What's new in production: Ionic liquids

Fig. 1. Ions (cations and anions) present in the most common ILs. From E. Kianfar. Ionic Liquids: Properties, Application, and Synthesis (Fine Chemical Engineering, December 2020).

What are ionic liquids? There are various definitions of what constitutes an IL but for our purposes, and simply speaking, ILs are synthesized salts that are liquids at low temperatures, compared to conventional, solid salt compounds. They are not naturally occurring. Melting points of ILs typically range from well below -100^oF to several hundred degrees F, but most typically up to the boiling point of water. In contrast, the melting point of common sodium chloride (NaCl) salt is 1,473.3^oF. Of specific interest and application, though, are ILs that are liquids at ambient conditions, or “room temperature ionic fluids” (RTILs).   

For those who enjoy chemical structures, Fig. 1 shows ions (cations and anions) present in the most common ILs. However, there are many more possibilities, including those that are yet to be created.  

What are special properties of ILs? Besides their low melting points and existence in liquid form at low temperatures, including at ambient conditions, other unique properties include, but are not limited to, the following: 

• High polarity 
• Very low, or negligible, vapor pressure  
• Non-volatile 
• Low-to-high viscosity 
• Surface tension typically lower than water 
• Stable to temperatures up to about 300^oC (572^oF) 
• Electrochemical stability 
• High electrical conductivity 
• Strong solvent powers 
• Green chemistry (can be synthesized as “green”). 

Fig. 2. A partial list of IL applications. From G. Kaur, H. Kumar, M. Singla. Diverse Applications of Ionic Liquids: A Comprehensive Review (Journal of Molecular Liquids, Vol. 351, April 2022).

General applications of ILs. There is a wide variety of industrial and commercial applications of ILs. Many are based on their solvent characteristics, but others are based on their unique properties. Because the cation and anion pairs in ILs can be changed, very specific, enhanced solvent properties, for example, can be established (or “tuned”). In Fig. 2 is a partial list of IL applications, mostly outside of the oil and gas industry.  

IL applications in oil and gas. There are quite a few RTIL applications in the oil and gas industry. The most commonly cited that are related to hydrocarbon production include the following:  

• Solvents 
• Demulsification 
• CO₂ capture and sequestration 
• Wax and asphaltene inhibition 
• Gas hydrate inhibition 
• Shale inhibition (drilling) 
• Extraction of heavy oil or bitumen 
• Wastewater treatment. 

Much can be written about each of these and other oil and gas applications. Two that are not listed above, but which are of special interest with respect to oil and gas production, are the potential use of ILs in well stimulation, specifically in acidizing and hydraulic fracturing.   

Acidizing application of ILs. Much like certain conventional compounds, certain RTIL formulations and combinations of RTILs can react with water to produce acids, including hydrochloric (HCl) and even hydrofluoric (HF) acid in a more controlled, and safer manner within the formation to be treated. Use of specific, environmentally favorable RTILs in acid stimulation can enable deeper penetration of live acids in the formation while avoiding corrosion to pumping equipment and to downhole tubulars.  

Hydraulic fracturing application of ILs. Current water-based fracturing fluids are generally classified as follows:  

• Slickwater: Low concentration of polymer friction reducer (no added viscosity). 
• Polymer gel: Viscosity generated with a high-concentration polymer friction reducer or an organic polymer (e.g., guar / guar derivative). 
• Surfactant gel: Viscosity generated with a viscoelastic surfactant (VES). 

With surfactant-gelled fracturing fluids, the worm-like micelle property of the VES creates the viscosity needed for fracture propagation and proppant transport. VES fracturing fluids can be advantageous in oil well fracturing, because they combine viscosity, water-wetting properties, solvency with oil, and thus the ease in breaking viscosity naturally within the formation. A disadvantage of VES fracturing fluids is that they are not typically sufficiently stable at higher temperatures and pressures. Also, because viscosity does reduce when in contact with oil, such fluids may “break” prematurely.  

Viscoelastic micelles for viscosity can also be prepared with certain ILs. Such ILs, or RTILs, can be potentially used either alone as the viscosity-generating fracturing fluid additive or in combination with a conventional surfactant (VES or non-VES). An advantage of such a RTIL is that it will be stable at high temperature and pressure, thereby enabling deeper fracture propagation and proppant transport. It can also have favorable formation wetting and environmentally friendly properties.  

Obviously, this month’s column barely touches upon the subject of ionic liquids. Readers are encouraged to explore the literature on ionic liquids and their applications, as well as the commercial suppliers of such chemistries. Ionic liquids represent a fascinating area of chemistry, especially with their current and future potential uses in oil and gas operations, including environmentally favorable production applications. Progress continues to be made in the research and development of ionic liquids for drilling, stimulation, EOR, and flow assurance. While current use is still relatively limited, these unique ionic liquid chemistries could or should grow in importance to the future of oil and gas production.  

About the Authors

Leonard Kalfayan

Contributing Editor

Leonard Kalfayan has 42 years of oil, gas and geothermal experience. He has worked for Hess, BJ Services, Unocal, and as a consultant. He is an SPE Distinguished Lecturer and Distinguished Member. He has authored numerous publications, and also holds 13 U.S. patents.