The ground you are boring through decides the drilling fluid. Sand and gravel need viscosity and a good filter cake to hold the hole and carry cuttings; reactive clay needs chemical inhibition, not just a thicker mud. Here is how to read the ground and build the fluid.
Load bentonite to a viscosity target, not to a fixed number. A basic mix for cuttings suspension runs around 10 to 15 pounds per 100 gallons; pushing toward 15 to 27 pounds per 100 gallons raises the funnel viscosity into the normal working band, and coarse or highly permeable ground (sand, gravel, cobble) takes more to build both viscosity and a competent filter cake. Mix to the viscosity you want for the ground, then check it with a funnel — the number on the funnel is what you are really setting.
Water runs through a Marsh funnel in about 26 seconds — that is the baseline. Most HDD work targets roughly 45 to 60 seconds per quart, higher in coarse sand and gravel where cuttings are heavy and the hole will not stand on its own. Reactive clay is the exception: there the answer is not simply a higher funnel number but chemical inhibition, because the problem is the clay swelling, not thin fluid.
| Ground | What the fluid must do | How |
|---|---|---|
| Sand / gravel / cobble | Hold the hole open and carry heavy cuttings | Higher viscosity and gel strength, plus a filter cake to seal the permeable wall |
| Reactive clay / shale | Stop the clay swelling and balling on the tooling | PHPA polymer to encapsulate the clay and tight filtration to keep water off it — not just thicker mud |
| Rock | Carry cuttings and seal any fractures | Viscosity for transport; lost-circulation material if the rock takes fluid |
The two polymers do different jobs. PAC is added to a bentonite mix for secondary filtration control, especially in sands and cobbles where the filter cake matters. PHPA is used in place of bentonite in reactive clay: it wraps the clay particles and stops them swelling, which bentonite cannot do. The sequencing rule from the mixing-order guide applies exactly here — add polymer only after the bentonite is fully mixed, or the bentonite balls up. And always add PAC before PHPA, and dry polymers before liquid ones.
Usually yes. HDD additives — bentonite and polymers alike — mix and perform best in water at about pH 8 to 9, alkaline and free of calcium hardness. Soda ash conditions the water to that range before the bentonite goes in, which is why it is the first thing added. The mixing-order guide covers why in full.
In sand and gravel the hole will not stand on its own and the cuttings are dense, so you need viscosity and gel strength to carry them and a filter cake to seal the permeable wall. In reactive clay the failure mode is the opposite: water invading the clay makes it swell, ball up on the tooling, and stick — so the fix is to keep water away from the clay with a PHPA polymer and tight filtration control, not simply a thicker mud. Reaching for more bentonite in reactive clay makes the balling worse.