Polyanionic Cellulose (PAC) in Oil Drilling Fluids: Complete Guide

Celulosa polianiónica (PAC) in oil drilling fluids is the preferred fluid-loss control and rheology additive for water-based mud systems, delivering stable filtration control in fresh water, seawater, and saturated brine at temperatures up to 120 °C. This guide explains how PAC-LV and PAC-HV differ, how they pass API/ISO 13500 testing, and how to dose them for optimized drilling performance in every water chemistry.

Introducción

Fluid loss is the quiet killer of drilling economics. When drilling mud leaks into permeable formations, the filter cake thickens, the hole begins to stick, formation damage sets in, and the mud bill climbs. Polyanionic cellulose (PAC) is the additive drilling-fluid engineers reach for when they need predictable, temperature-stable filtration control across the widest range of water chemistries — from fresh water to saturated brine.

Polyanionic cellulose (PAC) is a high-purity, high degree-of-substitution carboxymethyl cellulose (CMC). The carboxymethyl groups (up to DS 0.9–1.3, far higher than standard CMC) make the molecule strongly anionic, highly soluble, and resistant to the salt that deactivates ordinary CMC. PAC comes in two functional grades that solve different problems: PAC-LV (low viscosity) for filtration control with minimal viscosity contribution, and PAC-HV (high viscosity) for filtration control plus viscosity and cuttings suspension. Most field muds use both, tuned to the density, water chemistry, and temperature of the well.

En TENESSY Chemical, we manufacture PAC from premium cotton-derived cellulose on German process equipment, with production control aligned to API 13A / ISO 13500 testing methods. Our PAC grades are exported to 40+ countries and used in both land and offshore drilling operations. This guide distills the field experience behind PAC selection: the mechanism of filtration control, the LV vs. HV decision, performance in every water type, and practical dosage programs.

The Role of PAC in Drilling Fluids: Filtration, Viscosity, and Rheology

Fluid-Loss Control: The Filter Cake Mechanism

Fluid loss is the volume of mud filtrate that escapes into the formation under pressure. In a standard API filtration test (API 13B-1, LPLT, 690 kPa / 100 psi over 30 minutes), a good PAC-treated mud gives 5–8 mL filtrate, versus 15–40 mL or more for an untreated or starch-only mud.

Polyanionic cellulose (PAC)controls this loss through the filter cake. The polymer chains adsorb onto the surfaces of clay particles (bentonite) and drill solids, bridging them into a thin, low-permeability cake on the wellbore wall. Because the cake is thin and pliable, it limits both the volume of filtrate and the risk of a stuck pipe. Two properties make PAC the right polymer for this job:

• High anionic charge. The carboxymethyl groups adsorb strongly onto positively charged clay edges and onto the calcium/magnesium sites on formation rock, so the cake binds firmly even in brines.
• Chain extension. In solution, PAC’s charged chains repel each other and stay extended, so even a small dosage covers a large surface area of clay. This is why PAC outperforms starch (which is neutral and leaches out of the cake) and ordinary CMC (which coils up in brine and loses effectiveness).

Viscosity and Suspension

PAC-HV raises the mud’s viscosity by entangling in the water phase, which does three jobs:

• Cuttings transport. The annulus must carry drilled cuttings to the surface; a viscosity minimum is required to keep them moving. At low shear rates (5–100 s⁻¹, the range of the annulus), PAC-HV builds the gel-like structure that suspends cuttings when circulation stops.
• Hole cleaning in deviated wells. In horizontal and deviated sections, cuttings settle on the low side of the hole. The low-shear viscosity from PAC-HV is the main defense.
• Barite suspension. Weighted muds rely on a yield stress to keep barite (density 4.2 g/cm³) suspended when pumps are off. PAC-HV contributes to that yield stress.

Rheological Control and Temperature Stability

Polyanionic cellulose (PAC) is thermally robust for a biopolymer: it maintains its filtration and viscosity performance up to about 120 °C (some high-DS grades to 130 °C) in low-solids muds, and it is compatible with the common rheology modifiers — xanthan gum for low-shear viscosity, polyacrylamide-based shale inhibitors, and lignosulfonate thinners. Unlike xanthan, PAC does not shear-degrade catastrophically in the bit nozzles, and unlike starch, PAC does not ferment or thin out in hot, oxygenated mud. Its resistance to bacteria is another practical advantage: starch needs biocide treatment, PAC does not.

The typical field combination is PAC-LV for filtration + xanthan or PAC-HV for viscosity, with the ratio set by the mud density and hole angle. A straightforward rule of thumb is covered in the dosage section below.

PAC-LV vs PAC-HV: What’s the Difference and Which One Do You Need?

