At present, a significant number of well pumps in domestic oilfields use rod pumps, with most being plunger pumps. Conventional plunger pumps have become one of the primary methods for oil recovery. However, as many domestic oilfields have entered a phase of high water cut, along with factors such as high sand content, high-temperature steam-based heavy oil extraction, polymer injection, and exposure to corrosive environments, the wear and corrosion of pump barrels and plungers have become increasingly severe. As a result, the average maintenance cycle for these pumps is getting shorter, with some requiring servicing in less than 30 days. This significantly affects the overall economic efficiency of oilfield operations.
Numerous research institutions around the world have invested substantial effort into studying pump failure causes and protective measures. One of the most effective strategies is optimizing the materials and surface treatments of the pump barrel and plunger. The friction pair between the cylinder and plunger, along with their working environment, forms a tribological system. The total wear and corrosion in this system depend on the properties of the materials used, their relative motion, and operating conditions. Ultimately, the choice of materials and surface treatment processes plays a decisive role in determining the level of wear within the tribological system.
The goal of selecting and optimizing the material and surface treatment process for the pump barrel and plunger is to minimize corrosion and wear. However, due to the complexity of multiple influencing factors, there is still no comprehensive theoretical framework for material selection and matching. Currently, R&D teams focus on testing various combinations of materials and surface treatments under controlled lab conditions to determine which pairs perform best under specific scenarios.
In one test, different materials were evaluated: 45 steel with electroless nickel-phosphorus coating, 303 stainless steel with chrome plating, and 4%-6% chromium steel with nitriding. For plungers, options included carbon steel with chrome plating, laser-treated carbon steel, navy brass with electroless nickel-phosphorus coating, and Monel alloy with chrome plating. These combinations were tested using a ring-and-block setup, with water as the medium at 60°C and low viscosity. The MG-200 high-speed friction and wear testing machine was used, applying a load of 1 kN over a 20 km stroke. Based on the results, the best combination was found to be carbon steel with electroless nickel-phosphorus coating paired with carbon steel chrome plating.
Another test involved nine different material combinations, including carbon steel with chrome plating, electroless nickel-phosphorus coating, and spray-welded nickel-based alloys, as well as 38CrMoAl nitrided parts. The test medium was oily wastewater containing 5% oil, with high levels of chloride ions and other minerals. The same MG-200 machine was used, with a load of 100 N and a total friction distance of about 6 km. After analysis, the best pairing was determined to be carbon steel spray-plated plungers with chrome-plated cylinders, and 38CrMoAl nitrided plungers with chrome-plated cylinders.
Despite these findings, laboratory tests are conducted under controlled conditions that may not fully reflect real-world downhole environments. Changing test parameters would require extensive work, and results from different setups may not be directly comparable. Additionally, the materials tested are often limited to those already in use, making it difficult to explore new or potentially better alternatives.
With the advancement of material engineering and the development of new surface treatment technologies, many innovative materials and coatings could be applied to pump components. However, current laboratory testing is not sufficient to meet the growing demand for longer-lasting pump systems. Therefore, integrating principles from material science and other disciplines to design optimal material combinations for pump barrels and plungers has become an urgent challenge.
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