Wed, 28 June, 2023
A paper co-authored by number of TWI experts working on the H2020-funded Geo-Drill project was recently published by the Journal of Thermal Spray Technology.
The paper, ‘Feasibility Study of High-Velocity Oxy-Fuel (HVOF) Sprayed Cermet and Alloy Coatings for Geothermal Applications,’ investigated the use of high-velocity oxygen fuel (HVOF) spraying for cermet and alloy coatings.
Effective coating and drilling technologies can greatly reduce the costs associated with geothermal drilling operations, where the harsh environments reduce the lifetimes of the components used to access geothermal energy.
We took some time to find out more…
Can you start by telling us about the importance of reducing the cost of geothermal operations?
There has been an increasing challenge worldwide to meet the growing global demand for affordable, reliable and sustainable energy. Geothermal energy has been taking a more and more important role in the renewable energy mix, offering significant advantages such as a low carbon footprint, independent of variable climatic conditions compared to some other renewables (notably wind and solar), and the capability of providing more affordable energy. It is also one of the least utilised renewable energy sources due to high investment costs and the long development cycle.
A major factor of the cost in deep geothermal projects relates to the drilling processes, where costs increase in proportion to the drilling depth, tripping times, harsh environments (temperature, pressure and geothermal fluid composition), etc. It is estimated that the drilling costs contribute between 30-70% of the overall expenditure of a deep geothermal project. Therefore, the Geo-Drill project focuses on the development of ‘holistic’ drilling technologies that have the potential to drastically reduce the cost through DTH hammer, to advance drill monitoring through low-cost and robust 3D printed sensors, and to improve component life through advanced materials and coatings.
Why are effective coatings so important for this work – and why were alloy and cermet coatings chosen for this purpose?
The drilling of deep geothermal wells involves the breakage of rock into random-sized chippings, which are required to be lifted from the base of the well to the surface. This is generally achieved with a fluid (drill mud), which is alkaline and lifts the cuttings through a combination of viscosity and velocity. The flow in the annulus of a well bore is often turbulent, which would lead to erosion-corrosion of in-hole components through mechanical wear and chemical corrosion of fluids (both formation and introduced muds). Such effects can cause low mechanical efficiency, short component lifetime, and affect the stability and safe operation of the drilling system. Therefore, the application of surface coatings that can combat such effects is highly desirable.
Cermet coatings can provide good protection to drilling tools where high surface hardness and resistance to erosion/wear are required at elevated temperatures. The hard carbide particles in the coating provide hardness and wear resistance while its metal binders give necessary coating toughness. Some alloy coatings are being considered as alternatives of more cost-efficient systems with comparable resistance to corrosion environments.
HVOF spraying has been chosen as the method for applying these coatings – why was this and what does it offer?
High velocity oxy fuel (HVOF) spraying is widely used to deposit cermet, metal and alloy coatings with improved bond strength and higher density compared with some other thermal spray techniques. Characteristic features of HVOF sprayed coatings include their low level of porosity usually 1-2vol%, low oxide content in the range of 1-2wt%, and high bond strength of up to 150 MPa. This allows preparation of cost effective, high performance parts made from low cost materials with enhanced surface. High spray velocities offered by the HVOF process means carbide phases are less likely to degrade and alloys have a reduced level of oxidation during the deposition process.
- Greater particle impact velocities - higher density (lower porosity) coatings
- Harder, tougher coatings - better wear resistance
- Higher bond strength to the underlying substrate and improved cohesive strength within the coating
- Less in-flight exposure time (for metallic coatings) - lower oxide content
- Less residual stresses - thicker coatings
- Smaller powder sizes - smoother as-sprayed surface
You can find out more about HVOF coatings in our FAQ.
How did the coatings and the spray technology match up against existing materials and methods?
Low alloy steels and stainless steels are mostly used for manufacture of DTH bits. Studies carried out in Geo-Drill indicate that the application of cermet and alloy coatings through spray technology could highly improve wear and corrosion resistance of such components in simulated geothermal environments. Compared with benchmarking steel that has a hardness of 341HV0.3, cermet coatings offer hardness up to 1218HV0.3, and alloy coatings up to 827HV0.3. The combined erosion-corrosion resistance of most HVOF coatings (24hrs, 120rpm, 15wt.% solid load) developed in Geo-Drill was significantly improved compared with low alloy steels.
What is next for your work in geothermal?
Geothermal application is complex and can be very aggressive. In Geo-Drill, a wide range of materials/coatings have been studied and various types of corrosion and tribology testing were carried out to evaluate their performance in geothermal drilling environment. A few coating types have exhibited good corrosion and wear resistance compared with currently used hammer materials. However, some drilling components facing erosion-corrosion failures have complex geometries and the coating deposition process would need to be further developed to cope with various angles to obtain well-adhered uniform coatings. Material performance for different service environments is also worth further exploration - such as fluid type, chemical constituents, pressure, etc.
TWI is also currently undertaking a Joint Industry Project exploring the use of new coating technologies to generate effective wear and corrosion protection. This project aims to benchmark the performance of EHLA (extreme high speed laser application) and HVAF (high velocity air fuel) sprayed coatings against electroplated and HVOF alternatives. Both techniques show exciting potential to provide step changes in both performance and cost.
You can find out more about this coating technologies joint industry project here.
The Geo-Drill project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 815319