In technical terminology, a slurry is a mixture of solid particles in a liquid that can be readily pumped. The following article describes the use of tribology in understanding slurry-related wear prevalent in drilling operations.
Non-productive time (NPT) is an acronym that may be of no significance to us but is of major importance in oil and gas industry. NPT is generally referred to as a loss of money, mind, and time. NPT is the time when drilling operations do not occur by one or more reasons such as pipe sticking, lost circulation, tool failure, weather, wellbore instability or because of drill pipe failure. Drilling operators emphasize on minimizing NPT and maximizing drilling time. Long duration’s of NPTSs can cost the company billions of dollars and would be loss for the country’s economy as NPTs lead to “lesser supply than demand” scenario, shortage of fuel can cause the fuel prices to rise .
One of the reasons for NPTs mentioned above is drill pipe failure. The drill pipe is a hollow, thick-walled steel pipe and is a major component of the drill string. A drilling string includes all tubular equipment such as drill pipes, drill collars, and other accessories. The drill pipe occupies 90-95% of the drill string’s length. A drill pipe is used to pump drilling fluid down the hole and back from the annulus. Depending on drilling depth and other factors, drilling operators use a variety of pipes of different shapes, sizes, materials, strengths, and weight. Pipe failure can occur because of twist off, fatigue, fracture or presence of corrosion inhibitors in drilling fluids such as oxygen, carbon dioxide, acidic and organic salts. Apart from these corrosion inhibitors, sometimes it has been seen that the composition of drilling fluid itself can cause severe wear of the drilling pipe and even to the drilling tool .
Application in drilling fluid
A drilling fluid refers to a liquid, gas, or gasified liquid used to aid the drilling process and is often termed as a slurry in the oil and gas industry. Its main functions include cooling and lubrication of drill bit and drill string, removal of excessive heat, transportation of rock cuttings from the wellbore to the surface, hole stabilization and enhancement in rate of penetration. The drilling fluid can be classified into pneumatic and liquid. Pneumatic drilling fluids generally covers dry gas, mist, foam, and gasified mud whereas liquid drilling fluids include, clear water, water-based and oil-based mud. Drilling mud generally comprises of a combination of water and clay or oil and clay or sometimes all three (water, oil, and clay). If the continuous phase is oil then the mixture is called oil-based mud and when water is in continuous phase it is called water-based mud .
The performance of these drilling muds has to be tested first before being introduced into an actual drill pipe. The tests can be done on universal tribometers or multi-purpose tribometers with any setups such as disc-on-disc or pin-on-plate. The choice of mud additives such as weighting agents (barrite, iron ores, lead sulfides) or mud thinners (phosphate, lignite), or mud thickeners (lime, cement) can also be made wisely by tribological tests. In the case of oil-based muds, rheological and tribological tests can be performed on mineral oil, diesel oil, synthetic oil before being selected as the main oil for oil-based mud .
Some of the research work which has been done related to the subject is discussed below.
In a research work done by H.R.M. Saffari and team, Nano borate particles of Al, Ti, Zn and Mg elements were synthesized as lubricant additives. Results showed that Nano-lubricant produced a protective film (tribo-film) on the contacting surfaces which significantly increased extreme pressure property and decreased the COF and wear. 
The effects of modified NATT (Nano-Attapulgite) drilling fluids on the tribological properties mainly the coefficient of friction (COF) between the drillstring and the wellbore were studied by J. Abdo. They concluded that NATT in drilling fluid significantly reduces the COF between the drillstring and the wellbore thus improves the axial force transfer considerably. 
Pixiang Lan and team also concluded that for drilling fluid (mud) all additives decreased the COF at 75 °C. At extreme temperatures, abrasive particles in the drilling fluid played a critical role. 
To better simulate the elevated-temperature environment in the borehole, Pixiang Lan and team proposed a method to perform tribological studies of drilling fluids at temperatures higher than 100°C by conducting experiments in a high-chamber-pressure environment, which can suppress the evaporation of the drilling fluid at high temperatures. A prototype additive (Additive A) reduced the coefficient of friction (COF) significantly by 44.8%, whereas a commercial additive (Additive B) caused only a slight reduction of the COF by 4%. 
WC-10Co4Cr surface coatings prepared by high-velocity oxygen-fuel (HVOF) spraying technology were widely used to improve the anti-corrosion and wear resistance properties of drilling tools. Xiao-yuCui and team in their research presented that the friction coefficient of the coatings decreased, and the wear volumes of the coatings increased with increasing corrodibility of the saturated saltwater drilling fluid. The wear mechanism of the coating corroded by saturated salt drilling fluid include spalling, drop pits of the WC particles, and adhesion. 
