Research on the Interface Structure during Unidirectional Corrosion

Sep 15, 2016 - Research on the Interface Structure during Unidirectional Corrosion for Oil-Well Cement in H2S Based on Computed Tomography Technology...
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Research on the Interface Structure during Unidirectional Corrosion for Oil-Well Cement in H2S Based on Computed Tomography Technology XiaoWei Cheng, KaiYuan Mei, ZaoYuan Li, XingGuo Zhang, and XiaoYang Guo* Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, P.R. China ABSTRACT: The disruption of interface layer occurring in cement stones during corrosion was investigated using threedimensional (3D) X-ray computed tomography (CT). This study was conducted using this characterization technique by continuously examining the air-void within a sample. Examination of the obtained CT images shows that air-void distribution of the interface layer of the cement stone exposed in humid H2S gas exhibits a clear difference from the distributions found in unexposed samples. To confirm this difference, a dense layer was found using scanning electron microscopy, which was in good agreement with the CT image analysis showing that the air-void of the dense layer was much smaller than that in the unexposed sample. The compositions of the interface layer were further investigated using several characterization techniques such as scanning electron microscopy (SEM), SEM-energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD).

1. INTRODUCTION In previously reported studies, the basic evaluation of the microstructure of cement stone was usually based on the measurement of cement stone air-void.1−3 Quantitative characterization of air-void carried out using three-dimensional (3D) X-ray computed tomography (CT) provided a good understanding of the cement-based material.4,5 The use of this method enables both the size and distribution of the air-void in cement stone to be accurately identified. Although the pore size and distribution of the air-void in cement stone can also be identified by other stereological methods or other equipment, those require a massive sample preparation effort to examine a large number of sections in a single sample.6−10 For the case where a single phase needs to be identified from among some similar phases in complex materials, CT can increase the resolution ratio of the different phases with the electron density map and is much more useful than scanning electron microscopy (SEM).11 In the marine carbonate reservoir of the Sichuan Basin, cement sheathing in gas wells suffers from a fierce attack by humid H2S gas at the high temperature and pressure conditions of the formation.12−14 Additionally, cement stones are in direct contact with soil and groundwater that contain sulfate and are vulnerable to corrosion by these sulfates at the interface between the cement and the well wall. The effect of corrosion on cement stones gradually spreads and damages the inner structure. However, this process does not occur all the time and will stop with the formation of a low permeability interface layer in some cases.15−17 Researchers have discovered that © 2016 American Chemical Society

cement stone will build a protective layer with products in its inner structure in the carbonic acid environment while, in the gas fields with a high sulfur content, humid H2S gas is a potent substance.18−23 The purpose of this study is to characterize and compare the interface layer of the cement stones before and after the exposure to the humid H2S gas and to examine its corrosive effects on the microstructure of the cement stone using the X-ray computed tomography technique together with several auxiliary characterization instruments.

2. EXPERIMENTAL PROCEDURE 2.1. Materials. The cement slurry was mixed with the Class G high sulfate-resistant oil well cement, silica powder (15 wt % of the cement), and water with the water to cement ratio of 0.44. With these components, the slurry was prepared according to the standard of API Recommended Practice 10B24 and injected into the PVC pipes (Φ25*60 mm) for curing. The cement samples were then cured at 90 °C under a pressure of 20.7 MPa for 1 week. Cement stones after curing were shaped into cement bars with the size of Φ25*50 and Φ25*25 mm and inserted into PVC pipes with epoxy resin filling in the gaps around the stones to ensure that the humid H2S gas could only attack on one side of the sample (Figure 1b). Received: Revised: Accepted: Published: 10889

June 5, 2016 September 12, 2016 September 15, 2016 September 15, 2016 DOI: 10.1021/acs.iecr.6b02162 Ind. Eng. Chem. Res. 2016, 55, 10889−10895

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Industrial & Engineering Chemistry Research

Figure 1. (a) Corrosion reaction kettle; (b) sketch of the corrosion specimens.

