Oil-based Drilling Fluids Research Articles

Research and Application of New Technology of Bionic Enhanced Wellbore and Strong Lubrication Water-Based Drilling Fluid

After more than a century of development, drilling fluid technology has become capable of dealing with various extreme conditions. As the exploration and development targets shift towards complex oil and gas resources, however, the geological and surface conditions encountered get increasingly complex, which poses a greater challenge to drilling fluid. In this paper, bionics is introduced into the field of drilling fluids, imitating the characteristics, functions, structures, and principles of mussels and earthworms, and a bionic wall-fixing agent with side chains containing catechol functional groups to strengthen the wellbore is prepared. A bionic bonding lubricant that when making the direct friction between the two is changed to the sliding between the membranes is prepared. Compared with the advanced technology introduced from abroad, the strength of the rock is not only reduced but increased by more than 14%, the friction reduction rate is improved by 12.3%. Their mechanism of action and influencing factors are revealed from the macro and micro perspectives. Combined with the formation conditions encountered, other treatment agents are applied to develop a novel technology of bionic strengthened borehole and high lubricity water-based drilling fluid with comparable inhibition and lubricity to oil-based drilling fluid. In comparison with technology, the rate of well collapse is reduced by as much as 82.6%, the accident rate of stuck pipe is brought down by as much as 86.4%, the complication of stuck block is reduced by as much as 79.7%, and the overall cost is lowered by more than 30%. It is truly a safe, efficient, economic, environmentally friendly drilling fluid technology.

Self-destructive barite filter cake in water-based and oil-based drilling fluids

The complete removal of filter cake containing barium sulfate is a challenging and difficult task because of the involvement of various issues such as incompatibility and insolubility. Conventional filter cake removal techniques include the use of chelating agents, which are ineffective in completely removing the barite filter cakes. Barite is completely insoluble in water as well as in various acids, such as acetic acid, lactic acid, hydrochloric acid, and formic acid. In this study, a novel method is introduced to remove the barium sulfate filter cake by introducing a new formulation of both oil-based and water-based drilling fluids. The new formulation can prevent fluid loss as well as self-destruct the barium sulfate filter cake, which can be formed with oil-based and water-based drilling fluids. The new formulation is comprised of encapsulated thermochemical fluids in addition to the conventional drilling fluids additives. These thermochemical fluids can cause a strong exothermic reaction that can significantly increase the surrounding temperature and pressure. The generated heat and pressure can dissolve or dislodge the filter cake formed with drilling fluid. The laboratory experimental results showed that the new formulation could completely remove the filter cake in a single stage within 48 h. In addition to that, the new formulation resulted in the creation of micro-fractures, which can enhance the flowback and help in well cleanup process after drilling. The new formulation can be used as a promising formula to remove the filter cake before cementing operation.

Effect of Hydrophobic Iron Oxide Nanoparticles on the Properties of Oil-Based Drilling Fluid

AbstractLately, nanoparticles (NPs) have shown the potential to improve the performance of oil well fluids significantly. Several studies have reported the ability of NPs to produce improved properties of both water and oil-based drilling fluids. In this study, hydrophobic iron oxide NPs were synthesized by thermal decomposition of iron pentacarbonyl in an inert atmosphere, and its performance was tested in the oil-based drilling fluid with 90/10 oil-to-water ratio (base fluid). Oil-based drilling fluids treated with nanofluids were formulated by adding 0.5 wt% and 1.0 wt% iron oxide NPs in hexane solution to the base drilling fluid. The base fluid and the nanofluid-treated drilling fluids were evaluated by characterizing their rheological properties at different temperatures, viscoelastic properties, lubricity, filtrate loss, static and dynamic settling, and separation properties. Results showed that 0.5 wt% iron oxide dispersed in hexane reduced the high pressure high temperature (HPHT) filtrate loss by 70%, filter cake thickness by 55%, and the coefficient of friction by 39%. Moreover, the nanofluid based drilling fluid reduced the free oil layer caused by syneresis during aging at high temperature by 16.3% compared to the base fluid. This study has shown that hydrophobic iron oxide NPs have the potential to improve the properties of oil-based drilling fluid.