J.M.González and team studied the effects of a surfactant additive (SA) and its dissolution in diesel (SB), on the tribological and rheological properties of water-based drilling fluids (WBFs) formulated with two weighting materials (hematite and calcium carbonate). It was established that the evaluated surfactant additive can reduce significantly the CF and that SA formulation has a superior performance in CF reduction than SB. 
The addition of small concentrations of hydrocarbon additives to drilling fluid has shown significant positive effects on friction and wear properties of drilling fluid as per Mohammad Humood and team. The results showed that liquid friction reduction additives with long hydrocarbon tails and high metal affinity polar heads achieved better performance than additives that contained molybdenum dialkyldithiocarbamate. 
YongZheng and team investigated the tribological performance of potassium chloride polymer water-based muds (WBM) and artificial seawater (KCl brine) and comparing them with a reference OBM. Results showed 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 in fracturing fluid
Hydraulic fracturing is a process where large quantities of water, sand, and chemicals are pumped underground to break apart the rock and release the gas. The process is widely used in horizontal drilling. Sand and water constitute about 99.5% of the total composition of fracturing fluids. Additives cover the rest 0.5% and are generally used for lubrication and to prevent the formation of bacteria in cracks. Commonly used additives are acids, chlorides, ethylene glycol, and sodium carbomate. The fracturing liquid is transferred to the rocks via coiled tubing. Coiled tubing describes continuous lengths of small-diameter steel pipes used for well interventions and drilling operations .
The pumping of fracturing fluid through coiled tubing can cause considerable wear of tubing. High rates of pumping fracturing fluids often cause non-uniform erosion of tubing walls . The following problem can be solved as well by applying tribological knowledge. Erosive tests such as ASTM G65, dry sand rubber wheel abrasion test can be used to first check the effect of a particular composition of this fracturing fluid on the pipe material. Since sand is a major part of the fracturing fluids, the test will also allow experimenting with different types of sand and their impact on pipe surfaces at different impact angles. Studies show that at higher impact angles plastic deformation mechanism was more dominating whereas at lower impact angles plowing/cutting mechanism prevailed . Some of the work done on slurry erosion reveals other modes of wear such as combinations of erosion and corrosion which are more severe than individual erosion.
In one of the research work, Chao Zheng and team investigated the wear behaviors of coatings under dry and W/O fracturing fluid conditions. Results showed that compared to uncoated specimens, coated ones had a better wear resistance. 
New tribometers or modified versions of present tribometers can be utilized to match the actual flow rates, temperature, and pressure of these slurries. Computational tribological studies on slurries can prove to be a less expensive choice. Lubricant analysis tests such as ICP and FTIR can also reveal a lot of information about these slurries.
- Rig NPT: the ugly truth
- Basic Understanding about Drill Pipe, www.drillingformulas.com.
- Failure analysis and solution studies on drill pipe thread gluing at the exit side of horizontal directional drilling DOI: 10.1016/j.engfailanal.2013.05.017
- Functions of drilling fluid
- Slurry Tribology in Energy Production
- Mud engineering
- Tribological properties of water-based drilling fluids with borate nanoparticles as lubricant additives DOI:10.1016/j.petrol.2018.07.049
- Nano-attapulgite for improved tribological properties of drilling fluids DOI:10.1002/sia.5472
- The effect of lubricant additives on the tribological performance of oil and gas drilling applications up to 200 °C DOI:10.1016/j.triboint.2019.105896
- Elevated-Temperature and -Pressure Tribology of Drilling Fluids Used in Oil and Gas Extended-Reach-Drilling Applications DOI:10.2118/191380-PA
- Influence of the corrosion of saturated saltwater drilling fluid on the tribological behavior of HVOF WC-10Co4Cr coatings DOI:10.1016/j.engfailanal.2016.11.011
- Effects of interactions between solids and surfactants on the tribological properties of water-based drilling fluids DOI:10.1016/j.colsurfa.2011.04.034
- Influence of additives on the friction and wear reduction of oil-based drilling fluid DOI:10.1016/j.wear.2019.01.028
- Enhancements in the tribological performance of environmentally friendly water-based drilling fluids using additives DOI:10.1016/j.apsusc.2020.146822
- What Is Hydraulic Fracturing?
- Coiled Tubing
- Coiled Tubing Erosion During Hydraulic Fracturing Slurry Flow. DOI:10.2118/89479-MS
- Slurry Erosion Characteristics and Erosion Mechanisms of Stainless Steel DOI:10.1016/j.triboint.2014.05.014
- Wear behavior of HVOF sprayed WC coating under water-in-oil fracturing fluid condition DOI:10.1016/j.triboint.2017.05.002
- Cover image used is from pixabay “Free for commercial use No attribution required”
Harshvardhan Singh works as a Senior Service Engineer at a mining firm in India. He is currently working into oil analysis field. Has worked in the filed of tribology and lubrication and loves to write about the same.