How to evaluate the applicability of nickel alloy seamless pipes in deep well mining?

As an important resource development method, deep well mining faces extremely complex working conditions such as high temperature, high pressure, and high corrosion. Nickel alloy seamless pipes have shown potential application value in the field of deep well mining due to their excellent mechanical properties, corrosion resistance, and high temperature resistance. However, to ensure its safe, reliable, and efficient operation in deep well mining, its applicability must be comprehensively and systematically evaluated.

1. Analysis of deep well mining conditions

l Temperature environment

During deep well mining, the formation temperature will increase with the increase of depth. Generally speaking, the formation temperature gradient increases by about 3°C per 100 meters, but in some special geological areas, such as geothermally active areas, the temperature gradient may be higher. High temperature environment will have a significant impact on the mechanical properties of nickel alloy seamless pipes, which may cause the strength and hardness of the material to decrease, the plasticity and toughness to change, and even cause problems such as creep and fatigue fracture. Therefore, accurately understanding the temperature distribution and change law in deep well mining is an important prerequisite for evaluating the applicability of nickel alloy seamless pipes.

l Pressure environment

In deep well mining, nickel alloy seamless pipes are subjected to pressure from formation fluids, overburden pressure, and operating pressure during mining. With the increase of mining depth, the formation pressure increases linearly, and the high-pressure environment may cause the pipe to yield, deform, or even rupture. In addition, during different operation stages such as drilling, completion, and oil production, the pressure will also change dynamically, which puts higher requirements on the compressive and fatigue resistance of the pipe.

l Corrosive environment

The corrosion environment in deep well mining is relatively complex, mainly including formation water corrosion, oil and gas corrosion and microbial corrosion. Formation water usually contains a variety of electrolytes, such as sodium chloride, magnesium chloride, calcium carbonate, etc. These electrolytes will react electrochemically with nickel alloy seamless pipes, leading to corrosion. Oil and gas may contain corrosive gases such as hydrogen sulfide (H2S) and carbon dioxide (CO2), which will form an acidic corrosive environment under certain conditions, increasing the corrosion rate of pipes. In addition, the metabolic activities of underground microorganisms may also produce corrosive substances, causing corrosion damage to pipes.

l Mechanical load environment

In the process of deep well mining, nickel alloy seamless pipes are subjected to various mechanical loads such as tension, compression, bending, and torsion. During the drilling process, the pipe is subjected to the torque of the drill bit and the tensile load of the drill string; during the oil production process, the pipe is subjected to bending loads due to the inclination of the oil well and the deadweight of the casing; during the operation of downhole tools, such as the setting and unsealing of the packer, the pipe is subjected to cyclic loads of compression and tension. The combined effect of these mechanical loads may cause fatigue failure of the pipe.

2. Material performance evaluation of nickel alloy seamless pipe

l Mechanical properties

(I) Tensile properties​

Tensile properties are important indicators for measuring the mechanical properties of nickel alloy seamless pipes, including yield strength, tensile strength, elongation and cross-sectional shrinkage. Yield strength and tensile strength reflect the material's ability to resist plastic deformation and fracture, while elongation and cross-sectional shrinkage reflect the material's plastic deformation ability. In the high-pressure and high-stress environment of deep-well mining, nickel alloy seamless pipes need to have sufficiently high yield strength and tensile strength to prevent plastic deformation and fracture; at the same time, they also need to have good plasticity and toughness to adapt to the complex mechanical load conditions underground.

(II) Compression performance

Compression resistance refers to the ability of nickel alloy seamless pipes to resist deformation and damage under axial pressure. In deep well mining, pipes may be subject to the pressure of overburden and downhole fluid, so they need to have good compression resistance. Compression resistance is usually measured by short column compression test, in which the compressive strength, elastic modulus and Poisson's ratio of the pipe need to be measured.

( III ) Anti-fatigue performance

Fatigue resistance refers to the ability of nickel alloy seamless pipes to resist fatigue fracture under cyclic loads. In the process of deep well mining, the mechanical loads on the pipes are often dynamically changing, such as the cyclic changes of torque and tensile loads during drilling, pressure fluctuations during oil production, etc. These cyclic loads can easily lead to fatigue failure of the pipes. Fatigue resistance is usually evaluated through fatigue tests, which require the determination of parameters such as the fatigue limit, fatigue life and crack growth rate of the pipes.

