HDD drilling fluid calculator

Gallons per foot: the soil each pass moves

A pass cuts a cylinder. Its volume per foot of bore is the area of the circle times the length:

The Driller’s version rounds the constant to 24.5 and CETCO’s pocket toolbox prints 24.52; the three differ by less than 0.1 per cent. The metric form, used by Baroid and CETCO, is the same cylinder:

The calculator uses π/4 directly rather than either rounded constant. The square is the point Baroid’s paper makes about big bores: double the reamer and there is four times the soil to move, while the pump’s output is often fixed. Baroid’s table puts a 100 mm bore at 8 L/m and a 200 mm bore at 32 (its own formula gives 31.4).

Vermeer publishes the same table from 2 to 36 in; every entry in it is within 0.03 gal/ft of the exact figure. The reamer itself is usually sized from the product: Vermeer’s rule is 1.5 times the product diameter up to 9 in and 1.3 times from 10 in — the reamer size calculator covers that step.

How many gallons of fluid per gallon of soil

Cut soil will not flow on its own. It has to be mixed into a slurry thin enough to be pushed out of the hole, and the volume of fluid that takes for each volume of soil — the fluid-to-soil ratio, which Vermeer calls the soil safety factor — is the one number here that is judgement rather than geometry. CETCO’s toolbox sets the floor at 1:1. Above that, the four sources:

The spread is widest in sand, where Vermeer’s bottom figure is half The Driller’s minimum. Baroid’s reasoning for the order is the useful part: sand is inert and neither swells nor sticks, while clay swells and turns sticky, so the more clay in the ground the more fluid each gallon of it needs. Each soil in the calculator’s menu is labelled with the full spread the sources give for it, from the lowest figure any of them prints to the highest. Choosing one sets the ratio to the lowest figure that none of the sources calls too little: Vermeer’s low figure, which is where Vermeer’s own worked example sits (3:1 in sandy clay), CETCO’s for rock and for ground mixed with clay, and in sand The Driller’s 2:1 minimum rather than Vermeer’s 1:1. The summary then reports the job again at the top of the spread. Type any other ratio over it.

Too little fluid is the expensive mistake. Vermeer describes the drill outrunning its fluid: the mix gets too thick to flow out of the pits, and the result can be an inadvertent return, heaved ground, stretched product, damaged or stuck tooling. Baroid puts numbers on it. A 200 mm reamer pulling a 140 mm line in sand needs at least 32 L of fluid per metre; pump 12.5 L instead and 60 L of soil, pipe and fluid is being forced into the 32 L the soil used to fill. Too much fluid, in Vermeer’s words, is wasteful and adds cost — and Vermeer is careful to add that inadvertent returns can also come from the wrong type or amount of additive, not only from too little volume.

Minutes per rod and the pullback speed limit

The first line is Vermeer’s step 3. CETCO’s toolbox prints the same formula without the ratio, which is the 1:1 floor. The second is the same statement turned round: the bit or reamer can only advance as fast as the pump can supply ratio × soil for every foot it moves. Go faster and the hole is short of fluid, whatever the gauge on the pump says. The calculator gives both for every pass, because a pilot bore can run quickly and the last ream on the same pump cannot.

“Pump output” means what actually arrives downhole. CETCO’s toolbox tabulates how the output of a hydraulically driven pump falls as the fluid thickens: 95 to 100 per cent on water (26 seconds on the Marsh funnel), 80 to 90 per cent at 45 seconds, 60 to 70 per cent at 60 and 50 per cent at 80. The delivered-share field applies that derating to the rated figure.

Water, bentonite and bags

Bentonite is dosed per 100 gallons of fresh water, and the dose depends on the ground and on the product. These are CETCO’s figures for its Hydraul-EZ or Super Gel-X bentonite, from the no-recycle version of its HDD mixing chart (updated March 2024):

Every row starts with 0.25 to 0.5 lb of soda ash per 100 gal, mixed in first, and every row adds other CETCO products in small doses, none more than 1.5 lb or 1.5 qt per 100 gal, that this page does not count. In clay the bentonite is cut back to 7–10 lb. CETCO’s recycle version of the chart (July 2023) raises sand to 25–30 and gravel and cobble to 30–35, and CETCO’s older pocket toolbox (October 2015) listed 25 lb in sand, 30 in sand and gravel, 30 to 35 in gravel, 35 in cobble and 15 in clay. Picking a soil in the calculator sets the dose to 25 for sand, gravel and rock — the top of CETCO’s sand row and the bottom of its gravel rows; the chart has no rock row, and 25 is also the top of its unknown row — and to 10 for clays, the top of the clay rows. Another brand’s bentonite yields differently, so type in your supplier’s figure.

The fluid is water plus the bentonite in it. CETCO’s toolbox gives bentonite a specific gravity of 2.3 to 2.4, so 25 lb of it takes up about 1.3 gal; the calculator takes that out of the water figure (using 2.35) before counting bags. Bag size defaults to 50 lb, the bag CETCO counts Super Gel-X in.