2.2. Samples Loading. Cement samples were exposed in the liquid phase for a period of 7 days at a temperature of 100 °C and a total pressure of 10 MPa using N2 (8.3 MPa) and H2S (1.7 MPa) to simulate the humid acid gas environment of the shale (Table 1). All of the specimens were maintained in the Table 1. Designed Parameters of the Corrosion Experiment corrosion solution

PH2Sa/MPa

PTb/MPa

temperature/°C

time/d

distilled water

1.7

10

100

7

a

PH2S: Partial pressure of the H2S gas. bPT: Total gas pressure.

sealed Hastelloy kettle with water filled in as the liquid phase acting as an isolating environment for this test and also ensuring safety (Figure 1a). 2.3. Characterizations. After 7 days of curing, the PVC pipes and polymer-based sealant around the specimens were removed. Prior to the analysis, the specimens were placed in a clean desiccator at a temperature of 50 °C for 24 h to prevent the continuous corrosion of the inner structure. The exposed cement stones were shaped into rectangular blocks by sawing from the PVC pipes and polishing for characterization by the gas permeability test with nitrogen as the working gas. Prepared cement stone was divided into three sections for the gas permeability testing. The specimens prepared for CT testing were picked out from the region of interest (ROI) and polished. The specimens used for SEM were also prepared for CT testing, shaped, and dried in a drying oven. Prior to the examination, gold coating for specimens is required. The specimens need to be milled into powder for the X-ray diffraction (XRD) measurement, as shown in Figure 2a. The unexposed samples were prepared for analysis in the same way as the exposed samples. The well-prepared cement specimens were sampled in the profile scanning evaluation with the CT (Phoenix nanotom M, GE, USA), a powerful nonintrusive experimental method, that nondestructively measures the internal microstructure of the specimens of interest. X-rays passed through the object were absorbed and used to construct the CT images on all detectors forming a three-dimensional image of the object. The gas permeability of the specimens was measured by the gas

Figure 2. (a) Split of the unidirectional exposed specimen (b) and the option of the ROI.

permeability tester (DKS-3, Hai’an Petroleum Instrument, China) with nitrogen gas according to the Darcy’s law. SEM (JSM-6510, JEOL, Japan) equipped with an energy dispersive spectroscopy (EDS, X-Max, Oxford Instrument, UK) analyzer was employed to compare the changes of the microstructure and mineralogical composition of the specimens. The compositions of the mineral phase of the specimens were characterized using the qualitative XRD analysis (D8 ADVANCE, Bruker, German) with the X-ray diffractometer at a rate of 0.02° s−1 in the 2θ range from 10° to 70°.

3. RESULTS AND DISCUSSION 3.1. Permeability Analysis. Table 2 shows the permeability test results or the three sections of the samples with and without the exposure to the humid H2S gas. As shown in Table 2, gas permeability clearly changed between the top layers of exposed and unexposed samples, so that the region of interest (ROI) exists in the top layer that acts as insulation, stopping 10890

DOI: 10.1021/acs.iecr.6b02162 Ind. Eng. Chem. Res. 2016, 55, 10889−10895

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image indicate the presence of quartz, and the gray region corresponds to the cement matrix. Figure 3 shows a three-dimensional image of the differences of dispersal of the air-void within the specimens as obtained from the raw CT images, showing that the specimens exposed to the humid H2S gas contained many more pores than the unexposed specimens in this layer. The pore distributions of the specimens are shown in Figure 3. With the hydration of the cement matrix, cement phase crystals grew and the volume of these phases changed continuously with hydration, with all of these reactions leading to the appearance of inner pores. From the examination of the three 2D sliced images selected from the 3D cube images, it is observed that the pores of the unexposed specimen are uniformly distributed, as shown in Figures 4 and 5; this is in agreement with the previously reported studies. The most representative microstructures of the exposed specimen are presented in Figure 5. It could be seen that many more pores are present in the exposed specimen than in the unexposed specimen. Furthermore, as seen from Figure 5, the number of red spots or areas decreased from the top region to the bottom, indicating that the bottom region of the specimen contains fewer pores than the top region. This may be due to the weakening of the corrosion by the ROI. Furthermore, the structure of the top region shows a thin layer with fewer pores in this ROI. This also indicated that the density of this layer is higher than that of the other region, with this layer acting as a blocking layer in the cement matrix. To clearly compare the pore distribution, the front images of both exposed and unexposed specimens with only the pores remaining were obtained via thresholding, as shown in Figure 6. 3.3. Micrographs of the Interface Structure. The SEM analysis was performed on the interface structure to study the ROI of the exposed specimen. After investigation by the CT scanning, the specimen was imaged using an Environmental SEM system equipped with EDS. The SEM images revealed that the top of the ROI consisted of a needle-like phase that is denser than the other region below shown in Figure 7a, and the needle-like crystals were identified by the EDS equipment as containing mainly Ca (38.43 wt %), O (23.98 wt %), and S (18.54 wt %) (Figure 7). However, the SEM image of the corroding region presented in Figure 7b shows that the cement matrix had been badly damaged and the pores in this region expanded due to the loss of the cement matrix caused by the attack of the humid H2S, with the quartz left in these pores.

