-earthquakes suggest local stress change might be the cause to trigger existing faults. Analyzing two relatively large events with the injection history indicates they are likely induced by wastewater injection.

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Keywords

Micro-earthquakes, Location, Focal mechanisms, Seismic anisotropy, Induced seismicity

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