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In a water-based drilling fluid, filtration needs to be reduced, but viscosity also has to remain within the required operating range.

This creates a common formulation question:

Should you use PAC-LV or PAC-HV?

Both are polyanionic cellulose (PAC) grades used for fluid-loss control and rheology adjustment. However, they do not affect the drilling fluid in exactly the same way.

The correct choice depends on whether the system mainly needs filtration control, additional viscosity, or both.

The easiest way to understand PAC-LV and PAC-HV is to look at the effect they have on the fluid.

PAC-LV is generally selected when strong fluid-loss control is required but a large increase in viscosity is undesirable.

PAC-HV provides fluid-loss control while also contributing more strongly to viscosity.

This means the selection should not start with:

“Which grade is better?”

Instead, ask:

“What does this drilling fluid currently lack?”

If filtration is too high but viscosity is already suitable, PAC-LV is usually the more logical starting point.

If both filtration and carrying capacity need improvement, PAC-HV may be more appropriate.

PAC-LV is useful when the formulation needs better filtration control without making the drilling fluid unnecessarily thick.

Typical situations include:

• viscosity is already within target;
• the system contains enough bentonite or other viscosifiers;
• additional polymer viscosity would increase pumping pressure;
• solids content is already relatively high;
• filtration needs improvement without strongly changing rheology.

This can be particularly important in systems where adding more high-viscosity polymer would make circulation more difficult.

PAC-HV becomes more useful when the drilling fluid needs both fluid-loss control and additional viscosity.

Possible applications include situations where:

• the mud is too thin;
• cuttings suspension needs improvement;
• carrying capacity is insufficient;
• additional viscosity is required in a low-solids system;
• filtration also needs to be reduced.

PAC-HV can therefore perform two functions at the same time: reducing fluid loss and increasing viscosity.

However, this does not mean that more viscosity is always desirable.

If PAC-HV dosage is too high, the drilling fluid may become unnecessarily difficult to pump or show excessive rheological values.

The dosage should therefore be optimized according to the complete drilling-fluid system.

A common purchasing mistake is comparing PAC products only by their viscosity number. For drilling-fluid applications, the more useful question is:How does the product change the complete mud system?

A suitable comparison should include:fluid loss;apparent viscosity、plastic viscosity、yield point、gel strength、filter cake condition、salt tolerance、performance after aging.

PAC is used because it affects both rheological and filtration properties, so evaluating only one viscosity result gives an incomplete picture.

Two PAC products with similar laboratory viscosity can still behave differently in a bentonite drilling fluid.

When selecting a PAC grade, keep the base drilling fluid unchanged and compare both products under the same conditions.

The purpose of this table is to identify the first grade to test—not to replace laboratory evaluation.

Sodium carboxymethyl cellulose (CMC) is also widely used in oil-drilling fluids as a thickener, stabilizer, and fluid-loss-control polymer.

The decision between CMC and PAC should not be based only on price.

Important factors include:

• required fluid-loss level;
• salinity;
• viscosity target;
• degree of substitution;
• polymer purity;
• solids content;
• temperature;
• complete mud chemistry.

For demanding water-based drilling-fluid systems, the polymer should be selected based on actual mud performance rather than the polymer solution alone.

PAC-LV and PAC-HV are designed for different formulation needs.

Choose PAC-LV when the primary objective is fluid-loss control with limited additional viscosity.

Choose PAC-HV when the drilling fluid needs both fluid-loss control and stronger viscosity development.

But the final decision should always consider:

• water chemistry;
• solids content;
• rheology;
• filtration;
• pumping requirements;
• aging conditions.

For drilling-fluid formulators, the best PAC is not simply the grade with the highest viscosity.

It is the grade that achieves the required filtration control while keeping the complete drilling fluid within its target rheological range.