Laboratory Evaluation of Static and Dynamic Sag in Oil-Based Drilling Fluids

SummaryIn this paper, we present the results of barite sag measurements before and after hot-rolled oil-based drilling fluids (OBDFs) using different approaches for characterization. We characterized the barite sag of a liquid column under static condition using light-scattering (LS) measurements, hydrostatic pressure measurements, and gamma densitometry. Under the dynamic condition, we used a rheometer with a grooved bob-in-cup measuring system to characterize barite sag in rotational and oscillatory shear conditions. Extensive rheological characterization of the drilling fluid samples, before hot rolling (BHR) and after hot rolling (AHR), is carried out. It is found that barite sag decreased in hot-rolled fluid samples from the LS, rotational, and oscillatory shear measurements. The rheological characterization of the fluid samples showed that heat-activated chemicals in the hot-rolled fluid sample increased the viscosity and elasticity, which contributed to lower barite sag and longer suspension of particles than BHR. Both hydrostatic and gamma densitometry measurements reveal more or less uniform compaction of barite particles in the fluid sample below the liquid layer. Time-dependent oscillatory shear measurements provide new insights on the structural character of drilling fluids to predict barite sag tendencies during the fluid design phase.

Study on Reasonable Amount of Flushing Fluid and Flushing Time

Oil-based drilling fluid has many advantages, such as stable performance, strong anti-pollution ability, and strong inhibition, and it has been widely used in the world. However, at present, there is no unified evaluation standard for the effect of flushing fluid under this condition, and most of them are based on the flushing effect evaluation of water-based drilling fluid. However, there are few studies on wetting reversal phenomenon, reasonable dosage, and flushing time between oil-based drilling fluid and flushing fluid on the two-phase interface. Therefore, it is necessary to establish a comprehensive evaluation method for the effect of flushing fluid under oil-based drilling fluid, and explore the relationship between the amount of flushing fluid and flushing efficiency, so as to determine the reasonable range of flushing fluid consumption. In this paper, using the existing experimental instruments, a type A oil-based flushing fluid commonly used in oil fields was selected for the experiment, and on the basis of the high-temperature and high-pressure filtration meter, a flushing fluid evaluation device was developed. Based on the principle of equal shear rate, the flushing displacement and flushing time were determined, and then the flushing effect was evaluated. In order to comprehensively consider the influencing factors of the well wall flushing effect, five experimental factors, including different core, flushing displacement, flushing time, flushing fluid type, and surfactant concentration, were selected to formulate the flushing experiment scheme. The contrast method was used to test the cementation strength of core and cement slurry under different conditions, and the microscopic morphology of the interface was observed and analyzed to explore the reasons for the change of cementation strength between core and cement slurry due to the existence of oil-based drilling fluid. The conclusion obtained has very important practical significance to guide the field practice.

Risk assessment of human exposure to radionuclides and heavy metals in oil-based mud samples used for drilling operation

ABSTRACT This study investigates heavy metals and naturally occurring radionuclide materials (NORM) possible presence and pollution rates in oil-based drilling fluids system used to drill an oil and gas well. It also estimates the health risks of the drilling crew due to their exposure to these substances. Measurements from Atomic Absorption Spectrometry (AAS) revealed that, the concentrations of the metals present in the drilling mud samples varied significantly and decreased in the order of Zn > Al > Ni > Pb > Cr > Cu > As > Hg > Cd. Generally, amongst all the heavy metals considered, mud sample C had the highest heavy metal concentration when compared to samples A and B, respectively. When compared with the recommended maximum allowable limits, Cd and Ni were found to be higher than the International Reference Standard by factors of Cd (3 mg/kg) and Ni (50 mg/kg). The cancer risk obtained from this present study are 1.1 × 10−3 and 7.7 × 10−3 for the drilling crew, which is slightly above the acceptable risk range considered by the environmental and regulatory agencies. The concentrations of radioactive substances as obtained from analysis, show that K-40 is the dominant radionuclide in the samples with the highest value slightly twice the standard reference value. The concentrations of Ra-226 and Th-232 activity in the mud samples were found to be lower when compared with the International Reference Level. Also, the X-ray diffraction analysis helped to identify 16 very important/useful minerals in the three mud samples under consideration. The higher elemental concentrations of potassium and aluminum silicate found in sample C can be credited to the elevated heavy metal-content found in the mud samples. Significantly, these exposure risks found in this present study indicate that the potential health risks due to radiological activities may not pose short – but long-term risks to the drillers.

