Two numbers size an HDD job: how much fluid the borehole itself holds, and how much more you must pump above that to clean the hole. The first is a simple formula; the second depends on the ground. Here is the math, with a worked example.
The volume of an open hole, in gallons per foot, is:
D is the hole diameter in inches. So a 6-inch pilot hole holds 0.0408 × 6² = 1.47 gallons per foot; a 12-inch hole holds 0.0408 × 12² = 5.88 gallons per foot. Multiply by the bore length in feet for the total borehole volume.
When drill pipe is in the hole, the space around it (the annulus) is smaller. In barrels per foot that is (hole diameter² − pipe diameter²) ÷ 1029.4, with both diameters in inches.
| Hole size | Gallons per foot | Gallons per 100 ft |
|---|---|---|
| 6" | 1.47 | 147 |
| 8" | 2.61 | 261 |
| 10" | 4.08 | 408 |
| 12" | 5.88 | 588 |
More than the borehole volume — because the fluid has to carry cuttings out, not just fill the hole. How much more depends entirely on the ground, so treat it as a range, not a fixed multiple:
| Ground | Fluid to pump | Why |
|---|---|---|
| Sand / gravel / coarse | About 1 to 3 times the borehole volume (a minimum) | Coarse ground drains fluid and needs enough to keep a flowable, cuttings-carrying slurry |
| Mixed soils | About 3 to 5 times | The common working range for cleaning a bore |
| Reactive clay / shale | 5 times and up (reactive shale can reach 10–20 times) | Swelling clay demands far more fluid and strong inhibition |
A flat “two to three times” rule is only the lower bound for coarse ground — reactive clay and shale need more. Size to your worst ground, not your average.
Work it out in three steps: borehole volume, then total fluid pumped, then bentonite for that fluid.
These numbers are an example to show the method. Your hole size, bore length, ground, and target viscosity all change the result — send us the details and we help size the order.
Cuttings only leave the hole if the fluid moves up the annulus fast enough to carry them. If you advance the drill faster than the fluid can transport cuttings out — outrunning your mud — cuttings pile up in the annulus, the hole packs off, pressure spikes, and you risk stuck tooling or a frac-out to surface. The fix is to match the rate of penetration and the pump rate to how fast the fluid can actually clean the hole, especially on long bores. Note that the high annular-velocity minimums quoted for deep vertical wells do not apply to HDD: an HDD hole has a large annulus and limited pump pressure, so hole cleaning is managed by controlling advance and pump rate against the cuttings load, not by hitting a vertical-well velocity number.