Corrosion sensitivity analysis of pipelines in CO₂ and H₂S coexisting environment in the South China Sea

2.1 Experimental material and pretreatment

Considering the engineering application, N80 steel is used as the experimental material in the present work, and the chemical composition of N80 steel is shown in Table 1. The dimension of the test samples is designed as 30 × 15 × 3 mm. The test samples are firstly polished by using grit silicon carbide paper before the experiment; the samples are secondly rinsed with water; and the oil on the sample surface are removed by petroleum ether, and then dried by cold air; the samples are finally weighted and stored in a dry container.

To ensure the real reliability of the experiment results, a total of 25 Wells in the South China Sea are investigated by field measurement in this study, and the actual mining conditions (ranges data) for oil and gas field are given in Table 2. Meanwhile, a compositional analysis report of sampled water is shown in Table 3. Based on the above investigation, the present work focuses on the influence of the temperature, Cl⁻ concentration, CO₂ partial pressure, and H₂S partial pressure on corrosion sensitivity of pipelines of oil and gas wells. The simulated aqueous solution of formation fluid is the corrosive medium in the present study, which is configured through employing chemical reagents and deionized water. Therefore, the composition of simulated aqueous solution of formation fluid in the water in the South China Sea is shown in Table 4. K⁺+Na⁺ and Cl⁻ concentration are determined through experimental scheme, seen in Table 5.

2.2 Corrosion experiments

The 3 L high temperature and high pressure autoclave (HTHPA) is used as a platform for the implementation of corrosion simulation experiments, and the experimental schematic diagram is given in Figure 1. The detailed experimental procedures are as follows:

1. For ensuring the validity of the experiment results, the four test samples of N80 steel are used under the same experimental conditions. On the one hand, any three test samples are used to analyze the average corrosion rate. On the other hand, the remaining sample are used to observe the corrosion morphology and explore the composition of corrosion products.

2. The test samples after weighting are regularly placed on the hanger, and then the hanger is installed in the HTHPA. Next, the simulated aqueous solution of formation fluid is injected in the HTHPA, and then the HTHPA is sealed through using the rubber seal ring.

3. The temperature in the HTHPA is heated up 45 °C by adjusting the temperature controller, and the N² is injected into the HTHPA to dislodge O₂.

4. After 4 h,N² is discharged. Meanwhile, H₂S and CO₂ are injected into the HTHPA until partial pressures of H₂S and CO₂ are increased the specified pressure, respectively. Finally, the temperature is adjusted to the target temperature and waits this condition for 5 days.

5. All the test samples after finishing the above operations are immersed in petroleum ether to rinse surface oil, and then immersed ethanol to dehydrate. Surface corrosion products of the three samples are cleaned by using the cleaning solution, which consists of 10% hydrochloric acid, 0.5% hexamethylenetetramine, and deionized water. Next, the samples are washed with deionized water, dehydrated by absolute ethyl alcohol, and dried with cold air. Finally, an electronic balance with the accuracy of 0.1 mg is used to weight the three handled samples, and weight loss method is employed to analyze the average corrosion rate. The other sample is dried with cold air and sealed preservation for subsequent analysis experiments.

Figure 1:

The experimental schematic diagram.

The present work innovatively applies orthogonal experiment method to explore the corrosion sensitivity of N80 steel in H₂S partial pressure, CO₂ partial pressure, temperature and Cl⁻ concentration, so orthogonal array L₁₆(4⁴) is introduced to design the experimental scheme. Considering actual engineering environment of pipelines of oil and gas wells, the experimental scheme is shown in Table 5.

2.3 Corrosion products analysis

The scanning electron microscope (SEM) is used to analyze the surface and cross-section corrosion morphology of corrosive products. The energy disperse spectroscopy (EDS) is used to explore the composition of corrosion products.

Before the analysis of cross section of corrosion products morphology, the corroded samples are sealed with AB glue (Epoxy AB adhesive) for three days to protect corrosion product layer in cutting process. The sample after cutting should be polished to clearly observe the cross-section corrosion morphology.

3.1 Sensitivity analysis of corrosion

3.1.1 Average corrosion rate of N80 steel

The average corrosion rates of N80 steel in the different experiment conditions are irregularly fluctuated between 0.2 mm/a and 0.8 mm/a, as shown in Figure 2. In the condition of temperature 80 °C, Cl⁻ concentration 20,000 mg/L, H₂S partial pressure 0.5 Mpa and CO₂ partial pressure 1.0 Mpa, the average corrosion rate of N80 steel is minimum. In the condition of temperature 120 °C, Cl-concentration 25,000 mg/L, H₂S partial pressure 2.0 Mpa and CO₂ partial pressure 0.5 Mpa, the average corrosion rate of N80 steel is maximum.