Experimental study on water-based drilling fluid for horizontal wells

ABSTRACT High-yield shale gas flow has been discovered in the Cambrian Shuijingtuo group shale (the Shuijingtuo shale) of the first shale gas well of Yichang, West Hubei, China. However, there are very few papers or reports involving high-performance water-based drilling fluid for the Shuijingtuo shale. X-ray diffraction (XRD) analysis and scanning electron microscope (SEM) tests were conducted on the outcrop of the Shuijingtuo shale samples collected from Zigui County, West Hubei. Then, the screening experiments on mud-making materials, salt additives, nanoparticles, fluid loss additives, mixed metal hydroxides (MMH), and surfactants were carried out. A high-performance water-based drilling fluid for the Shuijingtuo shale (the SWDF drilling fluid) was proposed based on physical plugging by nanoparticles, electrical property inhibition, and wettability control theory. Finally, comprehensive appraisals on rheology, filtration, lubricity, temperature resistance, inhibition, anti-pollution capability, and expense of the SWDF drilling fluid were accomplished. XRD results showed that the Shuijingtuo shale samples had 55%~67% brittle minerals. Clay minerals were mainly illite (13%~27%), and no montmorillonite was observed, which indicated that the Shuijingtuo shale was a weak-expansive rock. SEM analysis results show that pore and micro-cracks on the surface of the Shuijingtuo shale could provide natural channels for the invasion of drilling fluid. Nanomaterials were found to reduce the fluid loss of drilling fluid and thermal stability of the nano-silica dispersion was better. As a whole, the SWDF drilling fluid had suitable viscosity and gel strength, a low API fluid loss of 7.2 mL, a lubricity coefficient of 0.21, temperature-resistance capability up to 120°C, and good inhibition to shale hydration. In addition, it had excellent resistance capability to inorganic salts (NaCl, MgCl2, and CaCl2) and low-yield mud-making materials (simulated cuttings). Also, the cost of the SWDF drilling fluid was competitive compared to common oil-based drilling fluids. As a consequence, the SWDF drilling fluid can meet the requirements of drilling horizontal wells in the Shuijingtuo shale of West Hubei, China.

Styrene butadiene resin/nano-SiO2 composite as a water-and-oil-dispersible plugging agent for oil-based drilling fluid

In this paper, styrene butadiene resin/nano-SiO2 (SBR/SiO2) composite was synthesized by the continuous emulsion polymerization method, which was used as a plugging agent for oil-based drilling fluid (OBDF) to enhance plugging efficiency in shale formations. The characterization of SBR/SiO2 was measured by Fourier transform infrared spectroscopy, thermal stability and scanning electron microscopy. The results implied that the structure of SBR/SiO2 was consistent with design. The dispersion of SBR/SiO2 in water and mineral oil were studied by particle size distribution and rheological analysis. The results indicated that SBR/SiO2 can be dispersed in water and mineral oil simultaneously, while most normal OBDF plugging agents can only be dispersed in water or mineral oil. Since OBDF is a water-in-oil emulsion, when the plugging agents can be dispersed in two continuous phases, the plugging efficiency can be maximized. And it’s the first time to explain plugging ability from dispersion perspective and produce the plugging agent for OBDF which can be dispersed in two phases. Then the plugging ability was evaluated by filtration volume, porosity physical adsorption instrument (BET), and pressure transmission test. The results showed that SBR/SiO2 can enter into the nanopores of shale formations and significantly reduce fluid invasion, thereby improving wellbore stability. Finally, the dispersion and plugging mechanism of SBR/SiO2 were analyzed. This study might provide an avenue to solve the wellbore instability and reservoir damage owing to OBDF during the drilling process by adding the plugging agent which can form a dense filter cake and reduce fluid loss in shale formations.

Application of SDS surfactant microemulsion for removal of filter cake of oil-based drilling fluid: influence of cosurfactant

After drilling oil wells using oil-based drilling fluid, there is the occurrence of a formation of a thin layer of solids adsorbed on the surface, called filter cake, which must be removed to obtain satisfactory well cementation. The present work developed microemulsion systems from sodium dodecyl sulfate, a biodegradable surfactant, with kerosene as oil phase, n-butanol as cosurfactant and distilled water, in order to evaluate its efficiency in removing the filter cake. The results showed that the microemulsion systems composed of inverse and bicontinuous micelles removed 100% of the filter cake regardless of the content of the constituents in the formulation, while the direct micelles only reached 100% with the formulation composed by 10% kerosene and cosurfactant/surfactant ratio = 4. It was observed that the cosurfactant/surfactant ratio is a determining variable for direct micelles to remove 100% of the filter cake and that they are compatible with cement, offering desirable thermal stability and improving the wettability of the rock in water.