Table 2. Changes of Gas Permeability in Each Section for Samples gas permeability (mDa)

section top layer transition layer bottom layer a

(mm)

before corrosion

after corrosion

0−20 20−40 40−60

0.035 0.035 0.035

0.005 0.036 0.030

1 mD is 1 millidarcy = 0.9869 × 103 mm2.

Figure 3. CT contrast 3D-images of the ROI of unexposed (a) and exposed (b) specimens.

the fierce attack by the humid H2S gas. Although the transition layer was attacked even after the exposure, the changes of gas permeability of this layer are negligible, breaking the progression of the corrosion process. With the further hydration of cement, the presence of the hydration products led to an increase of the gas permeability in the bottom layer that is hardly affected by the humid H2S gas. 3.2. Microstructure of Air-Void Investigated by Computed Tomography. To investigate this region more closely, testing specimens were stripped slightly from the top layer where the ROI exists, as shown in Figure 2b. The raw CT images had to be processed by a suite of image-processing applications and technique in advance, due to the large number of scanning images; the region of interest had been detected initially by the gas permeability testing. Therefore, 2D sliced images were chosen randomly from the top, front, and right portions of the 3D cube images. The differences of phases were evaluated via the threshold in the CT images, where red spots or regions show the presence of an air-void; bright spots in the

Figure 4. CT contrast 2D-images of the ROI of unexposed specimen. 10891

DOI: 10.1021/acs.iecr.6b02162 Ind. Eng. Chem. Res. 2016, 55, 10889−10895

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Industrial & Engineering Chemistry Research

Figure 5. CT contrast 2D-images of the ROI of exposed specimen.

hydrate (CSH) and the CSH matrix. Fewer pores are observed in this region compared to the other region of the ROI. All of these results showed that the microstructure of this region was well-preserved and did not exhibit any infiltration by the humid H2S gas, implying that the corrosion reaction was weakened by the denser region of the ROI. Furthermore, all SEM images of the three regions were in exact correspondence to the CT images of the inner pores distribution. All of these basic structures of the cement matrix are in agreement with the results reported in previous studies.25,26 3.4. XRD Analysis. Despite the EDS results, it was still necessary to precisely investigate the exact phases of the denser layer in ROI with the XRD analysis that can semiquantitatively characterize the unknown phases. The XRD samples were scraped from the CT examined samples of each region of the ROI, and the scraped powders were dried again prior to the XRD analysis measurements. Figure 8 shows the XRD patterns of the different regions of the exposed specimens. As shown in Figure 8, the XRD pattern of the denser layer of the ROI showed the disappearance of

Figure 6. CT contrast images of the pore distribution of the unexposed and exposed specimens.

Figure 7c shows the cement matrix that hardly suffered the corrosion with a large amount of the fibrous calcium silicate

Figure 7. (a, b, c) SEM images of each layer in the ROI specimen and the EDS elemental spectrum of the top layer of ROI. 10892

DOI: 10.1021/acs.iecr.6b02162 Ind. Eng. Chem. Res. 2016, 55, 10889−10895

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Figure 8. XRD patterns of exposed specimen in three sections of the ROI.

