Deep Eutectic Solvents as a New Frontier in Drilling Fluid Design: Opportunities and Challenges

Abstract

The chapter covers the topic of deep eutectic solvents (DESs) and their potential as drilling fluid additives, focusing on their use as mud rheology modifiers, shale swelling inhibitors, and hydrate inhibitors during drilling. The properties and characteristics of DES, as well as their advantages and disadvantages, are discussed in detail. The environmental benefits of DES-based drilling fluids, compatibility with other drilling fluid additives, and potential future applications are also examined. Additionally, the challenges and limitations of using DESs as a drilling fluid additive and the safety concerns associated with their use are highlighted. Overall, this chapter provides a comprehensive overview of the use of DES in drilling fluids and its potential to improve the efficiency and safety of drilling operations.

1. Introduction

1.1 Overview of drilling fluids and their importance in drilling operations

Drilling fluids, also known as drilling muds, are an essential component in drilling operations. They are used to lubricate and cool the drill bit, suspend cuttings and debris, and control pressure in the wellbore. Without drilling fluids, drilling operations would be impossible to carry out, as they play a critical role in ensuring the safety and efficiency of the drilling process. Drilling fluids are composed of various components, including a base fluid, which can be water, oil, or synthetic oil, and additives such as clays, polymers, and weighting agents. The type of drilling fluid used depends on the type of well being drilled, the formation being penetrated, and the conditions of the drilling operation. The composition of the drilling fluid is carefully selected to meet the specific requirements of each drilling operation [1].

One of the most important functions of drilling fluids is to provide stability to the wellbore. As the drill bit penetrates the formation, the drilling fluids help to keep the wellbore stable, preventing the formation from collapsing in on itself. This is particularly important when drilling through unstable or unconsolidated formations, where the pressure of the drilling fluid helps to support the formation and prevent it from collapsing. Another crucial function of drilling fluids is to cool and lubricate the drill bit. Drilling generates a significant amount of heat, which can cause damage to the bit and decrease drilling efficiency. Drilling fluids help to cool the bit by circulating around it and carrying away the heat generated by the drilling process. Additionally, they also provide lubrication, reducing friction between the bit and the formation, which further enhances drilling efficiency [2, 3].

Drilling fluids also help to suspend cuttings and debris generated by the drilling process, preventing them from settling in the wellbore and interfering with drilling operations. The suspended cuttings are then transported to the surface by the drilling fluid, where they are separated from the fluid and disposed of. Furthermore, drilling fluids are used to control pressure in the wellbore, which is essential in preventing blowouts and other dangerous situations. The weight of the drilling fluid is carefully controlled to maintain a balance of pressure between the wellbore and the formation being drilled, preventing the formation fluids from entering the wellbore [4].

In conclusion, drilling fluids play a critical role in ensuring the safety and efficiency of drilling operations. They provide stability to the wellbore, cool and lubricate the drill bit, suspend cuttings and debris, and control pressure in the wellbore. The selection of the appropriate drilling fluid composition for a specific drilling operation is crucial, as it ensures that the drilling process is carried out safely and efficiently.

1.2 The need for innovation in drilling fluid design

The oil and gas industry is constantly evolving, and with it, the technology used in drilling operations. In recent years, there has been a growing need for innovation in drilling fluid design. As drilling operations become more complex, the demand for drilling fluids that can meet the specific requirements of each operation has increased. The following are some of the main reasons why innovation in drilling fluid design is crucial [5]:

1. Increasing complexity of drilling operations

As drilling operations become more complex, the requirements for drilling fluids have become more stringent. Today’s drilling operations involve drilling in deeper waters, at higher pressures and temperatures, and through more challenging formations. This has led to a need for drilling fluids that can perform under extreme conditions, such as high-temperature and high-pressure environments, and can provide the necessary stability and lubrication required for efficient drilling.

1. Environmental regulations

Environmental regulations have become increasingly strict in recent years, leading to a need for drilling fluids that are more environmentally friendly. Traditional drilling fluids contain toxic chemicals that can harm the environment and wildlife if they are released into the surrounding ecosystem. Therefore, there is a need for drilling fluids that are less toxic and biodegradable, which can reduce the impact of drilling operations on the environment.

1. Cost-effective solutions

Drilling fluids can account for a significant portion of the total cost of drilling operations. As such, there is a need for cost-effective solutions that can meet the specific requirements of each drilling operation. Innovations in drilling fluid design can lead to the development of more efficient and cost-effective drilling fluids, which can help reduce the overall cost of drilling operations.

1. Increasing demand for energy

The world’s demand for energy is increasing, and the oil and gas industry is under pressure to meet this demand. This has led to an increase in drilling operations, and as such, a need for drilling fluids that can help increase drilling efficiency. Innovations in drilling fluid design can help develop fluids that can improve drilling efficiency, reduce the time required for drilling operations, and increase the overall output of oil and gas wells.

1. Safety and risk management

Drilling operations can be dangerous, and safety is always a top priority. Innovations in drilling fluid design can help develop fluids that are safer to handle, reducing the risk of accidents and injuries during drilling operations. Furthermore, drilling fluids that can provide better pressure control can reduce the risk of blowouts and other dangerous situations.

