Novel application of citric acid based natural deep eutectic solvent in drilling fluids for shale swelling prevention | Scientific Reports

Scientific Reports volume 14, Article number: 25729 (2024) Cite this article

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Swelling of shale in clastic reservoirs poses a significant challenge, causing instability in wellbores. Utilizing water-based drilling mud with shale inhibitors is preferable for environmental reasons over oil-based mud. Ionic liquids (ILs) have garnered interest as shale inhibitors due to their customizable properties and strong electrostatic features. However, widely used imidazolium-based ILs in drilling fluids are found to be toxic, non-biodegradable, and expensive. Deep Eutectic Solvents (DES), considered a more economical and less toxic alternative to ILs, still fall short in terms of environmental sustainability. The latest advancement in this field introduces Natural Deep Eutectic Solvents (NADES), renowned for their genuine eco-friendliness. This study explores NADES formulated with citric acid (as a Hydrogen Bond Acceptor) and glycerine (as a Hydrogen Bond Donor) as additives in drilling fluids. The NADES based drilling mud was prepared according to API 13B-1 standards and their efficacy was compared with KCl, imidazolium based ionic liquid, and Choline Chloride: Urea-DES based mud. A thorough physicochemical characterization of the in-house prepared NADES is detailed. The research evaluates rheological, filtration and shale inhibition properties of the mud, demonstrating that NADES enhanced the yield point to plastic viscosity ratio (YP/PV), reduced mudcake thickness by 26%, and decreased filtrate volume by 30.1% at a 3% concentration. Notably, NADES achieved an impressive 49.14% inhibition of swelling and improved shale recovery by 86.36%. These outcomes are attributed to NADES’ ability to modify surface activity, zeta potential, and clay layer spacing which are discussed to understand the underlying mechanism. This sustainable drilling fluid promises to reshape the drilling industry by offering a non-toxic, cost-effective, and highly efficient alternative to conventional shale inhibitors, paving the way for environmentally conscious drilling practices.

Shale, a versatile rock, serves a dual purpose as both a source and a reservoir for hydrocarbons, offering the potential to both generate and store these valuable resources within its porous structure1. However, shale’s composition, enriched with clay minerals like smectite, montmorillonite, kaolinite, and illite, renders it susceptible to swelling when exposed to water, causing instability issues in wellbores during drilling operations2,3. These challenges result in non-productive time (NPT) and a host of operational problems, including stuck pipes, lost circulation of mud, well bore caving, and bit balling, all of which lead to increased time and costs for remediation. Traditionally, oil-based drilling fluids (OBDF) have been preferred for shale formations due to their ability to counteract shale swelling4. However, their adoption comes with higher expenses and environmental risks. Synthetic-based drilling fluids (SBDF) have been considered as an alternative, but their suitability is compromised at elevated temperatures. Water-based drilling fluids (WBDF) present an attractive solution as they are safer, more environmentally friendly, and cost-effective when compared to OBDF5. In an effort to enhance WBDF’s shale inhibition capabilities, various shale inhibitors have been employed, including conventional options such as KCl, lime, silicates, and polymers. Nonetheless, these inhibitors have limitations in terms of effectiveness and environmental impact, especially the high concentration of K+ in KCl inhibitors and the pH sensitivity of silicates6. Researchers have explored the application of ionic liquids as additives in drilling fluids, yielding improvements in mud rheology and the inhibition of shale swelling and hydrate formation. However, these ionic liquids, particularly those containing imidazolium-based cations, are often toxic and associated with high costs, non-biodegradability, and intricate preparation processes. To address these challenges, the search for a more economical and environmentally7 benign alternative led to the introduction of Deep Eutectic Solvents (DES). DESs are combinations of Hydrogen Bond Donors (HBD) and Hydrogen Bond Acceptors (HBA) at specific molar ratios and temperatures, forming eutectic mixtures8. These eutectic blends exhibit lower melting points than their individual components, primarily due to charge delocalization facilitated by hydrogen bond formation. Several factors, including lattice energies, entropy changes, and interactions between anions and HBD, play pivotal roles in reducing the melting points of DES9,10.

In previous studies, various additives were incorporated into water-based drilling mud to address shale swelling. For instance, the introduction of 1-butyl-3-methylimidazolium chloride (BMIM-Cl) by Ofei et al. led to a remarkable reduction in mudcake thickness by up to 50% and a decrease in YP/PV across different temperatures11. Huang et al. employed ionic liquids, specifically 1-hexyl-3-methylimidazolium bromide and 1,2-bis(3-hexylimidazolium1-yl) ethane bromide, in conjunction with Na-Bt pellets, resulting in significant reductions of 86.43% and 94.17% in shale swelling, respectively12. Additionally, 1-Vinyl-3-dodecylimidazolium bromide and 1-Vinyl-3-tetradecylimidazolium bromide were utilized by Yang et al., achieving reductions of 16.91% and 5.81% in shale swelling, respectively13. Yang et al. also employed 1-Vinyl-3-ethylimidazolium bromide, which yielded a substantial 31.62% reduction in shale swelling while maintaining a 40.60% shale recovery rate14. Furthermore, Luo et al. utilized 1-octyl-3-methylimidazolium tetrafluoroborate, resulting in an impressive 80% reduction in shale swelling15,16. Dai et al. utilized ionic liquid copolymer in shale inhibition and achieved 18% improvement in linear recovery as compared to inhibition by amine inhibitor17.

Ionic liquids have certain disadvantages associated with it. It motivated scientists to explore greener alternative of ionic liquids, that’s when DES came into the picture. Han Jia pioneered the use of Propoanoic acid ChCl (1:1), 3-phenyl propanoic acid ChCl (1:2), and 3-mercapto propanoic acid + Itaconic acid + ChCl (1:1:2) based Deep Eutectic Solvents (DES), achieving substantial bentonite swelling inhibition rates of 68%, 58%, and 58%, respectively18. In a free-style experiment, M.H. Rasool utilized Glycerine: Potassium Carbonate DES at a 2:1 ratio, achieving a remarkable 87% reduction in swelling in shale samples19,20. Ma employed Urea: ChCl, obtaining a notable 67% reduction in shale swelling21. Rasool et al. utilized a combination of DES and polymer as a double action shale inhibitor and achieved excellent shale inhibition22.

