Andre Franzen

Cable development for distributed geophysical sensing with a field trial in surface seismic

Fibre-optic distributed sensing has the potential to revolutionize well and reservoir surveillance in the oil and gas industry. Benefits include the passive nature of optical fibre sensors, the potential for cost-effective installations, combined with the possibility of densely distributed measurements along the entire length of the fibre. Amongst a range of fibre-optic sensing technologies, Distributed Acoustic Sensing has the potential to provide a low cost alternative for conventional seismic technologies. To widen the geophysical application scope further, the fibre-optic sensing cable should be made more sensitive to incoming seismic waves that arrive at the cable perpendicular (“broadside”) to its axial direction. We introduce the development of such cable concepts, and present results of a successful cable deployment in a surface seismic field trial. Efforts continue to realize cost-effective directionally-sensitive cables for geophysical use, for deployment down-hole and on surface.

Microseismic Applications using DAS

Distributed Acoustic Sensing (DAS) is a rapidly maturing fibre optic technology with many applications for wellbore monitoring and geophysical surveillance. DAS transforms a fibre optic cable into a distributed array of acoustic sensors. Shell is developing DAS technology in partnership with OptaSense, a subsidiary of QinetiQ U.K. DAS has been proven to work for VSP applications (Mateeva et al., 2012). The technology has been improved through numerous field trials and it has been tested in a variety of installations, where it has been compared to geophones and sonic logs as a check-shot tool, and as an imaging tool for walk away VSP data. Signal to noise ratio, directionality and repeatability among other aspects have also been studied through these field trials and laboratory experiments. The performance of DAS for micro-seismic monitoring applications is still under evaluation. Although DAS offers the advantage of recording along the whole well at once, significantly increasing the number of receivers, it has several challenges with respect to geophone arrays: the current DAS system records data with a higher noise floor and with a more constrained angular sensitivity since it behaves as a doublet of one-component geophones. In this paper we report the current status of this technology regarding micro-seismic monitoring. A field trial specifically designed to test this application in a micro-seismically active area is described. The trial was jointly carried out with Petroleum Development Oman (PDO) in a field in Oman.

Development of a FBG vortex flow sensor for high-temperature applications

A robust fibre optic flow sensor has been developed to measure liquid or gas flows at ambient temperatures up to 300 ºC and pressures up to 100 bar. While such environmental conditions are typical in pressurized steam systems in the oil and gas industry (downhole and surface), wider applications are envisaged. The flow sensor uses a specially-designed bluff body to generate vortex-induced pressure fluctuations as a function of flow. The pressure fluctuations result in mechanical strain fluctuations in the sensor plate which is attached to the bluff-body. This is detected by means of a Fibre Bragg Grating (FBG). The frequency of the pressure fluctuations is proportional to the flow velocity and is measured by analyzing the spectrum of the FBG sensor signal. Flow velocity measurements ranging from ~1 m/s to ~25 m/s have been demonstrated. Special mechanical design, gluing and packaging processes have been developed to enable applications at high temperatures and high pressures (HPHT). Although the working principle is the same as for conventional vortex flow meters, this flow sensor does not require electronics, which is a great advantage at high temperatures.

Flow velocity tracking using Fibre Bragg Gratings

Experiments were performed with the objective to obtain the velocity of liquids, e.g. water and oil / water mixtures flowing through a pipe. Experimentally tested flow rates were between 0.75 and 39 m3/h, corresponding to velocities of 0.023 and 1.2 m/s. Temperature disturbances were created by injecting slugs of hot water through a side inlet. The temperature was measured at regular time intervals of 1.2 s by Fibre Bragg Gratings, which were located at 40 equidistant locations at 0.5 m spacing. Hot slug movement was clearly visible in nearly all data sets. The magnitude of the temperature rise (0.1 - 0.2 °C) at that flow rate had the same magnitude as the temperature noise. Travelling temperature waves showed a relatively strong deformation at low rates, due to relatively strong Taylor dispersion occurring at transitional flow (Reynolds number ~ 2500 at the lowest rate). Temperature disturbances travelling down the pipe were tracked by semblance processing of the temperature data taken at 34 locations after the side inlet; the data from 6 locations before the side inlet was not included. Semblance processing is a technique commonly applied in obtaining seismic wave velocities using an equidistant array of receivers, which was applied here for velocity tracking of hot liquid slugs. The best velocity estimates were obtained at flow velocities of 0.13 and 0.28 m/s, typical for a small oil well. Velocity errors were largely within a +/- 10% bandwidth.

Investigation of modulation formats in future SOA-facilitated optical networks

Semiconductor Optical Amplifiers (SOAs) are vital elements in future optical networks whether as amplifying elements to boost the optical signal. In addition to be used as amplifiers, SOAs can also be used as switching elements operating either as ON-OFF switches or as wavelength converters for wavelength routing switching. Additionally, their performance is compatible with nowadays and future transmission rates of 10, 40 and 80Gb/s and beyond. Hence, the impact they will have in all future optical networks is paramount and it is unimaginable that there will be any future optical network without them. With this knowledge a look into future optical networks reviles that the modulation format will be influenced by this development due to the physical impacts of the SOAs on the signal transmission. Hence the modulation format needs to be investigated. Here we pursue the investigation of different optical formats by means of optical simulation and do a comparison of the modulation formats with respect to the performance selected for this paper.

Buffering and scheduling of asynchronous variable-length packets

By eliminating optical synchronizers in optical IP routers, more complex scheduling algorithms are needed to schedule asynchronous packets. This will result in voids at the switch output thus reducing switch throughput. A novel scheduling algorithm had previously been proposed to reduce these excess losses by filling voids caused by asynchronous and variable length operation. Non-degenerate (i.e. non-uniform) buffer depth has previously been shown to improve the packet loss performance especially under bursty traffic. We investigated the performance of the void filling algorithm by combining non-degenerate and degenerate (uniform) fiber delay lines. Performance is studied for different threshold levels, i.e. the number of uniform delay lines in the feedback delay line before introducing the non-degenerate delay lines. The packet loss performance for combined non-degenerate and degenerate delay lines with void filling algorithm is presented for an optical router with a feedback delay lines buffer under self similar traffic. The recirculating delay lines buffer emulates a two-stage buffer where first stage buffers smaller packets whilst the second stage stores larger packets. This buffering mechanism is similar to SLOB (i.e. Switch With Large Optical Buffer).