Corrosion failure analysis of N80 oil pipe under w

Corrosion failure analysis of downhole N80 tubing in offshore oil fields

corrosion failure analysis of downhole N80 tubing in offshore oil fields

June 11, 2020

Introduction

an oil well was put into operation in June 2010, with a water cut of 1.5% at the initial stage of production. The well is highly deviated, with a maximum deviation of 74.98 º. In October 2015, the pump was stopped due to failure, and the water content was 81.7% before the pump was stopped. After the maintenance was normal, the production was stable, and the water content rose to 91% in September 2016. In March 2019, when replacing the string, it was found that many oil pipes of the original well string/2 "EU oil pipe were corroded, and each oil pipe had more than 20 1~2 mm perforations, with a corresponding depth of 234~369 M. The failure material oil pipe material is N80. In this study, a perforated oil pipe (recorded as pipe a) and an unperforated oil pipe (recorded as pipe b) are selected as the failure analysis objects in order to find out the cause of corrosion perforation

macro analysis and wall thickness measurement

1.1 macro morphology

the macro morphology of the outer wall of tube a and tube B is shown in Figure 1. For example, special vacuum devolatilization technology is adopted to reduce odor VOC; Show. The observation shows that there are many floating rust on the outer wall of pipe a, and there is no obvious adhesion of oily substances. A layer of oily substances is attached to the outer wall of pipe B as a whole. The macroscopic morphology of pipe ends of pipe a and B is shown in Figure 2. It can be seen from the figure that obvious corrosion pits are found on the inner wall and end of the male buckle of pipe a, and the corrosion products are reddish brown. Samples are taken for corrosion product analysis, while no obvious corrosion phenomenon is found on the outer wall of pipe B

Fig. 1 macro morphology of the outer wall of a and B tubes

Fig. 2 macro morphology of the end of a and B tubes

1.2 anatomical analysis

dissect the a tube along the axis, and further observe the corrosion of the inner wall of the tube, as shown in Fig. 3. In addition to the location of corrosion perforation on the inner wall of oil pipe a, most corrosion pits are also found, and the maximum depth of corrosion pit is about 4.9 mm, among which no obvious corrosion is found on the inner wall of the pipe end on the side with coupling. The nearest corrosion pit is located about 97 mm away from the pipe end. From this position, corrosion pits exist in multiple directions around the pipe body, extending axially to the middle of the oil pipe, and perforation has been formed in some positions (consistent with the externally observed perforation position)

further compare the inner and outer wall morphology of multiple perforation positions, as shown in Figure 4. The observation shows that the area of the corrosion pit on the inner wall is large, and the center of the corrosion pit extends radially to the outer wall until perforation. The corrosion feature of the oil pipe is internal corrosion

Fig. 3 the inner wall morphology of the pipe end on the side of the a oil pipe with the coupling

Fig. 4 the inner wall morphology of the corrosion perforation position

the distribution of the corrosion positions on the inner wall of the a oil pipe is observed as a whole, as shown in Fig. 5. It is found that the corrosion positions are located on both sides of the pipe end, and a total of five circumferential directions are linear to the middle, in which the inner wall corrosion pit (including the perforation position) on the side of the coupling extends to the farthest place 3 m away from the pipe end along the axial direction, The depth of the last 1m corrosion pit near the middle of the oil pipe becomes smaller and smaller until it disappears; The corrosion pit on the other side of the pipe end without coupling extends from the pipe end to the middle to about 1.5 m. with the extension to the middle, the corrosion degree becomes lighter and lighter until it disappears, as shown in Figure 6

Figure 5 distribution of corrosion position on the inner wall of pipe a

Figure 6 extended morphology of corrosion pit on the inner wall of oil pipe on the side of pipe band coupling a

the two sides of oil pipe B will be dissected according to their respective advantages along the axial direction. No obvious internal corrosion is found in the area of about 100 mm on both sides of the pipe end, as shown in Figure 7. Obvious corrosion pits were found in the middle, and a large area of ulceration was found along the axial direction. Taking the reddish brown corrosion products for analysis, the distribution of corrosion pits also has the characteristics of circumferential multiple linear distribution, and no obvious corrosion characteristics are found in some directions of the inner wall of some pipe sections, showing local corrosion characteristics as a whole, as shown in Figure 8

Figure 7 inner wall morphology of the pipe end on the side without coupling of B pipe

figure 8 inner wall morphology of the middle of B oil pipe

1.3 pipe body wall thickness detection

clean the outer walls of a and B oil pipes, then select a representative orientation along the circumference, and measure the remaining wall thickness with GE DM5E ultrasonic thickness gauge. The results show that the wall thickness thinning of a and B oil pipes is seriously uneven. For example, perforation occurs in some positions of a pipe, while the thinning in different directions of the same axial position is only 0.1 mm, with obvious differences

physical and chemical performance analysis

2.1 chemical composition analysis

the chemical composition of a and B tubing body samples was analyzed by spectro lablavm11 direct reading spectrometer. The results show that the chemical separation of the two oil pipes meets the technical requirements of ordering. However, in comparison, there are certain differences in the content of Mn, P and S elements between tube a and tube B, which should not belong to the same product batch. The market demand for sealing machines has a strong upward trend, and the content of elements in the middle and end of tube B is basically the same, and the content of P and S elements in the middle of tube a is higher than that at the end

