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Environ. Sci. Technol. 2008, 42, 282–288

Mineralogical Changes of a Well Cement in Various H2S-CO2(-Brine) Fluids at High Pressure and Temperature N I C O L A S J A C Q U E M E T , * ,† JACQUES PIRONON,† AND JÉRÉMIE SAINT-MARC‡ Faculté des Sciences, Université Nancy I, G2R-CREGU, BP 239, 54506 Vandoeuvre-lès-Nancy Cedex, France, and Centre Scientifique et Technique Jean Feger, TOTAL, avenue Larribau, 64018 Pau Cedex, France

Received April 11, 2007. Revised manuscript received September 07, 2007. Accepted September 10, 2007.

The reactivity of a crushed well cement in contact with (1) a brine with dissolved H2S-CO2; (2) a dry H2S-CO2 supercritical phase; (3) a two-phase fluid associating a brine with dissolved H2SCO2 and a H2S-CO2 supercritical phase was investigated in batch experiments at 500 bar and 120, 200 °C. All of the experiments showed that following 15–60 days cement carbonation occurred. The H2S reactivity with cement is limited since it only transformed the ferrites (minor phases) by sulfidation. It appeared that the primary parameter controlling the degree of carbonation (i.e., the rate of calcium carbonates precipitation and CSH (Calcium Silicate Hydrates) decalcification) is the physical state of the fluid phase contacting the minerals. The carbonation degree is complete when the minerals contact at least the dry H2S–CO2 supercritical phase and partial when they contact the brine with dissolved H2S–CO2. Aragonite (calcium carbonate polymorph) precipitated specifically within the dry H2S–CO2 supercritical phase. CSH cristallinity is improved by partial carbonation while CSH are amorphized by complete carbonation. However, the features evidenced in this study cannot be directly related to effective features of cement as a monolith. Further studies involving cement as a monolith are necessary to ascertain textural, petrophysical, and mechanical evolution of cement.

Introduction Capture and storage is an option for mitigating the anthropogenic release of carbon dioxide (CO2) into the atmosphere (1). After its separation from industrial and power plant sources, it is transported to a suitable storage site. One of the storage alternatives is geological storage. It is accomplished by injecting the gas into a geological reservoir (e.g., oil and gas depleted fields or deep saline aquifers) via wells penetrating the reservoir. Geological storage is the most promising and mature option for the long-term sequestration of CO2. Several industrial-scale injections of CO2 in geological reservoirs are currently operating: the Sleipner project in the North Sea, the Weyburn project in Canada, and the In Salah project in Algeria (1). * Corresponding author phone: +33-(0)2-38-64-34-65; fax: +33(0)2-38-64-37-19; e-mail: [email protected]. † Université Nancy I. ‡ TOTAL. 282

