Failure analysis on the oxygen corrosion of the perforated screens used in a gas injection huff and puff well
Introduction
Gas flooding is one of the most important means to improve the recovery of oil and gas. With exploitation, oilfields inevitably enter the secondary development stage where gas flooding is widely applied. In this process, injected gases include hydrocarbon gas, flue gas, CO2, N2, and air [1], [2]; however, the advantages of N2 are evident [3], [4], [5]. For example, it can be widely sourced, is not restricted by region, and yields no pollution. In addition, the physical properties of N2, including low viscosity, low thermal conductivity, low volatility (it is non-flammable and non-explosive), greater solubility in crude oil than in water, large compression coefficient, high expansion capacity, and large elastic energy, are excellent. Therefore, N2 is a superior source of injection gas.
A case of well failure in an oilfield in western China is an integrated opportunity for gas lift and injection. After six months of production, the nitrogen, which was produced by membrane nitrogen method, was injected into the well. After two months, a perforated screen fractured and the well broke down. H2S or CO2 could not be detected in this well. The depth of this well was 6600 m and the perforated screen was placed approximately 4500 m from the wellhead. The material of the perforated screen was C110 steel and its dimensions were 3 1/2′′×6.45 mm. The fracture of the failure screen was significantly corroded and damaged, therefore, fracture analysis could not be performed. Macroscopic examination, chemical composition analysis, mechanical properties testing, metallographic examination, scanning electron microscopy (SEM), X-ray diffraction (XRD), and electrochemical testing were used to comprehensively investigate the failure of the perforated screens.
Section snippets
Physical and chemical properties
The macroscopic morphology of the fracture of the failure-perforated screen is shown in Fig. 1. The hole of the perforated screen is observed around the fracture, which means that the fracture occurred near the perforation. The thickness of the failure-perforated screen near the fracture was measured by an ultrasonic thickness gauge and the results are shown in Fig. 2. The outer diameter near the fracture significantly decreases and necking occurs. Also, the thickness significantly decreases
Analysis of the corrosion product
The corrosion products are very abundant (Fig. 1). The surface morphologies of the corrosion products around the fracture were analyzed by SEM. The results, presented in Fig. 4, show that the even and dense corrosion products are on the outer surface while the loose and porous corrosion products are on the inner surface with many holes and cracks.
The corrosion products of the inner surface and the outer surface of the failure-perforated screen were collected separately and the corrosion product
Experimental method
Experiments were conducted at 1 atm in a glass cell including a water bath to control the temperature. A typical three-electrode cell was used with an Ag/AgCl electrode (in saturated KCl solution) as the reference electrode, a large piece of platinum plate with a surface area of over 4 cm2 as the counter electrode, and the C110 steel from the failure screen as the working electrode. The specimens with an exposed surface of 10.0 × 10.0 mm2 were machined from the failure screen and embedded in
Discussion
The physical and chemical properties of the failure perforated screens meet the requirements of API 5CT-2012.
The nitrogen was produced by the membrane method and was approximately 5% O2. Therefore, a large amount of oxygen was also entered the well during the injection of nitrogen. The residual gas in the well contained 1%–1.3% O2 after some time. Therefore, oxygen corrosion occurred in the well. There are a lot of previous studies [13], [14], [15], [16], [17], [18], [19], [20], [21] shown that
Conclusion
- 1)
The physical and chemical properties of the failure perforated screens meet the requirements of API 5CT-2012.
- 2)
The O2 is brought into the well during the injection of nitrogen, resulting in severe oxygen corrosion. High salinity formation water, especially with a large amount of Cl-, further accelerates the corrosion. The thickness of the perforated screen was decreased by the severe corrosion, resulting in fracture at the perforation.
- 3)
It is suggested that the oxygen content of the injected gas
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The project is supported by the Shanxi National Science Foundation (No. 2019JQ-213) and Basic Research and Strategic Reserve Technology Research Fund (project of China National Petroleum Corporation (2018Z-01)).
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Downhole corrosion inhibitors for oil and gas production – a review
2021, Applied Surface Science AdvancesCitation Excerpt :Operating conditions of the reservoir in terms of temperature and pressure, the amount of formation water production, the water-to-gas and condensate-to-gas ratios, the depth of the well are also important. The condition of the well in terms of the onshore or the offshore location, well design and completion procedure, the side effect of chemical injection on the downstream process, and the possibility of downhole corrosion monitoring and economic issues must also be considered [17-21]. Concerning all these factors, there are three methods of controlling corrosion in downhole coiled tubing, including the selection of corrosion-resistant alloys (CRA), implementing internal coatings, and injecting downhole corrosion inhibitors.