Polyanionic cellulose (PAC) is the purified cellulose polymer that controls filtration in water-based drilling fluids, and API Spec 13A covers it in two grades, PAC-LV and PAC-HV (18th edition (2010) text). In a drilling fluid, it builds a thin, resilient, low-permeability filter cake, so less filtrate and fewer mud solids enter permeable formations.
A premium grade controls fluid loss at 0.25 to 1 lb/bbl (0.71 to 2.85 kg/m3) in freshwater, seawater, KCl and salt muds and holds to about 300 degF (149 degC) circulating, typical values of POLYPAC UL (SLB) data sheet. Technical grades take more polymer, and HDD and water-well slurries take less, dosed per 100 gal of slurry.
Offshore discharge of a polyanionic cellulose mud rests on limits for the whole mud under 40 CFR 435.13, and water-well use rests on NSF/ANSI/CAN 60-2020 clause 8.3.1, which approves carboxymethyl celluloses with a degree of substitution greater than 0.4.
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.
The identity block below lists the names, registry numbers and constants of polyanionic cellulose.
| Attribute | Value |
|---|---|
| Canonical name | Polyanionic cellulose (PAC) |
| Synonyms | PAC polymer, high-substitution purified sodium carboxymethylcellulose, polyanionic cellulose LV, polyanionic cellulose HV |
| CAS number | None of its own; sold under 9004-32-4 (sodium carboxymethylcellulose) |
| EC / list number | ECHA list 922-572-9 (polyanionic cellulose); ECHA list 618-378-6 (sodium carboxymethylcellulose) |
| Formula | None (polymer) |
| Substance type | Polymer |
| Chemical class | Anionic cellulose ether (sodium carboxymethylcellulose, purified, high degree of substitution) |
| Appearance | White free-flowing powder |
| Specific gravity | 1.5 to 1.6 (typical value of POLYPAC UL (SLB) data sheet) |
| API Spec 13A grades | PAC-LV and PAC-HV (18th edition (2010) text) |
| Hazard line | No harmonised classification captured; see supplier SDS |
What Is Polyanionic Cellulose?#
Polyanionic cellulose is a sodium carboxymethylcellulose with a high degree of substitution, purified of most of its salt by-product, and it is sold as a white free-flowing powder for fluid-loss control in water-based muds.
Polyanionic cellulose is a fluid loss control additive first and a viscosifier second. Water-based muds rely on polyanionic cellulose and other fluid loss control additives, such as starch, lignite and AMPS copolymers, to keep filtrate out of the formation. Polyaluminum chloride and powdered activated carbon, both also abbreviated PAC, are water treatment chemicals and not drilling fluid polymers.
Why polyanionic cellulose has no CAS number of its own#
Polyanionic cellulose has no CAS number of its own: ECHA lists it as "Polyanionic Cellulose PAC" under list 922-572-9 without a CAS, and PubChem and supplier SDSs pair the name with 9004-32-4, the CAS of sodium carboxymethylcellulose (list 618-378-6). Both are ECHA list numbers, not EINECS numbers.
Structure and manufacture of polyanionic cellulose#
Polyanionic cellulose is a cellulose ether with no single formula: like carboxymethylcellulose, it is made from cellulose, monochloroacetic acid and sodium hydroxide, then purified to a higher degree of substitution and less residual salt than technical CMC. Some products still carry considerable sodium chloride.
What Does Polyanionic Cellulose Do in a Drilling Fluid?#
Polyanionic cellulose cuts fluid loss in water-based drilling fluids: it adsorbs on clay and plugs filter-cake pores, low-viscosity grades add little viscosity, regular and high-viscosity grades also viscosify, and all grades encapsulate shale and cuttings. The thin cake lowers the risk of differential sticking.
Polyanionic cellulose serves KCl-polymer, seawater and salt muds in oil and gas wells, HDD bores, water wells, mineral exploration and core drilling, and geothermal wells within its temperature range. In each of them it lowers the fluid loss of drilling fluids, the filtrate volume a mud loses through filter paper under pressure.
How does polyanionic cellulose work?#
Polyanionic cellulose works by adsorbing on clay particles and plugging the pores of the filter cake, which gives a thin, resilient, low-permeability cake and less filtrate.