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Around 40 acid gas (mixture of CO2 and hydrogen sulfide (H2S)) injections into geological reservoirs have operated in western Canada since 1990 (2). These operations provide practical experience for the operators/institutions that are/ will be in charge of the on-going and planned industrialscale CO2 geological storages. The features of a well are a casing made of steel and a sheath of cement (see Figure 1 in ref 3). The reactivity of these materials with the fluids (gas, brine) occurring in the host reservoir at high pressure and high temperature can result in their alteration (i.e., the change of their initial characteristics). The mineralogical changes (precipitation and/or dissolution of minerals) of cement induced by geochemical reactions can change the physical properties of the material (texture, porosity, permeability, diffusivity and mechanical resistance). Hence, the cement sheath may be modified by reactions and could provide a pathway for the H2S–CO2 fluids to leak to the surface. This could have impacts on human health and the environment since H2S is highly toxic. Leakage via deteriorated wells is therefore an issue for the long-term safety of acid gas geological storage (4). An understanding of the well cement reactivity in such an environment is thus essential to predict whether the cement sheath will retain its isolation characteristics over extremely long periods of time. Previous studies (mainly from the petroleum industry) deal with experimental aging of well cement in CO2containing fluids (5–9). The carbonation reaction is described as the main process of hydrated cement minerals alteration. The complete reaction is described as follows: •Carbonation of Calcium Silicate Hydrate (CSH): CaO · SiO2 · H2O + CO2 f CaCO3 + SiO2 · H2O •Carbonation of portlandite: Ca(OH)2 + CO2 f CaCO3 + H2O The aging of well cement in brine with dissolved H2S was studied by Lecolier et al. (10). Acid attack by the hydrosulfuric acid (H2S(aq)) caused the dissolution of portlandite and CSH of the cement matrix and subsequent increase of porosity and weakening of cement was observed. In Jacquemet et al. (11), crushed well cement was aged at 500 bar-200 °C within brine equilibrated with a mixture of H2S and CO2 during 15 days. The term “crushed cement” designates cured cement ground prior to experiments. It should not be confused with anhydrous cement powder (clinker powder). CO2 caused the partial carbonation of the 11Å-type tobermorite composing the starting cement to form a Ca-depleted 11Å-type tobermorite and calcite. The impact of H2S on cement mineralogy was limited since it was only responsible for the sulfidation of the ferrites (Ca2AlFeO5 or C4AF) into pyrite (FeS2). In the present study, we use the same cured cement in the form of crushed material and explore additional conditions of the aging process: aging within two-phase fluid or dry SuperCritical Phase (designated by “SCP”), lower temperature (120 °C) and time (to 60 days). The previous well cement aging studies have generally confined themselves to investigating cement as a monolith (5–7, 9, 10). By example, Barlet-Gouédart et al. (9) observed clear fronts with successive chemical and textural zones from surface to the core of cement. These fronts indicate a heterogeneous alteration caused by coupled reaction and transport (diffusion) processes. Precipitation of carbonates reduces porosity and diffusivity. The presence of a carbonate front decreases CO2 diffusion, creating heterogeneous reactivity areas in a plug. For these reasons, it is difficult to discriminate whether the carbonation degree (in terms of 10.1021/es070853s CCC: $40.75

 2008 American Chemical Society

Published on Web 12/04/2007

FIGURE 1. XRD patterns of the starting cement and of the aged cements. T: 11Å-type tobermorite; F: ferrite; Q: quartz; C:calcite; A: aragonite; P: pyrite; S: scawtite; w/o: without. See the text for the sample naming convention. The XRD patterns of the samples HW-200–15 and HW-200–60 are similar; only the HW-200–60 pattern is shown. The XRD pattern of the samples MW-120–15 and MW-200–15 are similar to the MW-120–60 and MW-200–60 patterns, respectively; only the two last patterns are shown. carbonates mass precipitation) of a given volume of cement can be more attributed to transport or to chemical processes effectiveness. The use of crushed cement avoids diffusionlowering phenomena by providing a homogeneous reactivity of cement with fluid discriminating chemical reactions impact.

Experimental Section Reactors; Gas Loading Operation; Cement Formulation and Cure; Brine, Steel and Gas Mixture Specifications. The employed reactors and the gas loading operation are described in Jacquemet et al. (11). The formulation of the cement is Portland cement (Class G, High Sulfate Resistant type, defined on API specification), silica flour (35% by weight of cement) and water (the mass ratio (water/(clinker + silica flour)) is 0.4). Silica flour is made of fine quartz grains. Cement is cured at 210 bar-140 °C during 8 days immersed in water in autoclaves. The characterization of the cured cement is shown below in the Results. It is finely ground in agate mortar

