If they are to give of their best, complex subsea wells with many zones demand permanent monitoring and control downhole. Several oil companies claim that such “smart wells” will on average yield a production increase of more than ten per cent of the original reservoir content. Even if this is only a possibility for certain Norwegian petroleum wells, there are billions of kroner at stake. Deficient reliability of the monitoring and control systems is, however, a critical factor, and a national push to improve this situation will yield great gains for the players on the Norwegian shelf. The last decade has seen a transition from traditional more or less vertical wells to complex horizontal or multi-branched wells that collect oil from a number of independent productive zones in the reservoir. Progress in drilling technique has made this possible, and the big finds in ever-deeper waters will make it pay.
Complex factors
If several zones with different pressures are interconnected, the risk of a powerful flow from a high-pressure zone to low-pressure zones is serious. And if the operator has really bad luck, he gets a water breakthrough. His super oil source is suddenly producing more oil than water!
Methods have been developed to prevent water production, but here we need to know where the water is being produced. In a subsea well at great depths, access to the well to log it and set in casings is expensive and dangerous.
An oil well is not static.
The oil industry is becoming more and more interesting in squeezing the last drops out of its reservoirs. The strategy for tapping is chosen on the basis of seismic surveys and well logging before the well is produced, but the wells do not necessarily remain stable. Instead of pressure sinking nicely and quietly, it can fluctuate suddenly as the rock fissures. Water production increases sharply and it can take months and cost tens of millions to do something about it – if anything can be done, that is.
If the well is equipped with sensors that continuously monitor the stream of oil, gas and water from the various zones and also install choke valves for each zone, the situation is very different. The reservoirs can then be tapped in a controlled fashion and dealt with quickly when unexpected things happen. Monitoring will also provide a good basis for changing drainage strategies related to initial weaknesses in the reservoir descriptions. Optimal production profiles may thus become a practical proposition.
Remote sensing and predictability
If, in addition to wellstream measurement, the operator has the equipment for permanent remote monitoring of the reservoir characteristics from the well, the operator can receive information about what is happening in real time. He can then take appropriate action before the water or gas breakthrough happens. Much of the technology for such instrumentation has been developed for well logging, typical devices being for resistance and acoustics measurement. If these reservoir characteristics are monitored over time, however, the operator will receive much more detailed information than an ordinary logging can provide. Production logging shows the immediate situation as a function of the position in the well. The trend in the measurements taken over a point in time, however, can be correlated with tapping and injection, and yield quite different information. In the same way, we can envisage local seismics done at short intervals. This can yield much better spatial resolution in the area close to the well than can traditional seismics. If the formation and the fronts around the well known, and also how they change over time, there is a basis for regular updating of production strategy.
Full three-phase measurement
These days, pressure and temperature sensors are being installed in several wells. On the basis of these measurements, the operator can guess what is really interesting, giving him the ability to perform much more interesting measurements. An example of this is the IPC system that Sintef Elektronikk og Kybernetikk has developed for Maritime Well Service A/S. With the aid of its own sensors and high-temperature electronics, this system measures – with reasonable precision – full three-phase streams independently for each production zone.
“Smart well systems” are still under development. Last year SINTEF initiated a research project, “Intelligent Oil Wells”, financed by the Research Council of Norway. The aim is to see how optimal tapping of reservoirs can be done in future by means of advanced instrumentation in the well.
In this programme, mathematicians are working together with petroleum researchers, instrument developers and specialists in regulation engineering to discover new methods. The main idea is that streaming models for reservoirs and wells can be combined into a single dynamic model that can be part of a model-based regulation system for the reservoir. This model can be updated by monitoring well-streams, pressures and temperatures and also measurements that describe the changes in the reservoir over time.
The problem of reliability.
The biggest obstacle to full use of the technology is lack of reliability. The environment down in an oil well is extremely difficult, and so far no manufacturer on the world market has been able to boast of a stable long lifetime, at any rate not in hot wells (>150oC).
There are many reasons for this. The usual faults have been related to leakages in cables and penetrations because the design solutions have not been sufficiently suited to conditions during installation. Electronics are also notoriously more unreliable as the temperature rises. Progress is, however, being made: ten years ago the main problem was making electronics that could work properly at 200oC. Sintef Elektronikk og Kybernetikk solved this to some extent by developing a family of special integrated circuits for high temperatures (HTASIC(). The design of these circuits was optimised for good specifications and the longest possible lifetime at 200oC, and laboratory tests have shown that they can survive the temperatures encountered in hot wells for many years. However, even this technology is based on the same materials (active silicon and metallised aluminium) and manufacturing techniques as other electronics Maximum achievable lifetime is therefore limited by the same fundamental failure mechanisms. Even so, it appears that it is other kinds of failure that prevail today. The difficulty of finding out what it is that really ruins the equipment is actually one of the biggest obstacles to progress in developing better manufacturing methods for well instrumentation. The IPC system is therefore made in such a way that it can easily be pulled out of the well for repair or upgrading without any disturbance worth mentioning to the well.
A national push will pay off
The days when it was easy to finance oil-related research are long gone. It is even more difficult to get finance for methods that appear so far from the oil wells as production of high-temperature electronics. Nevertheless, we do not think research in any other field will yield a bigger payoff for the oil industry and our entire economy.
In its report “Value Creation through Technology”, Conoco writes that investment in their Norwegian research programmes have provided a documented payoff of 15 times the investment. It is tempting to speculate what an improvement in the reliability of well instrumentation could bring about, but we shall pass that by. The figures are, however, gigantic.
What we need is a big open project that embraces all the Norwegian equipment manufacturers and the oil companies, the foreign ones too. A solid general foundation for reliable new sensors and electronics designs that will allow the ideas in Norwegian offshore industry to flower. We envisage a new generation of oil installations, in which sensors and electronics can be taken for granted as reliable, just as they are in modern process industry. A generation characterised by fewer nasty surprises, longer well lifetime and sound economics throughout the field’s lifetime.
The Integrated Production Control System
Control of the fluid streams in the well is the focus of the IPC system developed by Sintef Elektronikk og Kybernetikk for Maritime Well Service between 1997 and 2000. Three-phase streams from up to seven different productive zones can be monitored and controlled by the system, which is electronically operated and communicates with the outside world via a single downhole cable. The system is based on HTASIC( circuits and all data to and from the well are sent time-multiplexed over the same cable that sends power to the system.
The IPC system can be drawn up by so-called easy intervention: instead of having to pull up the entire production tube, the IPC system can be upgraded by a simple coil tube operation. This is achieved by a slim structure that can be paid out down inside the production tube together with an “electromagnetic plug”, an inductive connector, down in the well.
In addition to pressure and temperature in zone and well, each zone control module measures real three-phase streams from its individual zone.
On the basis of mathematical models of the reservoir and wellstream combined with new methods for well instrumentation, we wish to use modern model-based regulation theory to control the optimal tapping of oil wells. It is expected that this technology can enhance the economics of many fields, so that what are now regarded as marginal fields can become complete productive units in the years to come. “Intelligent wells” is a pure research programme financed direct by the Research Council of Norway, which will run the next four years.