( IV ) Hardness

Hardness is an indicator of the ability of a material surface to resist local plastic deformation, indentation or scratching. The hardness of nickel alloy seamless pipes is closely related to their strength, wear resistance and corrosion resistance. Higher hardness can improve the wear resistance and scratch resistance of the pipe and reduce damage during underground transportation and installation. Commonly used hardness test methods include Brinell hardness, Rockwell hardness and Vickers hardness.

l Corrosion resistance

(I) Uniform corrosion​

Uniform corrosion refers to the corrosion phenomenon in which the corrosive medium is evenly distributed on the surface of the pipe, resulting in a gradual decrease in the overall thickness of the pipe. The weight loss method is usually used to evaluate the uniform corrosion performance of nickel alloy seamless pipes, that is, the sample is exposed to a specific corrosive medium, and the mass loss of the sample is measured after a certain period of time, so as to calculate the corrosion rate. In addition, electrochemical methods such as polarization curve test and AC impedance test can also be used to study the corrosion behavior and corrosion mechanism of materials in corrosive media.

(II) Local corrosion

Localized corrosion refers to corrosion concentrated in local areas of the pipe surface, such as pitting, crevice corrosion and intergranular corrosion. Localized corrosion is often more harmful than uniform corrosion because it can easily cause leakage or rupture of the pipe before the overall corrosion damage. Pitting is a common form of localized corrosion, which usually occurs in corrosive media containing chloride ions. The pitting performance can be evaluated by pitting potential test and pitting depth measurement. Crevice corrosion usually occurs in the gap between the pipe and other components, such as flange connection gaps, threaded connection gaps, etc. The crevice corrosion test can be used to evaluate the crevice corrosion performance. By applying a certain gap width and corrosive medium to the sample, the corrosion of the sample is observed. Intergranular corrosion refers to the corrosion phenomenon along the grain boundary, which usually occurs in materials such as stainless steel and nickel-based alloys. Intergranular corrosion performance can be evaluated by intergranular corrosion tests, such as oxalic acid etching test, sulfuric acid-copper sulfate test, etc.

( III ) Stress corrosion cracking

Stress corrosion cracking refers to the brittle fracture phenomenon of materials under the combined action of tensile stress and corrosive media. In deep well mining, the tensile stress and corrosive media to which nickel alloy seamless pipes are subjected can easily cause stress corrosion cracking. The evaluation of stress corrosion cracking performance usually adopts test methods such as constant load method and slow strain rate method. By applying a certain tensile stress to the sample and exposing it to a specific corrosive medium, the cracking and fracture time of the sample are observed.

l High temperature resistance

(I) High temperature mechanical properties

Under high temperature environment, the mechanical properties of nickel alloy seamless pipes will change significantly. As the temperature increases, the yield strength and tensile strength of the material will gradually decrease, while the plasticity and toughness will increase. Therefore, it is necessary to evaluate the mechanical properties of nickel alloy seamless pipes at different temperatures to determine their applicability in deep well high temperature environments. High temperature mechanical property tests are usually carried out on high temperature tensile testing machines, and the test temperature range can be determined according to the actual temperature conditions of deep well mining.

(II) High temperature antioxidant performance

High temperature oxidation resistance refers to the ability of a material to resist oxidation corrosion in a high temperature environment. In deep well mining, if there is a high temperature oxidation environment, such as formation fluids containing oxygen or high temperature steam, nickel alloy seamless pipes need to have good high temperature oxidation resistance. High temperature oxidation tests can be used to evaluate high temperature oxidation resistance. The sample is exposed to a specific high temperature oxidation environment for a certain period of time, and the oxidation weight gain or oxide film thickness of the sample is measured to evaluate the oxidation resistance of the material.

( III ) High temperature creep performance

Creep refers to the phenomenon that a material slowly undergoes plastic deformation over time under constant stress and high temperature. Under the high temperature and high pressure environment of deep well mining, nickel alloy seamless pipes may undergo creep deformation, resulting in a decrease in the dimensional accuracy and mechanical properties of the pipes. Creep tests are usually used to evaluate high temperature creep performance. By applying a constant tensile stress to the sample and keeping it at a specific high temperature for a certain period of time, the creep deformation and creep rupture time of the sample are measured, thereby determining the creep limit and creep life of the material.

l Processing performance

(I) Cold working performance

Cold working performance refers to the ability of nickel alloy seamless pipes to undergo cold deformation processing at room temperature, such as bending, rolling, expanding, shrinking, etc. During the installation and connection of pipes in deep well mining, it may be necessary to perform cold working operations on the pipes, so it is necessary to evaluate their cold working performance. Cold working performance is usually evaluated through methods such as bending tests, expanding tests, and shrinking tests. During the tests, it is necessary to observe whether the pipes have defects such as cracks and fractures.