Viscosity depends on grade: ultra-low and low-viscosity grades add little viscosity, while regular and high-viscosity grades raise viscosity as they cut filtrate. The viscosity a grade generates also depends on the solids concentration, salinity and make-up water chemistry. In saturated salt systems, ultra-low-viscosity grades work better than regular-viscosity grades.
Encapsulation of shale and cuttings#
Anionic chains of polyanionic cellulose attach to exposed shale and drill cuttings and coat them, which inhibits the dispersion of cuttings and restricts water contact with the shale. Encapsulation makes the polymer one of the shale inhibitors such as encapsulators, a group that also includes PHPA, and it takes a higher dose than fluid-loss control alone.
What Do the API PAC-LV and PAC-HV Grades Require?#
API Spec 13A sets two polyanionic cellulose grades, PAC-LV and PAC-HV, and judges both on absence of starch, apparent viscosity and filtrate in a sea-salt test fluid (18th edition (2010) text, Tables 16 and 17).
The table below compares the requirements of both grades.
| Requirement | PAC-LV | PAC-HV |
|---|---|---|
| Starch and starch derivatives | Absent | Absent |
| Apparent viscosity in the test fluid | 40 cP max | 50 cP min |
| Filtrate | 16 mL max (corrected filtrate) | 23 mL max (API filtrate) |
| Sample mass for the filtrate test | 2.0 g | 1.0 g |
API Spec 13A, 18th edition (2010) text, Tables 16 and 17; purity and particle size are recommendations, not requirements.
PAC-LV and PAC-HV entered the standard as clauses 17 and 18 of the 18th edition (February 2010, effective 2010-08-01), as new clauses of ISO 13500:2008. That ISO document, with Cor 1:2009 and Amd 1:2010, is the ISO counterpart of the API text.
The governing text is API Spec 13A, 19th edition (2019) with Addendum 1 (2020), Addendum 2 (2022) and Errata 1 (2024), the current edition (API Standards Plan, checked 2026-10-27); a 20th edition is in development. API Spec 13A is a specification for manufacturers covering 13 materials, of which PAC-LV and PAC-HV are two.
The sea-salt and KCl test fluid#
API Spec 13A tests both grades in 358 g of sea-salt solution at 42 g/L with 35.0 g of potassium chloride, 1.0 g of sodium bicarbonate and 28.0 g of API standard evaluation base clay (18th edition (2010) text). Each grade goes in at its sample mass from the table above. Apparent viscosity is half the dial reading at 600 r/min, a shear rate of 1,022 s-1, and the PAC-LV filtrate is corrected by doubling the volume collected between 7.5 and 30 minutes.
Is PAC-R an API grade?#
No: PAC-R is a trade designation, like UL, ULV, ELV, technical grade and PAC-L, and only PAC-LV and PAC-HV are API Spec 13A grades (18th edition (2010) text, clauses 17 and 18). A quote for a trade grade therefore names the product data sheet and its test values instead of an API grade.
Purity and particle size recommendations#
API Spec 13A recommends, but does not require, a purity above 96 % sodium carboxymethylcellulose on a dry-weight basis and a particle size of 0.8 mm (20 mesh) or less for both grades (18th edition (2010) text). A buyer who needs either value writes it into the purchase specification.
How Much Polyanionic Cellulose Does a Mud Need?#
Polyanionic cellulose controls fluid loss at 0.25 to 1 lb/bbl (0.71 to 2.85 kg/m3) in freshwater, seawater, KCl and salt muds, typical value of POLYPAC UL (SLB) data sheet. The same treatment serves KCl-polymer muds, where potassium chloride inhibits the shale and the polymer keeps the filtrate low.
