Sensors look well ahead

UK researchers are developing intelligent oil wells that could spot advancing water from up to several hundred metres away and react before it contaminates production.

A team led by Dr Matthew Jackson, lecturer in the department of earth science and engineering at Imperial College, plans to use a combination of seismic and electrokinetic sensors to monitor fluid flow in reservoirs around oil wells. The two-year project is being backed by down-hole oil technology company Sondex, with funding from the EPSRC, the Royal Society and the Malaysian government.

Oil companies produce around three barrels of water for every barrel of oil, which reduces productivity and requires careful disposal. More oil could be removed if wells could prepare for an influx of water, according to the researchers. Jackson said most oil wells are very primitive and cannot react to potential contamination. Although oil companies use valves in well walls to control the flow of non-oil fluids, they cannot accurately predict the flow of such liquids beyond a few metres from the well.

’The way we manage a reservoir is very different to other complex industrial processes, such as an oil refinery or chemical plant, because of the high level of uncertainty and reactive nature of our response to changes in conditions,’ Jackson said. ’We’d like to see an entire oil field with down-hole sensors feeding back information into valves and continually monitoring wells to optimise their performance.’

Electrokinetic sensors can detect changes in electric potential for hundreds of metres in reservoirs around wells. As water passes through porous rock a current forms, caused by the flow of ions into the fluid relative to the stationary ions stuck to the rock grain surface. Water has a higher electric potential than oil or gas, and this will be detected by the sensors.

Seismic sensors measure the refraction and reflection of a sound wave when it encounters rock layers, yielding a picture similar to a geological cross-section.

The researchers claim that as well as providing details of the reservoir’s rock layers, seismic sensors could also extend the range of the electrokinetic sensors beyond several hundred metres, by using the sound waves to oscillate ions in the rock grain. The oscillation creates an alternating rather than direct current, releasing more ions from the pore wall of the rock and producing a higher potential difference – and a stronger signal. ’It’s incredibly novel and quite exciting,’ Jackson said. ’It would be an enormous step forward in down-hole measurement’.

The researchers admit that the oil industry needs to be convinced of the value of such monitoring. ’Intelligent oil wells are expensive and technologically challenging, which has put off the industry,’ Jackson said. ’We don’t know how reliable the technology is and we’ve not been sure how to use it. The industry is interested, but wants to see more results.’

One problem with oil well sensors and valves is that if they malfunction they are impossible to fix, forcing engineers to revert to traditional production methods.

’Technology for down-hole repairs is pretty much zero,’ Jackson said. But oil companies are investigating possible ways of carrying out such repairs, including the use of robots.

It is difficult to install intelligent oil well devices without damaging them and the technology has had a relatively high failure rate so far, Jackson said. Common problems his team will try to tackle include failures in sensor communication back to the surface and the build up of deposits, blocking valves.

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