Figure 2:

The average corrosion rate of N80 steel.

3.1.2 Corrosion sensitivity

The orthogonal experiment is used to explore the corrosion sensitivity of N80 steel in the different temperature, Cl⁻ concentration, CO₂ partial pressure, and H₂S partial pressure. The sensitivity analysis of corrosion factors is shown in Table 6, and the calculation process of parameters in Table 6 referred to Appendix A.

(1)RA=max{K1A,K2A,K3A,K4A}−min{K1A,K2A,K3A,K4A}RB=max{K1B,K2B,K3B,K4B}−min{K1B,K2B,K3B,K4B}RC=max{K1C,K2C,K3C,K4C}−min{K1C,K2C,K3C,K4C}RD=max{K1D,K2D,K3D,K4D}−min{K1D,K2D,K3D,K4D}

(2)Fe+H2S→FeS+2H

Table 6:

The sensitivity analysis of corrosion factors.

Test no.	A	B	C	D
Bit level (K1)	K 1 A (2.14)	K 1 B (1.35)	K 1 C (1.87)	K 1 D (1.89)
Bit level (K2)	K 2 A (2.07)	K 2 B (2.04)	K 2 C (1.72)	K 2 D (1.68)
Bit level (K3)	K 3 A (1.86)	K 3 B (2.25)	K 3 C (2.10)	K 3 D (1.97)
Bit level (K4)	K 4 A (1.86)	K 4 B (2.28)	K 4 C (2.23)	K 4 D (2.38)
Range (R)	R A (0.28)	R B (0.93)	R C (0.51)	R D (0.71)

The variation range of environmental factors (including the temperature, Cl⁻ concentration, CO₂ partial pressure, and H₂S partial pressure) can be determined by Eq. (1). As shown in Table 6, the range of different environmental factors from the largest to the smallest is given as RB>RD>RC>RA. In the corrosive environment within H₂S partial pressure, CO₂ partial pressure, temperature, and Cl⁻ concentration, it can be seen that H₂S partial pressure is the main factor of the corrosion of N80 steel, and the effect of CO₂ partial pressure on corrosion of N80 is weaker than Cl⁻ concentration and temperature. Therefore, in the South China Sea, H₂S partial pressure is major corrosion factor for N80 steel, and the corrosion mechanism can be shown in Eq. (2); besides, the influence of Cl⁻ concentration on the corrosion rate of N80 steel is greater than that of temperature under the experimental gradient designed in this paper. According to experimental results, the H₂S partial pressure and Cl⁻ concentration are considered as the main corrosion factors of pipelines of oil and gas wells in the South China Sea, and so an anticorrosive inhibiting the corrosion effect of H₂S partial pressure should be added to improve the resistant corrosion of the pipelines.

The variation trend of corrosion rate under different experimental conditions is shown in Figure 3. In the light of Figure 3(a), the sum of average corrosion rate of N80 steel is firstly decreased and then stabilized with the increment of CO₂ partial pressure in range from 0.5 to 2 Mpa. The reason may be that the solubility of CO₂ is close to saturation when CO₂ partial pressure is greater than 1.5 Mpa. In Figure 3(b), the sum of average corrosion rate of N80 steel is firstly decreased and then increased with the increment of Cl⁻ concentration in range from 10 to 25 g/L. Meanwhile, the minimum corrosion rate appears in Cl⁻ concentration 15 g/L, which the critical point under the experimental gradient of the present work. When Cl⁻ concentration is less than 15 g/L, the solubility of CO₂ and H₂S in an aqueous solution is decreased with the increment of Cl⁻ concentration, which causes the sum of corrosion rate is gradually decreased; When Cl⁻ concentration is greater than 15 g/L, the abilities of Cl⁻ reducing the formation of metal surface passivation film and accelerating the destruction of metal surface passivation film are gradually enhanced with the increment of Cl⁻ concentration, which causes the sum of corrosion rate is gradually increased. According to Figure 3(c), the sum of average corrosion rate of N80 steel is gradually increased with the increment of H₂S partial pressure. Meanwhile, the sum of average corrosion rate of N80 steel is rapidly increased when H₂S partial pressure is changed from 0.5 Mpa to 1.0 Mpa. While H₂S partial pressure is exceed 1.5 Mpa, the corrosion rate of N80 steel is slowly increased as the solubility of H₂S is gradually approached saturation. As shown in Figure 3(d), the sum of average corrosion rate of N80 steel is firstly decreased and then increased with the increment of environmental temperature. Meanwhile, the existence of the minimum corrosion rate is observed in 80 °C.