1.3 Insight into some tradition drilling fluid additives

Traditional drilling fluid additives have been used in the oil and gas industry for many years to optimize drilling operations. These additives are designed to improve the physical and chemical properties of the drilling fluid, which is crucial for efficient drilling and wellbore stability. Some of the commonly used traditional drilling fluid additives include [6]:

Bentonite clay: Bentonite is a natural clay mineral that is used as a viscosifier and filtration control agent in drilling fluids. It is effective in increasing the viscosity of the drilling fluid, which helps in maintaining the stability of the borehole. Bentonite clay also has good filtration control properties, which helps in preventing the loss of drilling fluid into the formation.

Barite: Barite is a mineral that is used as a weighting agent in drilling fluids. It is added to the drilling fluid to increase its density, which is necessary for controlling the formation pressure and preventing well blowouts. Barite also helps in maintaining the stability of the borehole by providing adequate hydrostatic pressure.

Caustic soda: Caustic soda (sodium hydroxide) is used as a pH control agent in drilling fluids. It helps in maintaining the alkalinity of the drilling fluid, which is important for preventing the corrosion of the drill string and other equipment. Caustic soda also helps in reducing the viscosity of the drilling fluid, which improves the efficiency of drilling.

Xanthan gum: Xanthan gum is a biopolymer that is used as a viscosifier in drilling fluids. It is effective in increasing the viscosity of the drilling fluid, which helps in maintaining the stability of the borehole. Xanthan gum is also shear-thinning, which means that its viscosity decreases under high shear conditions, such as when the drilling fluid is pumped through the drill bit.

Potassium chloride: Potassium chloride is used as a salt in drilling fluids. It is added to the drilling fluid to increase its density and provide sufficient hydrostatic pressure to control the formation pressure. Potassium chloride is also effective in stabilizing shale formations by inhibiting clay swelling.

These traditional drilling fluid additives have been used for many years and have proven to be effective in improving the properties of the drilling fluid. However, they also have some limitations and drawbacks. For example, some of these additives can be expensive and may have adverse environmental impacts. Additionally, they may not be effective in certain drilling conditions or formations. As a result, there is a growing interest in exploring alternative additives, such as deep eutectic solvents, that can provide comparable or superior performance while addressing some of these limitations.

2. Deep eutectic solvents

2.1 Definition of DES

Deep eutectic solvents (DESs) are a relatively new class of solvents that have been attracting increasing attention due to their unique properties and potential applications in various industries, including the oil and gas industry. DESs are typically formed by mixing two or more components, such as a hydrogen bond acceptor and a hydrogen bond donor, at a specific molar ratio, which results in a eutectic mixture that has a lower melting point than either of the individual components. The concept of eutectic mixtures has been known for over a century, but it was not until the early 21st century that the term ‘deep eutectic solvents’ was coined to describe these types of mixtures. The ‘deep’ part of the name refers to the fact that the melting point depression is significant and occurs at a temperature much lower than that of either of the individual components, which is not the case for traditional eutectic mixtures [7].

2.1.1 Eutectic mixtures

Eutectic mixtures are a type of mixture formed when two or more substances with different melting points are combined in certain proportions to form a new mixture that has a lower melting point than either of the individual components. The specific proportions at which the mixture will have the lowest melting point are called the eutectic point or eutectic composition. At this point, the mixture will solidify or melt at a single temperature instead of over a range of temperatures.

In the case of deep eutectic solvents (DESs), eutectic mixtures are formed between a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD). These mixtures form a new solvent with unique properties such as low vapor pressure, low flammability, high thermal stability, and a wide range of tunable physical and chemical properties. The formation of a eutectic mixture is dependent on the molecular structure and properties of the HBA and HBD. The HBA typically has a higher melting point and stronger electron-withdrawing ability than the HBD. When combined in the correct proportions, the HBD can disrupt the lattice structure of the HBA, leading to a reduction in the melting point of the mixture. The formation of DES is a delicate balance between the strength of hydrogen bonding and the size and shape of the molecules involved. Too weak of a hydrogen bond or a mismatch in size and shape can result in a mixture that does not have the desired properties of a DES.

Overall, the formation of eutectic mixtures and subsequent development of DESs have opened up a new frontier in solvent design, with potential applications in a wide range of industries, including drilling fluid design.

2.2 Properties and characteristics of DES

Deep eutectic solvents (DESs) are a class of solvents that have been gaining increasing attention in various industries, including the oil and gas industry. Here are some of the key properties and characteristics of DES:

Low toxicity and biodegradability: DESs are often composed of natural compounds that are non-toxic and biodegradable, making them more environmentally friendly than traditional solvents.

Low vapor pressure: DES typically have low vapor pressures, which reduce the risk of volatile organic compounds (VOCs) being released into the atmosphere.

High thermal stability: DESs have high thermal stability, which allows them to be used in high-temperature applications without breaking down.

