In oil drilling environments, CMC HV (high viscosity carboxymethyl cellulose) and PAC HV (high viscosity polyanionic cellulose) are commonly used drilling fluid additives, and each shows significant differences in performance and application. The following compares the advantages and disadvantages of the two from multiple dimensions:
1. Chemical structure and basic properties
CMC HV
It is made by carboxymethylation of cellulose and belongs to anionic cellulose ether. The carboxylmethyl group on its molecular chain gives it a certain salt resistance, but the degree of substitution is relatively low (usually ≤0.8), resulting in limited stability in high temperature and high salt environments.
Advantages: low cost, stable viscosity enhancement effect in fresh water or low salt environment, and can effectively control the filtration loss to form a thin and tough mud cake.
Disadvantages: poor temperature resistance (usually ≤150℃), prone to viscosity drop and increased filtration loss under high salt (such as saturated brine) or high temperature conditions.
PAC HV
is a polyanionic derivative of cellulose with a high degree of substitution (≥0.8) and uniform distribution. It carries a large number of negatively charged functional groups on the molecular chain, which significantly improves salt resistance and high temperature resistance.
Advantages: It can maintain stable viscosity and fluid loss reduction performance in high temperature (up to 180°C or above) and high salt (including saturated brine) environments, and is especially suitable for complex formations (such as shale and salt gypsum layers).
Disadvantages: The production cost is high, and the rheology may be difficult to control due to excessive viscosity increase in freshwater environments.
2. Drilling fluid performance comparison
2.1. Viscosity increase and rheology control
CMC HV
Advantages: It has a significant viscosity increase effect in freshwater or low solid phase mud, can effectively suspend drill cuttings, and has a low initial shear force, which is conducive to the discharge of gas and solid phase particles.
Disadvantages: Under high salt or high temperature conditions, the viscosity is easily destroyed and needs to be frequently replenished to maintain performance.
PAC HV
Advantages: It can maintain high viscosity in high-salt and high-temperature environments, and its rheology is controllable. It can inhibit the dispersion and expansion of clay and shale and stabilize the wellbore.
Disadvantages: In freshwater environments, the pump pressure may increase due to excessive viscosity growth, and the amount of addition needs to be precisely controlled.
2.2. Fluid loss reduction performance
CMC HV
Advantages: It can effectively reduce the fluid loss under normal conditions and form a dense mud cake. It is suitable for medium and shallow wells and non-complex formations.
Disadvantages: In high-salt or high-temperature environments, the quality of the mud cake decreases and the fluid loss increases. It needs to be used in conjunction with other fluid loss reducers.
PAC HV
Advantages: It has strong salt resistance and can still maintain low fluid loss in saturated salt water or seawater slurry. The mud cake is tough and has low permeability.
Disadvantages: It is expensive when used alone, and its performance may decrease at extremely high temperatures (such as >200℃).
2.3. Shear resistance and temperature resistance
CMC HV
Advantages: Stable performance under low-speed shear conditions, suitable for conventional drilling operations.
Disadvantages: Viscosity is easily degraded under high-speed shear (such as deep well turbine drilling) or high temperature (>150℃), requiring frequent maintenance.
PAC HV
Advantages: Strong shear resistance, viscosity can still be maintained under high shear rates, and excellent temperature resistance (up to 180℃), suitable for deep and ultra-deep wells.
Disadvantages: Thermal decomposition may occur under ultra-high temperatures (such as >200℃), and high-temperature resistant polymers are required.
3. Application scenarios and economic efficiency
CMC HV
Applicable scenarios: freshwater or low-salinity mud systems, medium and shallow wells, non-high-temperature formations (such as <120℃), and projects with limited budgets.
Economic efficiency: Low cost, but frequent replenishment is required, and the comprehensive cost may increase with long-term use.
PAC HV
Applicable scenarios: high-temperature and high-salt formations (such as deep wells, salt-gypsum layers), shale gas wells, seawater mud, and complex wellbore stability requirements.
Economical efficiency: The unit price is high, but the dosage is small, the performance is stable, and the long-term comprehensive cost is better.
4. Environmental protection and compatibility
CMC HV
Environmental protection: non-toxic, good biodegradability, but large-scale use may increase the solid content of the mud.
Compatibility: compatible with most water-based mud additives, but easy to flocculate in high calcium and magnesium ion environments.
PAC HV
Environmental protection: meets international environmental standards, has no harmful residues, and is suitable for environmentally sensitive areas.
Compatibility: has good compatibility with salts, polymers and surfactants, especially stable in high-salt systems.
Summary
CMC HV: The advantages are low cost and good freshwater viscosity enhancement effect; the disadvantages are poor stability at high temperature and high salt, and it is suitable for conventional drilling.
PAC HV: Advantages include salt resistance, high temperature resistance, and excellent fluid loss reduction performance; disadvantages include high cost and suitability for complex formations.
In actual applications, comprehensive selection is required based on formation conditions (temperature, salinity, lithology), well depth, and budget: CMC HV is preferred for conventional wells to control costs; PAC HV is a better solution for high-temperature, high-salt wells and shale gas wells, which can significantly improve operating efficiency and wellbore stability.
