Salt Resistance and High-Temperature Stability of HEC in Oil Extraction

Hydroxyethyl cellulose (HEC) is a nonionic, water-soluble polymer widely used in oil drilling, fracturing fluids, completion fluids, and oil production. Its excellent thickening, water retention, and rheological control capabilities make it an important additive in oilfield chemicals. In complex oilfield environments, especially under high-temperature and high-salt conditions, the performance stability of HEC directly affects the construction effectiveness and economics of drilling or fracturing fluid systems. Therefore, studying the salt resistance and high-temperature stability of HEC in oil extraction is of great significance for improving the reliability of oilfield chemical systems.

Molecular Structure and Performance Basis of HEC

HEC is prepared from natural cellulose through an ethylene oxide etherification reaction. Its main chain retains the β-1,4-glucosidic bond structure of cellulose, while hydroxyethyl substituents are introduced into the side chains. It is these hydrophilic hydroxyethyl groups that enable HEC to form a stable solution in water, exhibiting good thickening and rheological properties. Because HEC is a nonionic polymer, its solution properties are not significantly affected by pH and electrolyte concentration. This characteristic allows it to maintain good flow stability in drilling or fracturing systems with high salt content.

Application Scenarios of HEC in Oil Extraction

In oil extraction, HEC is mainly used in the following types of fluid systems:

Drilling fluids: As a viscosity modifier and filtrate control agent, it improves the rock-carrying capacity of drilling fluids and reduces filtrate intrusion into the formation.

Completion and workover fluids: Maintain wellbore pressure balance, prevent wellbore collapse, and reduce contamination of the oil reservoir.

Fracturing fluids: Enhance fracturing fluid viscosity, improve sand-carrying capacity, and ensure sufficient fracture extension and conductivity.

These systems are often located in complex formation environments with high temperatures (>100℃) and high salinity (NaCl, CaCl₂, etc. concentrations reaching tens of thousands of ppm), therefore, HEC is required to have excellent salt resistance and temperature resistance.

Analysis of HEC Salt Resistance

The salt resistance of HEC mainly stems from its nonionic molecular properties. Unlike anionic polymers (such as CMC), HEC molecules are uncharged and therefore do not undergo charge shielding or bridging reactions with cations in solution. Even at high concentrations of Na⁺, Ca²⁺, and Mg²⁺ ions, the molecular chains of HEC solutions maintain a good swelling state with minimal viscosity change.

However, at extremely high salt concentrations (especially in divalent salt systems), the increased ionic strength of the solution reduces the solubilizing ability of water molecules on the polymer, leading to partial shrinkage of the HEC molecular chains and a slight decrease in viscosity. To further improve salt resistance, the following improvements are commonly used industrially:

Introducing higher degrees of substitution (MS or DS): Increasing the number of hydrophilic groups on the molecular chain enhances solubility.

Optimizing compound systems: Using HEC with xanthan gum or polyacrylamide (PAM) can significantly improve salt tolerance and system stability.

Using modified HEC (MHEC, HEMC): Improving rheological retention under high salt conditions through methyl or hydroxypropyl substitution.

Experiments have shown that in 5% NaCl or 2% CaCl₂ solutions, the viscosity of high-quality HEC solutions decreases by less than 20%, still meeting the requirements for rock carrying and suspension in drilling fluids.

High-Temperature Stability of HEC

In deep wells or high-temperature reservoirs, drilling fluid and fracturing fluid temperatures can reach 120–160℃. At these temperatures, polymeric thickeners are prone to thermal degradation or molecular chain breakage. The stability of HEC under high-temperature conditions mainly depends on its molecular weight, degree of substitution, and solution pH.

4.1. Thermal Degradation Mechanism:

The β-1,4-glycosidic bonds in the HEC molecular chain are easily broken under high-temperature hydrolysis or oxidation conditions, leading to a rapid decrease in viscosity. The presence of oxidizing ions (such as Fe³⁺) also accelerates this process.

4.2. Methods to Improve Temperature Resistance:

Increasing the degree of substitution (DS): A higher degree of substitution reduces intermolecular hydrogen bonds and improves thermal stability.

Adding antioxidants: Such as sodium sulfite and thiosulfate, which can effectively inhibit oxidative degradation.

Compounding with temperature-resistant additives: Blending with polyethers or temperature-resistant polysaccharides (such as guar gum derivatives) can maintain high viscosity above 150℃.

Surface crosslinking modification: Mild crosslinking enhances the rigidity of the molecular chain, thereby improving thermal stability.

The modified HEC system can stably maintain a viscosity decay of less than 30% for more than 24 hours at 150℃, exhibiting excellent thermal stability.

Comprehensive Performance and Application Prospects

Due to its excellent salt resistance and high-temperature stability, HEC is widely used in deep well drilling, offshore oil production, and shale gas fracturing. Compared with other water-soluble polymers (such as PAM and CMC), the HEC system is more environmentally friendly, non-toxic, and has good biodegradability, meeting the sustainable development requirements of green oilfields. In the future, as oilfield development gradually extends to extreme high-temperature and high-salt environments, the molecular structure modification and compounding technology of HEC will become a research hotspot. Through molecular design and nanocomposite modification, its temperature and salt resistance limits are expected to be further improved, expanding its application in high-pressure deep oil and gas fields and unconventional energy extraction.

HEC, with its excellent salt resistance and good high-temperature stability due to its nonionic structure, has become a key polymer material in petroleum extraction systems. Through molecular modification and formulation optimization, HEC will maintain an important position in the future oilfield chemicals field, providing strong technical support for improving oil and gas extraction efficiency and environmental friendliness.