The table below compares polyanionic cellulose doses by system, role and source.
| System | Dose, oilfield (SI) | Role | Source |
|---|---|---|---|
| Saltwater and PAC-polymer polymer drilling fluids | 1 to 3 lb/bbl (2.85 to 8.56 kg/m3) | Encapsulation | typical value of POLYPAC UL (SLB) data sheet |
| Technical grade in freshwater to saltwater water-based mud | 0.5 to 3.0 lb/bbl (1.43 to 8.56 kg/m3) | Fluid loss | typical value of M-I PAC R (SLB) data sheet |
| Technical grade in fresh water and salt muds | 1.0 to 4.0 lb/bbl (2.85 to 11.4 kg/m3) | Viscosity | typical value of M-I PAC R (SLB) data sheet |
| HDD bentonite slurry and water-well bentonite slurry | 0.5 to 2.0 lb/100 gal (0.21 to 0.84 lb/bbl; 0.60 to 2.40 kg/m3) | Fluid loss, borehole stability | typical value of PAC-L (Baroid IDP, Halliburton) data sheet |
| Fresh or salt water in water-sensitive formations | 3 to 7 lb/100 gal (1.26 to 2.94 lb/bbl; 3.6 to 8.4 kg/m3); very salty water may need twice the fresh-water dose | Formation stability | typical value of PAC-L (Baroid IDP, Halliburton) data sheet |
| Gyp-lime fluid for water well drilling mud | 1.5 lb/100 gal (0.63 lb/bbl; 1.80 kg/m3) | Fluid loss | Water Well Journal (2023) |
| Gel foam for foam drilling fluids | 0.5 lb/100 gal (0.21 lb/bbl; 0.60 kg/m3) | Gel-foam slurry | Water Well Journal |
| Drill-in fluid in formate brines, North Sea field example | 4.6 lb/bbl (13 kg/m3) | Fluid loss | AADE paper on formate drill-in fluids |
Product and journal values per system, not class ranges; lb/100 gal is per 100 US gal of slurry.
Premium and technical grades in oil and gas muds#
Technical-grade polyanionic cellulose takes a higher dose than a premium grade for the same filtration control, and it contains salt as a by-product of processing. Water hardness and salt level change the amount either grade needs.
Technical grade also builds viscosity on its own: 2.5 lb/bbl (7.13 kg/m3) in fresh water without clay gives about 15 cP (15 mPa.s) of viscosity and a yield point of 30 lbf/100 ft2 (14.4 Pa), typical value of M-I PAC R (SLB) data sheet. That yield point helps a clay-free fluid carry cuttings for hole cleaning.
Doses in HDD, water-well and foam slurries#
Polyanionic cellulose goes into HDD and water-well bentonite slurries at the lb/100 gal rates of the dose table. The powder can be pre-dispersed in vegetable or mineral oil and poured into the drill string, and in very salty make-up water it works better when mixed in fresh water first.
Water Well Journal publishes two water-well recipes with the polymer: a gyp-lime fluid with gypsum, lime, PHPA and lignite that holds calcium above 600 ppm and fluid loss at 12 mL or less, and a gel foam of 10 to 12 lb bentonite per 100 gal (4.20 to 5.04 lb/bbl; 12.0 to 14.4 kg/m3) with soda ash and foamer. The same polymer serves HDD drilling fluids, where it controls filtration in sandy ground.
What Limits Polyanionic Cellulose in a Drilling Fluid?#
Three limits apply to polyanionic cellulose: circulating temperature, total hardness combined with high pH, and very salty make-up water, and each published limit belongs to one product, not to the class.
Bacteria are not among the limits. The polymer resists bacterial attack and needs no biocide or preservative in the mud.
Temperature limits by grade#
Premium polyanionic cellulose holds to about 300 degF (149 degC) circulating, typical value of POLYPAC UL (SLB) data sheet, and technical grade to 250 degF (121 degC), typical value of M-I PAC R (SLB) data sheet. A thermal stabilizer is needed as the technical grade approaches its limit, and hotter wells move to the synthetic alternatives listed at the end of this page.
Hardness, salt and pH#
Polyanionic cellulose stays effective below a total hardness of 1,000 mg/L as calcium and can precipitate where high hardness meets high pH, typical value of POLYPAC UL (SLB) data sheet. Salt is a smaller limit: grades work from fresh water to saturated salt muds, and very salty make-up water raises the dose.
Hardness above that level is precipitated as calcium and magnesium before the polymer goes in, so the filtrate readings that track the chlorides and hardness of drilling fluids decide whether a pretreatment comes first.
Which Mud Chemistry Is Polyanionic Cellulose Compatible With?#
Polyanionic cellulose is compatible with bentonite, xanthan gum, starch, PHPA, potassium chloride, lime and gypsum in water-based muds, and it precipitates only where high hardness meets high pH.
