Weak rocks demand strong solutions

08/01/2008 Personnel from three North Sea fields were flown ashore this autumn following forecasts of high waves and hurricane winds. That reflects fears aroused by seabed subsidence in the wake of oil and gas production.

Text: Eric Mathiesen, Steinar Njå and Ina Gundersen

The threat of high waves prompted the evacuation of more than 760 people from Valhall, Ekofisk and Eldfisk to land between 6-8 November.

Because the seabed on these fields has been subsiding, the gap between the bottom of their platform topsides and the waves has become too small to cope with such bad weather.

A number of North Sea installations were wholly or partly shut down for two days, reducing daily Norwegian oil production by roughly 200 000 barrels.

The NPD is constantly being asked what happens to reservoirs on the Norwegian continental shelf after close to 30 billion barrels of oil equivalent have been extracted from them.

Petroleum resources on the NCS are found in geological formations of sandstone or chalk which normally lie between two to four kilometres beneath the seabed.

These hydrocarbons fill small pores between the rock grains. As they are produced, the natural pressure in the reservoir declines.

But the overlying layers of sandstone, shale, clay and water still press down with the same weight as before, so removing oil and gas increases the load on the reservoir rocks.

Normally, the weight of these overlying rocks would cause no noticeable compression of the formation. And that means the seabed on the field would remain unaffected.

But there are exceptions. The best known case of subsidence on the NCS is Ekofisk.

Seabed subsidence on Ekofisk was first established in 1984, when the number of visible openings in the protective tank wall had been reduced from five-six to four. Photo: Norwegian Petroleum Museum/ConocoPhillips

Seabed subsidence on Ekofisk was first established in 1984, when the number of visible openings in the protective tank wall had been reduced from five-six to four. Photo: Norwegian Petroleum Museum/ConocoPhillips

When this field was discovered in 1969, it was uncertain how much of its oil and gas could be recovered because of the relatively weak formation rocks. Test production was accordingly launched in 1971, before an investment was made in developing the field.

The reservoir rock on Ekofisk consists largely of chalk, which is extremely porous – in some zones, the pores account for more than 50 per cent of the rock volume.

Oil under relatively high pressure helped to bear the weight of the overlying layers. As this was produced, however, a growing share of the burden had to be carried by the chalk – which failed to take the load.

But test production on Ekofisk had given little cause for concern. The possibility of subsidence was discuss­ed, but was expected to cause a sharp reduction in output as reservoir pressure declined.

That did not happen, and the field was accordingly developed with drilling, production and quarters install­ations and brought on stream in 1974.

At the same time, the 2/4-T concrete storage tank was installed on Ekofisk. It took a decade before people began to ask why the lowest openings in its shield wall were disappearing.

This confirmed that subsidence was happening, although it was not the first time that such a phe­nomenon had been recorded in connection with oil, gas or coal production.

The special feature of Ekofisk, however, was that its installations – which stood initially in 70 metres of water – had subsided by three metres in 13 years.

Seabed subsidence created major problems for both the operator and the authorities. Its cause was just one of the questions which required answers and proposals for a solution.

Another was the consequences this would have for the safety of people working on the field installations, and for continued production.

The joint chalk research programme devoted tens of millions of kroner to mechanical tests of the reservoir rocks. Modelling of rock properties and future production plans were then applied to predict the future course of the subsidence.

Each forecast showed that the rate of sinking would decline in the near future. But it was finally recognised that the models were wrong – and the problem continued.

The operator adopted a temporary solution in the 1980s by jacking up six Ekofisk installations and plac­ing a protective breakwater around the tank. But this was not enough.

With the seabed continuing to subside by up to 50 centimetres a year, the operator resolved in 1994 – under government pressure – to redevelop the field. Ekofisk II, built to cope with 20 metres of subsidence, came on stream in 1998.

More intensive research over the past decade into the interaction between pore filling and rock stabil­ity has improved theoretical understanding of the mechanism of chalk compression.

Water injection is used today as the primary means of preventing such compression. The seabed on Ekofisk is still sinking, but at only 20 centimetres a year. Total subsidence so far has been measured at almost nine metres.

Ekofisk is not the only Norwegian field to suffer from this phenomenon. It has been recorded on Eldfisk, Valhall and West Ekofisk.

All these reservoirs comprise the same weak reservoir rocks and a corresponding geological structure between the hydrocarbon-bearing formations and the seabed.

Compression of similar reservoirs along with associated surface subsidence has also happened elsewhere – as in Denmark, although the scale of the sinking there is much smaller than on the NCS.

Removing gas from the Groningen sandstone reservoir in the Netherlands has caused 25-40 centimetres of subsidence since production began in 1963.

That does not sound much by comparison with the Norwegian experience, but nevertheless represents a serious problem. Some of the land areas affected by subsidence were already more than six metres below sea level.

Nor is subsidence entirely unknown on Groningen’s Norwegian cousin, the Troll field in the North Sea, and the potential for this was taken into account when building the Troll A gas platform.

Every field could give rise to subsidence. The larger its area and the weaker the rocks in and above the reservoir, the greater the danger of it happening.

Shallow reservoirs also have a bigger subsidence potential than deeper ones. However, the extent of any sinking on other Norwegian fields is expected to be small – up to one metre.

Hydrocarbons have been produced from Wilmington, the USA’s third largest oil field, since the late 1930s. This shallow reservoir lies close to homes in Long Beach, California, and near one of the world’s largest international ports.

When subsidence was detected in 1941, oil flowed from more than 1 000 wells in the area. Certain areas in Long Beach subsided by 60-100 centimetres per year during peak production.

Exceeding eight metres by 1958, this sinking affect­ed an area of more than 65 square kilometres and has caused damage to the railway network, bridges and port facilities. In addition, large areas became exposed to flooding.

The Long Beach subsidence is being kept in check by the same method used on Ekofisk – injecting water to replace the oil being produced.

Such waterflooding has proved to have several positive effects on the Ekofisk reservoir. In addition to displacing and replacing oil, it has also accelerated production in some parts of the field through increased rock compression.

Production of oil and gas at Long Beach in California has caused land areas to subside – as here, where a fire hydrant now stands four metres above the ground.

Production of oil and gas at Long Beach in California has caused land areas to subside – as here, where a fire hydrant now stands four metres above the ground.
Photo: City of Long Beach

Updated: 14/12/2009