The Key Role of Polyanionic Cellulose in Drilling Fluid Systems

Polyanionic cellulose (PAC) is a highly efficient water-soluble polymer derived from natural cellulose through chemical modification. It possesses multiple functions, including excellent thickening, filtration loss reduction, wellbore stabilization, and improved rheological properties, making it an indispensable basic treatment agent in oil and gas drilling fluid systems. Due to its strong salt resistance, good temperature stability, and significant effects with low dosage, PAC has become one of the most commonly used filtration loss reducers in freshwater, seawater, brine, and high-salinity systems.

1. Significantly Reduces Filtration Loss and Improves Cake Quality

The core function of PAC is as a highly efficient filtration loss reducer. Its molecular chains contain numerous anionic groups, enabling adsorption with clay particles in the drilling mud, forming a dense filter cake structure. This dense, thin, and uniform cake effectively prevents the filtrate from permeating into the formation, reducing formation damage and lowering the risk of wellbore leakage. In freshwater systems, PAC exhibits significantly better filtration loss reduction capabilities than traditional carboxymethyl cellulose (CMC). Even in high-salt or high-valence ion environments, PAC maintains high levels of viscosity retention and filtration loss control, making it the preferred material for salt-water drilling fluids.

2. Improved Rheological Properties, Enhanced Propane Carrying and Circulation Efficiency

PAC's long and flexible molecular structure allows it to form a spatial network structure in water-soluble systems, contributing moderately to the apparent viscosity (AV), plastic viscosity (PV), and dynamic shear force (YP) of the drilling fluid system. Appropriate addition of PAC can improve the proppane carrying capacity of the drilling fluid, making it easier for cuttings to be carried out of the wellbore during circulation, reducing the risk of stuck pipe caused by proppant buildup at the bottom of the well. Simultaneously, PAC's thickening properties are mild, preventing excessively high viscosity that could affect pumping efficiency; therefore, it is widely used in drilling conditions requiring a balance between fluidity and proppane carrying capacity.

3. Improved Wellbore Stability, Preventing Collapse and Sticking

In unstable or fractured formations, PAC can control wellbore pressure by reducing filtration loss, thereby minimizing wellbore collapse caused by hydration expansion. Furthermore, the filter cake formed by PAC can seal formation microfractures, improving wellbore integrity and reducing wellbore enlargement. Especially in shale formations, where numerous microfractures and high permeability are common, PAC effectively reduces the formation of permeable channels, maintaining wellbore stability and reducing the risk of formation blockage and sticking during drilling.

4. Excellent Salt and High-Temperature Stability

Compared to ordinary CMC, PAC has a more regular molecular structure and a higher degree of substitution, thus exhibiting superior electrolyte tolerance. Even in high-salt or high-calcium environments, PAC maintains good solubility and stability without significant performance degradation. Under high-temperature conditions, PAC also exhibits good thermal stability, maintaining its effectiveness in medium-to-high temperature drilling conditions of 120℃–150℃, making it widely applicable in deep wells, ultra-deep wells, and offshore drilling fluid systems.

5. Wide range of applications, suitable for various drilling systems

PAC has strong compatibility and can be used in freshwater mud, seawater mud, saturated salt mud, polymer mud, and oilfield completion fluids and working fluids. Depending on the specific needs, PAC-LV (low viscosity) or PAC-HV (high viscosity) can be selected:

PAC-LV is primarily used to control filtration loss without significantly increasing viscosity, suitable for high-speed drilling and salt mud.

PAC-HV combines viscosity enhancement and filtration loss reduction effects, and can be used to enhance proppant carrying capacity and improve rheological properties.

PAC has become one of the most critical treatment agents in drilling fluid systems due to its significant reduction in filtration loss, improvement of rheological properties, enhancement of wellbore stability, and increased salt and temperature resistance. With its low dosage, strong effect, and wide range of applications, it not only improves drilling efficiency but also effectively ensures wellbore stability and drilling safety, making it an indispensable high-performance material in modern drilling engineering.