Effects of surface modified nanosilica on drilling fluid and formation damage

Despite the fact that hydrophobic nanosilica can improve the stability of water/oil emulsion, there exist controversies pertaining to its influence on oil-based drilling fluid rheological behavior and the subsequent formation damage. The present study addresses the above using a surface modified nanosilica, where the particles were functionalized with different silane-based groups to alter their hydrophilicity: 3-glycidoxypropyl-triethoxy silane (GPTS) and combined GPTS and propyl silane (PGPTS). The NPs were characterized through FTIR analysis, particle size, and zeta-potential measurements followed by flow behavior experiments, core-scale mud flow tests, Computed Tomography (CT) scanning and SEM imaging techniques to determine the mud penetration depth.We found that, depending on the type of functionalized group and the size of NPs, the surface modification of NPs can enhance the rheological properties and mitigate formation-damage. The most desirable behavior, both in terms of the plastic viscosity and yield stress, was achieved with 15 nm size NPs. The functionalized NP with the 3-glycidoxypropyl-triethoxy silane (GPTS@SiO2), which is less hydrophilic compared with bare NP, improved plastic viscosity of the drilling fluid, however slightly deteriorated the yield stress. This was compensated for by adding a double amount of GPTS on the surface of NP (DGPTS@SiO2), which was shown to be more effective than merely increasing the GPTS content in the drilling fluid. Further improvement of rheological properties was made by combining GPTS and propyl silane (PGPTS@SiO2); implying the positive effect of reducing hydrophilicity on both the plastic viscosity and yield stress. It is noteworthy that surface modifications were identified to be efficient below an optimal concentration of 1 wt% of NPs due to the lack of particle agglomeration. Moreover, a decreasing trend in plastic viscosity was observed against increasing temperature as a result of lowering viscosity of the continuous phase. Meanwhile, the yield stress experienced greater changes versus temperature due to the larger dependency of the interactions between drilling fluids components on temperature.In addition, the results were further validated through a series of core-flow tests which showed that with the aid of the nano-based drilling fluids (NDFs) containing DGPTS@SiO2 and GPTS@SiO2 the penetration depth of mud, thus the degree of internal formation damage can be significantly reduced.

Intercalation Inhibits the Surface Hydration of Sodium Montmorillonite: Experiments and Density Functional Theory Simulation

In order to use water-based drilling fluids to substitute for oil-based drilling fluids in the drilling industry, one of the main challenges is the inhibiting the surface hydration of clay to furth...

Enhancements in the tribological performance of environmentally friendly water-based drilling fluids using additives

Since their introduction in the 1960s, oil-based drilling fluids or muds (OBM) have been applied as drilling fluids in oil & gas exploration activities. However, they cause potential issues, such as contaminating immediate areas of freshwater aquifers around the well site, and causing permanent damage to the environment during the disposal of cuttings. In an effort to seek promising environmentally-friendly alternative drilling fluids with competitive or even better drilling performance than OBM, this paper investigates the tribological performance of potassium chloride polymer water-based muds (WBM) and artificial sea water (KCl brine), and comparing them with a reference OBM. To improve the OBM tribological attributes, select additives are adopted and added individually into the above-mentioned muds. Test results show that the tribological performance of both WBM and KCl brine could be drastically improved to levels equal or better than the widely-used OBM with the addition of a low percentage of certain friction-modifying additives. Chemical characterization showed the formation of an effective FeCl2/Fe3O4 tribofilm, which is the main reason for the significant enhancement.

Application of amorphous silica nanoparticles in improving the rheological properties, filtration and shale stability of glycol-based drilling fluids

The drilling process in shale formations and well stability in the presence of shale are among the most challenging issues in the drilling industry. Routinely, oil-based drilling fluids are used for drilling troublesome shale formations. However, oil-based drilling muds have a number of significant disadvantages, including very high cost, causing severe damage to the environment, and interfering with the well-logging process. To address this issue, a sustainable glycol-based drilling fluid is designed in this study to substitute the routinely used oil-based drilling fluids. In the first step, amorphous silica nanoparticles with different particle sizes (12, 22, and 54 nm) are prepared from rice husks. The prepared nanoparticles are then dispersed in water and added to the glycol-based mud as a liquid-based additive. Finally, the effect of silica nanoparticles on rheological properties, filtration, and shale stability of glycol drilling fluid is studied. The results show that the use of silica nanoparticles improves the rheological properties of glycol drilling fluid. This improvement is a function of the amount and size of nanoparticles. Adding silica nanoparticles also decreases fluid loss and increases the thermal stability of the drilling fluid. Moreover, silica nanoparticles can effectively plug nanoscale pores of Gurpi shale resulting in increasing the shale cutting recovery and decreasing the penetration rate of glycol drilling fluid into the Gurpi shale samples.