2.2 mechanical property analysis

► the plastic collected by Rockwell can be used for energy recovery (incineration) hardness

hardness test is carried out on a and B tubing samples with r574 Rockwell hardness testing machine. The results show that the hardness of two oil pipes meets the technical requirements of ordering

► tensile property

use Z600 double column universal material testing machine to conduct tensile test on a and B tubing samples. The results show that the tensile properties of the two oil tubes meet the technical requirements of ordering

► impact performance

use psw-750 pendulum impact testing machine to conduct impact test on a and B tubing samples. The results show that the impact test of the two oil pipes meets the technical requirements of the order

2.3 metallographic analysis

take samples at the non corroded end, corroded middle, non corroded middle and corroded end (the other side) of oil pipe a, and the numbers are A1, A2, A3 and A4 respectively; Take samples at the corroded and non corroded positions at the end and middle of oil pipe B, numbered B1, B2 and B3 respectively. Use the observer a1m metallographic inverted microscope to conduct metallographic analysis on the transverse samples of a and B tubing bodies respectively. The analysis results show that the metallographic structures of tubing a and B are tempered sorbite, and there is no decarburization structure from the internal and external surface structures of A2, A3 and B2 samples

take the longitudinal samples of a and B oil pipes to observe the banded structure. The research shows that there are different levels of banded structure in a and B oil pipes, and the higher the level of banded structure, the greater the tendency of corrosion, and the corrosion mostly extends along the direction of banded structure

a no banded structure is found in the samples taken from the non corroded side of the end of the oil pipe, while high-level banded structure is found in the samples taken from the end of the corroded side and the samples taken near the corrosion perforation, and the banded structure level of the samples taken from the middle is low, indicating that the higher the banded structure level, the more serious the corrosion is, which is consistent with the macro characteristics of the corrosion perforation of a pipe

b there is no obvious corrosion at the end of the oil pipe, and its banded level is small, while the banded structure in the middle of the severely corroded part is high. It can be seen that the degree of corrosion is also related to the level of banded structure

analysis of corrosion products

3.1 micromorphology and energy spectrum analysis

respectively take the corrosion products of the inner wall of the perforated part at the end of oil pipe a, the inner wall of the middle pipe section without local corrosion, and the inner wall of the corrosion position of oil pipe B, numbered 1, 2, 3. Observe their micromorphology with Zeiss Evo 18 scanning electron microscope, and analyze the micro components of the three corrosion products with Oxford energy spectrometer. The results show that the surface of the sample is granular and closely packed; The main components of corrosion products are C, O, Al, Si, Mn, Fe, Ni

3.2 XRD analysis

the corrosion products numbered 1, 2 and 3 described in Section 3.1 are dissolved with petroleum ether and alcohol for oil removal, filtered and dried, and then X-ray diffraction (XRD) test is carried out, with a scanning angle of 2 θ： 3~80 °, sampling step width is 0.02, wavelength λ= 1.540 56 nm。 Using search match software and EDS results, the composition of three groups of corrosion products was analyzed

the results show that the main components of 1 and 2 corrosion products are the same, both of which are Fe2O3 and FeOOH. Although the XRD pattern did not match the belonging substance of C, from the EDS data, the content of C in the two samples was very high, which could be speculated to be introduced by the corrosive environment in the medium. 3 the main components of corrosion products are FeCO3, feso3, FEC, fe6 (OH) 12co3, Fe2 (SO4) 3

in summary, the corrosion products of perforated pipes are mainly iron oxides, and FeCO3 is found in unperforated oil pipes. Since the partial pressure of carbon dioxide in this environment is 0.003 MPa, according to tnb111 regulations issued by DNV, when the partial pressure of carbon dioxide is lower than 0.021 MPa, there is no carbon dioxide corrosion, so carbon dioxide corrosion is excluded, and the existence of FeCO3 may be related to hco3- ions in fluid medium

failure cause analysis

this oil pipe is an ordinary oil pipe with poor corrosion resistance. At the initial stage of corrosion, corrosion pitting appeared in some parts of banded structure. With the progress of corrosion, the adjacent small corrosion pits develop into corrosion pits in larger areas, and then develop vertically into deeper corrosion pits. The structure of the corrosion products produced is relatively loose and unprotected. Once some points on the metal surface are corroded, the corrosion will continue. Due to the existence of corrosion products, the anode area lacking oxidant is formed inside the corrosion products, and the cathode area rich in oxidant is formed outside, which constitutes the continuous corrosion of concentration difference battery

conclusion

(1) the physical and chemical property test of the tubing body shows that the material meets the requirements of API Spec 5CT standard

(2) the corrosion resistance of the oil pipe in the medium environment under this working condition is poor, and oxygen corrosion occurs, especially under the scouring of large flow, the corrosion rate is further aggravated

(3) the corrosion characteristics of oil pipes belong to the form of internal corrosion, which is distributed in strips on the inner wall of the pipe body

(4) the relatively thick corrosion product layer formed on the inner wall surface of the oil pipe constitutes a concentration cell, which further intensifies the corrosion

references (omitted)

author Yu Xuan, Zhang Guoqing, Chen Wei

(Offshore Oil Engineering Co., Ltd.)