While Deep Eutectic Solvents (DESs) are often considered a greener alternative to ionic liquids, they, too, contain potentially toxic components like ammonium-based salts, raising questions about their environmental friendliness. This concern prompted the emergence of Natural Deep Eutectic Solvents (NADES), which are still classified as DESs but are composed of naturally occurring substances and salts, including potassium chloride (KCl), calcium chloride (CaCl2), Epsom salt (MgSO4.7H2O), and more. The vast number of possible DES and NADES combinations make this area ripe for exploration, with potential applications in various fields. Several researchers have successfully developed novel DES combinations that have proven effective in various applications. For instance, Naser et al. created a potassium carbonate-based DES in 2013 and investigated its thermophysical properties, subsequently finding utility in fields such as hydrate inhibition, drilling fluid additives, delignification, and nano-fibrillation23. Jordy Kim and colleagues formulated an Ascorbic acid-based NADES, assessing its antioxidant properties for diverse applications24. Krister et al. devised a citric acid-based NADES, identifying its potential as an excipient in collagen-based products25. Liu Y and collaborators summarized NADES applications as extraction and chromatographic media in a comprehensive review, while Misan et al. explored successful NADES applications in the agri-food sector. It was about time, drilling fluid researchers started noticing the efficacy of NADES in applied fields. Recently. In 2023, Rasool et al. employed various combinations of Natural Deep Eutectic Solvents based on Ascorbic acid26, Calcium Chloride27, Potassium Chloride28 and Epsom salt29 and achieved commendable shale inhibition and shale recovery. This study is one of the pioneer studies to introduce NADES, specifically a citric acid and glycerine formulation, as an eco-friendly and highly effective shale inhibitor in water-based drilling fluids, offering superior environmental sustainability, enhanced shale inhibition, and improved fluid performance compared to traditional inhibitors like KCl, imidazolium-based ionic liquids, and conventional DES.

The study will involve the in-house preparation of Citric Acid (CA) based NADES, followed by its detailed physicochemical characterization and their utilization as drilling fluid additives to assess drilling fluid properties and their ability to inhibit swelling In this research, CA will serve as the hydrogen bond acceptor, while glycerine (Gly) will act as the hydrogen bond donor, selected based on M.H. Screening criteria for NADES formation/selection in shale inhibition studies30. Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction Analysis (XRD), and Zeta Potential (ZP) measurements will shed light on the NADES-clay interaction and the mechanism behind clay swelling inhibition. Moreover the study will compare CA NADES based drilling mud with 1-Ethyl-3-methylimidazolium chloride [EMIM]Cl7,12,14,17,31, KCl and Choline Chloride: Urea (1:2) based DES32 to check its efficacy in shale inhibition and improving drilling fluid properties in general.

Citric acid (monohydrated) and Glycerine (99USP), Urea have been acquired from EvaChem, Kuala Lumpur, Malaysia. Choline Chloride > 98%, [EMIM]Cl 98%, KCl have been purchased from Sigma Aldrich Malaysia. Chemical structures of all chemicals are depicted in Fig. 1. A greenity chart comparing the key chemicals used in the study—imidazolium-based ionic liquids, Choline Chloride DES, citric acid, glycerine, KCl, and NADES (citric acid and glycerine). The greenity chart of chemicals employed in this study has been tabulated as Table 1. The chart assesses each chemical based on toxicity, biodegradability, cost, and environmental sustainability.

Chemical structures of materials employed in this study: (a) Citric acid, (b) [EMIM]Cl (c) Choline chloride, (d) Glycerine.

The hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) candidates for formulating CA-based NADES (Natural Deep Eutectic Solvents) have been meticulously selected using the M.H. screening criteria30, specifically designed for the development of NADES as effective shale inhibitors. According to the criteria, components possessing a high count of hydrogen bond donors and acceptors, along with polar functional groups, are deemed suitable choices for the composition of NADES.

In addition, for the purpose of comparison in this study, [EMIM]Cl ionic liquid and Choline Chloride: Urea based deep eutectic solvents (DES) were chosen due to their widespread use as additives in drilling fluids33,34,35,36. Furthermore, a comparative analysis involving potassium chloride (KCl) was conducted, as it serves as a conventional inhibitor.

Citric acid and glycerine were combined at varying molar ratios to create a eutectic mixture. Through visual inspection, the eutectic mixture was verified, characterized by a uniform and clear liquid without any cloudiness, indicating the successful blending of Hydrogen Bond Donors (HBD) and Hydrogen Bond Acceptors (HBA) at the eutectic composition. Initial experiments were conducted to observe the temperature-dependent behavior of the HBD and HBA mixing process. The eutectic mixture ratios were assessed at three specific temperatures: 50 °C, 70 °C, and above 100 °C, in accordance with existing literature, which suggests that the eutectic temperature typically falls within the range of 50–80 °C. The METTLER Digital Balance was used for accurate weighing of the HBD and HBA components, while a Thermo Fisher Hot Plate was employed to heat and stir the HBD and HBA at a rate of 100 rpm under controlled conditions.

The thermophysical properties, including density, surface tension, refractive index, and viscosity, of internally synthesized Deep Eutectic Solvents (DES) have been precisely measured within the temperature range of 289.15–333.15 K. It is essential to note that this temperature range was primarily chosen due to the limitations imposed by the available equipment. The comprehensive analysis encompasses a thorough investigation into the diverse thermophysical characteristics of this NADES formulation, shedding light on their behavior across a spectrum of temperatures. The focus on this specific temperature range allows us to capture essential insights into the NADES properties that are particularly relevant for a range of applications.