Novel Spherical Polymer Sealing Agent for Oil-Based Drilling Fluids to Overcome Borehole Instability Problem in Low Permeability Reservoirs

Due to borehole instability problem in low permeability formations, a new type of spherical polymer sealing agent (OLBS) for oil-based drilling fluids was prepared via suspension polymerization, and the sealing effect of polymer sealing agent on low permeability cores was evaluated. The results indicated that OLBS had no influence on the rheological properties of oil-based drilling fluids. The core displacement experiments indicated that OLBS could reduce the core permeability by a significant amount, and thus stop fluid invasion into the formations, so OLBS could provide an excellent sealing effect and improve wellbore strength. Using this polymer sealing agent to plug the low permeability formations is a very powerful approach to address borehole instability problem in troublesome low permeability formations. This sealing agent is suitable for the drilling of long sections of horizontal laterals. In the future, the oil-based drilling fluids containing OLBS might hold great promise to resolve borehole instability problem in low permeability formations.

Novel Cake Washer for Removing Oil-Based Calcium Carbonate Filter Cake in Horizontal Wells

An impermeable layer “filter cake” usually forms during the overbalanced drilling technique. Even though it helps in protecting the formation from a further invasion of drilling fluids, the removal of this layer is essential for a proper cement job and to avoid any reduction in wellbore deliverability. The design of the removal process is complicated and depends on the filter cake composition and homogeneity. This paper presents an experimental evaluation on the usage of a novel cake washer (NCW) in the removal of a filter cake formed by an invert emulsion oil-based drilling fluid that contains calcium carbonate as a weighting material while drilling a horizontal reservoir. The proposed NCW is a mixture of organic acid, mutual solvent and nonionic surfactant. It is designed to enable restored wellbore permeability for a sustainable production. Since the filter cake mainly consists of the weighting material, the solubility of calcium carbonate in NCW at different ranges of temperature, duration and concentration was investigated. An actual casing joint was used to test the corrosion possibility of the treating solution. High-pressure and high-temperature (HPHT) filtration tests on ceramic discs and Berea sandstone core samples were conducted to measure the efficiency of the filter cake removal and the retained permeability. Ethylene glycol mono butyl ether (EGMBE) was used as a mutual solvent and the solubility was higher compared to when the mutual solvent was not used in the washer formulation. A significant increase in calcium carbonate dissolution with time was observed for a duration of 24 h. The solubility was found to be proportional to the concentration of NCW with optimum results of 99% removal at a temperature of around 212 °F. At those conditions, no major corrosion problems were detected. Permeability of the core retained its pristine value after the treatment.

A Review of Recent Research on Contamination of Oil Well Cement with Oil-based Drilling Fluid and the Need of New and Accurate Correlations

Drilling fluids and oil well cement are important well barriers. Their compatibility affects the long-term integrity of the well. The mixing of drilling fluid with the oil well cement causes contamination of oil well cement. If the contamination is due to diesel/oil-based drilling fluid (OBF) it adversely affects the rheological and mechanical properties of oil well cement—in other words, the long-term integrity of the well. An initial study on OBF contamination of oil well cement was carried out two decades ago. In recent years, several research projects were carried out on the same topic to understand the reason for changes in the properties of oil well cement with OBF contamination. This literature review shows that using OBF eliminates several drilling problems, as the long-term integrity of the well depends on the amount of OBF contamination in the cement slurry. This paper compares the experiments performed, results and conclusions drawn from selected research studies on OBF contamination of oil well cement. Their shortcomings and a way forward are discussed in detail. A critical review of these research studies highlights the need for new and accurate correlations for OBF-contaminated oil well cement to predict the long-term integrity of wells.

