Polyanionic Cellulose (PAC) is a water-soluble cellulose derivative that is widely used in oil drilling, mainly for the preparation of drilling fluid. It has become an important additive in the drilling fluid system due to its superior properties, such as viscosity enhancement, fluid loss reduction, stability and environmental protection.
1. Reduce fluid loss
Fluid loss control is a key function in oil drilling. When the drilling fluid contacts the formation during the drilling process, it may cause mud cake formation and filtrate invasion into the formation, resulting in formation damage and affecting drilling efficiency. PAC effectively reduces fluid loss and filtrate invasion into the formation by forming a protective film in the drilling fluid, thereby reducing formation pollution. This property helps to improve wellbore stability and protect oil and gas formations.
Principle
PAC dissolves in water to form a colloidal solution with high viscosity. When the drilling fluid contacts the formation, the PAC molecules can form a dense mud cake on the surface of the formation to prevent further penetration of the liquid phase. This mud cake has good flexibility and toughness, and can withstand large pressure differences, thereby effectively reducing the filtration loss.
2. Increase the viscosity of drilling fluid
Viscosity enhancement is another important function of PAC in drilling fluid. Drilling fluid needs to have a certain viscosity to carry cuttings back, so as to ensure the cleanliness of the wellbore and maintain drilling stability. As a viscosity enhancer, PAC can increase the viscosity of drilling fluid, enhance the ability of drilling fluid to carry cuttings, and promote the return and discharge of cuttings.
Principle
PAC molecules dissolve in drilling fluid to form a polymer chain structure, which increases the internal resistance of the fluid. This structure can significantly increase the apparent viscosity and yield value of the drilling fluid, and enhance its ability to carry and suspend cuttings. At the same time, the viscosity enhancement effect of PAC is still effective under high temperature and high pressure conditions, and is suitable for deep well drilling and complex geological conditions.
3. Improve wellbore stability
Wellbore stability is an issue that requires special attention during drilling. The drilling fluid must be able to stabilize the wellbore wall to prevent the wellbore wall from collapsing. PAC’s combined effects of reducing filtration and increasing viscosity in drilling fluid can effectively enhance wellbore stability.
Principle
PAC prevents drilling fluid from entering the formation by forming a solid mud cake layer on the surface of the well wall. At the same time, its viscosity can enhance the adhesion of the well wall surface and reduce the generation of microcracks in the formation, thereby improving the mechanical stability of the wellbore. In addition, PAC can also improve the thixotropy of the drilling fluid, so that it forms a strong support force when it is stationary, and maintains appropriate fluidity when it flows, further stabilizing the well wall.
4. Environmental protection characteristics
With the improvement of environmental protection requirements, chemicals used in drilling fluids need to have good environmental protection performance. PAC is a modified product of natural cellulose, with good biodegradability and low toxicity, which meets environmental protection requirements.
Principle
PAC is a chemically modified product based on natural cellulose, does not contain toxic substances, and can be degraded by microorganisms in the natural environment. Compared with synthetic polymers, PAC has less impact on the environment and is more in line with the requirements of green drilling. This characteristic gives it a clear advantage in environmentally sensitive areas and offshore drilling.
5. Temperature and salt resistance
In high-temperature and high-salt environments, traditional clays and polymers often have difficulty maintaining the stability of drilling fluids, while PAC exhibits good temperature and salt resistance and can maintain the effectiveness of drilling fluids in complex environments.
Principle
Anionic groups (such as carboxyl groups) are introduced into the molecular structure of PAC. These groups can exchange ions with salt ions in a high-salt environment to maintain the stability of the molecular structure. At the same time, PAC has high thermal stability and will not undergo significant degradation under high temperature conditions, ensuring the viscosity and filtration control ability of the drilling fluid. Therefore, PAC has excellent application effects in salt water slurries and high-temperature wells.
6. Optimize drilling fluid rheology
Rheology refers to the flow and deformation characteristics of drilling fluids under shear force. PAC can adjust the rheology of drilling fluids to ensure that they have good rock carrying capacity and can flow freely in the wellbore during drilling.
Principle
PAC interacts with other components in the drilling fluid to form a complex network structure and adjust the yield value and shear thinning characteristics of the drilling fluid. This regulating effect enables the drilling fluid to show good rock carrying capacity and fluidity during the drilling process, especially in complex formations and high-pressure wells.
7. Case analysis
In practical applications, PAC is widely used in various drilling fluid systems. For example, in a deep well drilling project, a water-based drilling fluid containing PAC was used. The results showed that PAC significantly reduced the filtration loss of the drilling fluid, enhanced the stability of the wellbore, improved the drilling efficiency, and reduced the downhole accident rate caused by formation pollution. At the same time, PAC also performed well in marine drilling, and can still effectively control the performance of the drilling fluid under high salinity and high temperature conditions to ensure the smooth progress of drilling operations.
The application of polyanionic cellulose in oil drilling is mainly reflected in its excellent characteristics of reducing filtration loss, increasing viscosity, improving wellbore stability and environmental protection. Its application in water-based and oil-based drilling fluids not only improves drilling efficiency and reduces downhole accident rates, but is also environmentally friendly and helps achieve the goal of green drilling. Under complex geological conditions and high temperature and high pressure environments, PAC’s temperature and salt resistance further highlights its importance in oil drilling. Therefore, polyanionic cellulose occupies an indispensable position in modern oil drilling technology.