Causes and Suggestions for Corrosion Failure of Tubing in an Oilfield in the Eastern South China Sea

introduction

Crude oil from an oil field in the eastern South China Sea is a heavy crude oil with high density, high viscosity, low wax content, low freezing point, low sulfur, and low dissolved gas-to-oil ratio undersaturated naphthenic oil with high biodegradability

In January 2018, it was discovered that the tubing of Well 05 in the oilfield was corroded and perforated, and some sections of the tubing were very corroded, showing large areas of open perforation.

In order to find out the cause and mechanism of on-site tubing corrosion and perforation, timely protective measures were taken to avoid such accidents from happening again

Corrosion analysis

1.

Table 1 shows the relevant well conditions and produced fluid properties of corroded perforated tubing in service

Table 1 Related well conditions and produced fluid properties of corroded perforated tubing

After inspection, it was found that the tubing was very corroded

Figure 1 Macro morphology of corrosion perforation of tubing

1.

Regarding the irregularly distributed corrosion holes and extremely uneven wall thickness on the pipe fittings, the corroded area and the non-corroded area of ​​the pipe fittings are sampled separately for energy spectrum analysis (EDS) to determine whether the cause of the corrosion is uneven matrix material composition Cause

Table 2 Energy spectrum analysis results of substrate samples in the corroded area

Table 3 Energy spectrum analysis results of substrate samples in uncorroded areas

The results of the energy spectrum analysis of the samples in the corroded area and the uncorroded area can be seen that the matrix materials of the two areas are consistent in element types, and both contain only C, Mn, and Fe elements.

Corrosion product analysis

2.
1 Sample pretreatment

The sample to be tested is a dry corroded block and requires multiple cleanings to remove organic components
.
The cleaning method is: soaking and ultrasonic cleaning with ethanol 3 times (depending on the degree of cleanliness can be cleaned multiple times); after cleaning, put it in an oven and bake it at 50°C until completely dry
.
The state of the sample before and after cleaning is shown in Figure 2
.

Figure 2 Sample pictures before and after processing

2.
2 Microscopic morphology analysis

A scanning electron microscope (SEM) was used to observe the microscopic morphology of the corrosion product samples
.
The microscopic morphology of the corrosion product sample observed by the scanning electron microscope is shown in Figure 3
.
It can be seen from the figure that the particles are also amorphous, with no obvious crystal shape and no typical crystal plane
.
The particles are less aggregated and more dispersed
.

Figure 3 SEM image of corrosion product sample: (a) 100 times, (b) 1000 times

2.
3 Chemical composition analysis

The corrosion product samples were analyzed by energy spectrum analysis (EDS) and X-ray diffraction analysis (XRD) to determine the chemical composition.
The results of the energy spectrum analysis (EDS) are shown in Table 4.
The corrosion products are mainly composed of C, O, Cl, Cr, Mn, The composition of elements such as Fe and Si, and more Cl element proves the nucleation and growth process of corrosion
.
There is no S in the element, but a lot of C and O, indicating that the electrochemical corrosion that occurs under this condition is mainly carbon dioxide corrosion, and finally FeCO3 corrosion products are formed
.
The FeCO3 product generated under this condition is relatively loose and lacks effective protection of the matrix, so corrosion accelerates
.

Table 4 Energy spectrum analysis results of corrosion product samples

X-ray diffraction analysis (XRD) results are shown in Figure 4.
XRD analysis further confirmed that the corrosion products are mainly Fe2O3 and ferrous carbonate
.

Figure 4 X-ray diffraction analysis results of corrosion product samples

2.
4 Fluorescence microscope detection

Fluorescence microscope detection can understand whether there are sulfate reducing bacteria (SRB) on the surface of the corrosion product sample, so as to determine whether the pipe fails due to bacterial corrosion
.
The fluorescence microscope uses ultraviolet rays as the light source to illuminate the object under inspection to make it emit fluorescence, and then observe the shape and location of the object under the microscope
.
The corrosion product samples were placed on a glass slide for observation, and the results are shown in Figure 5
.
There is no fluorescence display in the fluorescence detection image of the corrosion product sample, indicating that there is no bacteria in the corrosion product, thereby eliminating the corrosion perforation caused by bacteria
.

Figure 5 Fluorescence detection diagram of corrosion product samples

Suggest

Therefore, in order to avoid recurrence of corrosion failure perforation, the following suggestions are made for on-site corrosion protection work:

(1) Pay close attention to the tubing corrosion from deeper downhole positions in the well, summarize the corrosion rules, and assist in judging the cause of tubing corrosion;

(2) It is recommended to use CO2 corrosion-resistant tubing materials, and at the same time use internal coatings and fill with appropriate downhole corrosion inhibitors.
High-quality internal coatings and applicable corrosion inhibitors can effectively inhibit CO2 corrosion damage on the inner wall of the pipeline
.

References (omitted)

Author | Jin Xi, He Yifan, Gao Dayi, etc.

(CNOOC Changzhou Coating Chemical Research Institute Co.
, Ltd.
)