HDD drilling fluid is a bentonite slurry in soda-ash-treated water, with polyanionic cellulose (PAC) or partially hydrolyzed polyacrylamide (PHPA) added where the ground needs it, pumped at 2 to 5 times the soil volume of the bore (Trenchless Technology).
The slurry carries cuttings, holds the hole open and cuts pullback load. Like every drilling fluid, HDD mud is chosen by the ground, then measured, mixed in a fixed order and, on FERC projects and in some states, governed by additive guidance.
The four key facts of an HDD bentonite fluid are listed below.
- Bentonite: 15 to 25 lb/100 gal (6.3 to 10.5 lb/bbl; 18 to 30 kg/m3) for normal drilling (typical value of MAX GEL (Wyo-Ben) data sheet).
- Make-up water: soda ash to pH 8.5 to 9.5 (The Driller).
- Fluid to soil in sand: 2:1 minimum (The Driller).
- Additive guidance: FERC HDD guidance (October 2019), for projects under FERC jurisdiction.
Standards and regulations are checked against their issuing bodies, product values against the named data sheet and figures against the dated source named in the sentence; see our methodology and fact-checking.
Which Ground Conditions Shape an HDD Drilling Fluid?#
Soil type sets the HDD drilling fluid: reactive clay needs 3 to 5 times the soil volume in fluid plus clay control, while sand and gravel need 2 to 3 times and more gel (Trenchless Technology).
Clay and reactive clay#
Reactive clay needs fluid volume and clay inhibition rather than more gel, with fluid-to-soil ratios up to 5:1 (The Driller).
Inhibition matters because reactive clay swells and balls on the bit and reamer. PHPA wraps the clay and prevents swelling. Low-molecular-weight polymers control reactive clay without a significant viscosity rise and suit recycling, while high-molecular-weight types raise viscosity and do not (Trenchless Technology, 2025).
Sand and silt#
Sand and silt need viscosity and gel to suspend cuttings, with 2 gal of fluid per gal of soil (2:1) as the minimum and 3:1 as the upper working ratio (The Driller). The gel has to build quickly, stay constant and break with rotation (Trenchless Technology). Polyanionic cellulose controls fluid loss in coarse non-reactive soils (Trenchless Technology).
Gravel, cobble and fractured rock#
Gravel, cobble and fractured rock need high gel strength and filtration control, with more bentonite than sand. Xanthan gum raises gel strength with less viscosity than extra bentonite (Trenchless Technology). Mixed metal hydroxide (MMH) with bentonite builds fragile high gels at about half the pump pressure of a bentonite fluid (Trenchless Technology) and works best with straight 90-yield bentonite (The Driller, 2017).
Coarse ground also loses fluid. PPI TR-46 lists fractured rock and coarse permeable soils among the high-risk situations for inadvertent returns.
Shallow cover and hydrostatic pressure#
Shallow cover limits the hydrostatic pressure an HDD bore can hold, and entry and exit points carry the thinnest cover.
An 8.5 lb/gal (1.019 sg) fluid under 60 ft (18.29 m) of true vertical depth exerts about 26.5 psi (182.8 kPa) of hydrostatic pressure, calculated on true vertical depth, not bore length.
What Is a Typical HDD Mud Formulation?#
A typical HDD mud formulation is make-up water treated with soda ash to pH 8.5 to 9.5 (The Driller), then high-yield bentonite at a dose set by the ground. One product lists 15 to 25 lb/100 gal (6.3 to 10.5 lb/bbl; 18 to 30 kg/m3) for normal drilling and 25 to 40 lb/100 gal (10.5 to 16.8 lb/bbl; 30 to 48 kg/m3) in gravel (typical value of MAX GEL (Wyo-Ben) data sheet).
The formulation table compares bentonite, polymer or LCM additions and fluid system by ground condition.