(giving the material designated by “crushed cement” in this paper) and dried for some days in oven at 60 °C prior to its introduction into the reactors. This ensures that no additional free water (interstitial water) than the water contained in brine is introduced in the reactors. The drying operation temperature is considerably lower than the cure temperature ensuring no mineral transformation occurs before experiments. The brine is a solution prepared with deionized water and NaCl with a concentration of 150 g/L. The steel is a low alloyed steel (C22E or XC18 type) made of 98 mol% of metallic iron and typical of well casing. It is machined into the form of small cylinders. The gas mixture is provided by Air–liquide and composed of 66 and 34 mol% of H2S and CO2, respectively. Studied Systems. Three systems: “Highly Wet” (HW), “Moderately Wet” (MW) and “Dry” (D), were studied. The mass of crushed cement, steel cylinder, and gas mixture are fixed respectively to 0.3, 0.07, and about 0.3 g in the three systems. A decreasing mass of brine and consequently of VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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combination of experimental P–T is based on in situ conditions of deep sour oil-fields (4 km depth and overpressured) of the Caspian sea, which are candidates for acid gas injection. These conditions are higher than the maximal ones encountered in Alberta acid gas sequestration sites (360 bar110 °C, (2)) and those encountered in the CO2-storage pilots Sleipner (100 bar-37 °C, (12) and Weyburn (150 bar-54 °C, (13)). The experimental temperature of 200 °C was also chosen to maximize mineral reaction kinetics as done by Kaszuba et al. (14). The duration of the treatments was 15 and 60 days for the systems HW and MW, respectively, and only 60 days for the system D. Identification of Samples. Samples are identified according to the following convention: the prefixes HW, MW or D designate the system where the cement is aged; the second group of numbers designates the temperature (°C) of aging; the suffix designates the duration (days) of the treatment. Thus, the sample aged in the system HW at 120 °C and during 15 days is called “HW-120-15”. Cement and Steel Analysis. After recovery, the crushed cement is prepared by regrinding in an agate mortar for XRD and TEM analysis. The steel corrosion precipitates are scraped off with a scalpel blade, and analyzed by TEM. The XRD and TEM analysis details are given in the Supporting Information. Residual Gas Analysis. The gas line designed for the study allows recovery of the residual gas (i.e., unreacted gas) after the experiments (11). Quantitative gas analysis by Raman spectrometry was developed by Dubessy et al. (15) and applied for this study by using a specific gas cell described in Jacquemet et al. (11). The amount of gas consumed by mineral reactions was determined by comparing the molar quantities of the initial gas with the residual gas. Fluid Sampling and Analysis during the Experiments: The Synthetic Fluid Inclusion Method. Quartz crystal lamella is loaded in each reactor adjacently to cement for fluid inclusion study. The Synthetic Fluid Inclusions (designated by “SFI”) method was previously described (11, 16, 17). We also used for this study a specific protocol to analyze quantitatively the dissolved sulfur in fluid inclusions solutions (16). In summary, the SFI method is an efficient technique for microsampling of fluids in contact with the cement under the experimental conditions and for the investigation of the state and chemistry of this fluid at experimental P–T.

Results FIGURE 2. TEM observations of CSH (A–D); CSH remnant (E); microthermometric observations of synthetic fluid inclusions (F-H). (A) CSH from starting cement exhibiting platelet morphology related to tobermorite; (B) CSH from starting cement exhibiting fibrous aggregate morphology related to a form of C–S–H gel; (C) CSH from sample HW-120-15 exhibiting well crystallized platelet morphology related to tobermorite; (D) CSH from sample HW-200-15 exhibiting well crystallized elongated platelet morphology related to tobermorite; (E) silica-rich particle exhibiting amorphous granule morphology; (F) synthetic fluid inclusion observed at 200 °C exhibiting homogeneous aqueous liquid phase; (G) synthetic fluid inclusion observed at 200 °C exhibiting a bubble of H2S-CO2 SCP (SuperCritical Phase) surrounded by aqueous liquid phase; (H) synthetic fluid inclusion observed at 200 °C exhibiting a H2S–CO2 SCP. See the text for samples identification. free water was introduced into the HW and MW systems: the system HW contains 0.29 g of water and the system MW 0.07 g. The system D is free of brine. The evacuation prior the gas loading operation ensures an optimal dryness of this system. Pressure, Temperature and Duration of Experiments. Two combinations of pressure (P) and temperature (T) are employed: 500 bar-120 °C and 500 bar-200 °C. The first 284