High solubility: DESs have high solubility for a wide range of organic and inorganic compounds, which makes them useful in various applications, including as drilling fluid additives.

Tunable properties: The properties of DES can be easily tuned by changing the composition of the solvent, allowing for customization to suit specific applications.

Non-flammability: DESs are typically non-flammable, which reduces the risk of fires and explosions in applications that involve high temperatures.

High viscosity: DESs often have higher viscosities than traditional solvents, which makes them useful as thickening agents or rheology modifiers in drilling fluids.

Good lubrication properties: DESs have good lubrication properties, which makes them useful in applications that involve friction or wear, such as drilling operations are free to decide how the main body will be structured. However, you are required to have at least one heading. Please ensure that either British or American English is used consistently in your chapter.

2.3 Utilization of DES in various fields

Deep eutectic solvents (DESs) are a type of ionic liquid that exhibit unique properties and have found diverse applications in various fields. Here are some details on the uses of DES and their mechanisms in different areas:

Green chemistry: DES can serve as environmentally friendly solvents for various chemical reactions, such as organic synthesis and catalysis [8, 9, 10].

Mechanism: DES provides a suitable medium for chemical reactions by dissolving reactants, stabilizing intermediates, and enhancing reaction rates through increased molecular mobility.

Extraction and separation: DES can be used as alternative solvent for the extraction and separation of organic compounds, metals, and biomolecules [11, 12, 13].

Mechanism: DESs can form strong interactions with the target compounds, leading to their selective extraction or separation from a mixture.

Electrochemistry: DES can act as electrolytes or electrode modifiers in electrochemical systems, including batteries, supercapacitors, and sensors [14, 15].

Mechanism: DES can facilitate ion transport, enhance electrode stability, and modulate the electrochemical behavior of the system.

Biocatalysis: DES can serve as reaction media for biocatalytic processes, enabling enzymatic reactions in non-aqueous or harsh conditions [16, 17].

Mechanism: DES can maintain the stability and activity of enzymes, improve substrate solubility, and enhance biocatalytic performance.

Pharmaceutical applications: DES can be utilized in drug delivery systems, formulation development, and synthesis of active pharmaceutical ingredients [18, 19].

Mechanism: DES can solubilize poorly soluble drugs, stabilize formulations, and provide a controlled release of therapeutic agents.

Material science: DES can be employed in the synthesis, processing, and modification of materials, including polymers, nanoparticles, and composites [20, 21].

Mechanism: DES can act as reaction media, dispersants, or template agents, influencing the morphology, properties, and self-assembly of materials.

The mechanisms underlying the properties and behavior of DES can vary depending on the specific composition and components involved. Generally, DESs operate through a combination of hydrogen bonding, ion-dipole interactions, and coordination between the components, leading to unique solvent characteristics and enabling their diverse applications.

3. Introduction to deep eutectic solvents and their potential as drilling fluid additives

In their pioneering work, Han Jia et al. (2019) investigated the use of deep eutectic solvents (DESs) as shale inhibitors. They tested three different DES compositions: Propoanoic acid ChCl (1:1), 3-phenyl propanoic acid ChCl (1:2), and a mixture of 3-mercapto propanoic acid, Itaconic acid, and ChCl (1:1:2). Their study revealed that these DES formulations provided 68%, 58%, and 58% inhibition of bentonite swelling, respectively [22]. In a subsequent study by M.H. Rasool et al. (2021), a Glycerine:Potassium Carbonate DES (2:1) was employed in a free-style experiment using swelling shale samples. Remarkably, this DES demonstrated 87% inhibition of swelling [23]. Moreover, M.H. Rasool et al. (2022) investigated a double-action approach using a Potassium carbonate-based DES in combination with a Poly (2-ethyl-2-Oxazoline) hydroxyl-terminated polymer in drilling mud. This innovative combination resulted in a remarkable 76% inhibition of swelling [24].

Deep eutectic solvents (DESs) have shown potential as drilling fluid additives for various applications, including mud rheology modification, shale swelling inhibition, and hydrate inhibition. A detailed discussion on each aspect has been carried out below.

3.1 As a rheology modifier

Drilling mud rheology refers to the study of the flow behavior of drilling fluids. Drilling mud is a critical component of the drilling process, used to cool and lubricate the drill bit, carry drill cuttings to the surface, and maintain pressure in the wellbore. The flow properties of the drilling mud, including viscosity, yield point, gel strength, and fluid loss, are crucial to ensure efficient and safe drilling operations. Viscosity is the most fundamental property of drilling mud rheology and refers to the fluid’s resistance to flow. The viscosity of the drilling mud must be carefully controlled to prevent excessive pressure buildup, which can cause wellbore instability, lost circulation, and other drilling problems. Yield point is another important rheological property, representing the minimum stress required to initiate fluid movement. Gel strength measures the shear strength of the drilling mud, and fluid loss measures the amount of fluid that is lost to the formation during drilling operations.

