- Author: Jin Xi, Han Fan, Gao Dai Yi, etc.
(Zhonghai Oil Changzhou Coatings and Chemical Research Institute Co., Ltd.)
Introduction
The crude oil in a certain oilfield in the South China Sea is a heavy crude oil with high density, high viscosity, low wax content, low pour point, low sulfur content, and low API gravity. The crude oil has a high degree of biodegradation of unsaturated cyclic hydrocarbons, and contains only a small amount of associated gas, almost no free gas, and the gas composition is mainly methane, with a content of 50.29%～95.64% (molar fraction), and also contains a small amount of CO.2、N2"Please note: The formation water in the oil field is CaCl."2.
In January 2018, corrosion was detected in well 05 of the oilfield, with significant corrosion and perforation in several sections of the well casing. The casing exhibited uniform thinning and large-area open perforations.
To determine the cause and mechanism of the corrosion perforation in the on-site pipeline, and to take timely protective measures to prevent similar incidents from occurring again. This work conducted a failure analysis of the recently retrieved corroded and perforated pipeline and its corrosion products, and determined that the electrochemical corrosion occurring under these conditions is primarily due to CO.2- Corrosion and corresponding protection recommendations.
Corrosion Condition Analysis
1.1 Macro Analysis
The well conditions and produced fluid properties, as shown in Table 1, are as follows. As indicated in the table, the produced water from this well has a high salinity level, moderate temperature and total pressure, and contains a high concentration of carbon dioxide and a small amount of hydrogen sulfide.
Table 1: Corrosive Pitting in Oil Wells and Associated Well Conditions and Produced Fluid Properties
After inspection, it was found that the oil pipe corrosion is very severe. There are uniform thinning and localized perforation, without obvious stress corrosion or erosion. The pipe has formed large, through-hole corrosion, with numerous corrosion holes, as shown in Figure 1. The initial assessment indicates that the failure of this well casing is typical of carbonation corrosion in high-salinity oilfield production fluids.
Figure 1: Macroscopic morphology of corrosion perforation in the oil pipe.
1.2 Chemical Composition Analysis
"Addressing irregular corrosion holes and highly uneven wall thicknesses on pipe fittings, samples were taken from both the corroded and uncorroded areas of the substrate for Energy Dispersive X-ray Spectroscopy (EDS) analysis. This analysis was performed to determine if the corrosion was caused by uneven composition of the base material. EDS analysis results for the corroded area are shown in Table 2, and results for the uncorroded area are shown in Table 3."
Table 2: EDS analysis results of samples from corrosion areas.
Table 3: Spectroscopic analysis results of samples from non-corroded areas of the substrate.
The analytical results of samples from the corroded and uncorroded areas show that the base material composition in both areas is consistent, containing only C, Mn, and Fe elements. No significant amounts of P or S, which can lead to inclusions, were detected. Therefore, from the perspective of chemical composition, corrosion is not caused by uneven distribution of the base material. The results indicate that the chemical composition of the pipeline complies with API Spec 5CT requirements, and material factors are not the primary cause of corrosion failure.
Corrosion product analysis
2.1 Sample Preparation
The tested samples are dry, eroded, and blocky, requiring multiple cleaning cycles to remove organic components. The cleaning method is as follows: immerse in ethanol and ultrasonically clean 3 times (cleaning frequency can be adjusted based on the cleanliness); after cleaning, place in an oven at 50°C until completely dry. The before-and-after states of the sample cleaning are shown in Figure 2.
Figure 2: Sample images before and after treatment
2.2 Microscopic Morphology Analysis
The microstructure of the corrosion products was observed using a scanning electron microscope (SEM). The microstructure of the corrosion products sample observed by SEM is shown in Figure 3. From the figure, it can be seen that the particles are amorphous and lack any obvious crystalline shape or typical crystal faces. The particles are relatively few in number and are dispersed.
Figure 3: SEM images of corrosion products: (a) 100x, (b) 1000x
2.3 Chemical Composition Analysis
To determine the chemical composition of the corrosion products using Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD), the EDS results, as shown in Table 4, indicate that the corrosion products are primarily composed of C, O, Cl, Cr, Mn, Fe, and Si. The high concentration of Cl elements confirms the nucleation and growth processes of the corrosion. There is no S present, but a higher concentration of C and O is observed, indicating that the electrochemical corrosion that occurred under these conditions is primarily due to carbon dioxide corrosion, resulting in the formation of FeCO.3Corrosion products. The generated FeCO under these conditions.3The resulting product is relatively porous and lacks effective protection for the substrate, leading to accelerated corrosion.
Table 4: EDS analysis results of corrosion product samples
X-ray diffraction analysis (XRD) results are shown in Figure 4. The XRD analysis further confirms that the corrosion products are primarily Fe2O3 and ferrous carbonate.
Figure 4: X-ray diffraction analysis results of corrosion products
2.4 Fluorescence Microscopy Detection
Fluorescence microscopy can be used to detect the presence of sulfate-reducing bacteria (SRB) on the surface of corrosion products, thereby determining whether bacterial corrosion has caused the failure of the pipe material. Fluorescence microscopy uses ultraviolet light as a light source, illuminating the object to be inspected, causing it to emit fluorescence, which is then observed under a microscope to determine the shape and location of the object. Corrosion product samples can be placed on slides for observation, as shown in Figure 5. The fluorescence detection image of the corrosion product sample shows no fluorescence, indicating that there are no bacteria in the corrosion products, thus ruling out the possibility of corrosion perforation caused by bacteria.
Figure 5: Fluorescence detection image of corrosion products sample
Recommendation
Therefore, to prevent further corrosion, cracking, and perforation, the following recommendations are proposed for on-site corrosion protection:
(1) Closely monitor the corrosion conditions of the wellbore at deeper locations, analyze the corrosion patterns, and assist in determining the causes of the corrosion.
(2) Recommend using CO-resistant materials.2Corroded oil pipe materials, while using both an inner coating and appropriate downhole corrosion inhibitors, a high-quality inner coating and appropriate corrosion inhibitors can effectively suppress CO buildup on the inner walls of the pipelines.2Corrosion and degradation.
References (omitted)
Article source: Coatings and Protection Magazine