If you’ve ever watched a crew fight a loop down a borehole, you’ll know the problem isn’t gravity. It’s the opposite. Polyethylene loop pipe wants to float, and the deeper and heavier the fluid in the hole, the harder it pushes back.
Why a water-filled loop still floats
Most closed-loop ground heat exchangers are U-bend loops made from polyethylene (PE) pipe. PE is a great material for the job — tough, flexible, chemically stable — but its density is around 0.95 g/cm³, slightly less than water.
That means even a loop completely filled with water is marginally buoyant in a water-filled borehole. And boreholes are rarely just water. Drilling fluid, bentonite muds and grouts are all denser than water, and the denser the fluid, the more upthrust the loop experiences.
Add the pipe’s natural “memory” — loop pipe comes off a coil and wants to curl back into shape — and a buoyant loop can snake against the borehole wall, building friction until it simply won’t go any further.
The three numbers in the calculation
- The weight of fluid the loop displacesThis is the upthrust.
- The weight of water inside the loop
- The weight of the loop pipe itself
The British Drilling Association’s geothermal drilling training sets out the basic calculation as those three quantities. If the first is bigger than the second and third combined, the loop needs added weight to sink — plus a margin for friction and pipe memory.
A worked example
| Per metre of borehole (two pipes) | Approx. |
|---|---|
| Fluid displaced (outer volume × SG 1.3) | 2.09 kg |
| Water inside both pipes | 1.08 kg |
| PE pipe wall (at 0.955 g/cm³) | 0.51 kg |
| Net upthrust | ≈ 0.5 kg / m |
A single U-loop of 32mm SDR11 PE pipe, 150m deep, in fluid with a specific gravity of 1.3.
One more thing that’s easy to miss: the weight is in the fluid too. Steel has a density of around 7.85 g/cm³, so in a fluid with an SG of 1.3 it loses about a sixth of its weight to buoyancy. In that example, a 56kg GeoWeight works like roughly 47kg, a 40kg like roughly 33kg, and a 20kg like roughly 17kg.
Why “whatever’s on the truck” isn’t good enough
Historically, crews have sunk loops with whatever was to hand — lengths of rebar, scrap steel, homemade bars. It works, sometimes. But improvised weights come with real problems:
- Snagging. Square-cut ends catch on the borehole wall and casing, and can damage the loop on the way down.
- Unknown weight. If you don’t know what’s on the loop, you can’t record it — and drilling logs increasingly record loop details, including the weight installed.
- Poor attachment. No rated lifting point means improvised fixings, and a weight that isn’t secure can be lost down the hole.
- Suitability. Scrap materials have unknown coatings and contamination — not what you want sealed into a borehole alongside groundwater.
What a purpose-made weight should do
A good loop weight should be predictable, easy to handle, and easy to scale. That’s what GeoWeights are built around: steel for consistent density; domed and coned ends so they don’t snag; certified M10 lifting eyes for secure handling; a uniform 80mm diameter across the range; and units that shackle together with standard hardware when a borehole needs more.
Not sure how much weight your boreholes need?
Send us your loop specification, borehole depth and drilling fluid and we’ll help you find the right combination.