1. Difference in salt resistance
1.1 Sodium carboxymethyl cellulose (CMC-Na) : In offshore drilling environments, the salinity of seawater is high, and the salt resistance of sodium carboxymethyl cellulose is relatively weak. High concentrations of salt ions (such as sodium chloride, etc.) will affect the extension and stability of the CMC-Na molecular chain. Salt ions will produce a charge shielding effect with the carboxylmethyl (-CH₂COONa) on the CMC-Na molecular chain, causing the molecular chain to curl up, resulting in a decrease in its thickening and fluid loss reduction properties. For example, in a high-salinity seawater environment, the viscosity of the CMC-Na solution may be greatly reduced, and it will not be able to effectively carry cuttings and maintain the stability of the drilling fluid.
1.2 Polyanionic cellulose (PAC) : Polyanionic cellulose has better salt resistance. Its molecular structure contains multiple anionic groups, and the interaction between these groups and salt ions enables it to maintain a good molecular chain extension state in a high-salinity environment. PAC molecules can maintain a certain spatial conformation in the salt solution through electrostatic repulsion, thereby maintaining its rheological properties and fluid loss reduction properties. In offshore drilling, facing the high salinity of seawater, PAC can effectively prevent the deterioration of drilling fluid performance and ensure the smooth progress of drilling operations.
2. Difference in temperature stability
2.1 Sodium carboxymethyl cellulose (CMC-Na) : During offshore drilling, the downhole temperature will increase with the increase in depth. CMC-Na may degrade at high temperatures, causing its performance to change. When the temperature rises, the thermal motion of the CMC-Na molecular chain intensifies, and the chemical bonds between the molecular chains may break, resulting in problems such as reduced molecular weight and decreased viscosity. Especially in the high temperature environment of deep formations, CMC-Na may not meet the performance requirements of drilling fluids.
2.2 Polyanionic cellulose (PAC) : Polyanionic cellulose generally has good temperature stability. It can maintain its chemical structure and performance relatively stable over a wide temperature range. This is because the molecular structure design of PAC enables it to maintain a certain molecular chain integrity and functionality at high temperatures. In the high temperature environment of offshore drilling, PAC can effectively maintain the viscosity, shear force and other properties of the drilling fluid, providing good rock carrying and wall protection functions for drilling.
3. Differences in the impact on formation damage
3.1 Sodium carboxymethyl cellulose (CMC-Na) : Under certain formation conditions, CMC-Na may cause certain damage to the formation. If the formation has a high permeability, CMC-Na may form a poor quality filter cake on the well wall after contacting the formation due to its insufficient salt resistance and stability. This filter cake may have poor sealing properties, which may easily lead to the intrusion of drilling fluid filtrate into the formation, causing problems such as formation clay expansion and pore blockage, thereby affecting the permeability and oil and gas production capacity of the formation.
3.2 Polyanionic cellulose (PAC) : Polyanionic cellulose can form high-quality filter cakes on the well wall. It can better adapt to the formation environment of offshore drilling, and through adsorption and aggregation on the surface of the well wall, it forms a dense filter cake, which effectively prevents drilling fluid filtrate from entering the formation. This helps to protect the original permeability of the formation, reduce damage to the formation, and improve oil and gas production efficiency.
