A gas field near the village of Sleen, in the east of Drenthe, is leaking. The methane from the gas field seeps into the groundwater, Dutch earth scientists discovered. It is the result of a drilling accident 52 years ago.
On December 1, 1965, something went wrong during a gas drilling at 't Haantje in Sleen, a village near Emmen in East Drenthe. The pressure in the drilling rig rose too quickly, an explosion followed, and the fifty-meter high derrick disappeared completely in the quicksand that formed. The staff managed to get themselves to safety in time, but the Nederlandse Aardolie Maatschappij (NAM) only had the matter under control again after a few months.
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The explosion (‘blowout ’) caused cracks in the subterranean soil layers, causing the gas to leak out of the gas field ever since. To this day, methane from the natural gas reservoir ends up in the groundwater. A team of earth scientists from Utrecht University and water research institute KWR discovered this after chemical analysis of the water. At the end of December, they published their findings in the scientific journal PNAS.
Airtight packed
Who wants to store something underground – CO2 for example, or waste water left over from oil pumping – this is often best done in a former oil or gas field. After all, these underground storage facilities have proven themselves for millions of years as reliable gas (or liquid) tight packaging. The natural gas in a gas field is contained in the pores of the porous rock, like water in a sponge, with an impermeable rock layer on top that acts as a lid.
But that cover must still be intact. “We know from international studies that leaks regularly occur along the outside of wells,” says earth scientist Gilian Schout of Utrecht University, first author of the PNAS article. “We are now showing that underground blowouts can also lead to persistent leakage.”
Incidentally, such accidents are rare, adds geochemist and co-researcher Jasper Griffioen. “The blowout at 't Haantje was a unique event in the Netherlands. It was world news at the time.”
Methane
The researchers found methane in the shallow groundwater, just above the two-kilometer deep gas field. They inferred that the methane came from this gas field from the ratio in which different carbon isotopes (see box) were present in this methane. This was comparable to the isotope ratio in the gas field. The ratio between methane, ethane and propane was also the same, which serves as additional proof.
It is not unusual for methane to occur in the groundwater, says Niels Hartog, geohydrologist at KWR, who participated in the study. It also arises from the decomposition of dead plants and algae in the soil, and in that case is called swamp gas or source gas. In the past, many farmers in the west of the country had their own, small-scale source gas installation on their property to extract this swamp gas for their own use.
Danger
But the buildup of methane, whether it be swamp gas or natural gas, can also be dangerous. If it is released to the earth's surface in an enclosed space, there is a risk of explosion and fire. “We still have to find out whether the methane from the gas field at Emmen is released to the earth's surface,” says Hartog. Because in that case it would escape into an open field, it wouldn't do much harm to Sleen.
The province does not check the surface water for the presence of methane. There is no problem for drinking water. Drinking water companies aerate the water as part of the purification procedure. This automatically removes methane from the water, if it is present. In Spannenburg in Friesland, the drinking water company Vitens has even been using this methane from the groundwater as a private energy supply since 2014.