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Starting Cement. XRD analyses revealed the presence of 11Åtype tobermorite (called simply tobermorite in this paper), ferrite and quartz (SiO2) (Figure 1). A summary of XRD analyses is also shown in the Table S1, Supporting Information. In hydrothermal cement pastes, the CSH (binding phase) can be of different varieties: “C–S–H gel” and hydrothermal CSH. C–S–H gel designates amorphous or poorly crystalline CSH while hydrothermal CSH designates more highly ordered phases (which crystallize especially at relatively high temperatures) like the tobermorite (18). The ferrites are anhydrous clinker remnants. The quartz present in the cement corresponds to silica flour (actually composed of quartz grains) residues, i.e., not consumed by the pozzolanic reactions during the cure. The TEM analyses revealed CSH as the major phase of the starting cement. Amorphous fibrous aggregates and crystallized platelets are the two observed morphologies of the CSH (Figure 2, A and B). The amorphous fibrous aggregates can be related to a form of C–S–H gel while the crystallized platelets can be interpreted as tobermorite (attested by the diffraction electron patterns). The mean (Si + Al)/Ca ratio of the CSH is 1.1 (Table 1) and slightly variable (standard deviation of 0.2). The TEM analyses of the CSH and related phases of this study are also plotted in the Al/Ca vs. Si/Ca graphs of the Figure S1, Supporting Information.

TABLE 1. Mean Composition, Morphology, and Crystallinity of the Starting and Aged CSH from the TEM Analysisa mean elemental composition (atomic %, excepted O) sample

Si

Ca

Al

mean (Si+Al)/Ca (standard deviation)

number of analyses

morphology and cristallinity

starting CSH HW-120–15 HW-120–60 HW-200–15 HW-200–60 mean HW MW-120–15 MW-120–60 MW-200–15 MW-200–60 mean MW D-120–60 D-200–60 mean D

49.0 50.5 53.6 51.3 52.6 52.0 86.4 92.5 83.6 91.9 88.6 94.1 92.7 93.4

48.0 44.9 41.5 45.3 42.2 43.5 7.9 2.2 10.0 2.0 5.5 2.2 3.7 3.0

3.0 4.6 4.9 3.4 5.2 4.5 5.8 5.2 6.3 6.1 5.8 3.7 3.7 3.7

1.1 (0.2) 1.3 (0.5) 1.4 (0.2) 1.3 (0.4) 1.4 (0.3) 1.3 41.5 (46.3) 63.3 (39.5) 34.0 (45.6) 71.0 (25.7) 52.4 56.0 (30.4) 39.0 (25.7) 47.5

34 26 18 24 19

CP and AFA (Figure 2, A and B) CP and AFA (Figure 2, B and C) CP and AFA (Figure 2, B and C) CEP (Figure 2, D) CEP (Figure 2, D)

24 15 21 14

AG (Figure 2, E) AG (Figure 2, E) AG (Figure 2, E) AG (Figure 2, E)

12 16

AG (Figure 2, E) AG (Figure 2, E)

a See the text for sample identification. CP: Crystallized Platelet; AFA: Amorphous Fibrous Aggregate; CEP: Crystallized Elongated Platelet; AG: Amorphous Granule.

