Polyanionic Cellulose (PAC) in Water-Based Drilling Fluid Additives

Polyanionic Cellulose (PAC) is a water-soluble anionic polymer obtained by chemical modification based on natural cellulose. It is widely used in water-based drilling fluids. As an efficient and environmentally friendly drilling fluid additive, PAC has excellent thickening, filtration reduction, and well wall stabilization properties, and can meet the strict requirements for drilling fluid performance under complex formation conditions.

Basic characteristics of PAC

PAC is an anionic cellulose ether with sodium carboxymethyl substituents on its molecular chain. It has excellent solubility and strong hydrophilicity and can exist stably in various water-based systems such as fresh water, seawater and saturated brine. According to the degree of substitution (DS) and degree of polymerization (DP), PAC can be divided into two types: low viscosity and high viscosity:

Low viscosity PAC (PAC-LV): Mainly used to control the water loss of drilling fluid, and almost no significant change in the rheological properties of drilling fluid.

High viscosity PAC (PAC-HV): It can not only effectively reduce filtration, but also increase the apparent viscosity and shear force of the drilling fluid and improve the carrying capacity.

The main role of PAC in water-based drilling fluid

Loss reduction effect: PAC can form a dense, low-permeability filter cake around the well wall and the drill bit, significantly reduce the filtration loss of the drilling fluid, prevent the drilling fluid from entering the formation, and reduce the risk of well wall collapse and stuck drill.

Improve rheological properties: PAC-HV can increase the viscosity and shear force of the drilling fluid, so that it has good rock carrying capacity, effectively remove the bottom of the well cuttings, and improve drilling efficiency.

Stabilize the well wall: PAC is adsorbed on the well wall to form a protective film to prevent the well wall from hydration expansion and collapse, especially suitable for shale, mudstone and other formations that are prone to hydration expansion.

Salt and calcium resistance: PAC has good stability in salt and calcium environments, and can maintain good filtration reduction effects in high-salinity drilling fluids, suitable for marine drilling and deep well operations.

Environmental protection and biodegradability: As a cellulose derivative, PAC is environmentally friendly and has a certain biodegradability, which is in line with the concept of green drilling.

Application advantages

Adaptable to a variety of water qualities: Whether it is fresh water, salt water or saturated salt water system, PAC can play a stable synergistic role.

Low addition and high efficiency: Usually the dosage is 0.2%~1.0%, which can significantly improve the performance of drilling fluid and has good economy.

Strong compatibility: PAC can be well matched with most commonly used drilling fluid treatment agents, such as bentonite, viscosity reducer and lubricant.

Simple construction: PAC is easy to disperse and dissolve quickly, not easy to agglomerate, and is suitable for rapid preparation and adjustment of drilling fluid on site.

Usage and precautions

The amount of PAC in water-based drilling fluid needs to be determined according to the formation conditions and drilling process. In general, PAC is evenly dispersed in clean water and stirred to dissolve, and then added to the drilling fluid system to adjust the performance. The following points should be noted when using:

Select the model reasonably: Low-viscosity PAC is suitable for situations where water loss is controlled and the viscosity of the system is not expected to be significantly increased; high-viscosity PAC is suitable for drilling operations that require enhanced carrying capacity and suspension.

Avoid excessive addition: Too high a concentration of PAC will make the drilling fluid viscosity too high, affecting pumping performance and drilling efficiency.

Compatibility with other treatment agents: During the preparation process, attention should be paid to the interaction with alkaline regulators and viscosity reducers to avoid performance degradation.

Storage and transportation: PAC should be stored in a dry and cool place to avoid moisture and agglomeration.

As an important additive for water-based drilling fluid PAC has become an indispensable material in modern drilling engineering with its excellent filtration reduction, well wall stabilization, rheological improvement and environmental protection performance. Selecting the appropriate model and addition amount according to different well conditions can significantly improve the comprehensive performance of the drilling fluid, improve drilling safety and operation efficiency.