The Interfacial Tension Meter (IFT700) is utilized to determine the surface tension of the freshly prepared NADES between 289.15 and 333.15 K. Using a capillary needle, a NADES droplet is formed in a chamber containing the bulk fluid under desired temperature and pressure conditions. An advanced image capturing and processing system inputs relevant geometric parameters to calculate interfacial tension via the Laplace equation.

To ascertain the refractive index of the recently prepared NADES within the temperature range of 289.15–333.15 K, the ATAGO refractometer is employed. This instrument evaluates the degree of light refraction, utilizing a thermo-module to regulate temperature, eliminating the need for a constant temperature water bath. The refractometer’s prism surface is cleaned, and a sample solution is evenly spread over it. Calibration is performed with a known reference solution, and the refractive index is then read from the screen.

The viscosity of the newly prepared NADES within the temperature range of 289.15–333.15 K is determined using the Brookfield Rotational Viscometer (Low Temperature model) with a shear rate of 30 rpm and spindle size 6. This viscometer measures viscosity by detecting the torque required to rotate a spindle at a constant speed within the sample fluid. After placing the sample on the screen beneath the spindle and tightening, the viscometer displays the viscosity in centipoise (cP), providing valuable insights into the fluid’s rheological properties.

To determine the density of recently formulated Natural Deep Eutectic Solvents (NADES) within the temperature range of 289.15–333.15 K, the DMA 35 Basic handheld density meter is employed. As the device lacks an in-built heating mechanism, the NADES is preheated to the specified temperature (± 2 °C) before utilizing the density meter. A minimal 2 ml sample is drawn through the pipe, and the density is promptly displayed on the screen. It is noteworthy that the absence of an in-built heating mechanism introduces a ± 2 °C margin of error.

For assessing the pH of the newly prepared NADES within the temperature range of 289.15–333.15 K, the Kenis desktop pH meter is employed. Due to the absence of an in-built heating mechanism, the NADES is initially heated using a hot plate to the required temperature (± 2 °C), after which the pH meter is directly utilized. The pH meter’s probe is fully immersed in the NADES, and the final value is recorded once the reading stabilizes.

A Thermogravimetric Analyzer (TGA) was employed to assess the thermal stability of Natural Deep Eutectic Solvents (NADES). The sample provided underwent analysis as it was subjected to heating. Employing a high-precision balance and meticulous control over the heating process allowed the plotting of mass loss against temperature. The NADES underwent heating from 0 to 500 °C at a rate of 1 °C per minute.

To initiate the process, the NADES sample was prepared through thorough mixing, homogenization, and the removal of any surface moisture. Subsequently, the prepared sample was placed in a TGA pan, typically constructed from an inert material like aluminum. Calibration of the TGA instrument was carried out using reference material, often a weighed standard, to ensure result accuracy. Following calibration, the TGA experiment commenced by heating the sample in a controlled manner, usually at a constant rate. The continuous monitoring of the sample’s weight in relation to temperature was a critical aspect of the experiment. The TGA instrument gathered data on temperature, weight, and additional parameters such as gas flow or sample temperature. After the completion of the TGA experiment, the collected data was analyzed to ascertain the sample’s weight loss or gain as a function of temperature. This information proved valuable in identifying the temperature range associated with the sample’s physical and chemical transformations, including processes such as melting, evaporation, oxidation, or decomposition.

The water-based drilling mud was meticulously prepared in accordance with the API 13B-1 standards, employing the specific composition outlined in Table 2 as the reference. To craft the Natural Deep Eutectic Solvents (NADES), Citric acid and Glycerine (99 USP) were procured from Sigma Aldrich, Malaysia. Additionally, KCl, a traditional shale inhibitor, was acquired from Sigma Aldrich, Malaysia. 1-ethyl, 3-methyl imidazolium chloride ([EMIM]Cl) with a purity exceeding 98% was selected due to its demonstrated effectiveness in enhancing mud rheology and inhibiting shale, as noted in previous studies. Both KCl and ([EMIM]Cl) will be employed for comparative analysis to evaluate the shale inhibition properties of the NADES.

Numerous researchers favor the use of bentonite wafers for studying shale swelling since bentonite contains the same ‘smectite’ group responsible for shale swelling. Obtaining authentic shale core samples is challenging because the coring process destabilizes shale, resulting in samples that are not entirely shale, often containing mixtures of sandstone and limestone layers. Furthermore, shale outcrop samples are unsuitable for swelling inhibition experiments as they typically lack the smectite group responsible for shale swelling.

In this particular investigation, refurbished bentonite pellets with an approximate diameter of 2.54 cm were employed. These pellets were manufactured through the compression of 11.5 g of sodium-bentonite powder at a hydraulic press with a pressure of 1600 psi. Prior to their introduction into the Linear Swell Meter (LSM) environment, the thickness of these pellets was precisely gauged. Subsequently, these pellets were immersed into drilling mud samples, encompassing both the foundational sample and samples infused with inhibitors designed to prevent shale swelling. The alteration in pellet thickness was then closely monitored utilizing a linear swell meter, with measurements being recorded at 60-second intervals throughout a 24-hour period.

The XRD shows that the composition of Bentonite, specifically its 47% smectite component, is a crucial factor in understanding its geological characteristics. Within the smectite composition of Bentonite, Montmorillonite is the predominant element, constituting a substantial 88.6% of the overall composition. Additionally, Quartz makes up 29%, Illite accounts for 7%, and Carbonates contributes 9%. A minor portion, approximately 3.2%, consists of Illite and smectite combined. There are also trace elements such as Fe2O3 at 4.7%, Silver alumino-silicates at 1.2%, Hatrurite at 4%, and Phosphates at 2.3%. Furthermore, insignificant amounts of Na2O at 1.83% and Iron silicates at 2.17% are present, offering a comprehensive overview of the Bentonite’s constituent elements and their respective proportions.

