Groningen: de grote schoonmaak

It is winter 2025. From the NAM headquarters in Assen, Operations Manager Robbert Kroes steers the car towards the Annerveen gas field, located on the border of the Dutch provinces of Drenthe and Groningen. Along the way, the production sites are hard to miss. Little clusters of curling pipes in the wide‑open farmland, partly shielded by shelterbelts—vegetational barriers of trees and bushes. For now, they stand as visible proof that Mother Earth contributed generously to the extraordinary rise in Dutch prosperity during the last century.

Since 1973, NAM has extracted a total of 71 billion cubic metres (BCM) of natural gas from the Annerveen field. That is a fraction of the 2,300 BCM drawn from the Groningen gas field, Europe’s largest. Annerveen was the second-biggest natural gas field in the Netherlands. The production was halted when the field became depleted.

Clean‑up operation more advanced

Gas extraction at Annerveen stopped in 2021. As a result, the clean‑up operation has progressed further than at the Groningen field to the north, where NAM began dismantling the first sites in 2019. The Annerveen field is the ideal place to get an initial impression of the immense and complex operation that must be carried out over the coming decades. The clean-up is an obligation embedded in the Mining Act of the Netherlands.

Dismantling oil and gas wells

NAM is not doing the clean-up alone. Engineering company Arcadis is part of the team, as is Wellgear. The latter is specialised in the global decommissioning of oil and gas wells. Arcadis takes care of the work above ground, while Wellgear handles the underground activities. The three parties together are doing the work under the name Siturn. This consortium will in turn rely on dozens of subcontractors. Like industrial cleaning firm Reym, the Institute for Safety and Environment (IVM), heavy‑lifting specialist Mammoet and scaffolding company Kaefer.

Before work begins, NAM consults all stakeholders: the State Supervision of Mines, landowners and permitting authorities. Discussions are also held with local residents to minimise inconvenience as much as possible. “Those are quite intensive interactions,” says Robbert Kroes, who was born and raised in Oosterhesselen, about 35 kilometres (22 miles) from the gas field. “In beautiful little villages we sometimes go door‑to‑door. It’s easy to imagine that people aren’t waiting for a lorry with floodlights coming down their street,” Kroes says.

Flare tower

We drive across the provincial border, entering the province of Groningen. In the distance, a flare tower signals the location of NAM’s Wildervank site. This is one of the gas production locations of the Annerveen field, situated right on the outskirts of Veendam. At this so‑called cluster, the first phase of the clean‑up process has been completed. The remaining gas in the pipelines and processing installations has been burned off in the flare tower, after which the gas wells could be disconnected. The next step is the careful cleaning of the installations above ground and pipelines.

Steel and equipment for reuse

After changing into overalls, a hard hat, gloves, safety boots and protective goggles, we are allowed onto the site. Everything here revolves around the major clean‑up, with NAM aiming to prepare as much steel and equipment as possible for reuse.

The clean-up work itself is not without risk. Naturally, Groningen natural gas contains small amounts of mercury. During processing, the gas is stripped of impurities under pressure and at a temperature of minus 12 degrees Celsius (+10.4 Fahrenheit). Traces of mercury that remain in the pipes and processing units are jet‑cleaned at extremely high pressure (2,500 bar). Then the rinse water is collected, reused and ultimately transported to the waste processing terminal in Moerdijk (ATM). Any remaining contamination is removed with a chemical treatment.

Respirator

A rubber protective suit with respirator hanging over a fence shows that someone has just been working with it. It is heavy work, with the worker continuously monitored from a control room via cameras and medical equipment.

Anyone leaving the site must step into a tray of water to rinse off any traces of mercury from their safety boots. Robbert Kroes: “With these strict safety measures, the risk of mercury poisoning is under control.” The greatest danger during the work, he adds, comes from gravity combined with the enormous steel components that need to be moved.

After the cleaning operation, the above‑ground dismantling begins. Recyclable steel is cut into pieces with scrap shears. The giant compressors (23 megawatts) are destined for a second life. They were needed to keep the steadily shrinking gas reserves under pressure over the years, and to get the gas into the Dutch transport network (run by Gasunie) at the correct pressure.