Figure 9. Schematic of interface reaction in cement.

silicate (C2S), and tetracalcium aluminoferrite (C4AF). In addition to these phases, gypsum at 3−5 wt % of the cement is always added to prevent flash set before leaving the factory. Following cement hydration, the major phases of the set cement are CSH and CH; these provide sufficient products when the humid H2S gas seeped into and attacked the cement matrix through the pores. The reason why the humid H2S gas can corrode the cement is that the H2S dissolved into water and lowered the pH, making the hydration of the cement unstable and leading to the initiation of the cement dissolution in the acid pore water by the H2S. For pyrite, 2θ peaks are 33°, 38°, 44°, 47°, and 68°, which are consistent with some unidentified peaks at the top layer of the ROI in Figure 8. Therefore, pyrite is formed at the top layer of the ROI, as described by eqs 4−7.

ettringite and the appearance of gypsum, in good agreement with the results of the elemental EDS analysis. The obtained XRD pattern of the loose region in the exposed specimen clearly showed broad peaks at positions corresponding to quartz (SiO2), portlandite (CH), and tricalcium silicate (C3S). This means that the cement matrix in this region was consumed by the corrosion and the corrosion products can be transported due to the ionic migration through the pores filled with acid water, causing the lowering of pH when the H2S dissolved in water. As the low pH acid water diffused into the cement matrix, the portlandite, CSH, and ettringite crystal disintegrated and dissolved in the pore water, according to eqs 1−3.27−29 Ca6[Al(OH)6 ]2 · (SO4 )3 ·26H 2O

H 2S ↔ H+ + HS− ↔ 2H+ + S2 −

→ 6Ca 2 + + 2Al(OH)4 − + 4OH− + 3SO4 2 − + 26H 2O

(4)

(1)

CSH → Ca 2 + + SiO32 − + H 2O

(2)

Ca(OH)2 → Ca 2 + + 2OH−

(3)

0.43(3CaO ·Al 2O3 ·3CaSO4 ·31H 2O) + 1.3H 2S = 1.3 CaS + 0.86Al(OH)3 + 1.3CaSO4 · 2H 2O + 10.83H 2O (5)

Oil-well cement consists mainly of four components: tricalcium aluminate (C3A), tricalcium silicate (C3S), dicalcium

SO4 2 − + Ca(OH)2 + 2H+ = CaSO4 ·2H 2O 10893

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DOI: 10.1021/acs.iecr.6b02162 Ind. Eng. Chem. Res. 2016, 55, 10889−10895

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Industrial & Engineering Chemistry Research Fe2 + + S2 − = FeS

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The dissolution of the cement matrix leads to the increase in porosity and changed the distribution of the pores in the ROI.30 As shown in the CT images presented in Figures 3−6 and the SEM images in Figure 7, the ions released from the dissolution of hydration products of cement can migrate to the upper layer of the cement matrix. To summarize the reaction process, there a schematic of this process shown in Figure 9.

4. CONCLUSION This paper presents an original attempt to investigate the interface microstructure for an oil-well cement unidirectionally corroded by humid H2S gas by employing the computed tomography. Prior to the use of computed tomography, the variation of the gas permeability at three different sections was discovered and the permeability data showed that the corrosion was stopped by the denser layer in the ROI. Then, the pores distribution of the exposed and unexposed specimens was extracted from the CT images, and the denser layer was accurately determined to be located on the top of the ROI. Furthermore, the components and the formation mechanism of this denser layer were characterized and identified by the SEM, EDS, and XRD analyses. The results showed that the denser layer was grown by the gypsum crystal when the cement matrix dissolved at the low pH and the ions migrated from the loose layer to the top layer of the ROI in the pore water. Previous studies also confirmed this formation mechanism of cement dissolution and ions migration during corrosion. Due to the presence of the dense gypsum layer, the cement matrix was not attacked further and was preserved by this layer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the support provided by the National Science and Technology Major Special Projects of China (2011ZX05021-004), the Education Department projects of Sichuan Province, China, 2013, 2015 (13ZA0182, 15ZB0062), and the National Key Research and development Plan, China (2016YFB0303602).

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02162 Ind. Eng. Chem. Res. 2016, 55, 10889−10895