Abstract
Distributed acoustic sensing (DAS) has great potential for monitoring natural-resource reservoirs and borehole conditions. However, the large volume of data and complicated wavefield add challenges to processing and interpretation. In this study, we demonstrate that seismic interferometry based on deconvolution is a convenient tool for analyzing this complicated wavefield. We also show the limitation of this technique, in that it still requires good coupling to extract the signal of interest. We extract coherent waves from the observation of a borehole DAS system at the Brady geothermal field in Nevada. The extracted waves are cable or casing ringing that reverberate within a depth interval. These ringing phenomena are frequently observed in the vertical borehole DAS data. The deconvolution method allows us to examine the wavefield at different boundary conditions and separate the direct waves and the multiples. With these benefits, we can interpret the wavefields using a simple 1D string model and monitor its temporal changes. The velocity of this wave varies with depth, observation time, temperature, and pressure. We find the velocity is sensitive to disturbances in the borehole related to increasing operation intensity. The velocity decreases with rising temperature. The reverberation can be decomposed into distinct vibration modes in the spectrum. We find that the wave is dispersive and the fundamental mode propagates with a large velocity. This interferometry method can be useful for monitoring borehole conditions or reservoir property changes using densely-sampled DAS data.
Highlights
Fiber-based sensors have been applied in the oil and gas industry for borehole monitoring since early 1990s [1]
The PoroTomo project was a four-week experiment conducted during March 2016 in which the team performed vibroseis experiments under varying pumping operations and collected a variety of geophysical data including surface Distributed acoustic sensors (DAS) (DASH), borehole DAS (DASV), nodal geophones, InSAR, GPS, pressure, and temperature (DTS) data
We focus on the DASV, the distributed temperature sensors (DTS) temperature, and the pressure data from the PoroTomo project [21,22]
Summary
Fiber-based sensors have been applied in the oil and gas industry for borehole monitoring since early 1990s [1]. The PoroTomo project was a four-week experiment conducted during March 2016 in which the team performed vibroseis experiments under varying pumping operations and collected a variety of geophysical data including surface DAS (DASH), borehole DAS (DASV), nodal geophones, InSAR, GPS, pressure, and temperature (DTS) data. Previous studies have analyzed the DASV, DTS, and pressure data. Miller et al [14] investigated the DASV data to find the signatures of earthquakes, vibroseis sweeps, and responses to different borehole processes They suggested that reverberations on the upper half of the DASV are due to ringing of the casing and the DAS cable. We focus on the DASV, the DTS temperature, and the pressure data from the PoroTomo project [21,22]. The DC and RMS amplitudes are calculated using a min time window with 50% overlap
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