In oil drilling projects, carboxymethyl cellulose (CMC) and polyanionic cellulose (PAC) are two commonly used drilling fluid additives. They primarily optimize drilling fluid properties (such as viscosity and fluid loss) to ensure safe and efficient drilling. The following is a detailed introduction to their structure, properties, roles in drilling, and differences:

1.1. Carboxymethyl cellulose (CMC)

1.1 Full name : Carboxymethyl Cellulose, abbreviated as CMC.

1.2. Core performance (in drilling fluid)

· Water solubility : easily soluble in water to form a uniform, transparent, viscous solution; insoluble in organic solvents.

· Viscosity increase : The molecular chains stretch in the solution and form a network structure through hydrogen bonds and intermolecular forces, which significantly increases the apparent viscosity and plastic viscosity of the drilling fluid and enhances the ability to carry cuttings (insufficient viscosity will cause cuttings to settle and block the wellbore).

· Fluid loss reduction : The carboxylmethyl groups in the molecules are adsorbed on the surface of solid particles (such as clay) in the drilling fluid to form a stable colloidal system; at the same time, a thin and tough "filter cake" is formed on the well wall, reducing the amount of drilling fluid lost to the formation (excessive fluid loss will cause the formation to absorb water and expand, especially shale formations are prone to collapse).

· Stability : Stable under neutral to alkaline conditions (optimal pH 7-10), easily hydrolyzed and ineffective under acidic conditions; moderate temperature resistance, usually suitable for shallow wells or medium-deep wells with temperatures ≤120°C.

1.3. Role in oil drilling

· Carrying cuttings : By increasing the viscosity of the drilling fluid, the suspension capacity is improved to ensure that the cuttings are carried out of the ground in time to avoid drill sticking.

· Stabilize the wellbore : reduce filtration loss, reduce formation water absorption, and prevent shale expansion and wellbore collapse.

· Protect reservoirs : Reduce the intrusion of drilling fluid filtrate into reservoirs (especially low permeability reservoirs) and reduce damage to oil and gas formations.

· Adjust flow pattern : Optimize the rheological properties of drilling fluid (such as dynamic shear force and static shear force) to meet the needs of different drilling stages (such as preventing rock cuttings from settling during drilling).

1.4. Common types:

CMC-LV (low viscosity)

CMC-HV (high viscosity)

1.5. Dosage recommendations:

It is generally between 0.2 and 1.5%, and is adjusted according to the well conditions.

2. Polyanionic cellulose (PAC)

2.1.Full name : Polyanionic Cellulose, abbreviated as PAC.

2.2. Core performance (in drilling fluid)

· Water solubility : It dissolves faster than CMC, and the resulting solution is more stable. It can maintain good solubility even in high salt (such as NaCl, CaCl₂) solutions (salt tolerance is much better than CMC).

· Viscosity increase and fluid loss reduction : The viscosity increase efficiency is comparable to that of CMC, but the fluid loss reduction performance is better (the filter cake is denser); and the performance degradation is smaller in high temperature and high salt environments.

· Stability : It has a wider applicable pH range (pH 3-11), better temperature resistance (can be used in deep wells above 150°C), and strong resistance to salt pollution (can still function effectively in saturated brine drilling fluid).

2.3. Role in oil drilling

· Adaptable to complex formations : Especially suitable for high-salt formations (such as salt domes and gypsum salt layers), deep wells (high temperature) and complex lithologic formations (such as the interaction between shale and salt layers), solving the problem of CMC performance failure under extreme conditions.

· Enhanced wellbore stability : In formations with high fluid loss risks (such as those with developed fractures), the filter cake formed by PAC is tougher, with more significant fluid loss reduction effects, reducing the risk of lost circulation and well collapse.

· Anti-pollution ability : It is not sensitive to calcium and magnesium ions (common ions in formation water) in drilling fluid and is not prone to flocculation, ensuring stable drilling fluid performance.

2.4. Common types:

PAC-LV (low viscosity) : Mainly used to control filtration loss without significantly increasing viscosity.

PAC-HV (high viscosity type) : It can control filtration loss and significantly increase mud viscosity.

PAC-R (food grade/environmentally friendly) : used in areas with high environmental protection requirements.

2.5. Dosage recommendations:

The general usage is 0.3~1.0% (adjusted according to the drilling fluid system and purpose).

3. Comparison and Application Scenarios between CMC and PAC

4. Practical application in drilling fluid

1. In saline or high-temperature formations, PAC-LV is preferred for fluid loss control because of its strong salt tolerance and more stable fluid loss control effect.

2. If the drilling fluid system requires higher viscosity and good carrying capacity, PAC-HV can be used .

3. For cost- sensitive formations with low performance requirements, CMC-LV or HV can be selected , which have a higher cost-effectiveness.

4. PAC and CMC can be used together to balance cost and performance.

5. Summary

CMC and PAC are cellulose derivative additives for drilling fluids. Their core function is to optimize drilling fluid performance by increasing viscosity and reducing fluid loss, thereby ensuring drilling safety. PAC , with its advantages of temperature resistance, salt tolerance, and efficient fluid loss reduction, has become the first choice for drilling in complex formations (deep wells, high-salt formations) . In addition, PAC's advantages such as dissolution efficiency, resistance to extreme environments, and low usage are gradually replacing CMC as the first choice for high-performance drilling fluids . CMC, on the other hand , is suitable for shallow wells or non-salt formations with simple conditions due to its low cost and moderate performance . CMC still has application value in cost-sensitive scenarios . Both are highly biodegradable and have little impact on the environment, making them indispensable key chemicals in oil drilling. Users need to optimize the amount added based on parameters such as well depth, salinity, and temperature to achieve a balance between drilling safety and efficiency .