Abstract
This study investigates a microseismic swarm induced by injection operations in the Arkema-Vauvert salt field. The seismic activity in this field is monitored only by two permanent 3-component stations deployed in two wells. This study focuses on a period of 21 months (2004 January-2005 September) during which 1214 seismic events are located. The seismic activity is divided into three periods correlating with the water injection operations, highlighting a migration of the seismicity toward a thrust fault connecting the injection well and the production well. A waveform analysis reveals S-wave anisotropy, and focal mechanisms are computed using P, Sv and Sh amplitudes manually measured on anisotropy-corrected seismograms. First, synthetic resolution tests assess the reliability of the focal mechanisms determination from the two 3-component stations deployed in the field. Synthetic data are generated for 1056 earthquakes with various focal mechanisms and are perturbed with noise. The results indicate that the type of focal mechanism is correctly retrieved for 74 per cent of the synthetic earthquakes, but the uncertainties of the strike and rake are significant (from 15 to 45). Next, the focal mechanisms are computed for 532 real earthquakes. The solutions primarily correspond to a dip-slip/thrust fault type with subvertical NE-SW and subhorizontal N-S to NW-SE nodal planes. Correlations between the focal mechanisms and the spatio-temporal distribution of the seismic activity are noteworthy. The study shows it is possible to reliably retrieve double-couple focal mechanisms for some faulting geometries with two 3-component seismological stations. However, the reliability of the focal mechanism retrieval depends on the station configuration. Therefore, the addition of further stations would improve the results.
Highlights
In the last 15 years, seismological monitoring of reservoirs has steadily increased
We focus on the spatio-temporal variability of a microseismic swarm (1200 events during 21 months) induced by the exploitation of Wells PA22 and PA23 in the NE portion of the field
We studied a microseismic swarm induced by injection operations in the Arkema-Vauvert salt-field (Wells PA22 and PA23)
Summary
In the last 15 years, seismological monitoring of reservoirs has steadily increased. The potential applications of this monitoring are numerous (Maxwell & Urbancic 2005). One very common monitoring network consists of a set of sensors deployed in wells to decrease the magnitude detection threshold, improve the signal-to-noise ratio and increase the sensitivity. Such a design has two main consequences: (1) it often depends on the availability of existing wells, and (2) the deployment cost is high. These factors limit the number of sensors used for such surveys
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