Field testing of modular borehole monitoring with simultaneous distributed acoustic sensing and geophone vertical seismic profiles at Citronelle, Alabama

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 velocity‐converted 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.

On the Broadband Instrument Response of Fiber‐Optic DAS Arrays

“…To determine the DAS instrument response, we must first convert the DAS strain and seismometer velocity values into a consistent format. Daley et al () showed how the array‐nature of DAS can be used to convert DAS array strain values into particle velocity values in the frequency‐wavenumber (FK) domain utilizing the phase velocity relation . This FK‐rescaling technique reformulates as …”

“…Generally, DAS studies have focused on seismic wave phase information, which is sufficient to model seismic wavefield velocities, for example, in vertical seismic profiling (Daley et al, ; Mateeva et al, ), ambient noise velocity inversions (Ajo‐Franklin et al, ; Dou et al, ; Zeng et al, ), and earthquake phase identification (Ajo‐Franklin et al, ; Jousset et al, ; Lindsey et al, ; Yu et al, ). However, true ground motion amplitudes are necessary for many other seismological processing tasks, including full‐waveform inversion, AVO analysis, moment tensor inversion, and attenuation analysis, which the DAS community will likely investigate in the near future (Cole et al, ; Paitz et al, ).…”

Progress in monitoring strategies for risk reduction in geologic CO2 storage

“…Borehole seismic methods such as vertical seismic profiles (VSP), 3D-VSP, and continuous active-source seismic monitoring (CASSM) (Daley et al, 2008(Daley et al, , 2015aCheng et al, 2010) have application for higher spatial resolution, better signal to noise characteristics, easier direct calibration, and more rapid time-lapse monitoring near wellbores (Coueslan, 2013). These methods require wellbore access for receiver placement and provide information at generally higher spatial resolution in a more limited subsurface region.…”

“…A minimum number of such seismic imaging surveys might consist of one baseline followed by a repeat survey, although as the CGS site activity progresses additional surveys might be beneficial. Advances in technology, such as distributed acoustic sensing with fiber optics, have potential to greatly reduce the cost of repeat surveys via low-cost permanent sensing cables (Daley et al, 2015a).…”

Vertical Seismic Profile Complete Session

“…Despite the success of these DAS field trials, there remain challenges and problems for current DAS acquisition approaches and systems, such as low S/N, lack of transverse wave motion sensitivity and uncertainty in receiver depth, which still limit the application of DAS technology (Daley, 2015). There is also a need for further validation and improvement of DAS recording systems as well as DAS processing workflows, in addition to tests of DAS VSP recording in active wells and full evaluation of their utility and business value.…”