The selection logic is simple once you know the mud program:

• PAC-LV is the filtration specialist. It is chosen when the mud already has enough viscosity from bentonite, xanthan, or solids, and the only gap is fluid loss. This is the common case in weighted muds, where adding a high-viscosity polymer would push yield stress and pump pressure too high.
• PAC-HV is the dual-purpose additive. It is used in low-solids, unweighted muds (where there is no bentonite to build viscosity) and in vertical-hole sections where the mud needs viscosity but no extra density. PAC-HV adds viscosity and filtration control with a single product.

Most mud systems use both. A typical field recipe for a fresh-water low-solids mud starts with 4–6 kg/m³ PAC-LV plus 1–3 kg/m³ PAC-HV; a saturated-brine completion brine uses 5–8 kg/m³ PAC-LV alone, because no polymer viscosity is wanted in a completion fluid.

PAC Performance in Fresh Water, Seawater, and Saturated Brine

he defining test for a fluid-loss polymer is its performance in salt. Ordinary CMC loses most of its viscosity and filtration control in brines because salt shields the polymer’s charges and coils the chains. PAC, built with higher DS and higher purity, keeps its chains extended even in saturated NaCl.

Fresh-Water Muds

In fresh water, Polyanionic cellulose (PAC) disperses and hydrates quickly. A typical fresh-water mud with 4% bentonite + 4 kg/m³ PAC-LV gives an API filtrate of 6–8 mL/30 min and a plastic viscosity increase of only 1–3 mPa·s — filtration control with almost no rheology penalty. This is the classic application for PAC-LV in low-solids muds, where the filtration spec is met while the mud stays thin enough to drill fast.

Seawater and High-Salinity Muds

Seawater (about 3.5% total salts) and mixed-salt muds are where PAC separates from CMC. At 3.5% NaCl, standard CMC retains only 40–60% of its fresh-water viscosity; PAC retains 70–90% because its higher DS keeps the chains ionized. Filtration control remains in the 6–9 mL range. Field practice is to raise PAC-LV dosage by 20–30% when the mud system switches from fresh to seawater to compensate for the remaining viscosity loss.

Saturated Brine (NaCl ≥ 36%)

Saturated-brine muds and completion brines are the harshest environment. The mud is nearly a dead salt solution, and most polymers either precipitate or gel. PAC at 5–8 kg/m³ delivers an API filtrate of 8–12 mL/30 min in saturated NaCl — acceptable for most drilling operations and far better than starch, which gives 15–25 mL and degrades. For completion brines (where no filter cake on the producing formation is desired), PAC-LV is used specifically because it builds a thin, acid-soluble cake that cleaning fluids can remove. Note that PAC is used in monovalent brines; for divalent brines (CaCl₂, CaBr₂, ZnBr₂) above about 10%, PAC’s performance degrades and specialist polymers are required.

API and ISO Testing Standards for PAC

Drilling-fluid additives are bought to standards, not to datasheets. PAC grades are tested and certified according to:

• API 13A — Specification for Drilling Fluids Materials, which covers PAC under section 8 (carboxymethyl cellulose and polyanionic cellulose), setting requirements for moisture, degree of substitution, viscosity of a 1% solution in 4% salt water, and fluid-loss control measured as filtrate volume.
• ISO 13500 — the international standard (identical technical content to API 13A) used for procurement outside North America.
• API 13B-1 / ISO 10414-1 — the field and laboratory procedures used to measure mud properties: the LPLT filtration test (100 psi, 30 min), plastic viscosity and yield point from the Fann viscometer readings, and the HTHP filtration test (typically 500 psi, up to 150 °C) for deep wells.

Key numbers for TENESSY PAC grades, tested per these standards:

When specifying Polyanionic cellulose (PAC), always require the certificate to show the API 13A / ISO 13500 test results, not just a viscosity number. A product that fails the filtrate test under standard conditions will fail your well, no matter what its 2% viscosity says.

Recommended Dosage and Best Practices

Dosage Programs by Mud System

Best Practices from the Field

• Pre-hydrate before the pit. Add Polyanionic cellulose (PAC) to water with good agitation before adding salt. PAC hydrates poorly if dumped into brine — always build the polymer pill in fresh water first, then mix in the salt.
• Add slowly. Pouring the bag into the suction line causes fish-eyes and viscosity spikes. Meter the powder through a mixing hopper or eductor.
• Titrate by test, not by habit. Run the LPLT filtration test after each addition. The target is a 30-minute filtrate in your working range (5–10 mL for most muds), and the correct dosage is the minimum that achieves it.
• Monitor temperature. Above 120 °C, PAC’s filtration control weakens. For deep hot wells, pair PAC with sulfonated polymers (e.g., SMP or lignite derivatives) or switch the base to synthetic/ester-based mud.
• Check compatibility before blending. PAC is compatible with most water-based additives, but verify with a pilot test when adding surfactants, scale inhibitors, or HTHP defoamers — anionic PAC can precipitate with heavy-metal salts in hard make-up water.
• Keep it dry in storage. PAC is hygroscopic; opened bags must be resealed or the moisture content will drift out of the API spec and dosing will be inaccurate.