Super-amphiphobic, strong self-cleaning and high-efficiency water-based drilling fluids

Abstract Based on the amphiphobic theory on underground rock surface, a super-amphiphobic agent is developed and evaluated which can form nano-micro papilla structure on rock, filter cake and metal surface, reduce surface free energy, prevent collapse, protect reservoir, lubricate and increase drilling speed. With this super-amphiphobic agent as the core agent, a super-amphiphobic, strong self-cleaning and high-performance water-based drilling fluid system has been developed by combining with other agents based on drilled formation, and compared with high-performance water-based drilling fluid and typical oil based drilling fluid commonly used in oilfields. The results show that the super-amphiphobic, strong self-cleaning and high-performance water-based drilling fluid has better rheology, and high temperature and high pressure filtration similar with that of oil-based drilling fluid, inhibiting and lubricating properties close to oil based drilling fluid. Besides, the super-amphiphobic system is non-toxic, safe and environmentally friendly. Field tests show this newly developed drilling fluid system can prevent wellbore collapse, reservoir damage and pipe-sticking, increase drilling speed and lower drilling cost, meeting the requirement of safe, high efficient, economic and environmentally friendly drilling. Compared with other drilling fluids, this new drilling fluid system can reduce downhole complexities by 82.9%, enhance the drilling speed by about 18.5%, lower drilling fluid cost by 39.3%, and increase the daily oil output by more than 1.5 times in the same block.

Optimization and Characterization of Highly Stable Nanoemulsion for Effective Oil-Based Drilling Fluid Removal

Summary In this study, an oil-in-water nanoemulsion for the effective removal of an oil-based drilling fluid has been developed by means of the phase-inversion concentration method. The influence of four factors on the droplet size and removal efficiency were tested, including the mass ratio of mixed surfactants (Rm), the surfactant/oil ratio (SOR), the mass concentration of cosurfactant (cco), and the salinity of the saline solution (cs). Considering the application environment of displacement spacers, the long-term stability and temperature resistance were investigated. The results showed that Rm and SOR had an obvious influence on the removal efficiency and droplet size of nanoemulsions. Because of the synergy effect of the surfactants, the nanoemulsion possesses remarkable storage stability and temperature resistance. Moreover, the removal mechanisms of the nanoemulsion were analyzed by the dynamic interfacial tension (IFT), dynamic wetting angle, and solubilization tests. The results indicated that the nanoemulsion could spread rapidly and thoroughly on the oil-wetting surfaces, and the nanoemulsion can contain more oil while the system is still stable, which is beneficial for the removal of an oil-based drilling fluid.

Experimental Determination of Drilling Fluid Thermal Parameters When Calculating APB

Thermal parameters of the drilling fluid affect the prediction of annular pressure buildup (APB) and play an important role in the type selection of casing. The high-temperature and high-pressure PVT tester is used to measure the volume increment of the salt water, water-base drilling fluid, and oil-base drilling fluid. According to the equation of state, the thermal expansion coefficient and compressibility coefficient of annular fluid at different temperatures and pressures are obtained. The results show that the expansion coefficient and compressibility coefficient of the salt water and oil-base fluid are directly proportional to temperature and inversely proportional to pressure; the expansion coefficient of the water-base fluid is directly proportional to temperature, but the compressibility coefficient is inversely proportional to temperature; the expansion coefficient of the water-base drilling fluid is inversely proportional to the density of the drilling fluid, while the density has little effect on the compressibility coefficient of the water-base drilling fluid; the expansion coefficient of the oil-base drilling fluid with oil-water ratio of 8:2 tends to be stable for temperatures over 50°C, and the thermal expansion coefficient of the oil-base drilling fluid with oil-water ratio of 7:3 is stable for temperatures over 75°C. Determination of the drilling fluid thermal parameters provides a theoretical basis for the APB prediction and the type selection of casing.

Clay minerals in drilling fluids: functions and challenges

AbstractThe addition of clay minerals in drilling fluids modifies the dispersion's viscosity. In this article, scientific advances related to the use of clays and clay minerals (bentonite, palygorskite, sepiolite and mixtures of clay minerals) in drilling fluids are summarized and discussed based on their specific structure, rheological properties, applications, prevailing challenges and future directions. The rheological properties of drilling fluids are affected by the temperature, type of electrolytes, pH and concentration of clay minerals. Bentonites are smectite-rich clays often used in drilling fluids, and their composition varies from deposit to deposit. Such variations significantly affect the behaviour of bentonite-based drilling fluids. Palygorskite is suitable for use in oil-based drilling fluids, but the gelation and gel structures of palygorskite-added drilling fluids have not received much attention. Sepiolite is often used in water-based drilling fluids as a rheological additive. Dispersions containing mixtures of clays including bentonite, kaolin, palygorskite and sepiolite are used in drilling fluids requiring specific features such as high-density drilling fluids or those used in impermeable slurry walls. In these cases, the surface chemistry–microstructure–property relationships of mixed-clay dispersions need to be understood fully. The prevailing challenges and future directions in drilling fluids research include safety, ‘green’ processes and high-temperature and high-pressure-resistant clay minerals.