The gas reservoir of PUGUANG contains high levels of H₂S and CO₂ with 15% and 8% by volume respectively. As to the corrosion of CO₂/H₂S mixture to cement, people seldom involve in research. Cement stone samples corroded by H₂S/CO₂ mixture under different temperature and pressure are tested to probe the change of compressive strength and permeability. Microstructure and corroded products of corroded samples were observed by SEM and XRD. The result shows the corroded products of CO₂/H₂S mixture to cement are similar to those by single-component H₂S or CO₂ gas, except that the amount of expansive crystal produced by H₂S is reduced. Combination of H₂S and CO₂ accelerates the corrosion progress, the recession of strengthen and permeability is more serious than that of single action by H₂S or CO₂ simultaneously, but CO₂ dominates the whole corrosion process after the long duration. Fly ash and Clay have benefits to resist corrosion of combination of H₂S and CO₂.

sour gas, cement stone, corrosion, H₂S, CO₂, puguang

The PUGUANG gas field is the biggest sour gas field in China which contains 15% and 8% by volume of H₂S and CO₂ respectively.¹Acidic gas and alkaline cement ring will react in wet condition to cause compressive strengthen reduce and damage seal function of cement ring.^{2 ̶ 3} Previous studies focused on the single corrosion mechanism of CO₂ or H₂S to cement stone, as to the corrosion of CO₂/H₂S mixture to cement, people seldom involve in research. Such as Yaoxiao⁴ & Zhou Shiming⁵ has studied the change of compressive strengthen of cement by CO₂, Ma kaiHua⁶ has made systematic studies on the H₂S corrosion to cement. What will happen on the cement after the combinatorial action by H₂S and CO₂ mixture? In this paper test methods are established under well whole condition of PUGUANG gas field,⁷ the compressive strength and permeability of cement stone are measured before and after corrosion according to API Spec10B. SEM and XRD are used to study the change of micro structure and reaction product before and after corrosion.

Experimental process

Experimental parameter: Test parameters are determined according to well condition of PUGUANG gas field as follows:

1. Experimental temperature: 95˚C, 130˚C, 150˚C.
2. Corrosive media: xH₂S = 65.2% xCO₂ = 34.8%.
3. Pressure: 15MPa, the partial pressures of H₂S and CO₂ are 10MPa and 5MPa respectively.
4. Conservation Time: 21 days.
5. Test water: Simulated formation water.

Experimental process

Slurry is prepared according to API Spec10B and poured into test mold and cured in HTHP cell for 24hours. The half of set samples are removed from molds and numbered and putted into the corroding chamber and cured for 21days, then seal the chamber and inject H₂S & CO₂ mixture and apply heat and pressure according to required temperature and CO₂ & H₂S partial pressure. The rest of samples are cured in HTHP cell as comparative analysis samples. During the test the temperature and pressure in the chamber are hold constant at all time by refilling mixture gas and auto temperature controlling.⁸

The compositions of slurry

Two groups of slurry composition are prepared for test listed in Table 1 & 2. The ingredients of group 2 contain 35% silica by weight of cement when the temperature is more than 110˚C. The number of composition matches the number of sample in the following text.

Change of compressive strengthen and permeability after corrosion

Table 3 shows the change of compressive strengthen and permeability of group 1 cement stone before and after corrosion at 95˚C, and Table 4 & 5 show those changes of group 2 at 130˚C and 150˚C. Table 3 ̶ 5 shows most of samples present compressive strengthen reduction and permeability increase after corrosion. Sample 3 and 4 show comparatively good corrosion resistance with other samples.

Information in tables also shows that as the temperature rises, the recession of strengthen and permeability is more serious, which presents a complete opposite rule with that of single corrosion by H₂S or CO₂.

Analysis on reaction products of corroded cement samples

Corroded products analysis at 95˚C: Figure 1 is XRD result of sample 1 and Figure 2 is SEM picture of sample 1and 6 of group1. Figure 1 shows that there is lots of CaCO3 crystal in outer layers of sample 1, which are the products of CO₂ reaction. Little CaSO₄ Crystal is founded in the inside of the sample which is the products of H₂S A large amount of Ca (OH) ₂ is founded in the inner of sample 1. Figure 2 shows that there are lots of cracks and pores in both samples of 1 and 6 which verifies the recession of strengthen and permeability. The picture also indicates there is almost no hydrated calcium silicate (CSH) in the cement stone.

Corroded products analysis at 130℃: Figure 3 & 4 are of XRD results of the corroded samples from No.1to NO.6 of group 2 at 130˚C. Figure 3 analyzes products in the outer layer of samples and Figure 4 analyzes products of in the core of samples. Figure 3 shows there are large amount of mini-crystal calcium carbonate (CaCO₃ (I)) and calcite (CaCO3 (II)) and a little gypsum in all the samples. Figure 4 shows almost no Ca(OH)₂ in the core of samples. For lack Ca(OH)₂, CSH lost stability by transforming to C₂SH, which can explain why the recession of strengthen and permeability becoming more serious as the temperature rising. Figure 5 is the SEM pictures of sample 3 and 4. The picture indicates there are lots of cracks and pores in both samples. The picture also indicates There are almost no hydrated calcium silicate (CSH) and Ca(OH)₂ in the cement stone

Corroded products analysis at 150˚C: Figure 6 & 7 are of XRD results of the corroded samples from No.1to NO.6 of group 2 at 150˚C. Figure 6 analyzes products in the outer layer of samples and Figure 7 analyzes products of in the core of samples. Figure 6 shows there are large amount of mini-crystal calcium carbonate (CaCO₃ (I)) and calcite (CaCO₃ (II)) and a little gypsum in all the samples. Figure 7 shows there is CaCO₃ in the core of cement stone, which indicates the corroding reaction makes deeper. Figure 8 is the SEM pictures of sample 3 and 5. The picture indicates there are lots of cracks and pores in both samples, and almost no hydrated calcium silicate (CSH and Ca(OH)₂ in the cement stone. The crystal of CaCO₃ and CaSO₄ is founded in the core of cement which verifies the further corroding reaction with temperature rising.

