Distributed Acoustic Sensing for Geophysical Monitoring

Abstract

Vertical seismic profiling (VSP) is commonly used in the oil and gas industry for better subsurface imaging and characterisation, as well as for providing depth calibration for surface seismic. The use of VSP in mineral exploration and mine planning is not very common mostly due to the small diameter and stability of the boreholes, as well as the relatively high cost of such surveys. These issues can be mitigated by using cheap and potentially disposable borehole sensors, such as fibre-optic cables utilised in distributed acoustic sensing (DAS). The questions we want to answer in this work are how the quality of DAS data compares to other types of borehole measurements and what are the operational benefits and constraints for the use of this technology in mineral exploration settings. To this end, we have tested performance of DAS measurements in one of the boreholes of the Mineral Systems Drilling Program in South Australia and compared them to hydrophone measurements. The DAS measurements provide data quality that is much better than a hydrophone string, in particular it has consistent amplitudes at different depths, shows less cable and tube waves, and the reflections are much clearer. The acquisition of DAS data is quicker than any other borehole measurements that require multiple pulls of the receivers. The reduction of the acquisition time increases with the depth of the borehole. This case study demonstrates that DAS measurements show big potential for mineral exploration and exploitation.

Vertical seismic optical profiling on wireline logging cable

“…In VSP processing (c), the direct arrival is removed and seismic reflections are converted to two-way time (TWT) and then stacked in a single trace (corridor stack) to be compared with surface seismic data. (Mestayer et al 2011(Mestayer et al , 2012Barberan et al 2012) and with a cable designed for permanent deployment that was installed temporarily (Miller et al 2012); what is shown in the present paper is the ability to perform this type of borehole seismic survey on wireline cable as an intervention service.…”

“…, Madsen et al and Mateeva et al ) and on fibre cemented behind casing (Mestayer et al . , ). While of considerable interest, this prior work addresses a different seismic application (reservoir monitoring versus exploration borehole seismic in the work presented here).…”

“…Results of borehole seismic acquisition with distributed optical sensors have previously been reported in permanently C 2014 European Association of Geoscientists & Engineers, Geophysical Prospecting, 62, [693][694][695][696][697][698][699][700][701] Vertical seismic optical profiling on wireline logging cable 701 installed optical cables on production tubing (Barberan et al 2012, Madsen et al 2013and Mateeva et al 2013) and on fibre cemented behind casing (Mestayer et al 2011(Mestayer et al , 2012. While of considerable interest, this prior work addresses a different seismic application (reservoir monitoring versus exploration borehole seismic in the work presented here).…”

“…The technology was previously demonstrated in permanently installed cables (Mestayer et al . , ; Barberan et al . ) and with a cable designed for permanent deployment that was installed temporarily (Miller et al ); what is shown in the present paper is the ability to perform this type of borehole seismic survey on wireline cable as an intervention service.…”

Monitoring hydraulic fracturing using distributed acoustic sensing in a treatment well

“…The interrogator unit emits short laser pulses into the fiber and analyzes the Rayleigh back-scattering to probe the strain along the fiber. Several field trials conducted recently in onshore wells (Mestayer et al, 2011(Mestayer et al, , 2012Miller et al, 2012) as well as offshore wells (Mateeva et al, 2013) demonstrate that DAS is a feasible technology for VSP. In this paper, we describe an experiment where time-lapse seismic data were recorded on a DAS fiber in a treatment well for a hydrofrac in a Shell-owned gas field and discuss the time-lapse changes due to stimulation.…”