In this comprehensive section, the research thoroughly explores the rheological and filtration characteristics of mud samples formulated with NADES (Natural Deep Eutectic Solvents) and utilizes it as drilling fluid additive at different concentrations (1%, 3%, and 5%). These NADES-based mud samples are then subjected to a comparative analysis with mud samples composed of potassium chloride (KCl), CC: Urea DES (Choline Chloride: Urea Deep Eutectic Solvent), and ionic liquid. This investigation encompasses a range of crucial parameters, including viscometry readings taken using a FANN Viscometer before and after exposure to aging conditions at 100 °C and 150 °C. These measurements are obtained at different RPM values (3 rpm, 6 rpm, 300 rpm, and 600 rpm), allowing for a comprehensive analysis of the mud’s behavior. The acquired data is subsequently employed to deduce essential characteristics such as the Yield Point (YP) and Plastic Viscosity (PV), providing valuable insights into the fluid’s performance under diverse conditions. Filtration properties (mudcake thickness and Filtrate volume) have been found using High Pressure High Temperature (HPHT) filtration test at 400 psia at 150 °C which is usually the temperature of High Temperature wells.

In this section, state-of-the-art equipment is employed, namely the Grace HPHT Linear Swell Meter (M4600), to carefully evaluate the characteristics of shale swelling inhibition in our formulated water-based drilling fluid. The LSM is an advanced device that consists of two integral components: the Wafer Compactor and the Linear Swell Meter (Model: M4600). For our analysis, bentonite wafers are created using the Grace core/wafer compactor. Following this, the LSM provides immediate data on the swelling behavior of these wafers, enabling a thorough assessment of shale swelling inhibition properties. Shale swelling tests are conducted in ambient conditions, i.e., 25 °C and 1 psia.

The examination of shale stability involves a critical test, often referred to as the shale recovery test, shale immersion test, or shale dispersion test. To initiate this evaluation, shale cuttings undergo separation via a 6 BSS mesh sieve before being positioned onto a 10 mesh sieve. These cuttings are subsequently introduced into aging cells, where they are mixed with both the base fluid and drilling fluids containing NADES (Natural Deep Eutectic Solvents). The next step entails subjecting this amalgamation to an extensive hot rolling process within an oven, ensuring a thorough interplay between the mud and the cuttings. Following a 16-hour period, the cuttings are extracted from the mud, causing the shale to disintegrate, leading to a decrease in the cuttings’ weight. Shale recovery tests are conducted after aging the shale cuttings in drilling mud at 150 °C, 1000 psia for 24 h in oven.

To gauge the retrieval of shale cuttings, a finer mesh screen (40 mesh) is employed for filtration, followed by meticulous washing with water and subsequent drying in an oven. This meticulous procedure allows for a comparative assessment of the recovered cuttings against their initial weight, ultimately yielding the percentage of successfully reclaimed shale cuttings. The source of the shale sample lies in the Niah region of the Miri District in Sarawak, Malaysia. Before conducting the dispersion and recovery test, the shale specimen underwent rigorous XRD (X-Ray Diffraction) analysis, quantifying its clay composition to confirm its suitability for the examination. The clay mineral composition of the sample is as follows: Illite comprises 18%, Kaolinite constitutes 31%, Chlorite accounts for 22%, Vermiculite makes up 10%, and Mica contributes 19% to the overall composition.

Surface tension, a critical factor governing the invasion of water cations into shale micropores through capillary action, is meticulously investigated within this section. The role of surface tension in the drilling fluid’s cohesive forces is explored, underscoring its profound impact on the drilling process particularly in shale inhibition. We employ an Interfacial Tensiometer (IFT700) to precisely determine the surface tension of our drilling fluid samples, shedding light on an essential aspect of fluid behavior in context of shale inhibition.

In this section, there is a detailed exploration of d-spacing, which denotes the interlayer separation between alumino-silicate layers in clay and a single alumino-silicate layer. The analysis covers wet drilling mud samples containing 1%, 3%, and 5% CA NADES, along with 3% KCl, 3% [EMIM]Cl, and 3% CC: Urea-based DES for comparative purposes. The state-of-the-art benchtop X-ray diffractometer (D2 phaser), operating at 40 mA and 45 kV with Cu-Kα radiation (λ = 1.54059 Å), plays a crucial role in capturing XRD peaks for both wet samples and dry Na-Bt. The application of Bragg’s equation enables the precise determination of d-spacing, providing valuable insights into clay behavior.

In this section, precise measurements of Zeta Potential are conducted using the advanced Malvern Zetasizer Nano ZSP. This assessment provides valuable insights into the electric charge properties of our diluted drilling mud samples, which include 1%, 3%, and 5% CA NADES, along with 3% KCl, 3% [EMIM]Cl, and 3% CC: Urea-based DES for comparative analysis. These findings contribute to our comprehension of colloidal stability and interactions within the fluid.

The examination of clay samples before and after exposure to Natural Deep Eutectic Solvents (NADES) was conducted using a Zeiss Supra 55 VP model Field Emission Scanning Electron Microscope (FESEM) equipped with Energy Dispersive X-Ray (EDX) analysis capabilities. The imaging process occurred at a resolution of 500 nm, employing electron beam energies of 30 kV and 50 kV. The FESEM facilitated high-resolution visualization of the surface morphology and structural characteristics of the clay samples. This study aimed to gain microscopic insights into the impact of NADES on clay samples by comparing images captured before and after exposure.

Through the utilization of the FESEM technique, this study seeks to investigate the NADES influence on clay samples at a microscopic level. This exploration sheds light on the potential applications and effects of NADES on clay morphology and average size, contributing valuable information to the field.

In this study, error bars are employed to visually depict the variability and uncertainty associated with the Average Mean Percentage Error (AMPE) across different experimental conditions. Rather than plotting individual AMPE values, which can obscure trends and exaggerate minor fluctuations, error bars are calculated using the 5% rule. This approach ensures that each error bar represents the range within 95% confidence and where 100% AMPE values are expected to fall, providing a clearer and more concise summary of the data distribution for each experimental condition. The use of error bars based on the 5% rule thus enhances the interpretability and reliability of our graphical representations, facilitating a more nuanced understanding of the results and their implications.