Deep eutectic solvents (DESs) have shown potential as drilling fluid additives for various applications, including mud rheology modification. One of the most common components of drilling fluids is sodium bentonite, which is a clay mineral that provides viscosity and filtration control properties to the drilling fluid [25]. The interaction between DES and sodium bentonite in drilling mud can improve the rheology of the fluid and provide other benefits. The interaction between DES and sodium bentonite is believed to involve the formation of hydrogen bonds between the HBA and HBD components of DES and the hydroxyl groups of sodium bentonite. The hydrogen bonding can lead to changes in the interlayer spacing of the sodium bentonite and improve the rheological properties of the drilling mud. DES can act as both a thickener and a thinner of drilling fluids depending on the HBA and HBD components used [26].

For example, DES based on choline chloride and urea can increase the viscosity of sodium bentonite-based drilling fluids. The interaction between DES and sodium bentonite can cause an increase in the interlayer spacing of sodium bentonite, leading to an increase in the viscosity of the drilling fluid. In addition, DES can reduce the gel strength of the drilling mud, which can improve the flow properties of the fluid and reduce the risk of differential sticking. On the other hand, DES based on choline chloride and ethylene glycol can decrease the viscosity of sodium bentonite-based drilling fluids. The interaction between DES and sodium bentonite can cause a decrease in the interlayer spacing of sodium bentonite, leading to a decrease in the viscosity of the drilling fluid. This can be useful in situations where a lower-viscosity drilling fluid is required. Furthermore, DES can also improve the thermal stability of sodium bentonite-based drilling fluids. The interaction between DES and sodium bentonite can help to prevent thermal degradation of the fluid and improve the thermal stability of the fluid. However, there are still challenges that need to be addressed when using DES as drilling fluid additives, including issues related to the compatibility of DES with other drilling fluid components and the need for further research to fully understand the behavior of DES under different drilling conditions.

In conclusion, the interaction between DES and sodium bentonite in drilling mud can improve the rheology of the fluid and provide other benefits. The mechanism of DES interaction with sodium bentonite is believed to involve the formation of hydrogen bonds between the HBA and HBD components of DES and the hydroxyl groups of sodium bentonite. DES can act as both a thickener and a thinner of drilling fluids depending on the HBA and HBD components used. Further research is needed to optimize the use of DES in drilling fluids and address compatibility issues. DES can also be used as shale swelling inhibitors in drilling fluids. Shale swelling is a common issue in drilling operations, which can lead to significant challenges, including reduced drilling efficiency and wellbore instability. DESs can inhibit shale swelling by penetrating the shale formation and interacting with the clay minerals, preventing them from absorbing water and swelling. The mechanism of DES as a shale swelling inhibitor is believed to involve the disruption of the hydrogen bonding between the clay minerals and water molecules. This can prevent the swelling of the shale formation and improve the stability of the wellbore.

3.2 As a hydrate inhibitor

During deepwater drilling operations, the high-pressure and low-temperature conditions can cause the formation of gas hydrates, which are crystalline structures composed of water and gas molecules. These hydrates can cause significant problems in drilling operations, such as plugging of equipment and pipes, reducing the efficiency of the drilling process, and even leading to safety hazards. Gas hydrates form when gas molecules (typically methane, ethane, propane, and butane) are trapped within the lattice structure of water molecules, under conditions of high pressure and low temperature. In deepwater drilling, the temperature can drop below the hydrate formation temperature of the gas, and the pressure can be high enough to stabilize the hydrate structure. Once formed, these hydrates can accumulate in pipes, valves, and other equipment, reducing the flow of drilling fluid and increasing the risk of blockages.

Deep eutectic solvents (DESs) have shown potential as drilling fluid additives for various applications, including the inhibition of hydrate formation during deepwater drilling operations. DES can act as hydrate inhibitors by forming a hydrate-inhibiting film on the surface of gas hydrate crystals, which prevents further growth and agglomeration of the crystals as shown in Figure 1. The mechanism of DES inhibition of hydrate formation involves a combination of thermodynamic and kinetic effects.

Figure 1.

DES as a hydrate inhibitor [27].

The thermodynamic effect is due to the ability of DES to reduce the driving force for hydrate formation by lowering the equilibrium temperature and pressure for hydrate formation. This is because DES can dissolve in water and reduce the concentration of free water molecules, which are required for hydrate formation. As a result, the formation of hydrates is suppressed, and the stability of existing hydrate crystals is reduced. The kinetic effect is due to the ability of DES to adsorb onto the surface of hydrate crystals and form a film that prevents further growth and agglomeration of the crystals. The adsorption of DES onto the surface of hydrate crystals is believed to be facilitated by the presence of hydrogen bonding sites on the surface of the crystals. The hydrogen bonding between the HBA and HBD components of DES and the hydrogen bonding sites on the surface of hydrate crystals can lead to the formation of a stable film that inhibits further growth of the crystals.

One of the most promising DES for hydrate inhibition is based on choline chloride and urea. This DES has been shown to be effective in inhibiting hydrate formation in laboratory experiments and in field trials. In addition, this DES has the advantage of being environmentally friendly and biodegradable, which is important for reducing the environmental impact of drilling operations. However, there are still challenges that need to be addressed when using DES as hydrate inhibitors in drilling operations, including issues related to the compatibility of DES with other drilling fluid components and the need for further research to fully understand the behavior of DES under different drilling conditions.