| Ground condition | Bentonite | Polymer or LCM addition | Fluid system | Source label |
|---|---|---|---|---|
| Make-up water (every bore) | No bentonite yet | Soda ash 0.33 to 0.5 lb/100 gal (0.14 to 0.21 lb/bbl; 0.40 to 0.60 kg/m3) for pH control; up to about 1 lb/100 gal (0.42 lb/bbl; 1.20 kg/m3) in very hard water | Treated make-up water | typical value of CETCO SODA ASH (Minerals Technologies) data sheet; Water Well Journal 2018 |
| Normal drilling, sand and silt | 15 to 25 lb/100 gal (6.3 to 10.5 lb/bbl; 18 to 30 kg/m3) | Polyanionic cellulose 0.5 to 2.0 lb/100 gal (0.21 to 0.84 lb/bbl; 0.6 to 2.4 kg/m3) | Bentonite water-based mud | typical value of MAX GEL (Wyo-Ben) data sheet; polyanionic cellulose range product-specific |
| Gravel and poorly consolidated ground | 25 to 40 lb/100 gal (10.5 to 16.8 lb/bbl; 30 to 48 kg/m3) | Xanthan gum or MMH, product directions | Bentonite water-based mud | typical value of MAX GEL (Wyo-Ben) data sheet |
| Reactive clay | Product directions | Polyanionic cellulose 3 to 7 lb/100 gal (1.26 to 2.94 lb/bbl; 3.6 to 8.4 kg/m3) for water-sensitive formations; PHPA per the container directions | Polymer-extended bentonite or polymer drilling fluids | Product-specific |
| Lost-circulation control | 35 to 45 lb/100 gal (14.7 to 18.9 lb/bbl; 42 to 54 kg/m3) | Mineral fiber or swellable polymer LCM, product directions | Bentonite water-based mud | typical value of MAX GEL (Wyo-Ben) data sheet |
Oil-based, synthetic and pneumatic systems are other types of drilling fluids and stay out of this table. The table is a starting point, adjusted by measured Marsh viscosity and by the returns. Conversions use 0.42 lb/bbl and 1.1983 kg/m3 per lb/100 gal.
How much fluid does an HDD bore need?#
An HDD bore in sand needs at least 2 gal of fluid per gal of soil cut: 8.16 gal/ft (101.3 L/m) behind a 10-in reamer (The Driller).
The calculated results for a 1,000-ft (304.8 m) bore in sand are shown below.
| Result (calculated) | Value | Inputs |
|---|---|---|
| Soil cut per foot | 4.08 gal/ft (50.7 L/m) | 10-in reamer |
| Fluid per foot | 8.16 gal/ft (101.3 L/m) | 2:1 fluid to soil, sand |
| Fluid for the bore | 8,160 US gal (194.3 bbl; 30.9 m3) | 1,000 ft, 8.16 gal/ft |
| Bentonite for the bore | about 1,943 lb (881 kg) | 194.3 bbl at a 200-bbl/ton yield |
The reamer is 1.2 to 1.5 times the product pipe diameter (PPI Handbook of PE Pipe, 2008), and the fluid volume calculator scales the fluid volume with reamer size and bore length.
Target properties: viscosity, density, sand content and pH#
The HDD crew checks Marsh viscosity, density, sand content and pH, the parameters FERC HDD guidance (October 2019) lists for monitoring together with the additives used.
The property table sets each check against its HDD value and its test.
| Property | HDD value or what the check shows | Source | Test or property page |
|---|---|---|---|
| Density | Fluid weight, monitored on FERC projects | FERC HDD guidance (October 2019) | mud balance |
| Marsh funnel viscosity | Clays 35 to 40, coarse soils 50 to 90, unknown ground 50 to 60 s/qt (The Driller); water 26 s/qt | The Driller; water per the funnel calibration | Marsh funnel viscosity |
| PV and YP | Read on the rotational viscometer | API RP 13B-1 (6th edition, 2026) | 600 and 300 r/min readings |
| Gel strength | Builds quickly to the needed level, stays constant, breaks with rotation | Trenchless Technology | gel strength |
| Sand content | Volume % of particles above 75 µm (No. 200 sieve), a FERC monitoring parameter | FERC HDD guidance (October 2019) | sand content |
| pH | 8.5 to 9.5 | The Driller | pH of drilling fluids |
| Filtrate and filter cake | Measured on the filter press | API RP 13B-1 (6th edition, 2026) | filter press tests |
Trenchless Technology puts clays in the low 40s and sand in the mid 60s s/qt.
Which Additive Classes Go into an HDD Bore?#
HDD bores draw on seven additive classes: viscosifiers (bentonite, MMH, xanthan gum), fluid-loss additives (polyanionic cellulose), shale inhibitors (PHPA), pH control (soda ash), lost circulation materials (fibers, swellable polymers), surfactants (drilling detergents) and lubricants.
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The additive table compares each material by role, dose and source.
