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 a three letter acronym which 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 time when drilling operations do not occur by one or more reasons such as pipe sticking, lost circulation, tool failure, weather, well bore 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 of NPTs mentioned above is drill pipe failure. The drill pipe is a hollow, thick walled steel pipe and is a major component of drill string. A drilling string includes all tubular equipment’s such as drill pipe, drill collars and other accessories. The drill pipe occupies 90-95% of drill string’s length. A drill pipe is used to pump drilling fluid down the hole and back from annulus. Depending on drilling depth and other factors, drilling operators use a variety of pipes of different shapes, size, material, strength 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 .
A drilling fluid refers to a liquid, gas or gasified liquid used to aid the drilling process and is often termed as slurry in 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 well bore to the surface, hole stabilization and enhancement in rate of penetration. 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 combination of water and clay or oil and clay or sometimes all the three (water, oil and clay). If the continuous phase is oil than 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 sulphides) or mud thinners (phosphate, lignite) or mud thickers (lime, cement) can also be made wisely by tribological tests. In 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 .
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 constitutes to 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 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 to experiment with different types of sand and their impact on pipe surface 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 is more severe than individual erosion .
New tribometers or modified versions of present tribometers can be utilized to match actual flow rates, temperature and pressure of these slurrys. 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 slurrys.
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- Miller, John E., and Frederick Schmidt. “Description” Slurry Erosion: Uses, Applications, and Test Methods: A Symposium. Philadelphia, PA: American Society for Testing and Materials, 1987. Web http://www.asminternational.org/materials-resources/results/-/journal_content/56/10192/ASMHBA0002268/PUBLICATION
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