POLY‑SAL, DEXTRID, N‑DRIL, and IMPERMEX are brand names for modified drilling starch — the oldest and usually the lowest-cost filtration-control chemistry in water-based drilling. One procurement list often carries two or three of these names for what is, at class level, the same material family. Ironstone supplies drilling-grade modified starch with a certificate of analysis for every batch.
All of them are starch — from potato, corn, or tapioca — that has been processed so it hydrates in cold water without cooking (pregelatinized). Premium grades are additionally chemically modified or cross-linked for higher temperature and brine service; the base commodity product is pregelatinized starch. In the mud, the swollen starch particles are soft and deformable: they lodge in the pores of the filter cake and squeeze into shape, which is what cuts the filtrate. Because starch is non-ionic — the molecule carries no charge — salt does not switch it off, and that is the reason starch remains the default filtration additive in saturated-salt and high-hardness systems where charged polymers struggle.
Habit and history more than chemistry. The names on one list can even come from competing service companies — POLY‑SAL is an M‑I SWACO product while DEXTRID is its Baroid counterpart — because mud programs inherit whichever vocabulary the engineer trained on, even when the underlying class is the same. The differences that do matter are grade-level: base starch source, degree of modification, temperature stability, and whether the product is preserved against bacterial attack. Those show up on a COA — not in the brand name.
| Grade | Choose it when |
|---|---|
| Standard modified starch | Normal-temperature sections and short exposure times — the lowest-cost filtrate control available. |
| Temperature-stabilized grade | Deeper or hotter sections where standard starch degrades and filtrate climbs mid-section. If your current brand carries an HT suffix, this is the class to match. |
Starch is organic material: every grade has a temperature ceiling and, especially in lower-salinity systems, can ferment if the system is left uncirculated without preservative. Both limits belong in the mud program, whatever brand or generic is in the sack.
| Parameter | Why it decides the match |
|---|---|
| Filtrate at stated dosage in the relevant brine | The core job — and starch must be compared in the brine you actually run, from fresh water to saturated salt. |
| Viscosity contribution at dosage | Some grades thicken more than others; a mismatch changes pump pressure and dosing tables. |
| Temperature stability | Decides whether the product survives your section or quietly degrades downhole. |
| Preservation status | Whether the product ships with preservative, and what that means for storage and for systems left static. |
| Moisture and purity | True starch content per sack — where cheap material hides filler and water. |
Starch test methods are covered by API Specification 13A. We report COA results against the 13A test procedures and state the edition on the document.
Test the candidate on the bench in your worst-case brine, not in fresh water — starch that matches in fresh water can perform differently from your current product in saturated salt. Compare filtrate and viscosity at equal dosage, check the product mixes cleanly through your hopper, and hold a sample at your bottom-hole temperature if your lab can. Then trial one interval before switching the program.