In our pursuit of synthesis of Natural Deep Eutectic Solvent (NADES), an in-house preparation scrutinized several crucial parameters. These paramount factors included temperature, molar ratios, and mixing speed. Our experiments revealed that the eutectic mixture was achieved at precisely 50 °C when HBA (Citric Acid) and HBD (Glycerine) were blended at a molar ratio of 1:4. The hallmark of a eutectic mixture is its transparent, homogenous, and precipitation-free appearance. Consequently, this pivotal milestone highlights the significance of molar ratios, temperature, and mixing speed, with molar ratio emerging as the most influential factor in DES and NADES preparation as shown in Fig. 2.

Mixing of HBD and HBA with varying CA: Gly molar ratios.

The refractive index (n), indicating the ratio of light velocity in a vacuum to that in a second, denser medium, assumes particular significance in the context of Natural Deep Eutectic Solvents (NADES) when considering applications sensitive to optics, such as biosensors. The determined refractive index of the studied NADES at 25 °C is recorded at 1.452, which intriguingly falls below that of glycerine.

Notably, the refractive index of NADES exhibits a decline with temperature, a trend accurately depicted by Eq. (1) and Fig. 3, boasting a remarkable 0% Absolute Mean Percentage Error (AMPE). This temperature-dependent behavior is attributed to the reduction in viscosity and density at elevated temperatures, causing light to traverse the medium at a faster pace and consequently yielding smaller refractive index values (n). These findings contribute valuable insights for the strategic utilization of NADES in optics-sensitive fields, underscoring their potential in biosensor applications.

Refractive index trend of CA: Gly NADES between 25 and 60 °C.

Surface tension, representing the tendency of liquid surfaces to minimize their area, holds profound significance in assessing the suitability of Natural Deep Eutectic Solvents (NADES) for applications reliant on capillary pressure. The investigation of surface tension within the temperature range of 25–60 °C provides valuable insights. At 25 °C, Citric acid-based NADES exhibits a surface tension of 55.42 mN/m, notably lower than that of water and glycerine. Figure 4 illustrates a discernible reduction in surface tension with an increase in temperature. This phenomenon can be attributed to the heightened kinetic energy of molecules and a concomitant decrease in intermolecular attraction.

The observed linear decrease in surface tension across the studied NADES is aptly represented by Eq. (2), elucidating the underlying mathematical relationship for a temperature range of 25–60 °C. The graphical representation in Fig. 4 serves to visually capture the temperature-dependent trend in surface tension, with an accompanying Absolute Mean Percentage Error (AMPE) of 1.4%, providing a quantified measure of accuracy in the reported surface tension values. These findings contribute significantly to the understanding of NADES behavior and their potential applications.

Surface tension trend of CA: Gly NADES between 25 and 60 °C.

Understanding the density dynamics of Natural Deep Eutectic Solvents (NADES) holds paramount importance in facilitating their application in numerous studies. At 25 °C, the density of Citric acid-based NADES is recorded at 1.361 g/cc, surpassing the density of raw glycerin. This disparity can be attributed to the incorporation of a Hydrogen Bond acceptor (Citric acid) in glycerine.

In the case of Citric acid-based NADES, the density diminishes to 1.19 g/cc at 60 °C. The increase in kinetic energy upon heating prompts NADES molecules to disperse, causing the NADES molecules to occupy more volume and consequently leading to a decrease in density. The observed decline in density exhibits a somewhat linear correlation with the rise in temperature, aptly represented by Eq. (3). These insights into the density variation of NADES are graphically depicted in Fig. 5, with an Absolute Mean Percentage Error (AMPE) of 1.12%, providing a quantified measure of accuracy in the reported density values.

Refractive index trend of CA: Gly NADES between 25 and 60 °C.

Viscosity, denoting the force of attraction between distinct fluid layers in motion, plays a pivotal role in comprehending the applicability of Natural Deep Eutectic Solvents (NADES) across diverse domains. At 25 °C, the viscosity of NADES was measured at 951 cp., surpassing that of glycerine.

The observed reduction in viscosity as temperature ascends is primarily attributed to a diminishing intermolecular attraction. This phenomenon results in a less viscous liquid, a trend vividly illustrated in Fig. 6 and quantified by Eq. (4). Notably, the viscosity decreases to 898 cp. at 60 °C, show casing an overall Average Mean Percentage Error (AMPE) of 1.4%. This nuanced understanding of the viscosity trend in NADES, as influenced by temperature, holds significant implications for their practical utilization.

Viscosity trend of CA: Gly NADES between 25 and 60 °C.

The pH values of a solution, determined by the negative log of hydrogen ion concentration, hold paramount importance, particularly in pH-sensitive applications like DNA synthesis, necessitating a thorough understanding of NADES pH before utilization. In the case of citric acid-based NADES, a notably acidic pH of 1.91 was observed, in stark contrast to the relatively neutral pH of Glycerine.

Interestingly, the pH of Citric acid-based NADES exhibits a nonlinear decline with increasing temperature. This phenomenon is attributed to heightened molecular vibrations disrupting the H + equilibrium in the solution, leading to the generation of [H] + ions and subsequently changing the pH. Although the natural pH range of citric acid falls between 3 and 5, the presence of acidic hydrogen in glycerin further decreases the pH to 1.91.

The pH behavior of Citric acid-based NADES within the temperature range of 25–60 °C is aptly represented by Eq. (5), providing a mathematical expression for the observed pH trend. This intriguing relationship is graphically depicted in Fig. 7, highlighting the impact of temperature on NADES pH, as reported with an AMPE of 1.4%.

pH trend of CA: Gly NADES between 25 and 60 °C.

Thermogravimetric Analysis (TGA) was systematically conducted on Citric Acid based Natural Deep Eutectic Solvent (NADES) over a temperature range extending from room temperature to 500 °C. From Fig. 8a and b, it is discerned that the initial mass loss, experienced up to 100 °C, is predominantly attributed to absorbed moisture and the water of hydration associated with Citric acid and pure glycerine. A notable retention of approximately 88% of mass is observed until 180 °C, primarily linked to the decomposition of citric acid into aconitic acid, followed by the subsequent formation of methyl maleic anhydride (III) upon further heating (Fig. 8b). Beyond 180 °C, the discernible emergence of acrolein (propenal) from glycerine is also observed, as delineated in Fig. 8b37.