What comes back out of the hole

The soil a pass cuts does not add to the returns. It leaves a hole of exactly its own volume behind, and as long as that hole stays full of slurry, what comes out at the pits is the fluid pumped in, plus the volume of the product on the pullback. That is the calculator’s lower figure. Baroid points out that where the bore dips well below the entry or exit, the hole cannot be anything but full of slurry. If the hole drains instead — Baroid describes an air gap above the flowing slurry as the best case in line-and-grade work — up to the whole finished hole’s volume can come out on top of the fluid. That is the upper figure: fluid plus every gallon of soil. Baroid’s 60 L example above is this same balance, run for the case where the pump fell behind.

Fluid that disappears into open gravel or a fracture never reaches the pits, so real returns can fall below both figures. Enter a tank capacity to count vacuum truck loads.

Pressure at the bottom of the hole

The imperial form is from The Driller and CETCO’s toolbox, the metric form from Baroid; the calculator uses the unrounded constants. The Driller’s example is clean mud at 8.5 lb/gal at 60 ft. It prints 26.6 psi; 8.5 × 0.052 × 60 is 26.52, and 26.49 with the exact constant, so the answer is 26.5. Baroid’s are fresh water at 3 m, 29.4 kPa, and a slurry of SG 1.2 at 3 m and 10 m, 35.3 and 117.7 kPa. The calculator reproduces all four.

That is the pressure with the pump off. Baroid’s paper is about what happens with it on: the circulating pressure is the hydrostatic pressure plus the friction loss of pushing slurry along the annulus, and the thicker the slurry the bigger that loss. Its example supposes that the SG 1.2 slurry circulates at an equivalent SG of 2.0, which gives 59 kPa at 3 m — more than the 49 kPa Baroid gives for 3 m of clay at SG 1.7 (1.7 × 9.81 × 3 is 50), so the slurry is likely to break out upward. The figure here is the floor of that calculation, not a frac-out check.

Annular velocity

This is CETCO’s toolbox formula (it prints 24.52), and the constant is the same 24.51 as in the hole volume. The calculator applies it to the pullback, with the final reamer as the hole and the product as the pipe. It assumes every gallon goes one way along the product; where the returns split between the two pits, the figure along the product is lower.

What it leaves out

• The ground changes along the bore. The Driller points out that soil reports are commonly taken at or near the entry and exit pits, not along the whole path, and Baroid that very often no two bores are alike even on the same street. A ratio chosen for the pits may be wrong in the middle.
• Losses and dead volume. Fluid lost into permeable ground, and the fluid needed to fill the hoses, the drill string and the bottom of the tank, are not counted.
• Recycling. With a reclaimer most of the fluid is cleaned and pumped again, so the fresh water and bentonite fall a long way below the figures here. The fluid pumped does not.
• Whole circle or new ring. Vermeer’s and Baroid’s examples count the whole circle of each pass. The Driller notes that its 10 in figure takes no account of the pilot or any pre-ream; taking account of them is what the box that counts only the new ring does, giving each pass fluid only for the soil outside the hole before it. With a pilot and several reams that cuts the total by roughly a third to a half, and it assumes the slurry already in the hole still flows.
• Rig size. Vermeer introduces its recommendations as being for HDD rigs up to 100,000 lb. The hole volume is geometry and holds at any size; Vermeer does not give its ratios for bigger rigs.
• Drilled length. Enter the length along the path, rods times rod length, not the horizontal distance between pits; the entry and exit curves make the path longer. The bore path profile page works out that geometry.
• Pressure. Static only, as above. No annular pressure loss and no comparison with what the ground can hold.

Sources

• Vermeer ProTips, “Making sense of HDD drilling fluids, Part 2”, 10 May 2019 — hole volume table, soil safety factor chart, minutes per rod, the 300 ft worked example, reamer sizing rule, the 100,000 lb rig limit.
• Mario Brunet (CETCO), “Calculating Fluid Volumes, Pressures for Horizontal Drilling”, The Driller, 27 June 2018 — D² ÷ 24.5, ratios for sand and clay, the 0.052 hydrostatic formula and its example.
• Robert Petrie and Frank Canon, “Downhole Pressure: Friend or Frac-out?”, Baroid Industrial Drilling Products — D² ÷ 1273, soil volume table, ratios for sand and clay, the 200 mm example, hydrostatic and circulating pressure.
• CETCO, HDD Mixing Chart, no recycle (updated March 2024) and recycle (updated July 2023) — bentonite and soda ash doses, funnel viscosities, bag size.
• CETCO, Pocket Toolbox (updated October 2015) — hole and annular volume, minutes per rod, annular velocity, hydrostatic pressure, fluid-to-soil ratios including rock, pump efficiency against viscosity, specific gravity of bentonite, its earlier mixing doses.