PAC-LV vs PAC-HV vs Technical CMC in Water-Based Drilling Fluids
How polyanionic cellulose and technical CMC control fluid loss and rheology, how salt, calcium and temperature change the choice, and which grade fits fresh-water, polymer, weighted and saturated-salt muds.
Sentai Applications Lab · · 7 min read
Use PAC-LV when you need to cut filtrate loss without adding viscosity, typically in weighted or high-solids muds where rheology is already high. Use PAC-HV when you want fluid-loss control plus viscosity and cuttings-carrying capacity, typically in low-solids polymer muds. Technical CMC does the same two jobs at lower cost in fresh-water systems, but it loses efficiency faster as salinity and hardness rise, and that is where PAC's higher, more uniform substitution earns its price.
Same backbone, different substitution
Polyanionic cellulose is a carboxymethyl cellulose, made by the same alkalisation and etherification route. What sets PAC apart is a higher and more evenly distributed degree of substitution, achieved through tighter process control. More uniform substitution means fewer unsubstituted stretches of chain that can collapse and associate when salts screen the polymer's charge, so PAC keeps its viscosity and fluid-loss performance in brines and hard water where technical CMC weakens. Sentai PAC-LV and PAC-HV are specified at DS ≥ 0.9.
How cellulose polymers control fluid loss
When drilling fluid is pushed against a permeable formation, solids build a filter cake and water filtrate passes into the rock. Excessive filtrate damages productive zones and hydrates shales, and a thick cake raises torque, drag and the risk of differential sticking. PAC and CMC reduce filtrate in three ways: they adsorb on bentonite and drilled-solid particles and improve their dispersion, so the cake packs more tightly; the hydrated polymer itself plugs pores in the cake; and the polymer raises the viscosity of the filtrate. The result is a thin, slick, low-permeability cake.
How fluid loss and rheology are measured
The test concepts in API RP 13B-1 for field testing water-based fluids are widely used. The low-temperature API filtrate test applies 100 psi (690 kPa) across a 7.1 in² (45.8 cm²) filter paper for 30 minutes at ambient temperature and reports the filtrate in mL. The high-temperature, high-pressure (HTHP) test uses a 500 psi differential at elevated temperature on a half-area cell, so the collected volume is doubled.
Rheology is read on a direct-indicating viscometer at 600 and 300 rpm. Apparent viscosity is the 600 rpm reading divided by 2, in mPa·s; plastic viscosity is the 600 rpm reading minus the 300 rpm reading; and yield point is the 300 rpm reading minus the plastic viscosity. China's GB/T 5005 specification for drilling-fluid materials applies the same ideas, with apparent viscosity, dial readings and filtrate measured in defined test fluids, and it is the standard Sentai uses to specify and release its drilling grades.
Sentai drilling grades at a glance
| Grade | Type | Viscosity specification | Filtrate loss | DS | Main role |
|---|---|---|---|---|---|
| PAC-LV | Polyanionic cellulose | Apparent viscosity ≤ 40 mPa·s | ≤ 16 mL | ≥ 0.9 | Fluid-loss control with minimal viscosity |
| PAC-HV | Polyanionic cellulose | Apparent viscosity ≥ 50 mPa·s | ≤ 16 mL | ≥ 0.9 | Fluid-loss control plus viscosity |
| LV-CMC | Technical CMC | 600 rpm reading ≤ 90 in deionised water | ≤ 10 mL | — | Economical fluid-loss control in fresh water |
| HV-CMC | Technical CMC | 600 rpm reading ≥ 30 in deionised water, 4 % brine and saturated brine | ≤ 10 mL | — | Viscosity and fluid loss from fresh water to saturated brine |
| CMC-5000 / 6000 / 7000 | High-viscosity CMC | 1 % Brookfield: 5,000–6,000 / 6,000–7,000 / 7,000–8,000 mPa·s | — | ≥ 0.8 | Primary viscosifier |
Salt, calcium and temperature
In fresh water, technical CMC and PAC both work well, and CMC is cheaper. As sodium chloride rises towards saturation, about 26 % by weight at room temperature, charge screening coils the polymer chains: technical CMC loses viscosity and fluid-loss efficiency, while PAC retains much more of both. Sentai HV-CMC is nevertheless specified with a 600 rpm reading of at least 30 in saturated brine, which makes it a cost option for some salt sections.
