Special aluminum alloys appear to be promising materials for manufacture of high-strength light-alloy drill pipes (HSLADP) that can be used in areas with a severe climate and challenging geology. The effect of using light-alloy drill pipes (LADP) depends directly on the properties of the aluminum alloys from which such pipes are made. As the wells become deeper and horizontal wellbores get longer, use of LADPs becomes more relevant. Since light-alloy pipes are 2.8 times softer than steel pipes, LADPs offer the same performance as steel drill pipes of the lowest strength grade even in the case of rotary drilling. The materials from which such pipes are made have a number of unique advantages: extra light weight in the drill mud, allowing the coefficient of sliding friction between the pipe surface and the borehole wall to be reduced; high corrosion resistance in aggressive media with A high concentration of hydrogen sulfide and carbon dioxide; and high magnetic inductive capacity that allows LADPs to be used as a housing for MWD (measurement while drilling) and LWD (logging while drilling) telemetry systems during well-drilling operations. This study suggests methods for industrial production of submicrocrystalline (SMC) structure in aluminum alloys with the help of severe plastic deformation. Through the example of model aluminum-lithium alloys 1420 (Al-Mg-Li-Zr) and 1460 (Al-Сu-Li-Zr), the researchers demonstrate that SMC structure helps significantly increase resistance to wear and reduce the rate of corrosion depending on the pH value. The research team also states that severe plastic deformation methods may be used to develop highly promising technologies for manufacture of high-strength LADPs with advanced strain-stress properties for use during operations in the Arctic.

light-alloy drill pipes, tribotechnical tests, corrosion of aluminum alloys, Arctic shelf, horizontal drilling

Current evolution of drilling technologies and aggravation of geological and climatic conditions for drilling operations caused by the development of hydrocarbon deposits in the Arctic shelf leads to an increase in operating and maintenance costs associated with well-site construction. Reliable performance of drilling equipment and tools, specifically drill pipes, is an essential aspect that well drilling and operating performance indicators depend on. Drill pipes play a key role in the stress behaviour of rock cutting tools. The current trend of increasing the length of wells also leads to a higher probability of complications and accidents, which increases the overall cost of construction. Reliable performance of drill pipes depends on their proper use during drilling and transportation, which is achieved by timely quality control and withdrawal of deficient pipes from service. One effective way of improving performance of drill pipes, including light-alloy drill pipes (LADPs), involves development of advanced technologies for production of improved structure of aluminum alloys used to manufacture LADPs, as well as modification of drill fluids with special doping agents for the entire drilling cycle.

Implementation and improvement of the techniques used for production of fine-grain materials of such alloys, as well as assessment of the state of light-alloy drill pipes by simulating their performance in the well, is essential for higher credibility and better forecasting of their performance under real-life conditions and for meaningful estimation and planning of construction of long horizontal wells.

Real-life experience shows that such wells cannot be drilled without using high-strength light-alloy drill pipes (HSLADP) manufactured from special aluminum alloys (Fain 1990, Basovich et al. 2003, Basovich et al. 2015, Shammazov et al. 2000). Pipe strips are manufactured by extrusion from the Al-Cu-Mg D16T alloy, with subsequent hardening and natural ageing (Т). Where increased strength and corrosion resistance requirements apply, the pipes are manufactured from the Al-Zn-Mg 1953Т1 alloy subject to hardening and artificial ageing (Т1). Based on specific requirements, pipe strips are manufactured from the АК4-1Т1 alloy.