Figure 3:

Variation trend of corrosion rate under different conditions: (a) CO₂ pressure, (b) Cl⁻ concentration, (c) H₂S pressure, (d) temperature.

3.2 Micro morphology of corrosion products surface

The micro morphology of the surface of N80 steel samples formed on different experimental conditions is shown in Figure 4, which is clearly observed the coverage scale, crystal shape, and accumulation pattern of corrosion products by the SEM. According to the micro morphology of the surface corrosion products with 100 times magnification, the local corrosion and pitting may occur on the surface of N80 sample, but hydrogen induced stress cracking is less found on the surface. Though observing the coverage scale of surface corrosion products on the surface of N80 sample, the local corrosion of the metal surface area can be found in the testing area of experiment 1; meanwhile, the pitting of the metal sample can be observed in the testing area of all experiment; besides, the exfoliation phenomenon of corrosion product layer can be seen in the testing area of experiment 11. In the light of accumulation pattern of the surface corrosion products, the accumulation pattern of the surface corrosion products under experiments 4, 9, and 16 is relatively compact, and the accumulation pattern of the surface corrosion products under experiments 1–3, 5–8, and 10–15 is relatively loosened. Under the microscopic morphology of 2000 times magnification, the crystal shape of surface corrosion products is mainly divided into four kinds, such as the sheet-like crystal, grainy crystal, polyhedral crystal and columnar crystal. The corrosion products of the sheet-like crystal are mainly discovered in experiments 4, 7, 8, 12, and 15; The corrosion products of the grainy crystal are mainly discovered in experiments 2, 3, 10, and 16; The corrosion products of the polyhedral crystal are mainly found in experiments 1, 5, 6, 9, and 13; The corrosion products of the columnar crystal are mainly found in experiments 11 and 14.

Figure 4:

The morphology of surface corrosion products of experiment 1–16, (a) the morphology of corrosion products at 100 times magnification, (b) the morphology of corrosion products at 2000 times magnification.

3.3 Corrosion product composition

The EDS analyses of the corrosion production of N80 steel surface under different experimental conditions are shown in Figure 5. Through the composition analysis of corrosion products, the validity of corrosion sensitivity analysis for N80 steel is further verified. According to point number 5–1 in Figure 5(a), chemical elements Ca, C, O, and S are mainly included in the square crystal, and the corrosion product on the surface should be CaCO₃ because CO₃²⁻ is combined with Ca²⁺ to form CaCO₃. As shown in point number 7 in Figure 5(c), chemical elements Fe and O are mainly included in the granular crystal, which indicates that the N80 steel may be oxidized. Considering component analysis of the other corrosion products in Figure 5, the surface corrosion products under different experimental conditions are mainly formed by chemical elements Fe and S, which may be FeS. The experimental results show that the H₂S partial pressure is the most sensitive factor to the corrosion of N80 steel. Meanwhile, the content of element C in the corrosion products under different experimental conditions is few, which explains that the influence of CO₂ partial pressure on corrosion of N80 steel is very weak in the South China Sea. The reason may be that the corrosivity of H₂S dissolved in water is greater than CO₂. Additionally, the enrichment of Cr element on the surface of corrosion products is different under different corrosion conditions, which indicates that the anticorrosion effect of Cr element could be related to the experimental environment.

Figure 5:

The EDS figure of corrosion products: (a) experiment 5, (b) experiment 6, (c) experiment 7, (d) experiment 8, (e) experiment 13, (f) experiment 14, (g) experiment 15, (h) experiment 16.