In conclusion, DES has the potential to be effective hydrate inhibitor in deep-water drilling operations. The mechanism of DES inhibition of hydrate formation involves a combination of thermodynamic and kinetic effects, which reduce the driving force for hydrate formation and prevent further growth and agglomeration of hydrate crystals. Further research is needed to optimize the use of DES as hydrate inhibitors in drilling fluids and address compatibility issues.

3.3 As a shale inhibitor

Shale is a fine-grained sedimentary rock composed of clay minerals such as illite, kaolinite, and smectite. Shale is a common rock encountered in oil and gas drilling, and it poses several challenges to drilling operations due to its unique properties. One of the major issues with shale is its tendency to swell when exposed to water-based drilling fluids. This phenomenon is known as shale swelling, and it can result in significant problems such as stuck pipe, lost circulation, and formation damage. Shale swelling is primarily caused by the interaction between the clay minerals in shale and water-based drilling fluids. When shale is exposed to water-based drilling fluids, the clay minerals in the shale absorb water and begin to swell. This swelling can cause the shale to expand and reduce the permeability of the formation, making it difficult for the drilling fluid to circulate through the wellbore [25, 28].

Deep eutectic solvents (DESs) have shown potential as drilling fluid additives for inhibiting shale swelling during drilling operations. The mechanism of DES inhibition of shale swelling involves a combination of physical and chemical effects. The physical effect is due to the ability of DES to reduce the hydration of clay minerals in shale formations. Clay minerals have a layered structure that consists of negatively charged sheets of silica and alumina, with positively charged ions such as sodium and potassium located between the layers. When water is present, the positively charged ions interact with the negatively charged sheets, causing the layers to swell and expand. DES can interact with these positively charged ions, reducing the amount of water that can access the clay mineral layers and reducing the amount of swelling as shown in Figure 2 [7, 29]. The chemical effect is due to the ability of DES to interact with the clay minerals at a molecular level. DES can penetrate into the interlayer space of clay minerals and disrupt the interactions between the layers. This can lead to the delamination of the clay minerals, reducing their ability to swell and expand.

Figure 2.

DES as a shale inhibitor [7].

One of the most promising DES for shale swelling inhibition is based on choline chloride and urea. This DES has been shown to be effective in reducing shale swelling in laboratory experiments and in field trials. In addition, this DES has the advantage of being environmentally friendly and biodegradable, which is important for reducing the environmental impact of drilling operations. However, there are still challenges that need to be addressed when using DES as shale swelling inhibitors in drilling operations, including issues related to the compatibility of DES with other drilling fluid components and the need for further research to fully understand the behavior of DES under different drilling conditions [30, 31].

In conclusion, DESs have the potential to be used as drilling fluid additive for various applications, including mud rheology modification, shale swelling inhibition, and hydrate inhibition. The mechanism of DES in each application is believed to involve specific interactions with the components of the drilling fluid and the geological formations. Further research is needed to fully understand the behavior of DES under different drilling conditions and to optimize their use in drilling fluids.

3.4 Advantages and disadvantages of using DES as drilling fluid additives

Deep eutectic solvents (DESs) have gained attention as potential additives for drilling fluids due to their unique properties. However, as with any new technology, there are advantages and disadvantages to their use in drilling operations [23, 32, 33].

3.4.1 Advantages of using DES as drilling fluid additives

Environmentally friendly: DESs are typically composed of natural compounds that are non-toxic and biodegradable, making them a more environmentally friendly alternative to traditional solvents.

Low toxicity: DESs have been shown to have low toxicity, which can reduce the risk of negative health effects for workers who come into contact with the drilling fluid.

High solubility: DESs have high solubility for a wide range of organic and inorganic compounds, making them useful as a means of controlling drilling fluid properties.

Customizable: The properties of DES can be easily tuned by changing the composition of the solvent, allowing for customization to suit specific drilling applications.

High thermal stability: DESs have high thermal stability, which allows them to be used in high-temperature applications without breaking down.

3.4.2 Disadvantages of using DES as drilling fluid additives

Limited research: DESs are a relatively new technology and there is limited research on their long-term effectiveness and environmental impact.

Limited compatibility: DESs may not be compatible with all types of drilling fluids, which could limit their use in certain applications.

Limited availability: DESs may not be readily available in all regions, which could limit their use in certain applications.

Potential for increased viscosity: DES can have high viscosity, which could lead to increased pressure drop and potential problems with equipment [24].

In conclusion, DESs have unique advantages as drilling fluid additives due to their environmental friendliness, high solubility, and tunable properties. However, their limited research and higher cost, as well as potential compatibility issues and increased viscosity, may limit their use in certain drilling applications. Further research is needed to fully understand the benefits and limitations of DES as drilling fluid additives.