Figure 1 XRD analysis of core and out layer of the corroded sample 1.

Outer layer SEM photos of No.3 and No.5 corroded sample (the left is No.3).

Inner layer SEM photos of No.3 and No.5 corroded sample (the left is No.3)

Figure 2 Core and outer layer SEM photos of No.1 and No.6 sample.

Figure 3 XRD results of out layer of the corroded samples 1-6 at 130℃.

Figure 4 XRD results of core of the corroded samples 1-6 at 130℃.

Outer layer SEM photo of No.3 and No.4 corroded sample (the left is No.3) inner layer SEM photo of No.3 and No.4 corroded sample (the left is No.3)

Figure 5 SEM photo of samples 3 and 4.

Figure 6 XRD analysis of out layer of the corroded samples 1-6 at 150℃.

Figure 7 XRD results of core of the corroded samples 1-6 at 150℃.

Outer layer SEM photos of No.3 and No.5 corroded sample (the left is No.3).

Inner layer SEM photos of No.3 and No.5 corroded sample (the left is No.3)

Figure 8 Inner layer and outer layer SEM photos of No.1 and No.6 sample.

• Corrosion mechanism of CO₂ to cement

Corrosion mechanism of CO₂ to cement complies with reaction formula (1) and (2).^7,8

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃^- (1)

Ca(OH)₂ + H⁺ + H₂CO₃^- → CaCO₃ + 2H₂O (2)

There is almost no CSH gel in the corroded cement stone sample, but there is large volume of C₂SH, which shows that CSH gel begins to react with CO₂ and produce CaCO₃ and C₂SH, its reaction complies with the reaction formula (3). ^{5,9 ̶11}

CSH + H⁺ + HCO₃^- → C₂SH + CaCO₃ (3)

• Corrosion mechanism of H₂S to cement

Firstly H₂S reacts with Ca(OH)₂ to produce CaSO₄.2H₂O (gypsum) and the volume of solid substance expands, producing fractures in cement stone, then it makes corrosion expanding into the cement until all cement gelatin is corroded and collapsed.

The reaction of H₂S with cement stone is as following.¹²

Ca(OH)₂(S) + H₂S(g) + H₂O(1) → CaSO₄ + 2H₂O (S) (4)

The density of Ca(OH)₂ is 2.24 g/cm³, while the density of CaSO₄·2H₂O is 2.30 g/cm³. Therefore, when corroded by H₂S, the volume cement stone will expand, and producing fractures in it. CSH gel of cement stone also reacts with H₂S solution to produce CaSO₄·2H₂O (gypsum).

The reaction formula is as following.⁶

CSH + H₂S + H2O → CaSO₄·2H₂O + C(m)S(n)H(x) (5)

CaSO₄·2H₂O will continues to react with C3A to produce Ettringite (AFT) catalyzed by Ca(OH)₂,

The reaction formula is as following.¹²

C3A+3(CaSO₄·2H2O) +2Ca(OH)₂+24H₂O－3CaO·Al₂O3·3CaSO₄·32H₂O (6)

The density of Ettringite is 1.73g/cm³, too much ettringite generate will cause cement stone expanding split.

Corrosion mechanism of CO₂ and H₂S mixture to cement

The products corroded by H₂S and CO₂ mixture to Cement stone are similar to those by single-component gas.¹³ CO₂ dominates the whole corrosion process in the long duration, because its’ corroding products are more than the products by H₂S. For the small quantity of products by H₂S, expanding split cannot be founded in whole process by combination of H₂S and CO₂.

Combination of H₂S and CO₂ accelerates the corrosion progress, the recession of strengthen and permeability is more serious than that of single action by H₂S or CO₂ simultaneously. As temperature rises, the recession of strengthen and permeability is more serious, which presents a complete opposite rule with that of single corrosion by H₂S or CO₂.

The temperature of bottom well bore of PUGUANG gas field is 150˚C,and the partial pressures of H₂S and CO₂ are 10MPa and 5MPa respectively, which will bring serious corrosion on cement ring, and damage its’ seal ability. The higher of the temperature rises, the more severe of the recession of strengthen and permeability, which presents a complete opposite rule with that of single corrosion by H₂S or CO₂. The composition of cement slurry is the predominant factor affecting cement corrosion resistance. The introduction of Latex and Fly ash and clay into system will reduce the alkalinity in the cement slurry system and improves the corrosion resistance of set cement. The hydration products are crystallized after corrosion under combination of H₂S and CO₂, and loose arrangement of that crystal is the reason of strength decline and permeability rise. Measures against composite corrosion of H₂S and CO₂ mixture: decline the alkalinity and reduce the porosity of cement slurry.

None.

The authors declare no conflict of interest.

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