Relying on the existing classification, all pipe materials can be formally divided into three classes: ultra-fine grain (UFG) with grain size of 1 to10 um, submicrocrystalline (SMC) with grain size of 0.1 to 1 um, and nanocrystalline (NC) with grain size below 100 nm. This classification is justified because the strain-stress behaviour of the UFG, SMC and NC alloys varies markedly. For instance, alloys with UFG structure demonstrate structural superplasticity in certain sections of the temperature-strain rate curve, while reduction of the grain size to nanocrystalline values in composite and mechanical alloys leads to the effect of high-strain rate superplasticity (Valiev et al. 1988, Valiev and Tsenev 1990, Tsenev 1996, Valiev et al. 1997). It has been proven that SMC and NC structures can be used to manufacture high-strength materials (Furukawa et al. 1996, Furukawa et al. 1997). Even such structurally insensitive parameters as Curie and Debye temperatures, modulus of elasticity, saturation magnetic moment, etc. are prone to change in these materials. These findings indicate that scientists can develop new advanced technologies for production of aluminum alloys used in the manufacture of HSLADPs with improved mechanical properties that will demonstrate better performance in longer horizontal leads from the vertical borehole.

This study uses an example of model commercial aluminum-lithium alloys to demonstrate the techniques of producing pipe strips with SMC structure. Experimental results obtained during the research are used to suggest that HSLADP manufacturers jointly develop advanced technologies for the production of such drill pipes with improved mechanical properties and corrosion resistance for use in different geological settings, including in the Arctic.

The material used for the research was represented by model commercial aluminum-lithium alloys 1420 (Al-5.5% Mg-2.2% Li -0.12% Zr) and 1460 (Al-2.5% Сu-2.2% Li -0.12% Zr). Originally, after casting and prior to severe plastic deformation (SPD), the alloy had a coarse-grain structure. Mean grain size in the alloy was 5 mm (Fig. 4a)

Submicrocrystalline structure was obtained by equal channel angular pressing (ECAP) method (Wang et al. 2003, Kaibyshev et al. 1984). The diagram of the pressing process based on this method is provided in Figure 1. The method relies on shear deformation, with severe plastic deformation along the surface of channels intersection. To obtain the SMC structures, the ECAP process must be repeated eight times on one sample.

Fig. 1.

Diagram of equal channel angular pressing: 1) piston, 2) pipe strip

Transmission electron microscope (TEM) JEМ-2000 EX was used for optical microscopic evaluation of samples produced with the help of this technique.

The study demonstrated that the severe plastic deformation technique (ECAP method) can be used not only to make the structure of alloys significantly (by several orders of magnitude) more compact, but also to obtain grains with different states of grain boundaries. Detailed studies of the structural changes that occur during the severe plastic deformation process showed that fragmentation of the alloy’s structure takes place as a result of formation of an equilibrium recrystallised structure.

Preliminary tribotechnical and corrosion tests proved a substantial increase in wear and corrosion resistance of the material with the finer structure. It is important to note that resistance to wear and corrosion also depends markedly on the state of the grain boundaries. Fig. 3 shows that the alloy’s wear immediately after severe plastic deformation is at its minimum. A metallographic examination of the alloy’s structure indicates that this is caused by high inner stresses, non-equilibrium of the grain boundaries, and the size of the grains. Fig. 4 demonstrates that the alloy becomes more resistant to corrosion if appropriate material treatment methods are used to make the material suitable for service in aggressive media with a high pH value. Numerous studies have proved that hardness, breaking strength, hot strength, microstructure and corrosion resistance are the key criteria for the material’s wear resistance (Makarov and Korshunov 2004, Luzhnov et al. 2016).

Thus, the study used model alloys 1420 and 1460 to demonstrate the prospects for controlling the structure of materials with the help of one of the methods of severe plastic deformation. Combining SPD methods with subsequent heat treatment, we can formulate technologies for production of light-alloy drill string elements that can be successfully employed in a variety of challenging climatic and geological conditions.

Today, aluminum alloys D16T, 1953Т1 and АК4-1Т1 are widely used in the production of light-alloy pipes. The mechanical properties of these materials are described in Table 1. We believe that different combinations of SPD and heat treatment techniques can help increase corrosion resistance of aluminum alloys by a factor of five and wear resistance by 30%.

To summarise the above, the relevance of the matter described in this study is growing. The authors made an assumption concerning the prospects for development of new domestic substitutes of technologies for production of high reliability light-alloy drill pipes that can be used in different geological settings, including in the Arctic.