Introduction
One of the most important current developments in the exploration and production (E&P) world is the introduction of the permanently instrumented reservoir (sometimes called the ‘smart field’ or the ‘electric oilfield’). The long-term goal is to install a network of sensors within a reservoir, in the wells themselves and on the surface/seabed above. These sensors will monitor a range of parameters, including temperature, pressure, acoustics and flow within the wells, and seismic within the reservoir. They will allow faster, more efficient and longerterm recovery from the reservoirs through a greater understanding of the dynamics of the reservoir over time.
The realisation of this vision is expected to bring great economic and operational benefits to the reservoir operators. However, to achieve this goal, a wide range of technical hurdles must be overcome, ranging from the provision of reliable, economic sensors, through the handling of the huge amounts of data generated, to data interpretation and visualisation. One of the key elements in this jigsaw is the sensors and, equally importantly, the associated telemetry, which must cost-effectively gather a large amount of high-quality data in a hostile environment and transmit it to a point where it can be interpreted.
Much progress has been made in developing sensors for downhole temperature, pressure and flow. One of the most exciting approaches here has been the use of fibre optic sensors, which are now becoming widely adopted for these applications, and have also recently been demonstrated for downhole seismic. The next challenge is to produce a solution for life of field seismic (LOFS), using permanent seabed seismic monitoring systems. Fibre optic sensing technology also offers a highly attractive solution to this problem, although the technical challenges are substantially different. This article describes a fibre optic solution developed by QinetiQ Ltd and shows how this can help to recognise the smart field vision, moving from the electric oilfield to the optical oilfield.
The Challenge
The installation of a full-scale permanent reservoir monitoring system requires a significant capital outlay, which may well be in excess of US$20 million. Such systems will only be widely adopted if they can show very clear benefits to the operators, both in absolute terms and in comparison with other techniques such as repeat streamer surveys. The advantages of permanent systems include the following:
- better imaging in difficult seismic conditions (under gas clouds or basalt);
- better measurement repeatability;
- allow short interval repeat surveys (six-monthly or less);
- allow permanent monitoring of microseismic; and
- allow better coverage of field where existing infrastructure (platforms, etc.) make streamer surveys difficult.
These benefits together mean that good economic cases for permanent systems can be made for many fields. However, if the full benefits of permanent systems are to be realised, a number of significant technical challenges need to be overcome.
These include the following:
- manufacturing costs of current systems are too high;
- cables and sensor packages are large and heavy, requiring very expensive deployment systems;
- electrical systems require large amounts of electronics underwater, leading to potential longterm reliability problems;
- electrical cables are also prone to leak-induced failure; and
- large permanent systems will generate very large amounts of data, which requires efficient storage and processing capabilities.
Table 1: Requirements for Full-scale Seabed Seismic Array
| Parameter |
Specification |
| Package Type |
Three orthogonal geophones/
accelerometers plus one hydrophone |
| Number of packages |
1,000–12,000 |
| Package separation |
25 metres – 500 metres |
| Total cable length |
100km – 300km |
| Operating water depth |
500 metres typically (3,000 metres in
some fields) |
Table 1 summarises the likely requirements for a fullscale system. These include the use of large numbersof permanent 4-C packages, large lengths of cable (> 100km) and operation in very deep water (> 3,000 metres in some cases). The requirements are also demanding in terms of sensitivity and vector fidelity.
Category:
Reservoir Engineering
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