| Class | Material | Role in an HDD bore | Dose (basis) | Source |
|---|---|---|---|---|
| drilling fluid viscosifiers | Bentonite | Viscosity, gel, filter cake | 15 to 45 lb/100 gal (6.3 to 18.9 lb/bbl; 18 to 54 kg/m3) by ground | typical value of MAX GEL (Wyo-Ben) data sheet |
| Viscosifiers | mixed metal hydroxide (MMH) | Fragile high gel with bentonite, about half the pump pressure | Product directions | Trenchless Technology; The Driller |
| Viscosifiers | xanthan gum | Gel with less viscosity in gravel | Product directions | Trenchless Technology |
| fluid loss control additives | polyanionic cellulose (PAC) | Filtration control | 0.5 to 2.0 lb/100 gal (0.21 to 0.84 lb/bbl; 0.6 to 2.4 kg/m3); 3 to 7 lb/100 gal (1.26 to 2.94 lb/bbl; 3.6 to 8.4 kg/m3) in water-sensitive formations | Product-specific |
| shale inhibitors | PHPA | Wraps reactive clay | Container directions | Product-specific |
| pH and alkalinity control additives | soda ash | Removes hardness, sets the pH | 0.33 to 0.5 lb/100 gal (0.14 to 0.21 lb/bbl; 0.40 to 0.60 kg/m3) | typical value of CETCO SODA ASH (Minerals Technologies) data sheet |
| lost circulation materials (LCM) | LCM fibers | Mineral fiber bridge | Product directions | Product-specific |
| Lost circulation | Swellable polymer LCM | River crossings, cobble, gravel | Product directions | Product-specific |
| Surfactants | Drilling detergents | Reduce clay sticking | Product directions | Product-specific |
| Lubricants | Lubricants | Torque and drag | Product directions | Product-specific |
Buyers can compare additive suppliers for every material in the table.
FERC HDD guidance asks for non-petrochemical, non-hazardous additives, so HDD fluids rest on clays, minerals and water-soluble polymers. Oil and gas wells add further drilling fluid additives, such as weighting agents and thinners.
Bentonite grades and yield for HDD#
Bentonite yield is the number of barrels of 30-cP mud one US ton of clay makes; good bentonite yields at least 85 bbl/ton.
Standard 90-yield (single-sack) bentonite, sold as API Spec 13A bentonite, yields about 90 bbl/ton (14.3 m3 per 907 kg) against 150 to 220 bbl/ton (23.8 to 35.0 m3 per 907 kg) for high-yield HDD grades (The Driller). One product reaches 220 bbl/ton (typical value of MAX GEL (Wyo-Ben) data sheet). A ton of 200-bbl bentonite makes about 8,400 to 8,500 US gal (31.8 to 32.2 m3) of slurry (The Driller).
Which polymers does an HDD bore use?#
An HDD bore uses three polymers: polyanionic cellulose (PAC) controls filtration, PHPA wraps reactive clay, and xanthan gum adds gel with less viscosity.
API Spec 13A grades exist for bentonite, polyanionic cellulose and xanthan gum, not for PHPA. PHPA goes in by the container directions. Residual acrylamide monomer in PHPA has been on the REACH Candidate List since 2010-03-30 as carcinogenic and mutagenic; PHPA itself is not a substance of very high concern. The base fluid of a bore is a choice between bentonite vs polymer drilling fluids, set by the ground along the bore.
How Is HDD Drilling Mud Mixed and Handled?#
HDD drilling mud is mixed in a fixed order: soda ash into the make-up water first, bentonite next until it is hydrated, then polyanionic cellulose before PHPA and dry polymers before liquid ones.
The eight mixing steps are listed below.
- Treat the make-up water with soda ash to the target pH.
- Add the bentonite through the hopper.
- Mix until the bentonite is hydrated.
- Add polyanionic cellulose, then PHPA.
- Add dry polymers before liquid polymers.
- Check Marsh viscosity, density and pH.
- Record the FERC parameters on FERC projects.
- File an SDS for every additive other than bentonite and water.
Soda ash and bentonite arrive as dry powders, and the SDS filed for each product sets the protective equipment for handling it.
Which Rules Govern HDD Fluid Products and Spent Mud?#
HDD fluid additives fall under project guidance by jurisdiction, led by FERC HDD guidance (October 2019), for projects under FERC jurisdiction, and Ohio EPA HDD contingency plan guidance (August 2023).
The rules table compares the instruments that set HDD fluid requirements.
| Jurisdiction | Instrument and date | Fluid requirement |
|---|---|---|
| US, projects under FERC jurisdiction | FERC HDD guidance (October 2019) | NSF/ANSI 60 compliant, non-petrochemical, non-hazardous additives; SDS for all additives but pure bentonite and water; monitor fluid weight, viscosity, sand content, additives and pH; plans for normal drilling, loss of circulation and inadvertent returns; real-time downhole annular pressure monitoring in the pilot hole |
| Ohio | Ohio EPA HDD contingency plan guidance (August 2023) | Additives consistent with materials used in drinking water distribution; petroleum-based products strictly prohibited; disposal testing by SW-846 8260B (VOC) and 8270C (SVOC); on-site burial at 1:1 with overburden |
| Pennsylvania | PA DEP HDD safety data sheet repository | The agency publishes the SDS of HDD additives |
| Standard | NSF/ANSI/CAN 60-2026 | Current edition (60-2025 superseded) |
| Standard | ASTM F1962-25 | Current edition (F1962-22 historical) |
On-site burial is one route of spent mud treatment for HDD returns, and the state guidance decides where it is allowed. This page describes rules and gives no legal advice.