(II) Hot working performance

Hot working properties refer to the ability of nickel alloy seamless pipes to undergo hot deformation processing at high temperatures, such as forging, rolling, extrusion, etc. Hot working technology has an important influence on the structure and performance of the pipe, so its hot working properties need to be evaluated. Hot working properties are usually evaluated by methods such as hot compression test, hot tensile test and hot torsion test. The test requires studying the deformation behavior and organizational evolution of the material at different temperatures and strain rates.

( III ) Welding performance

Welding performance refers to the ability of nickel alloy seamless pipes to be connected into an integral structure by welding. In deep well mining, pipes are usually connected by welding, so welding performance is an important indicator for evaluating their applicability. Welding performance evaluation includes welding process evaluation and welding joint performance evaluation. Welding process evaluation mainly examines whether welding defects such as cracks, pores, and slag inclusions are prone to occur during welding; welding joint performance evaluation mainly includes tests on the mechanical properties, corrosion resistance, and high temperature resistance of welding joints.

3. Structural design evaluation of nickel alloy seamless pipe

l Pipe diameter and wall thickness design

(I) Pipe diameter selection

The choice of pipe diameter needs to be determined based on the production requirements of deep well mining, fluid delivery efficiency, and underground space limitations. A larger pipe diameter can improve fluid delivery efficiency, but it will increase the weight and cost of the pipe, and may also be limited by underground space; a smaller pipe diameter may have a lower cost, but it may increase fluid flow resistance and affect mining efficiency. Therefore, it is necessary to optimize the balance between production requirements, fluid delivery efficiency, and cost to select the appropriate pipe diameter.

(II) Wall thickness design

Wall thickness design is a key link in the structural design of nickel alloy seamless pipes. It is necessary to consider factors such as the various pressures, mechanical loads and corrosive environments that the pipes are subjected to in deep well mining. If the wall thickness is too thin, the pipe may not be able to withstand the pressure and mechanical loads underground, and it is easy to deform and break; if the wall thickness is too thick, it will increase the weight and cost of the pipe, and may also affect the processing performance and installation efficiency of the pipe. Wall thickness design usually adopts the strength verification method to calculate the required minimum wall thickness based on the maximum pressure, mechanical load and mechanical properties of the material that the pipe is subjected to, and consider a certain safety factor.

l Connection structure design

(I) Threaded connection​

Threaded connection is one of the commonly used connection methods for nickel alloy seamless pipes, which has the advantages of convenient connection and easy disassembly. The design of threaded connection needs to consider factors such as thread type, pitch, tooth angle, thread length and matching accuracy. Common thread types include API thread, trapezoidal thread, etc. The strength and sealing of threaded connection are key indicators for evaluating its applicability, which need to be verified by methods such as tensile test, compression test and sealing test of threaded connection.

(II) Flange connection

Flange connection is a rigid connection method with the advantages of high connection strength and good sealing. It is suitable for high pressure, high temperature and highly corrosive working conditions. The design of flange connection needs to consider factors such as flange type, specification, material and selection of sealing gasket. Common flange types include flat welding flange, butt welding flange, socket welding flange, etc. The installation and disassembly of flange connection is relatively complicated, requiring high installation accuracy and construction cost.

( III ) Welding connection

Welding connection is a way of directly connecting nickel alloy seamless pipes into a whole by welding. It has the advantages of high connection strength, good sealing and low cost. The design of welding connection needs to consider factors such as the selection of welding process, optimization of welding parameters and design of welding joints. The disadvantage of welding connection is that welding defects may occur during the welding process, affecting the performance and service life of the pipe, so strict quality inspection of welding joints is required.

l Anti-buckling design

In deep well mining, nickel alloy seamless pipes may be subjected to axial compression loads. When the compression load exceeds a certain critical value, the pipe will buckle and deform, resulting in structural instability. Therefore, anti-buckling design is required to ensure the stability of the pipe under compression load. Anti-buckling design usually uses the Euler formula to calculate the critical buckling load of the pipe, and selects the appropriate pipe size and material properties based on the calculation results to improve the anti-buckling ability of the pipe. In addition, the anti-buckling performance of the pipe can be improved by increasing the wall thickness of the pipe and using a reinforcing rib structure.

