08-01-2026

Why would a vertical well operation use an HDD mixing unit?

A jobsite visit with WellGear that shows why HDD equipment can be a surprisingly effective solution on a vertical operation when the conditions demand it.

Where we would normally scan the horizon for a drilling setup, the installation rising above us in the Dutch town of Honselersdijk looks different. This is not a drilling rig, but a Hydraulic Workover Unit (HWU) used for well intervention and abandonment work. Among the vertical well equipment on site, the familiar bright orange unit seems like an unusual addition to this vertical intervention spread. What is a SiteTec mix tank doing here?

Need for flexibility
WellGear is a Dutch company specialised in the maintenance, repair and abandonment of geothermal, salt, gas and oil wells. From their headquarters in Westerbork they operate internationally, using modular units they can assemble and adapt per project. That flexibility fits the sectors they work in. They bring more than equipment: their crews have the technical experience to work safely in wells where very little is standard. It is no surprise they are open to equipment from outside their own discipline, including an HDD mixing unit.

As Allistair Bredenkamp, project engineer for WellGear, puts it with a smile: “On the odd occasion, we need the brilliant bright orange mix tank from SiteTec.”

The real beauty of the M2000E
Geothermal and salt projects are often similar to HDD in terms of duration and limited space. Ease of use matters, and so does the ability to mix drilling fluid in small, accurate batches. A Hydraulic Workover Unit (HWU) sometimes needs more mixing flexibility than its built-in systems can offer.

WellGear’s collaboration with SiteTec began in April 2024 during a section milling project in Enschede. The operations program required special chemicals to be mixed both to carry metal shavings (swarf) to surface and to maintain fluid balance inside the well for proper well control.

“The SiteTec mix tank can condition the chemicals exactly to the mud engineer’s requirements.”

“The reason we used the HDD mix tank is simple,” Allistair explains. “It’s easy to work with, easy to transport and integrates quickly into our active fluid system. But the real beauty of the M2000E is how it mixes chemicals. The chemicals we use nowadays are highly advanced. Thanks to the nozzle-injection agitation system, the SiteTec mix tank can condition them exactly to the mud engineer’s requirements, within minutes.”

Plugging the vertical well
The well on this site is a geothermal source that supplied heat to the greenhouses right next to the location. Situated in the heart of the Dutch horticultural region, deep geothermal wells play a key role in providing sustainable energy. This one has reached the end of its technical life, so it needs to be emptied and permanently sealed – a process known as plugging.

A geothermal well consists of several layers of steel casing, each placed deeper and with a smaller diameter. Every layer is cemented in place to stabilise the formation and prevent fluids or gas from moving between geological zones.

For the WellGear crew, the work starts with cleaning the wellbore and circulating out old fluids. Next, cement plugs are placed at several depths to isolate water-bearing zones and prevent upward flow. The cement must bond perfectly with both casing and formation. After plugging, a pressure test confirms the integrity of the seal. The team has two weeks to complete the operation.

Mud behaviour: vertical versus horizontal
Intervention work also uses water and bentonite, but the fluid behaves differently than in HDD. HDD relies on large volumes with relatively low density, designed to transport cuttings horizontally over long distances. In well intervention, the requirements are different. The mud is often heavier, viscosity changes between phases of the work and the volumes are smaller but must be controlled precisely. Christian, mud engineer for Halliburton on the WellGear jobsite, explains: “Here we work with viscosities from 50 up to sometimes 100 seconds.”

The right level of control
So why bring in an HDD mixer on this specific job? The decision was made early in the planning phase with WellGear. Allistair explains: “Much like in 2024 in Enschede, our client had requested to prepare this geothermal well for abandonment. This well had some technical challenges to overcome, and with the use of the mix tank we were able to mix special chemicals that would help repair the leaks in the well – like pumping a fluid into your bicycle tyre to seal it. We also needed to mix milling fluid to bring swarf back to the surface and out of the well, and to maintain well control throughout the entire operation.”

A different, but necessary workflow
Christian joins us as he has a moment to talk. Originally trained as a petroleum engineer in Romania, he prefers working directly in the field. Here he is responsible for the fluid properties. During our visit the operation is on hold because tubing is stuck deep inside the well as a result of corrosion or deformation over time. The crew is attempting to cut the string at depth to continue the job.

“Vertical installations usually have a simple built-in mixer.”

Working with an HDD mixer is new for the experienced mud engineer. As Christian explains: “Vertical installations usually have a simple built-in mixer, but this well requires more flexibility. Heavy pills – small batches of fluid with specific properties – are chemically demanding and need to be mixed accurately, and viscosity has to be adjusted per batch.”

He lists several benefits of the SiteTec rental unit, such as the well-sealed hopper and the easy-to-operate valve. He also appreciates that the lines are simple and connect quickly to their setup. At the same time, he notes the differences from his usual workflow. Mixing can take longer when no operator is dedicated to it full time.

Too popular
An HDD recycling or mixing unit on a vertical well may seem unusual, but this jobsite shows exactly where it makes sense. When an installation has no built-in hopper, when heavy pills are required, when viscosity must be adjusted per batch or when mixing precision is critical, an HDD unit can be the right solution. Whether it fits depends on the situation. For this project, it was exactly what the job required.

“It’s the perfect solution to the need of mixing chemicals, its design and functionally make the unit easy to work with and even though its big and bulky in shape, we have still been able to squeeze this orange machine into some tight spaces.” Allistair laughs as he adds one final remark: “This tank is so popular, it’s almost never available. How are we going to fix that for the next job, SiteTec?”