The Thermogravimetric Analysis (TGA) of glycerine unfolds in a dual-stage mass loss process. The initial stage, spanning from 180 to 220 °C, involves the generation of propenal, succeeded by a pronounced mass loss at elevated temperatures between 230 and 300 °C (Fig. 8a). With increasing temperature, a sequential generation of acetaldehyde, carbon dioxide, methane, and hydrogen gas are witnessed. Notably, by 300 °C, a mere 28% of the mass is retained, indicating a potential compromise of the intrinsic characteristics of NADES 8(a)38,39.

(a) TGA of pure glycerine. (b) TGA of CA: Gly NADES.

The FTIR (Fourier-Transform Infrared) analysis of the freshly prepared NADES (Natural Deep Eutectic Solvents) mud was conducted to gain insights into the formation of new chemical bonds. This was achieved by comparing the NADES mud spectrum with spectra obtained from pure CA (Citric Acid) and Gly (Glycerine). In the CA spectrum, the distinct peaks are observed at 1752 1/cm and 1673 1/cm, which are indicative of C = O stretching vibrations and are characteristic features of CA. Additionally, in the fingerprint region, a notable shift in the OH bending vibration was observed at 1360 1/cm as shown in Fig. 9.

Similarly, in the case of glycerine, shifts in the OH stretching and bending vibrations at 3291 1/cm and 1414 1/cm wavenumbers are identified, respectively. Now, by analyzing the spectra of newly prepared NADES, significant shifts in the spectrum are observed. The C = O stretching vibration shifted from 1752 to 1720 1/cm, and the -OH bending vibration from glycerine shifted from 1414 to 1359 1/cm as shown in Fig. 7. These shifts in wavenumbers indicate a change in electronegativity, suggesting the formation of new chemical bonds within the NADES structure.

In particular, the observed shift in the C = O stretching vibration and the -OH bending vibration from glycerine implies an interaction between the hydroxyl group (-OH) and the carbonyl group (C = O). Given that this interaction involves a hydrogen atom as a donor and oxygen as an acceptor, it is recognized as the formation of a hydrogen bond. This finding is significant as it sheds light on the molecular interactions and bonding within the NADES, providing valuable insights into its chemical properties and potential applications.

FTIR spectra of Citric acid (blue), Glycerine (orange), NADES (red).

Delving into the realm of mud rheology, we explored Yield Points (YP) and Plastic Viscosity (PV) ratio (YP/PV) to discern the critical forces governing colloidal interactions within drilling mud. YP signifies colloidal attraction forces, while PV denotes the resistance exerted by solid particles and the liquid medium. The YP/PV ratio plays a pivotal role in defining mud flow behavior, and we discovered that higher YP/PV values create an ideal mud flow profile, enhancing cuttings’ transport capacity while avoiding excessive annular frictional pressure losses. Our research identified the optimal YP/PV range (0.75–1)lbm/100ft²/cp for efficient cuttings transportation7,40,41,42. Furthermore, our findings underscored that increasing temperature correlated with a decline in YP and PV, primarily due to multiple concurrent phenomena, including changes in electrical double-layer thickness, reduced hydration levels, enhanced thermal energy, and increased clay particle dispersion. At elevated temperatures, bentonite underwent dehydration, mechanical shearing, and degradation, ultimately affecting mud rheological properties.

Figure 10 presents a comparative analysis of the YP/PV (Yield Point/Plastic Viscosity) values for NADES-based mud in contrast to KCl, DES, and Ionic liquid-based muds, both before and after aging at temperatures of 100 °C and 150 °C. The results highlight significant findings: in non-aged samples, all inhibitors contributed to notable enhancements in the YP/PV values of the mud, bringing them closer to the optimal range. However, in the case of aged samples, a decrease in rheological properties was observed across the board. Particularly noteworthy was the performance of CA-based mud, which exhibited the most substantial improvement in YP/PV when compared to DES, KCl, and Ionic liquid-based muds. Intriguingly, DES-based mud also demonstrated impressive performance, closely approaching that of NADES-based mud, as depicted in Fig. 10. These outcomes shed light on the effectiveness of different inhibitors and their impact on mud rheology, offering valuable insights into mud formulation for various applications.

YP/PV of CA NADES mud in comparison with KCl, [EMIM]Cl and Choline Chloride based DES before and after aging.

The effectiveness of CA-NADES-based mud under HTHP conditions proved remarkable. In comparison to the base sample without NADES, the inclusion of NADES yielded notable enhancements in filtration properties. These improvements were evident through the reduction in mud cake thickness and filtrate volume, as depicted in Figs. 11 and 12. Thick mud cake can lead to operational challenges, such as drill pipe blockages, resulting in Non-Productive Time (NPT) and wellbore issues. Typically, thinners are introduced to mitigate mud cake thickness. Intriguingly, NADES exhibited properties akin to a thinner by decreasing both mud cake thickness and filtrate volume. This phenomenon can be attributed to NADES’s ability to bond with clay platelets and modify their wettability, thereby augmenting filtration properties. The results indicate.

The findings reveal that CA-based mud exhibited superior performance when compared to all other mud compositions, delivering a substantial 32% reduction in mudcake thickness as shown in Fig. 12. In contrast, KCl, Ionic liquid, and DES-based muds resulted in reductions of 12%, 20%, and 24% in filtrate volume, respectively as shown in Fig. 11. Likewise, CA NADES mud demonstrated outstanding performance at the optimized 3% concentration, achieving a notable 27.1% reduction in mudcake thickness. In comparison, KCl, Ionic liquid, and DES exhibited reductions of 5.2%, 25.12%, and 22.11% in mudcake thickness, respectively. The mechanisms underlying these improvements are elaborated upon in subsequent sections.