"Christmas tree" in Kiel‑Windeweer

For the next phase of the clean‑up operation, the journey continues over the Annerveen field, 6 kilometres (3.7 miles) northwards, to NAM’s Kiel‑Windeweer site. This cluster is also marked by a tower — not a flare tower, but a so‑called plug‑and‑abandonment unit. It is a mobile decommissioning tower, based on the principles of a drilling rig but with the opposite function. You could call it a “filling tower”.

The only things still hinting at the site’s former purpose are the asphalt and the so‑called “christmas tree”. This installation, which with a bit of imagination resembles a Christmas tree, is the above‑ground arrangement of pipes and valve wheels used to regulate the gas well.

Set in motion by gas production

In this phase, Wellgear plays the leading role — a relatively small and new player in this specialised industry. It is a multinational company with branches in Scotland, Abu Dhabi, the United States and Australia. Its headquarters is the former town hall of Westerbork (’t Aole Gemientehoes), a village of 4,700 inhabitants that was merged into the municipality of Midden-Drenthe in the year 2000. The company is a prime example of the local and regional businesses that were set in motion by gas production and later grew into global enterprises.

Upon arrival at Kiel‑Windeweer, the clean‑up tower developed by Wellgear is in full operation. The production tube, or inner tube, through which the natural gas was brought to the surface, reaches a depth of 3,190 metres here. Now, it is being hoisted out of the well in sections of about eleven metres, one after another. Wellgear’s employees work in shifts to carry out this task.

Inner tube reused in construction

Supervisor Hiddo Schikker explains that the steel pipes, chrome‑plated on the inside, are thoroughly cleaned and then reused in the construction industry as foundation piles. After the inner tube has been removed, the so‑called casing will be cleaned and inspected. This outer pipe — effectively the wall of the gas well — will remain in the ground.

Robbert Kroes gives an admiring explanation of a method developed by Wellgear, which relies on existing techniques but applies them in a new combination. Acoustic equipment is lowered into the well to detect whether the cement anchoring the outer pipe in the rock is properly bonded. If it is not, an explosive charge can be used to perforate the pipe — even kilometres underground if necessary. The hole is then filled with cement.

Gas well and cement plugs

Only once the outer pipe is firmly secured all the way down, the gas is well sealed. A thick layer of cement is poured at the bottom of the well to prevent gas from rising from a depth of three kilometres. But halfway up the well, there may still be reservoirs of hydrocarbons. The well therefore receives several so‑called cement plugs at different depths.

Wellgear needs between one and four weeks to complete a well. The tower — made of stacked containers — is then dismantled and reassembled on top of the next well. Pressure in the sealed well is monitored for at least three months afterwards to ensure that no leaks occur. If everything remains stable, the outer pipe is cut off three metres below ground level. The well is then sealed with a fifty‑metre‑thick layer of cement, with a steel plate as the final lock.

Soil remediated where necessary

Once all wells at a location have been closed, Arcadis takes charge of the final phase of the operation. The soil is examined for contamination and remediated where necessary. Concrete and asphalt are removed by NAM, as are the pipelines connecting the various sites.

The land is only returned to the owners once it has been restored to the original condition in which NAM once leased it. This goes quite far: not only is the original soil type reinstated, even the shelterbelts around the site and along the access road are removed — though this can be adjusted in consultation with the landowners. In most cases, the land is restored for agricultural use.

No trace left behind

Robbert Kroes gives a cautious estimate of the overall duration of the clean‑up operation. He expects that in fifteen to twenty years, no trace will remain of gas extraction in the Groningen and Annerveen fields. Over the years, the cooperating parties will gain increasing experience with this specialised work.

Kroes: “A delegation from Oman that visited us was amazed that we manage to do this with so few people.” He expects that the experience and expertise gained in the coming years will prove valuable not only in Groningen and the Netherlands, but also elsewhere in the world. “It’s work with a strong future.”