PREGUNTAS FRECUENTES

P1: ¿Cuál es la diferencia entre PAC-LV y PAC-HV?

A: PAC-LV (low viscosity) is designed for fluid-loss control with minimal viscosity increase, typically 30–120 mPa·s at 1% in brine. PAC-HV (high viscosity) provides fluid-loss control plus significant viscosity build, 700–1,200 mPa·s at 1% in brine, for cuttings transport and suspension. Most mud systems use both: PAC-LV as the filtration specialist and PAC-HV (or xanthan) as the viscosity source.

Q2: What is the difference between CMC and PAC in drilling fluids?

A: Polyanionic cellulose (PAC) is a higher-quality, higher-DS version of CMC. Both are carboxymethyl cellulose, but PAC has a degree of substitution ≥ 0.9 (versus 0.7–0.8 for drilling-grade CMC) and higher purity. This lets PAC stay extended and effective in seawater and saturated brine where CMC coils up and loses viscosity and filtration control. PAC also tolerates higher temperatures and builds a more acid-soluble filter cake, which is why it is preferred for demanding wells and completion operations.

Q3: What is the typical PAC dosage in drilling mud?

A: For fluid-loss control, PAC-LV at 3–6 kg/m³ covers most fresh-water, seawater, and brine muds; saturated brine needs 5–8 kg/m³. For viscosity plus filtration, PAC-HV at 1–4 kg/m³ is typical. The exact figure is set by the API LPLT filtration test (target 5–10 mL/30 min) and by the rheology needed for hole cleaning — always titrate to the test, and re-check when temperature or mud density changes.

Q4: What standards govern PAC for drilling fluids?

A: PAC is specified under API 13A (section 8) and its international equivalent ISO 13500, which define moisture, degree of substitution, viscosity in 4% salt water, and API filtrate requirements. Mud property tests follow API 13B-1/ISO 10414-1: the LPLT filtration test (100 psi, 30 min), Fann viscometer rheology (plastic viscosity, yield point), and HTHP filtration for deep wells. Insist on certificates showing these test results when buying PAC.

Q5: Does PAC work in saturated brine?

A: Yes — this is its defining advantage. PAC’s high DS keeps the polymer chains ionized and extended in saturated NaCl, where ordinary CMC coils up and fails. At 5–8 kg/m³, PAC-LV in saturated brine gives an API filtrate of 8–12 mL/30 min and a thin, acid-soluble filter cake. For divalent brines (CaCl₂, CaBr₂, ZnBr₂) above about 10%, PAC performance drops and specialist polymers are needed.

Q6: Can PAC be used with xanthan gum in the same mud?

A: Yes, and it is common practice. PAC supplies filtration control and moderate viscosity; xanthan adds the strong low-shear viscosity needed for cuttings suspension in deviated wells. The combination is standard in low-solids polymer muds. Add both through the mixing hopper in fresh water before salting, and verify with a pilot test that the two polymers do not over-thicken the mud at your target density.

Conclusión

Polyanionic cellulose in oil drilling fluids solves the two problems that dominate mud engineering: fluid loss into permeable formations and rheology control across changing water chemistries. PAC-LV delivers filtration control with no viscosity penalty, PAC-HV adds the structure needed for hole cleaning, and the high DS chemistry behind both keeps them effective in fresh water, seawater, and saturated brine at temperatures up to 120 °C. Specified and verified per API 13A / ISO 13500, PAC is the workhorse fluid-loss polymer of water-based drilling — and the right choice whenever the mud program must stay simple, stable, and cost-effective.

TENESSY Chemical manufactures PAC-LV and PAC-HV from premium cotton cellulose on German equipment, certified to API 13A / ISO 13500 test methods and exported to 40+ countries. Request a free 500–3000 g sample, an API test certificate, or a dosage recommendation for your mud system — production lead time is just 7–14 days.

Productos recomendados: PAC (celulosa polianiónica) | CMC (carboximetilcelulosa)
Related Reading: CMC (carboximetilcelulosa): Guía de aplicaciones industriales | HEC in Water-Based Paints: Thickener and Stabilizer Guide | HEMC (MHEC) en la construcción: aplicaciones y ventajas