3.5 Potential combination of DES as drilling fluid additives

There are numerous combinations of deep eutectic solvents (DESs) that have been successfully used as drilling fluid additives. Some of these include [22, 25, 34, 35]:

Choline chloride and urea – This combination has been shown to effectively inhibit shale swelling and improve the thermal stability of the drilling fluid.

Choline chloride and glycerol – This combination has been used as a lubricant in drilling fluids and has shown to be effective in reducing the coefficient of friction.

Choline chloride and ethylene glycol – This combination has been shown to have excellent hydrate inhibition properties and can be used in deep water drilling applications.

Choline chloride and lactic acid – This combination has been used as a pH buffer and has been shown to improve the stability of the drilling fluid.

Betaine and ethylene glycol – This combination has been shown to have good hydrate inhibition properties and can be used in deep water drilling applications.

Betaine and glycerol – This combination has been used as a lubricant in drilling fluids and has shown to be effective in reducing the coefficient of friction.

Betaine and urea – This combination has been shown to effectively inhibit shale swelling and improve the thermal stability of the drilling fluid.

Betaine and lactic acid – This combination has been used as a pH buffer and has been shown to improve the stability of the drilling fluid.

Levulinic acid and dimethyl sulfoxide – (DMSO) as a deep eutectic solvent for improving the thermal stability of water-based drilling fluids at high temperatures.

Eucalyptol and 1,3-propanediol as a deep eutectic solvent for improving the lubricity of oil-based drilling fluids.

N-methyl-2-pyrrolidone (NMP) and choline chloride as a deep eutectic solvent for improving the wettability of shale surfaces and preventing shale hydration.

Glycerol and malonic acid as a deep eutectic solvent for inhibiting the formation of gas hydrates in deepwater drilling operations.

Imidazole and choline chloride as a deep eutectic solvent for reducing the fluid loss of oil-based drilling fluids and preventing differential sticking.

Glucose and levulinic acid as a deep eutectic solvent for improving the fluid loss control and filtration properties of water-based drilling fluids.

N-methylacetamide and choline chloride as a deep eutectic solvent for enhancing the thermal stability and rheological properties of water-based drilling fluids at high temperatures and pressures.

Overall, the use of DES as drilling fluid additive has shown great promise in improving the performance and efficiency of drilling operations. As research continues, it is likely that new combinations of DESs will be developed and tested for their potential as drilling fluid additives.

3.6 Ionic liquids vs. DES as drilling fluid additives

Ionic liquids and deep eutectic solvents (DESs) are both types of solvents that have gained attention for their potential applications in various fields, including drilling fluid additives. While both are considered “green solvents” due to their low toxicity and biodegradability, there are key differences between them. Ionic liquids (ILs) are salts that are liquid at room temperature, typically composed of large, asymmetric cations and small, inorganic anions. The ionic nature of these compounds means that they have a high degree of polarity and can dissolve a wide range of materials, including inorganic salts, polymers, and gases. However, their high cost and potential toxicity have limited their widespread use [36].

In contrast, DESs are formed through the interaction of two or more solid components, typically a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), that undergo a eutectic reaction to form a liquid. This results in a solvent that has a lower melting point than either of its individual components and exhibits unique properties such as high polarity, low viscosity, and tunable solubility. DES can be composed of a variety of HBAs and HBDs, allowing for a wide range of possible solvents with different properties. One of the main advantages of DESs over ILs is their lower cost, as the solid components used to form DESs are typically more readily available and less expensive than the chemicals used to produce ILs. Additionally, DESs are generally less toxic and have a lower environmental impact than ILs.

In terms of their applications as drilling fluid additives, both ILs and DESs have shown promise in improving drilling fluid properties such as viscosity, lubricity, and stability. However, DESs are considered to be more suitable for use in this field due to their lower cost, lower toxicity, and compatibility with other drilling fluid additives. In summary, while both Ils and DES are considered “green solvents” with potential applications in a variety of fields, including drilling fluid design, DESs have certain advantages over ILs in terms of cost, toxicity, and compatibility with other additives.

4. Environmental benefits of DES-based drilling fluids

Deep eutectic solvents (DESs) are an emerging class of solvents that have gained attention as potential drilling fluid additives due to their unique properties. In addition to their tunable properties and high solubility, DES-based drilling fluids also offer several environmental benefits that make them an attractive alternative to traditional drilling fluids [37].

Biodegradability: DESs are typically composed of natural compounds that are biodegradable, meaning that they break down into harmless substances in the environment. This is in contrast to traditional drilling fluids, which can contain toxic chemicals that can persist in the environment for long periods of time.

Reduced toxicity: DESs have been shown to have low toxicity, which can reduce the risk of negative health effects for workers who come into contact with the drilling fluid. This can also reduce the risk of harm to wildlife and ecosystems in the event of spills or leaks.

Reduced environmental impact: The use of DES-based drilling fluids can help to reduce the environmental impact of drilling operations by reducing the amount of toxic chemicals that are released into the environment. This can help to protect water sources, wildlife, and ecosystems in the surrounding areas.