The six mud components below meet polyanionic cellulose in drilling practice.
- Bentonite: adsorbs the polymer on its clay and forms the base of HDD and water-well slurries
- Xanthan gum and starch: used with it in a North Sea formate drill-in fluid
- PHPA: added after polyanionic cellulose in HDD mud
- Potassium chloride: part of the API test fluid and of KCl muds
- Lime and gypsum: part of the gyp-lime water-well fluid
- Mixed metal hydroxide: bentonite-MMH fluids are incompatible with anionic deflocculants and small anionic polymers, and they need no polyanionic cellulose
In HDD mud the polymer goes in after the bentonite is in and hydrated and before PHPA, and dry polymers go in before liquid ones.
Cement contamination drives the filtrate pH above 11.5, so in a mud that already carries high hardness, mud contamination by cement is the case in which the polymer can precipitate.
Which Tests Verify Polyanionic Cellulose?#
Three API Spec 13A tests verify a polyanionic cellulose lot (absence of starch, apparent viscosity of the test fluid and its filtrate), and API RP 13B-1 (6th edition, 2026) checks the mud's own response.
The table below compares the lot tests and the mud tests.
| Test | What it measures | Method and edition |
|---|---|---|
| API Spec 13A tests for starch | Absence of starch and starch derivatives | API Spec 13A, 18th edition (2010) text |
| Apparent viscosity on the drilling fluid viscometer test | Viscosity of the grade in the test fluid, cP | API Spec 13A, 18th edition (2010) text |
| Test-fluid filtrate in the API filter press test | Corrected (PAC-LV) or API (PAC-HV) filtrate, mL | API Spec 13A, 18th edition (2010) text |
| API filtrate of the mud, per API RP 13B-1 | Filtrate at 100 psi (690 kPa), 30 min, 7.1 in2 (45.8 cm2) | API RP 13B-1, 6th edition, 2026 |
| HTHP filtrate of the mud | Filtrate at 500 psi (3,447 kPa) differential, doubled from the 22.6 cm2 cell | API RP 13B-1, 6th edition, 2026 |
| PV and YP of the mud, in drilling fluid rheology | Viscosity added by regular and high-viscosity grades, cP and lbf/100 ft2 | API RP 13B-1, 6th edition, 2026 |
Annex B of API Spec 13A carries no precision table for polyanionic cellulose (18th edition (2010) text). On the rig, PV and YP rise with regular and high-viscosity grades but move little with ultra-low grades.
Is Polyanionic Cellulose Hazardous? Hazard Line and SDS#
Polyanionic cellulose carries no harmonised EU classification on record, and its hazard line reads: No harmonised classification captured; see supplier SDS. Supplier SDSs list the polymer under CAS 9004-32-4, and sodium carboxymethylcellulose under that CAS is on the US TSCA Inventory and active in the 2024 Chemical Data Reporting (EPA Substance Registry Services, checked 2026-10-26).
A cellulose polymer is neither safe nor unsafe as a class: the hazard statements, dust handling and protective equipment for a polyanionic cellulose product are those on its SDS. Product data sheets direct handling as an industrial chemical with protective equipment.
Which Discharge and Water-Well Rules Apply to Polyanionic Cellulose?#
Three kinds of rules apply to polyanionic cellulose, none written for the additive alone: US offshore limits on the whole water-based mud, product-by-product assessment in the UK and the Netherlands, and NSF/ANSI/CAN 60 for potable water wells.
The table below compares the three by jurisdiction.
| Jurisdiction | Instrument | What applies to polyanionic cellulose |
|---|---|---|
| US Outer Continental Shelf, under US offshore discharge rules (40 CFR 435) | 40 CFR 435.13 (BAT), eCFR checked 2026-10-26 | Limits on the whole water-based mud and cuttings |
| UK and Netherlands, under the Offshore Chemical Notification Scheme (OCNS) | OCNS (Cefas), through the HOCNF | Ranking assigned per product |
| US potable water wells, for water well drilling fluids | NSF/ANSI/CAN 60-2020 clause 8.3.1 | Carboxymethyl celluloses above a set degree of substitution approved |
Discharge of polyanionic cellulose muds in US waters#
40 CFR 435.13 allows no discharge of water-based fluids and cuttings within 3 mi (4.8 km) of shore, Alaskan facilities excepted (eCFR checked 2026-10-26). Beyond 3 mi, the whole mud must pass a 96-hour LC50 of at least 3 % by volume of the suspended particulate phase, show no free oil and contain no diesel oil. The rule sets no additive-specific limit, so compliance belongs to the formulated mud.