Divalent ions are harder on carboxymethyl polymers than sodium, because calcium and magnesium can bridge carboxylate groups between chains. Technical CMC can flocculate in hard make-up water or when drilling anhydrite and cement, so treat calcium with soda ash and magnesium with caustic soda where the mud system allows, and prefer PAC in seawater, lime and gypsum muds. Cellulose polymers are typically used up to around 120–150 °C (250–300 °F) bottom-hole temperature, depending on exposure time, pH and oxygen; above that, synthetic fluid-loss polymers are usually needed.
Selecting by mud system
| Mud system | Typical first choice | Why |
|---|---|---|
| Fresh-water bentonite or spud mud | LV-CMC or HV-CMC | Lowest-cost fluid-loss control; HV-CMC adds viscosity |
| Low-solids non-dispersed polymer mud | PAC-HV, trimmed with PAC-LV | Viscosity and carrying capacity at low solids |
| Weighted or high-solids dispersed mud | PAC-LV | Reduces filtrate without raising plastic viscosity |
| Seawater, KCl and other inhibitive muds | PAC-LV and PAC-HV | Salt and hardness tolerance |
| Saturated-salt muds | PAC-LV and PAC-HV, or HV-CMC where cost-driven | Performance specified in saturated brine |
| Workover, completion and coring fluids | PAC-LV | Filtrate control at low solids |
| HDD, piling and tunnelling | HV-CMC or CMC-5000 to CMC-7000 | Viscosity and borehole stability at low cost |
Typical field treatment for cellulose polymers is 1–6 kg/m³ (0.1–0.6 %), equivalent to about 0.35–2.1 lb/bbl. Treat these as typical ranges and set the final concentration from pilot tests on your mud.
Field mixing tips
- Pre-hydrate bentonite in fresh water before adding polymers, because the polymer competes for water and slows clay yield.
- Feed PAC and CMC steadily through the mixing hopper; a 25 kg sack should take several minutes and should never be dumped.
- Where possible, hydrate the polymer before the fluid is salted up; PAC still yields in brine but needs more shear time.
- Treat out hardness before adding CMC, and keep the mud alkaline within the range your system specifies.
- Run a pilot test with every new lot or fluid: measure rheology and API filtrate, and hot-roll at bottom-hole temperature where relevant.
- On remote or low-shear sites, consider surface-treated instant CMC, which disperses without lumping.
- Store sacks dry and off the ground, because caked polymer disperses poorly and yields slowly.
How Sentai can help
Sentai's Oil-Drilling PAC & CMC family covers PAC-LV, PAC-HV, technical LV-CMC and HV-CMC, and the ultra-high-viscosity viscosifiers CMC-5000, CMC-6000 and CMC-7000. Tell us your mud system, salinity, hardness and bottom-hole temperature, and the applications team will recommend a starting grade and dosage for your pilot tests. Free 100–300 g samples are available, and industrial grades ship from a 1 t minimum order.
FAQ
What is the difference between PAC-LV and PAC-HV?+
Both are polyanionic cellulose fluid-loss reducers. PAC-LV controls filtrate with minimal viscosity increase, for weighted or high-solids muds; PAC-HV also adds viscosity and cuttings-carrying capacity, for low-solids polymer muds. Sentai specifies PAC-LV at an apparent viscosity of ≤ 40 mPa·s and PAC-HV at ≥ 50 mPa·s, both with filtrate loss ≤ 16 mL.
Is PAC better than CMC for drilling mud?+
PAC performs better in salt water, seawater and hard water because its substitution is higher and more uniform (DS ≥ 0.9). In fresh-water systems, technical CMC often does the job at lower cost. The right choice depends on salinity, hardness, temperature and budget.
How much PAC is added to drilling mud?+
Typical field treatment for PAC and CMC is about 1–6 kg/m³ (0.1–0.6 %, roughly 0.35–2.1 lb/bbl), depending on the fluid system and the filtrate target. Pilot tests on the actual mud should set the final concentration.
Does PAC work in saturated salt water?+
Yes. PAC is the usual cellulose fluid-loss reducer for saturated-salt and seawater muds because it tolerates high electrolyte levels. Sentai tests its drilling grades in fresh water, 4 % brine and saturated brine to GB/T 5005.
What temperature can PAC and CMC withstand in drilling fluids?+
Cellulose polymers are typically used up to around 120–150 °C (250–300 °F) bottom-hole temperature, depending on exposure time, pH and oxygen. Hotter wells normally need synthetic high-temperature fluid-loss polymers.
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