CGG’s Distributed Acoustic Sensing measures sound from inside a well bore using fibre-optic cable
CGG and its subsidiary Sercel have released a new down-hole seismic solution – Distributed Acoustic Sensing, or DAS. DAS is designed to overcome problems created by traditional seismic surveying in which both transmitter and geophones are located on the surface.
These problems have already been partially addressed by vertical seismic surveys, in which sensors are located in the well bore, and in micro-seismic surveys of frack jobs.
However, micro-seismic surveys require the deployment down-hole of a large quantity of complex equipment. If 4D results are required then that equipment also has to survive well-bore conditions of heat, pressure and fluids for many months, sometimes years.
CGG’s DAS system replaces this down-hole kit with a fibre-optic cable. Apart from being simpler, this also provides durability (the fibre has been extensively tested in SAGD wells at 260°C).
Light and sound
Light is transmitted into the fibre-optic cable and its backscatter is analysed for changes to its characteristics caused by the arrival of acoustic energy. Those changes arise because sound causes localised strain changes. In effect, the cable is the sensor.
The time-base tells the system how far down the cable the acoustic energy has arrived, and the changes to backscattered light reveal the magnitude and spectral characteristics of the acoustic energy at each point along the fibre.
Each light pulse is actually transmitted twice, mimicking a measurement gauge or an interferometer. Strain in the fibre (caused by arriving seismic energy) changes the separation of the pulses slightly, and the backscatter transmits this change, generating a recordable measurement.
Ideally each pulse would be an instantaneous spike (like the two points of a physical gauge) but the gauge pulses need enough energy to achieve the target sensitivity, and that means that each pair of pulses needs to be slightly longer. The physical distance between the transmitted pulses defines the length of cable over which a given seismic strain will be measured, so the shorter the gap the finer the seismic data acquired. DAS has to be tuned to the expected wavelength of the seismic energy being used, to generate an optimum pulse gap. If the pulse gap is too long then seismic data smears out.
The amplitude of the transmitted pulse is limited by the point at which the fibre is overloaded, and starts losing transmitted energy into “sideband energy” – effectively unwanted noise. So at the start of a job the cable is tuned to detect its maximum possible amplitude.
The third tuneable factor is pulse repetition frequency. At its limit this is a simple function of cable length (as each pulse must wait for the last reflection to arrive before the next pulse can be transmitted). With a cable length of 2 km PRF is around 50 kHz, falling to 2 kHz at 50 km. These compare with typical seismic sampling rates of 500 Hz, so even with a long cable DAS delivers high signal-noise ratios. Obviously, the higher the pulse repetition frequency, the better the S/N ratio. Typical cable lengths are 5-10 km, allowing pulse rates of 10-20 kHz, though CGG has found that S/N ratio improvements flatten out at 5 db.
A feature of DAS is that these settings can be tuned in real time, allowing the cable to be “aimed” at different depths by tuning it to different seismic frequencies and velocities.
CGG has various methods for deploying DAS. One option is to clamp the DAS fibre to the external diameter of the casing, allowing it to make good physical contact with seismic energy via the cement job.
In this conformation the fibre sits at the core of a combination of an outer jacket, armour wire, a stainless steel tube and a layer of hydrogen-scavenging gel. Where installation behind the casing is impractical, or ruled out by choice, DAS can be clamped to well tubing or deployed inside coiled tubing. However, these options create the risk that seismic data will be obscured by production noise, and also reduce the coupling of DAS’ fibre with the formation. A fourth solution is to use a carbon-fibre rod to carry the fibre into the well bore – less than 20 mm in diameter, but rigid enough to be deployed without difficulty.
CGG is working on new and better deployment methods, and also on post-processing techniques to overcome the problems of existing methods. For vertical wells one solution is to use FLI (see InnovOil issue 66).