Energy efficiency: DESs have a lower vapor pressure than traditional solvents, which means that they require less energy to maintain their liquid state. This can help to reduce the energy consumption and associated greenhouse gas emissions of drilling operations.

Renewable resources: Many DESs are derived from renewable resources such as plants, which means that they can be produced in a sustainable and environmentally friendly manner.

Overall, DES-based drilling fluids offer several environmental benefits over traditional drilling fluids. By reducing the environmental impact of drilling operations, these fluids can help to protect the environment and promote sustainable drilling practices.

5. Compatibility with other drilling fluid additives

The compatibility of deep eutectic solvents (DESs) with other drilling fluid additives is an important factor in determining their overall effectiveness in drilling operations. Here we will discuss the compatibility of DES with various additives commonly used in drilling fluids, including soda ash, caustic soda, xanthan gum, barite, KCl, and cellulose starch.

Soda ash: Soda ash can be used to remove hardness in water-based drilling fluids. Hardness in water is caused by the presence of dissolved calcium and magnesium ions, which can negatively affect the performance of drilling fluids by causing the precipitation of certain additives and reducing their effectiveness. Soda ash reacts with the calcium and magnesium ions in the water to form insoluble carbonates, which can be easily removed through filtration or settling. This process is known as water softening and can improve the performance of drilling fluids by reducing the amount of scale buildup and improving the efficiency of certain additives.

Caustic soda: Caustic soda, or sodium hydroxide, is used in drilling fluids to adjust pH levels, as well as to inhibit the swelling of clays. DES can be compatible with caustic soda, but again, it is important to ensure that the pH is within the desired range for both additives. In addition, caustic soda can increase the salt content of the drilling fluid, which can impact the solubility of the DES.

Xanthan gum: Xanthan gum is a commonly used viscosity modifier in drilling fluids. DES can be compatible with xanthan gum, but it is important to ensure that the concentrations of both additives are within acceptable ranges. High concentrations of xanthan gum can lead to decreased effectiveness of the DES, as well as potential precipitation or separation of the DES from the fluid.

Barite: Barite is a weighting agent used in drilling fluids to increase density. DES can be compatible with barite, but it is important to ensure that the concentrations of both additives are within acceptable ranges. High concentrations of barite can lead to decreased effectiveness of the DES, as well as potential precipitation or separation of the DES from the fluid.

KCl: KCl, or potassium chloride, is used in drilling fluids as a shale inhibitor and to increase fluid density. DES can be compatible with KCl, but again, it is important to ensure that the concentrations of both additives are within acceptable ranges. High concentrations of KCl can lead to decreased effectiveness of the DES, as well as potential precipitation or separation of the DES from the fluid.

Cellulose starch: Cellulose starch is a commonly used viscosifier in drilling fluids. DES can be compatible with cellulose starch, but it is important to ensure that the concentrations of both additives are within acceptable ranges. High concentrations of cellulose starch can lead to decreased effectiveness of the DES, as well as potential precipitation or separation of the DES from the fluid.

In general, the compatibility of DES with other drilling fluid additives is dependent on the specific composition of the DES and the other additives used. It is important to carefully consider the concentrations and pH levels of all additives in the drilling fluid, and to test the compatibility of the DES with other additives before using it in drilling operations.

6. Challenges and future outlook

6.1 Use of DES in high-temperature and high-pressure drilling environments

Deep eutectic solvents (DESs) have shown great potential as drilling fluid additives in high-temperature and high-pressure drilling environments. These conditions are often encountered in deepwater and ultra-deepwater drilling, where temperatures and pressures can exceed 150°C and 20,000 psi, respectively. Traditional drilling fluids may not be able to withstand these extreme conditions and can break down, resulting in decreased drilling efficiency and safety hazards. One of the key advantages of using DESs in high-temperature and high-pressure drilling environments is their excellent thermal stability. DESs have been shown to remain stable at temperatures up to 200°C and pressures up to 1000 bar. This makes them ideal for use in deepwater and ultra-deepwater drilling applications, where temperatures and pressures are at their highest. DESs also have good lubricity and can reduce the coefficient of friction between the drill string and the borehole wall. This can help to reduce wear and tear on the drill string and improve drilling efficiency [38].

6.2 Cost-effectiveness of DES-based drilling fluids

Deep eutectic solvents (DESs) have gained attention as a potential alternative to conventional drilling fluid additives due to their unique properties and potential advantages. One advantage that has been explored is the cost-effectiveness of using DES-based drilling fluids. The cost-effectiveness of drilling fluids is an important factor to consider in the oil and gas industry, as the cost of drilling can be a significant expense. DES-based drilling fluids have the potential to be more cost-effective than traditional drilling fluids for several reasons.

First, DES can be synthesized from relatively inexpensive starting materials, such as choline chloride and urea. This means that the cost of producing DES can be lower than that of traditional drilling fluid additives. Additionally, DES can be used at lower concentrations than traditional additives, further reducing the cost of the fluid. DES can also potentially reduce the overall cost of drilling by improving drilling efficiency. The unique properties of DES can improve drilling performance by reducing friction between the drill bit and the formation being drilled. This can lead to faster drilling times and less wear on the drilling equipment, reducing maintenance costs.