Product-by-product assessment in the UK, Netherlands and OSPAR area#
The Offshore Chemical Notification Scheme assesses each polyanionic cellulose product through its Harmonised Offshore Chemical Notification Format (HOCNF), so the ranking belongs to the product, not to the polymer. OCNS covers the UK and the Netherlands, and a buyer for those waters asks for the product's HOCNF.
Polyanionic cellulose in potable water wells#
NSF/ANSI/CAN 60-2020 clause 8.3.1 approves carboxymethyl starches and celluloses with a degree of substitution greater than 0.4 for water well construction, while natural starches, including pregelatinized starch, and guar are not approved. Polyanionic cellulose, a carboxymethyl cellulose of high degree of substitution, belongs to the approved class.
Polyanionic Cellulose Suppliers, Grades and Packaging#
A polyanionic cellulose quote is specified by API Spec 13A grade (PAC-LV or PAC-HV) or by a trade grade with its data-sheet values, plus quantity, packaging, delivery point and documents. Buyers compare additive suppliers on the grade and documents each one confirms in writing.
API Spec 13A judges conformity on a composite sample of no more than one day's production. A certificate of analysis (CoA) reports the lot against the requirement table above, and the API Monogram is voluntary licensing that needs an API Spec Q1 quality system.
Grade choice follows the well: a premium grade for hot or saturated-salt muds, and a technical grade where the mud stays below the technical grade's temperature limit.
The five items below define a polyanionic cellulose specification, the same fields buyers use for how to specify drilling fluid additives in a quote.
- Grade: PAC-LV or PAC-HV, or a trade grade with its data-sheet values
- Quantity and unit: sacks, t or bbl
- Packaging: sack or bulk, as agreed
- Delivery point: country and port or site
- Documents: CoA, SDS and API Monogram where relevant
How polyanionic cellulose is named on SDS and quote documents#
An SDS identifies polyanionic cellulose by CAS 9004-32-4 and, in the EU, by list number 922-572-9, so a quote names the full material name and grade to avoid the abbreviation clash. The CoA then reports the lot against the grade quoted: the API Spec 13A table for PAC-LV or PAC-HV, or the data-sheet values for a trade grade.
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How Does Polyanionic Cellulose Compare with Other Fluid-Loss Additives?#
Polyanionic cellulose shares fluid-loss control with technical CMC, modified starch, AMPS copolymers in hotter wells and resinated lignite in dispersed muds. Fluid-loss additives for cement slurries belong to well cementing, a separate field.
Each alternative comes from a different family of drilling fluid additives, from celluloses and starches to humates and synthetic polymers.
CMC, starch, lignite and AMPS copolymers as alternatives#
API Spec 13A treats the two celluloses differently: the PAC grades carry the purity recommendation above, while CMC-LVT and CMC-HVT are technical grades not normally purified of their by-products (18th edition (2010) text).
The table below compares the four alternatives.
| Material | Chosen where |
|---|---|
| technical CMC (CMC-LVT) | A purified grade is not needed |
| modified starch | Starch-based filtration control in water-based muds |
| AMPS copolymers | Hotter wells beyond the cellulose limits |
| resinated lignite | Dispersed muds |
Purity and salt by-product decide PAC vs CMC in drilling fluids.
Polyanionic cellulose in formate reservoir fluids#
A North Sea field drill-in fluid of 16.2 lb/gal (1.94 sg) carried polyanionic cellulose at 302 degF (150 degC), according to an AADE paper on formate drill-in fluids. The same fluid held 1.05 lb/bbl (3.0 kg/m3) of xanthan gum, 2.45 lb/bbl (7.0 kg/m3) of starch and 21.0 lb/bbl (60 kg/m3) of calcium carbonate.