3.4 Cross-section analysis of corrosion products

The SEM images of cross-section of N80 steel corrosion products are shown in Figure 6, which is observed obviously the corrosion product layer. As shown in Figure 6(a), corrosion products of two layers are observed clearly. Meanwhile, a crack between the middle layer and surface layer is found, which indicates that corrosion products of the two layers are loosely bonded and easily scraped off. The accumulation pattern of the middle layer crystals and surface layer crystals is compact, which can effectively reduce the chance of anion and cation contact inside and outside the product layer and decrease corrosion rate. In Figure 6(b), corrosion products of three layers are observed obviously. Meanwhile, the thickness of inner layer, middle layer and surface layer in the different positions is also different, which explains that the pitting of N80 steel can be found in the corrosive experiment 11. Besides, the accumulation pattern of corrosion products of the inner layer and surface layer is very loose, and the pore of corrosion products is discovered in the middle layer. Therefore, the layers of corrosive products are difficult to protect the N80 steel matrix.

Figure 6:

Morphology of cross-section corrosion products of N80 steel: (a) experiment 5, (b) experiment 11, (1-matrix, 2-inner layer, 3-middle layer, 4-surface layer, 5-epoxy AB adhesive, 6-creack, 7-pore).

The multilayer structure of corrosion products is closely related to the anodic reaction under different experimental environments. In present work, the experimental environments mainly include H₂S partial pressure, CO₂ partial pressure, temperature, and Cl⁻ concentration, and so several possible anodic oxidation mechanisms are as follows (Morse et al. 1987; Smith and Miller 1975; Sun 2006).

In the initial stages of corrosion experiments, CO₂ and H₂S are dissolved in the corrosive medium to react directly with the N80 steel matrix, and form FeS and FeCO₃, as shown in Eqs. (4)–(8). These corrosion products are gradually precipitated on N80 steel surface and the primary corrosion product scale is formed. Meanwhile, acidic solution formed after CO₂ and H₂S is dissolved in corrosive medium, which makes N80 steel matrix dissolve generate a lot of Fe²⁺. When the concentration of Fe²⁺, S²⁻ and CO₃²⁻ in corrosive medium is exceeded the solubility of FeS and FeCO₃, FeS, and FeCO₃ could precipitate on the steel surface as a crystal. In this case, the secondary corrosion product scale is formed. As time goes on, these corrosion products cover the N80 steel surface together, which is the reason of the formation of the middle layer, as shown in Figure 6.

When the steel surface is completely covered by corrosion products, the anions in the solutions can directly contact the steel matrix through the pores between the corrosion products. Therefore, when the anions in the solutions directly contact the steel matrix, FeS, and FeCO₃ are formed on the steel matrix, therefore, the formation of the inner layer is appeared as shown in Figure 6. Additionally, in the pretreatment stage, some corrosion products (such as FeOOH and FeCO₃) could be generated due to the reaction of O₂ and CO₂ dissolved in simulated aqueous solution with N80 steel samples, as shown in Eqs. (4), (5) and (10). Finally, these corrosion products could adhere to the surface of N80 steel to become a part of the inner layer of corrosion products. As time goes on, corrosion products of inner layer are gradually thicker, and the pores between the crystals of corrosion product are blocked. Later, Fe²⁺ can only contact the anions in the solutions to generate FeS and FeCO₃ by microscopic channels between the crystals of corrosion products, then FeS and FeCO₃ gradually accumulates on the middle layer and then forms the surface layer as shown in Figure 6(b).

The thickness of cross-section corrosion product layer is shown in Figure 7, and the composition of corrosion products of cross-section of experiments 5 and 11 is given in Figure 8, which describes the content of Fe, S, C, and O elements by the depth of color. The darker the color in the corrosion product layer, the greater the element content in the corrosion product layer. According to the distribution area of Fe, S, C, and O elements in experiment 5, the content of S element is the greater than the content of C element, which further verifies that the influence of the H₂S partial pressure on corrosion of N80 sample is greater than CO₂ partial pressure. Meanwhile, the corrosion products may be FeS and FeCO₃. Hence, the anode reaction of experiment 5 can be described in Eqs. (6)–(8). As shown in experiment 11, the content of C element of corrosion products is obviously higher than experiment 5. Meanwhile, the content of O element of the inner layers is obviously highest than middle layer and surface layer, which indicates that the carbonate content of corrosion products is gradually accumulated in the inner layers, which leads to the fact that the corrosion products are loose and easy to fall off.

Figure 7:

Composition of cross-section corrosion products: (a) experiment 5, (b) experiment 11, (1-epoxy AB adhesive, 2-corrosion product layer, 3-matrix).

Figure 8:

Corrosion products of cross-section on the N80 steel on experiments 5 and 11: (a) distribution area of Fe, (b) distribution area of C, (c) distribution area of S, (d) distribution area of O.