Another potential cost-saving benefit of DES-based drilling fluids is their ability to be reused. Traditional drilling fluids can become contaminated with rock cuttings and other debris, which can render them unusable and require disposal. DES-based fluids, however, have been shown to be more stable and less prone to contamination, making them potentially reusable and reducing the need for frequent fluid changes. Overall, the cost-effectiveness of DES-based drilling fluids is a promising advantage that warrants further exploration and development. As research into the use of DES in drilling fluids continues, it is likely that more cost-saving benefits will be identified [39].

6.3 Potential future applications of DES in drilling fluid design

Deep eutectic solvents (DESs) have shown promising results as drilling fluid additives due to their ability to modify the rheology of the drilling fluid, inhibit shale swelling, and prevent hydrate formation. However, the potential applications of DES in drilling fluid design go beyond these uses.

One potential application is in enhancing the lubricity of the drilling fluid. Lubricity is a critical property for reducing friction and wears on the drill bit and the drilling equipment. DESs have been found to exhibit good lubricating properties, and their use in drilling fluid design could lead to improved drilling efficiency and reduced wear on equipment. Another potential application of DES is in enhancing the thermal stability of the drilling fluid. In high-temperature and high-pressure drilling environments, the drilling fluid is subjected to extreme conditions that can cause thermal degradation and loss of viscosity. DESs have been shown to have good thermal stability, and their incorporation into the drilling fluid could improve the fluid’s performance in such environments [39].

DESs also have the potential to act as surfactants in drilling fluids, which could enhance their ability to wet and disperse solids and improve their emulsifying properties. This could lead to improved suspension of drill cuttings and better removal of solids from the wellbore. In addition, DESs have been investigated for their potential as environmentally friendly drilling fluid additives. They are biodegradable and have low toxicity, which makes them an attractive alternative to conventional drilling fluid additives that can have harmful environmental impacts.

Overall, DESs have shown great potential as versatile and innovative drilling fluid additives with a wide range of potential applications beyond their current uses. Further research and development are needed to fully explore their capabilities and optimize their performance in drilling operations [40].

6.4 Challenges and limitations

Despite their potential benefits, there are still several challenges and limitations associated with the use of deep eutectic solvents (DESs) as drilling fluid additives. Some of the major challenges are discussed below:

Limited availability: The production of DESs on a commercial scale is still limited, which makes them relatively unsuitable compared to other drilling fluid additives.

Limited understanding of DES behavior: There is still limited understanding of the behavior of DESs under various drilling conditions, which can make it difficult to optimize their use as drilling fluid additives.

Compatibility issues: Although DESs have shown compatibility with many drilling fluid additives, there are still some additives with which DES may not be compatible, which can limit their use in certain applications.

Stability at very high temperatures: Some DES may not be stable at very high temperatures, which can limit their use in high-temperature drilling environments.

Environmental concerns: While DESs have been touted as more environmentally friendly alternatives to traditional drilling fluid additives, there are still concerns about their long-term impact on the environment.

Health and safety concerns: The use of DES as drilling fluid additives may pose certain health and safety risks to workers in the drilling industry. It is important to properly handle and dispose of DES to minimize these risks though risks associated will be less severe than those posed by ionic liquids and oil-based mud.

Regulatory approval: The use of DES as drilling fluid additives may require regulatory approval in some jurisdictions, which can limit their use in certain applications.

In conclusion, while DESs offer many potential benefits as drilling fluid additives, there are still several challenges and limitations that need to be addressed before they can be widely adopted in the drilling industry. Further research is needed to better understand their behavior and optimize their use under various drilling conditions.

7. Conclusion

In conclusion, deep eutectic solvents (DESs) are a promising alternative to traditional drilling fluid additives due to their unique properties and potential benefits. They can serve as mud rheology modifiers, shale swelling inhibitors, and hydrate inhibitors during drilling operations. DES can improve the rheological properties of drilling mud by interacting with sodium bentonite, which increases the stability of the fluid and prevents solids from settling. DES can also inhibit hydrate formation during deep-water drilling by disrupting the formation of hydrate crystals. Furthermore, they can act as shale swelling inhibitors by coating the shale surface and preventing the absorption of water.

However, there are some limitations and challenges associated with the use of DES as drilling fluid additives, including their high cost and potential environmental impact. Compatibility issues with certain drilling formations and other additives must also be considered. Health and safety concerns should also be taken into account when handling DES. Despite these challenges, DES has shown potential in improving the efficiency and effectiveness of drilling operations. Further research is needed to explore the potential of DES-based drilling fluids in high-temperature and high-pressure drilling environments, as well as their compatibility with a wider range of drilling formations and additives. As the oil and gas industry continues to evolve and adapt to new environmental and regulatory requirements, DES may prove to be a valuable tool for sustainable and effective drilling fluid design.

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Acknowledgments

The authors would like to thank YUTP grant 015LC0-326 for providing the support.

Conflict of interest

The authors do not have any conflict of interest to disclose.