Silixa’s iDAS (intelligent Distributed Acoustic Sensor) offers a solution for permanent and continuous downhole production monitoring and reservoir monitoring by overcoming the cost and technology challenges encountered with traditional point sensors. The iDAS enables wide-aperture acoustic array observation even in harsh environments, like high-pressure, high-temperature wells, and negates the need for irreversible pre-completion sensor placement.
The iDAS measures the full acoustic field along the unmodified optical fiber up to tens of kilometres in length with a spatial resolution down to 1 m, capturing the full amplitude and phase of the incident wave on the sensing optical fiber up to frequencies >100 kHz with a wide dynamic range (>120 dB). The system enables high-quality synchronized coherent digital recording of acoustic waves with no cross-talk at every location along the fiber. These unique attributes make the iDAS technology particularly well-suited to the demanding applications encountered in oil and gas exploration and production.
Principle of Measurement
The iDAS functions by using a method based on optical time-domain reflectometry. When light is emitted into a fiber, the signal is reflected throughout the fiber length. These reflections are the result of a variety of mechanisms, including Rayleigh, Brillouin, and Raman scattering. By monitoring this scatter as a function of time following the emission of each sensing pulse, it is possible to build a map of the time variance of light-scattering properties throughout the fiber. This information can be analyzed to give a measure of the acoustic signal at each sensing zone along the fiber. Silixa’s iDAS uses a new digital optoelectronics architecture to rapidly and accurately measure this backscattered signal with a precision and speed that allows acoustic measurements from very low frequencies (<10 mHz) to high frequencies (>100 kHz). This sensing system can be retrofitted on existing standard optical-fiber cable installations, although superior performance can be achieved by using Silixa’s proprietary cables for logging operations and new permanent installations.
Applications
Silixa’s iDAS technology provides benefits through every stage of the life of a well. Since iDAS makes it possible to map acoustic propagation along the entire length of a borehole, the system creates the opportunity to perform real-time, continuous production and well-integrity monitoring. Further, distributed acoustics can facilitate seismic profiling in harsh, high-temperature downhole environments.
Silixa has also developed a number of installation techniques for permanent as well as logging applications.
The ability to synchronously measure the acoustic signals at every location along the well opens the door to a wide range of array-processing techniques to be used to extract the maximum value from the data. For example, iDAS can be used to determine the speed of sound in the material surrounding a sensing cable, enabling iDAS to profile the fluid composition. This facilitates, for instance, the detection and quantification of gas in oil; a necessary step toward distributed multiphase-flow measurement. It can also be used in enhanced oil recovery to monitor the flow of injection fluids (such as steam) entering the production wells.
The limitations of conventional technology have made routine vertical seismic profiling economically unfeasible. Further, point-sensing technology requires an array of seismic sensors to be lowered down the well, and this cannot be done without shutting off production. Silixa’s iDAS facilitates an unprecedented opportunity by offering an unobtrusive method of acquiring seismic data on demand without interrupting production.
iDAS can record the acoustic signal at a multitude of elevations and detect different events along the entire borehole. This makes it possible to detect leaks behind the casing, and further enables continuous monitoring of downhole components, such as ICVs, ICDs, gas-lift valves, and electrical submersible pumps). Other applications include perforation characterization, fracture mapping, and cementing evaluation.
The system can also be deployed in different configurations both inland and offshore. As all sensing points are phase synchronized, the acoustic response along the fiber can be coherently combined to enhance the detection sensitivity and/or directivity. These capabilities can be further enhanced by forming the sensing cable into a giant acoustic lens to image the whole oil and gas field over a wide area.
Field Trial
The system has been successfully used for seismic and flow monitoring in multiple offshore exercises, including a recent exercise involving multiple North Sea wells. The multizone completions observed during this trial featured existing fiber installations for downhole permanent-pressure and -temperature gauges. Acoustic measurements were made along the existing fiber infrastructure without any fiber optimization.
During this trial the existing downhole sensors were disconnected from their topside instrumentation. iDAS measured the phase and amplitude of the acoustic signal coherently along the fiber-optic cable. It was thus possible to use a variety of methods to identify the presence of propagating acoustic waves. Digital signal processing helped transform the time and space data into a diagram showing frequency and wavenumber in the frequency-space (f-k) domain.
Fig. 1 shows the acoustic time and space signal and the corresponding space-frequency domain. Using the f-k plot, the speeds of sound can be calculated by measuring the slope of the ridges. The frequency band over which the speed of sound can be determined is more than sufficient for compositional and flow characterization. With the iDAS system the speed of sound can be evaluated over a large section of the well and, therefore, the distributed variations of the flow composition and characteristics along the well can be measured. The technique is particularly powerful for determining the fluid composition of the flow; for example, gas has a speed of sound of around 600 m/s whereas oil and water have speeds of sound around 1,100 m/s and 1,500 m/s, respectively. This f-k analysis is facilitated by the unique ability for iDAS to determine the full coherent acoustic signal (amplitude and phase over a wide frequency band).
The slopes of the two ridges in the f-k space correspond to the speed of sound traveling up and down the well. The difference in the two speeds of sound corresponds to the Doppler-shift induced by the velocity of the moving fluid. Fig. 2 shows the distributed flow determined in a gas injector based on Doppler-shift measurements for a 30-s sampling. The determined flow speed varies with depth in the well corresponding to the change in hydrostatic pressure for a section of tubing with a uniform inner dimension and a gradually sloped well trajectory. In total, the instantaneous and locally determined flow is roughly within +/–0.3 m/s (that for this well is 10%) of the actual flow speed.
The signal quality and the presence of propagating acoustic waves enabled a detailed analysis of the recorded acoustic field, including the quantification of distributed composition and flow velocity along the wellbore. This trial also demonstrated that the signal quality holds potential for developing further diagnostic techniques in the future (SPE 149602).
The carbon footprint of Silixa’s iDAS is no greater than that of a standard high-performance desktop computer. This sensing system has clear benefits to health and safety as it can be used to identify possible equipment failure, and even detect downhole disruptions in advance of their manifestation at the wellhead. The iDAS system is highly robust, as the sensing fiber requires no moving components or sensitive splice points.
For more information, contact Gina Elesztos Gina.Elesztos@silixa.com +44 (0) 208 327 4210 or Lovynash Dookhee Lovynash.Dookhee@silixa.com +44 (0) 208 327 4210.
Reference
Johannessen, K., Drakeley, B., and Farhadiroushan, M. 2012. Distributed Acoustic Sensing—A New Way of Listening to Your Well/Reservoir. Paper SPE 149602 presented at the SPE Intelligent Energy International Conference, Utrecht, Netherlands, 27–29 March.