Filtrate Volume of CA NADES mud in comparison with KCl, [EMIM]Cl and Choline Chloride based DES before and after aging.

Mudcake Thickness of CA NADES mud in comparison with KCl, [EMIM]Cl and Choline Chloride based DES before and after aging.

Shale swelling inhibition was a focal point of this study, with various inhibitors tested against a base drilling fluid sample. Figure 13 elucidates the percentage increase in swelling for different inhibitors, highlighting their effectiveness. Notably, the base sample exhibited a 63% increase in shale swelling, a trend our study sought to mitigate. Upon the addition of inhibitors such as KCl, [EMIM]Cl, and Choline Chloride based DES, we observed significant reductions in swelling percentages, with CA based NADES emerging as the most effective inhibitor, achieving a remarkable 74.35% reduction as shown in Fig. 13. KCl primarily operates via cationic exchange between clay layers, expelling water cations and stabilizing the clay. However, KCl’s efficacy is hampered by the necessity for high concentrations, offsetting its advantages. Ionic liquids, with their robust electrostatic attractions and hydrophobic alkyl chains, interact effectively with clay, neutralizing charges, and impeding shale hydration. Despite their efficacy, these ionic liquids are costly and pose environmental concerns. CA NADES exhibited superior results, attributed to their exceptional hydrogen bond-forming capabilities. NADES effectively interacts with negatively charged clay, neutralizing clay charges and replacing water between clay layers, as supported by d-spacing results. Furthermore, as depicted in Fig. 14, it is evident that the 3% NADES formulation exhibited superior performance, surpassing all other inhibitors with an impressive 86.36% improvement in overall shale recovery. In contrast, the KCl-based mud demonstrated the least improvement in shale recovery, whereas the results for the Choline Chloride DES mud showed a recovery improvement of 85.8%, approaching the performance level of the NADES-based mud.

Linear swelling caused by CA NADES mud in comparison with KCl, [EMIM]Cl and Choline Chloride based DES.

Shale recovery caused by CA NADES mud in comparison with KCl, [EMIM]Cl and Choline Chloride based DES.

Surface tension’s pivotal role in capillary pressure, clay swelling, and inhibitor performance became apparent in this study. A substantial 12.69% reduction in surface tension with 3% NADES inhibitor is observed as shown in Fig. 15, signifying its ability to modify the clay’s behavior in the presence of water. NADES’s profound impact on surface activity is attributed to its robust hydrogen bonding capabilities, altering contact angles and capillary behavior, aligning with our LSM results. All error bars have been drawn with 5% error margin rule.

Surface tension CA NADES mud in comparison with 3% KCl, [EMIM]Cl and Choline Chloride based DES.

The examination of Zeta Potential unveiled a significant factor influencing behaviour of NADES modified clay by impacting the thickness of the electrical double layer. This parameter plays a crucial role in regulating clay swelling. The introduction of inhibitors had the effect of diminishing the thickness of the electrical double layer, thereby impeding cationic exchanges between the inhibitor and clay particles. Notably, the 3% NADES formulation exhibited the most substantial reduction in Zeta Potential, presenting an impressive decrease of 26.91%, as illustrated in Fig. 16. This decline in Zeta Potential aligns with the superior performance of NADES in inhibiting shale swelling.

Zeta Potential refers to the electrical charge at the surface of clay particles. It influences the thickness of the electrical double layer around these particles, which, in turn, affects how they interact with other substances. In the context of clay swelling, a thicker electric double layer can promote cationic exchanges with the clay, leading to increased swelling. Inhibitors introduced into the mud formulations were able to reduce this thickness, effectively limiting the interactions between the inhibitors and the clay particles.

The statement highlights that the 3% NADES formulation showed the most significant reduction in Zeta Potential, indicating that it was particularly effective in reducing the electrical double layer thickness around the clay particles. This reduction in Zeta Potential aligns with the formulation’s superior performance in inhibiting shale swelling, suggesting that by decreasing the electrical double layer thickness, NADES-based muds can mitigate shale swelling and improve shale recovery.

Zeta Potential CA NADES mud in comparison with 3% KCl, [EMIM]Cl and Choline Chloride based DES.

The examination of d-spacing, representing interlayer spacing in clay, further substantiated our findings. Inhibitors effectively intercalated between clay layers, displacing water, and enhancing clay stability against hydration. Of all the inhibitors, 3% NADES displayed the most affinity towards clay, achieving optimal concentration for shale stabilization as shown in Fig. 17. These results concurred with Linear Swell Meter (LSM) findings, affirming NADES as a promising shale inhibition solution. Through a comprehensive investigation, our study not only elucidates the effectiveness of NADES in improving drilling fluid properties but also unravels the underlying mechanisms governing these enhancements, paving the way for more efficient and environmentally friendly drilling practices.

d-spacing of CA NADES mud in comparison with 3% KCl, [EMIM]Cl and Choline Chloride based DES.

The influence of DES or NADES on the size of sodium bentonite particles is contingent upon the specific nature of the employed DES and the particular conditions during its introduction. Diverse investigations have demonstrated that certain DES types prompt the aggregation of bentonite particles, culminating in the formation of larger particles. Conversely, alternative studies posit that the addition of particular DES or NADES varieties can result in a reduction in the size of bentonite particles43,44. Upon introducing a DES or NADES into a suspension of bentonite particles, a modification in the particles’ surface charge and size occur. This alteration enhances their mutual attraction, fostering the creation of larger clusters or flocs. This clustering effect precipitates an overall reduction in the size of bentonite particles within the suspension. Additionally, NADESs can influence the rheological properties of the mud by modifying the structure of clay platelets and its size.

The mean particle size of both untreated and modified bentonite wafers has been approximated and is depicted in Fig. 18. The inclusion of inhibitors such as EMIM-Cl ionic liquid and KCl leads to a general diminution in particle size, consistent with the findings related to d-spacing. Notably, the most pronounced modification is observed in the 3% CA NADES-based mud, as illustrated in Fig. 18.