l Fatigue life design

As mentioned above, the mechanical loads on nickel alloy seamless pipes in deep well mining often change cyclically, which can easily lead to fatigue failure. Therefore, fatigue life design is needed to predict the service life of the pipe under cyclic loads. Fatigue life design usually adopts fatigue cumulative damage theory to calculate the fatigue life of the pipe based on the cyclic load spectrum and fatigue performance parameters of the material. During the design process, it is necessary to consider the effects of load size, frequency, number of cycles and environmental factors on fatigue life, and take corresponding measures to improve the fatigue life of the pipe, such as optimizing the structural design of the pipe, reducing stress concentration, and performing surface treatment.

4. Manufacturing process evaluation of nickel alloy seamless pipe

l Smelting process

The smelting process of nickel alloy has a crucial influence on its chemical composition and microstructure properties. Common nickel alloy smelting processes include electric arc furnace smelting, vacuum induction furnace smelting and electroslag remelting. Electric arc furnace smelting has the advantages of high production efficiency and low cost, but impurities are easily introduced during the smelting process, affecting the purity and performance of the alloy; vacuum induction furnace smelting can melt and refine the alloy under a vacuum environment, effectively remove gases and impurities in the alloy, and improve the purity and performance of the alloy; electroslag remelting is a process for further purifying and refining the alloy, which can improve the crystal structure of the alloy and improve the density and mechanical properties of the alloy. Therefore, it is necessary to evaluate whether the smelting process of nickel alloy seamless pipes can ensure the uniform chemical composition, high purity and good microstructure and performance of the alloy.

l Rolling process

The rolling process is one of the key processes for producing nickel alloy seamless pipes, including perforation, tube rolling and sizing. The perforation process is to perforate a solid round steel billet into a hollow rough tube; the tube rolling process is to roll the rough tube into a steel tube of the required size and shape through a rolling mill; the sizing process is to perform finish rolling on the steel tube to improve the dimensional accuracy and surface quality of the steel tube. The parameters of the rolling process, such as rolling temperature, rolling speed, rolling pressure and deformation, will affect the structure and properties of the steel tube. For example, too high a rolling temperature will lead to coarse grains and reduce the mechanical properties of the steel tube; too fast a rolling speed will cause cracks and defects on the surface of the steel tube. Therefore, it is necessary to evaluate whether the rolling process is reasonable and whether it can ensure that the dimensional accuracy, surface quality and organizational properties of the steel tube meet the requirements.

l Heat treatment process

Heat treatment process is an important means to improve the structure and performance of nickel alloy seamless pipes. Through heat treatment, the residual stress generated by the steel pipe during rolling can be eliminated, the grains can be refined, and the mechanical properties and corrosion resistance can be improved. Common heat treatment processes include annealing, normalizing, quenching and tempering. The annealing process can reduce the hardness of the steel pipe and improve the plasticity and toughness; the normalizing process can refine the grains and improve the uniformity of the structure; the quenching and tempering process can improve the strength and hardness of the steel pipe while maintaining a certain plasticity and toughness. The parameters of the heat treatment process, such as heating temperature, holding time and cooling rate, need to be reasonably selected and optimized according to the composition and performance requirements of the alloy. Therefore, it is necessary to evaluate whether the heat treatment process can effectively improve the structure and performance of the steel pipe to meet the working conditions of deep well mining.

l Surface treatment process

The surface treatment process of nickel alloy seamless pipe mainly includes pickling, passivation, plating and coating, etc. Its purpose is to remove the oxide scale, oil stains and impurities on the surface of the steel pipe and improve the surface quality and corrosion resistance of the steel pipe. Pickling can remove the oxide scale and rust on the surface of the steel pipe; passivation can form a dense oxide film on the surface of the steel pipe to improve the corrosion resistance of the steel pipe; plating and coating can form a metal or non-metal protective layer on the surface of the steel pipe, further improving the corrosion resistance and wear resistance of the steel pipe. The quality of the surface treatment process directly affects the service life and reliability of the steel pipe, so it is necessary to evaluate whether the surface treatment process can effectively improve the surface quality and corrosion resistance of the steel pipe.