| Title | Field testing of modular borehole monitoring with simultaneous distributed acoustic sensing and geophone vertical seismic profiles at Citronelle, Alabama |
| Publication Type | Journal Article |
| Year of Publication | 2015 |
| Authors | Daley, TM, Miller, D, Dodds, K, Cook, P, Freifeld, BM |
| Journal | Geophysical Prospecting |
| Pagination | n/a - n/a |
| Date Published | Jan-10-2015 |
| Keywords | Acquisition, Borehole Geophysics, Fibre-optic DAS., Seismics |
| Abstract | A modular borehole monitoring concept has been implemented to provide a suite of
well-based monitoring tools that can be deployed cost effectively in a flexible and robust
package. The initial modular borehole monitoring system was deployed as part
of a CO2 injection test operated by the Southeast Regional Carbon Sequestration
Partnership near Citronelle, Alabama. The Citronelle modular monitoring system
transmits electrical power and signals, fibre-optic light pulses, and fluids between the
surface and a reservoir. Additionally, a separate multi-conductor tubing-encapsulated
line was used for borehole geophones, including a specialized clamp for casing clamping
with tubing deployment. The deployment of geophones and fibre-optic cables allowed
comparison testing of distributed acoustic sensing. We designed a large source
effort (>64 sweeps per source point) to test fibre-optic vertical seismic profile and
acquired data in 2013. The native measurement in the specific distributed acoustic
sensing unit used (an iDAS from Silixa Ltd) is described as a localized strain rate.
Following a processing flow of adaptive noise reduction and rebalancing the signal
to dimensionless strain, improvement from repeated stacking of the source was observed.
Conversion of the rebalanced strain signal to equivalent velocity units, via
a scaling by local apparent velocity, allows quantitative comparison of distributed
acoustic sensing and geophone data in units of velocity. We see a very good match
of uncorrelated time series in both amplitude and phase, demonstrating that velocityconverted
distributed acoustic sensing data can be analyzed equivalent to vertical
geophones. We show that distributed acoustic sensing data, when averaged over an
interval comparable to typical geophone spacing, can obtain signal-to-noise ratios of
18 dB to 24 dB below clamped geophones, a result that is variable with noise spectral
amplitude because the noise characteristics are not identical. With vertical seismic profile
processing, we demonstrate the effectiveness of downgoing deconvolution from
the large spatial sampling of distributed acoustic sensing data, along with improved
upgoing reflection quality. We conclude that the extra source effort currently needed
for tubing-deployed distributed acoustic sensing vertical seismic profile, as part of a
modular monitoring system, is well compensated by the extra spatial sampling and
lower deployment cost as compared with conventional borehole geophones. |
| URL | http://doi.wiley.com/10.1111/1365-2478.12324http://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2F1365-2478.12324 |
| DOI | 10.1111/1365-2478.12324 |
| Short Title | Geophysical Prospecting |
