Report reveals earlier problems at North Sea gas site

The Elgin drilling platform in the North Sea has been leaking natural gas since 25 March, and is still at risk of catching fire or even exploding.

Total, the company in charge of the platform, had already experienced problems at the site. A report written by one of its drilling managers and published last year describes how difficult it is to pump gas from the deep Elgin field. It even explains exactly how drilling operations can damage the wells in ways that may have led to this week's leak.

Demand for natural gas has risen steadily over the last 30 years, and global gas production has more than doubled in that time. Gas producers like Total have turned to increasingly difficult reserves to meet those demands.

The Elgin field is one of those difficult reserves. At 5500 metres it is among the deepest active gas fields in the North Sea. Temperatures there reach 200 ?C and the gas is under 1100 bars of pressure. Last year's report explains that working with this kind of high-pressure/high-temperature (HP/HT) field is a particular challenge (Journal of Petroleum Technology, vol 63, p 116).

Once gas extraction begins, the pressure within any gas field will naturally begin to fall, but the drop is particularly acute in HP/HT reservoirs. This means that pressure at depth can vary dramatically and unpredictably once extraction begins, which makes it difficult to sink additional wells at a later date without damaging the expensive drilling equipment. To extract as much gas as possible, companies normally drill several wells into the reservoir before opening any of them. This ensures that they have backup wells, should some fail.

Wells failed

In fact, many of the wells in the Elgin field and the neighbouring Franklin field have failed since Total began extracting gas there in 2001, according to the report. The casings of some of the wells became badly deformed and their internal diameters shrank by 60 per cent, like damaged drinking straws. Others may have sheared off completely.

That damage was probably a direct result of gas extraction activities. The drop in pressure as gas was removed deformed the chalk layers above. Because the wells must pass through the chalk to reach the gas fields below, they too become deformed.

The damage to the well casings may explain this week's leak. We now know that its source is not the Elgin gas field itself, but rather a pocket of methane gas trapped within the chalk layers above. This methane somehow entered the casing of one well and travelled up to the Elgin platform.

The leaky well in question was actually closed over a year ago, and sealed at depth. But because Total continued to extract gas from the Elgin field, shearing forces within the chalk above may have cracked open the abandoned well-casing 4000 metres down ? 1500 metres above the main gas field ? allowing methane within the chalk to seep into the well and escape to the surface.

"Based upon the information we have available, I think it is highly probable this has happened," says Jon Gluyas of Durham University in the UK, who has 28 years of experience in the petroleum industry.

At the time of publication, Total had not responded to a request to comment on the relevance of last year's report to the current leak.

"Because of the demand for oil and gas, we will see this type of field exploited more often," Gluyas says. However, that does not necessarily mean we will see many more accidents along the lines of Elgin. Rather, the gas industry may improve its practices, for instance monitoring compaction in chalk layers more closely. "The industry will learn from this substantially," Gluyas says.

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