What Goes Wrong with HDD Drilling Fluids?#
HDD drilling fluids fail in four ways: inadvertent returns, losses into coarse ground, clay balling and cuttings settling, the four rows of the table below. FERC HDD guidance (October 2019) plans every HDD bore for three operating conditions: normal drilling, loss of circulation and inadvertent returns.
The problem table sets each problem against its fluid-side response.
| Problem | Ground or cause | Fluid-side response | Page |
|---|---|---|---|
| Inadvertent return | Thin cover, fractures, voids, old borings | Full returns, LCM under the contingency plan | this page, below |
| Losses | Coarse permeable soils, fractured rock | 35 to 45 lb/100 gal (14.7 to 18.9 lb/bbl; 42 to 54 kg/m3) bentonite (typical value of MAX GEL (Wyo-Ben) data sheet), fiber or swellable LCM | lost circulation |
| Clay balling | Reactive clay | PHPA or low-molecular-weight polymer, more fluid volume | bit balling |
| Cuttings settling | Low gel in sand and gravel | Marsh near the mid 60s s/qt, steady gel profile | hole cleaning |
Inadvertent returns (frac-out)#
An inadvertent return (frac-out) is an unauthorized discharge of drilling fluid to the ground surface or to surface waters, including wetlands.
Returns occur near entry and exit points with thin cover and along faults, fractures, voids, geotechnical borings and edges of subsurface structures. The Delft method (USACE CPAR-GL-98-1, 1998) evaluates hydrofracture risk and is invalid when full returns are not maintained (Trenchless Technology).
PPI TR-46 names five high-risk situations, listed below.
- Fractured rock
- Coarse permeable soils
- Elevation differences
- Insufficient cover
- Existing boreholes
Contingency plans allow LCM during an inadvertent return and when circulation is diminished or lost, and FERC adds real-time annular pressure monitoring in the pilot hole. HDD frac-out is unrelated to hydraulic fracturing, which is completion work.
Losses into gravel and fractured rock#
Losses into gravel and fractured rock are treated with more bentonite and with LCM sized to pass the pumps.
LCM selection weighs environmental impact, sizing to pass pumps, valves and bit orifices, particle size against pore throats, and the sequence of treatments.
Mineral fiber LCM bridges openings in bentonite HDD fluid at the dose its product directions give. Swellable polymer LCM absorbs hundreds of times its weight in water and targets river crossings, cobble and gravel.
How Are HDD Fluid Products and Services Sourced?#
HDD fluid products are bought one product at a time, each with its SDS, because FERC HDD guidance (October 2019) requires an SDS for every additive other than pure bentonite and water. HDD contractors use 500 to 2,000 US gal (1.9 to 7.6 m3) of slurry a day (The Driller).
NSF/ANSI/CAN 60 certification is stated per product on its data sheet, so the buyer checks it for each product, the LCM fiber included. Crews that want a fluid program instead scope mud engineering services.
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How Do HDD Fluids Differ from Fluids for Other Drilling Applications?#
HDD fluids differ from other drilling fluids in how close they run to the surface, where thin cover lets mud escape through fractures, voids and old borings.
Returns control and on-site disposal shape the HDD recipe, while the other drilling fluids by application answer to their own ground, depth and rules. The table compares HDD with seven of them.
| Application | How the fluid differs from HDD | Page |
|---|---|---|
| Water wells | Fluid removed by well development; drinking-water additive rules | water well drilling mud |
| Mud rotary | Vertical direct circulation | mud rotary drilling |
| Foundations | Support fluid for an open excavation, checked before concreting | support fluids for bored piles |
| Geothermal | Heat degrades biopolymers | geothermal drilling fluids |
| Mining | Clay-free polymer fluids and rod grease | diamond core drilling fluids |
| Offshore | Discharge rules for drilling fluids and cuttings | offshore drilling fluids |
| HPHT | Pressure and temperature limits of additives | HPHT drilling fluids |
Is bore mud only clay and water?#
Mostly, but not only: bore mud is bentonite in soda-ash-treated water, plus polyanionic cellulose or PHPA where the ground needs it. Bentonite in waterproofing and grouting belongs to construction chemistry.
Can plain water replace HDD drilling fluid?#
No: in a 2008 University of Texas at Arlington pullback test, water needed 19,815 lbf (88.1 kN) of pullback force against 4,425 lbf (19.7 kN) with drilling fluid (Trenchless Technology). The test pulled two 4-in fused PVC pipes, and pressure was 67 psi (462 kPa) with water against 18 psi (124 kPa) with drilling fluid.
Water builds no gel and no filter cake, so it fails two core functions of drilling fluid: suspending cuttings and sealing the bore wall.