The FESEM of: (A) Hydrated sodium bentonite wafer (Base Sample) (B)3% KCl based wafer (C) 3% CA NADES based wafer at 30kx magnification (D) 3% EMIM(Cl) based wafer (E)3% Choline Chloride: Urea DES based wafer (F) 3% CA NADES based wafer at 10kx magnification.

The utilization of Citric Acid: Glycerine based NADES in drilling mud formulations has exhibited promising outcomes, offering multiple advantages to drilling operations. In the realm of drilling fluids, Citric Acid-based NADES has proven its capacity to enhance fluid performance and tackle key challenges associated with drilling operations. By incorporating Citric Acid-based NADES, mudcake thickness can be effectively reduced, mitigating these challenges, and enhancing overall drilling efficiency. The efficacy of Citric Acid: Glycerine-based NADES as a shale inhibitor is a critical aspect of their application in drilling muds. Shales, with their complex mineral composition, are prone to swelling and dispersion when exposed to water-based drilling fluids, leading to wellbore instability and poor borehole cleaning. NADES, through their interactions with clay particles, have demonstrated the ability to mitigate these issues.

The mechanism underlying the shale inhibition properties of NADES involves their capability to form hydrogen bonds with clay particles. These bonds modify the clay’s face-to-edge orientation, reducing its affinity for water and minimizing swelling and dispersion. Moreover, NADES possess a stronger affinity for clay particles compared to water, displacing water molecules between clay layers, stabilizing shale formation, and improving drilling performance. Various analytical techniques, including X-ray diffraction (XRD) analysis, zeta potential measurements, and surface tension measurements, have been employed to support these findings. XRD results indicate structural changes in clay minerals upon interaction with NADES, consistent with observed shale inhibition effects. Zeta potential measurements reveal neutralization of negative charges on clay surfaces in the presence of NADES, confirming interaction. Surface tension measurements indicate NADES’ ability to lower surface tension, contributing to improved wetting, fluid flow characteristics, and shale inhibition in drilling mud.

The exploration of shale swelling inhibition and drilling fluid improvement has highlighted the superior performance of Citric Acid and Glycerine-based NADES as novel drilling fluid additives. At a 3% concentration, NADES achieved the lowest filter cake thickness and filtrate volume, surpassing KCl and ionic liquid (EMIM-Cl) based mud. NADES’ unique properties, including their ability to form hydrogen bonds with clay particles and modify surface characteristics, have played a pivotal role in enhancing drilling fluid properties and inhibiting shale swelling. The thermal stability and overall performance of NADES position them as promising alternatives, offering flexibility and potential advantages over traditional inhibitors. Analytical techniques such as XRD, zeta potential, and surface tension measurements provide robust support for the observed effects of NADES on drilling fluid properties. Continued research and development in this area holds substantial potential for advancing the efficiency and sustainability of drilling operations in the oil and gas industry.

The following conclusions can be drawn from this study.

In addition to the previously investigated inhibitors, a newly prepared Citric Acid (CA): Glycerine-based NADES has been introduced in this study. Notably, this NADES variant exhibits a highly acidic character, presenting a unique aspect for further exploration in the context of drilling fluid optimization and shale stabilization.

In contrast to previous studies that primarily utilized toxic and non-biodegradable shale inhibitors like KCl and imidazolium-based ionic liquids, this research introduces Natural Deep Eutectic Solvents (NADES) from citric acid and glycerine, demonstrating significantly enhanced environmental sustainability and superior shale inhibition performance, including a 49.14% reduction in swelling and an 86.36% improvement in shale recovery.“.

Among the inhibitors evaluated, the CA-NADES-based mud has emerged as a standout performer, displaying significant improvements in Yield Point to Plastic Viscosity (YP/PV) ratios and filtration properties. These enhancements are attributed to the establishment of hydrogen bonds between clay and NADES, resulting in structural modifications and improved rheological properties.

The reduction in surface tension by NADES effectively curtails capillary action, mitigating water cation infiltration. This finding further strengthens the argument for NADES as an effective shale inhibitor against hydration, contributing to improved drilling fluid performance.

The notable decrease in d-spacing within the NADES-based mud provides additional evidence of its heightened affinity for clay compared to water. This structural modification reinforces the potential of NADES as a reliable and effective solution in countering shale hydration.

This research not only consolidates the position of NADES as a superior shale inhibitor but also opens new avenues for harnessing NADES as a sustainable solution in the realm of drilling fluid optimization and other fields. The findings of this study have significant implications for advancing drilling practices in the oil and gas industry.

The comprehensive exploration of various inhibitors and the introduction of a newly prepared CA: Gly-based NADES have contributed to a nuanced understanding of their roles in shale inhibition. The remarkable efficacy of CA-NADES and the superior performance of NADES, in general, underscore their potential as key components in the development of sustainable and effective solutions for drilling fluid optimization and shale stabilization.

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

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This work has been conducted under YUTP FRG 1/2021, grant no. 015LC0-363.

Department of Petroleum Geosciences, Universiti Teknologi Petronas, Bandar , Seri Iskander, Malaysia

Muhammad Hammad Rasool, Maqsood Ahmad & Husnain Ali

Department of Chemical and Biological Engineering, The Hong Kong University of Science & Technology (HKUST), Clear Water Bay, China

Numair Ahmed Siddiqui

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M.H.R. wrote the original draft and devised the methodology. M.A. porvided funding and supervised the whole project. N.A.S. helped with the methodology section and analyzing results. H.A. helped with the concept and designing NADES solutions.

Correspondence to Muhammad Hammad Rasool, Maqsood Ahmad or Numair Ahmed Siddiqui.

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Rasool, M.H., Ahmad, M., Siddiqui, N.A. et al. Novel application of citric acid based natural deep eutectic solvent in drilling fluids for shale swelling prevention. Sci Rep 14, 25729 (2024). https://doi.org/10.1038/s41598-024-76182-7

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DOI: https://doi.org/10.1038/s41598-024-76182-7

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