System Highly Wet. The XRD analyses on samples aged at 120 °C show the preservation of tobermorite and quartz, the presence of newly formed calcite (CaCO3) and the disappearance of ferrite, independent of the duration of the experiment (Figure 1). The cristallinity of tobermorite is improved as shown by the increase in the intensity of its peaks (Figure 1). At 200 °C, the XRD analyses show also the preservation of tobermorite and quartz, the neoformation of calcite, scawtite (Ca7Si6(CO3)O18 · 2H2O) and pyrite (FeS2). Ferrite, too disappears, independent of the duration of the experiment. Similar to its behavior at 120 °C, the crystallinity of tobermorite is improved as shown by its increased peak intensity. However, quartz is partially dissolved as reflected in its decreased peak intensity. The XRD analysis of the equivalent of the sample HW-200-60 aged without steel shows also the neoformation of pyrite. This indicates the iron contained in the pyrite is provided by destabilization of the iron-bearing minerals (ferrites) of the cement. The TEM observations and microanalyses revealed CSH as the major phase and calcite and pyrite (only detected at 200 °C) as the minor phases. At 120 °C, independent of the duration, the CSH have morphologies of well-crystallized platelets (Figure 2, C) and of amorphous aggregates similar to the starting cement (Figure 2, B). At 200 °C and as well at 15 and 60 days they appear only in the form of elongated crystallized platelets (Figure 2, D). The CSH compositions are similarly variable as in the starting cement. No significant difference of composition is noticed between 120 and 200 °C but there is a small evolution with time: the mean (Si + Al)/Ca ratio is greater at 60 than at 15 days (Table 1). The overall CSH from the system HW is slightly Si-enriched, Alenriched, and Ca-depleted (by a few atomic %) in comparison with the starting CSH giving an overall (Si + Al)/Ca ratio increasing from 1.1 to 1.3. Traces of H2 (less than 5 mol%) are detected in the residual gas and roughly 40% of the analyses showed the presence of COS (carbonyl sulphide). The percentage of H2S and CO2 consumed by mineral reactions is in the range of 5–15 and 15–25 mol%, respectively (Figure 3). Typical microthermometric observations at 120 and 200 °C of the SFI created during the 15 days experiments are shown in Figure 2, F. All the inclusions brought to the experimental temperature exhibited a monophasic fluid composed of aqueous liquid. The microRaman analysis identified HS- and H2S(aq) (in order of abundance) which are stable in reductive conditions. No significant concentrations of carbon species were noted. The predominance of HSrelative to H2S(aq) indicates a basic pH under experimental

conditions. The aqueous sulfur concentration ranges between 0.1 and 1.4 mol/L. System Moderately Wet. At all temperatures and durations, the minerals detected by the XRD in the aged samples are quartz and calcite (Figure 1). The TEM investigations show the aged cement is mainly composed of calcite and silica-rich particles. The silica-rich particles exhibit similar amorphous granules morphologies for any duration and temperature (Figure 2, E). The amorphous nature of the silica-rich particles is proven by the electron microdiffraction performed with the TEM. They are more Ca-poor at 60 than at 15 days giving an increase of (Si + Al)/Ca ratio between these two durations (Table 1). Their compositional range at 15 days is broad but decreases after 60 days of treatment. No significant difference of composition is noted between 120 and 200 °C. The overall (Si + Al)/Ca ratio is greatly higher and variable than in the starting cement and in the system HW. The residual gas analysis showed traces of H2 (up to 1.8 mol% for the system MW-200-60) and, on some occasions (roughly 30% of the analyses), traces of COS. The percentages of mineralized H2S and CO2 range from 1 to 15 mol% and from 65 to 75 mol%, respectively (Figure 3). The amount of mineralized CO2 is thus 4 times higher than in the system HW; the amount of mineralized H2S is nevertheless not clearly different than in the system HW. Three populations of SFI are characterized under the experimental conditions: the first population exhibits a monophasic fluid composed of aqueous liquid as in Figure 2, F; the second population exhibits a diphasic fluid composed of aqueous liquid + H2S–CO2 SCP with various volumic phase

FIGURE 3. Relative amount of mineralized CO2 and H2S for the diverse experiments. See the text for sample identification. VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ratios (Figure 2, G); the third population exhibits a monophasic fluid composed of H2S–CO2 SCP (Figure 2, H). The aqueous liquids contain HS-, H2S(aq), HCO3- and CO2(aq). Some aqueous liquids have a predominance of HS- relative to H2S(aq) and of HCO3- relative to CO2(aq). These have a basic pH under experimental conditions. On the other hand, some aqueous liquids have a predominance of H2S(aq) relative to HS- and of CO2(aq) relative to HCO3-. These have a pH close to the neutral under our experimental conditions. The aqueous liquids have a range of aqueous sulfur concentration between 2.3 and 5.6 mol/L. The H2S-CO2 SCP phases contain traces of H2. System Dry. The XRD analysis shows the disappearance of tobermorite and ferrite, the preservation of quartz and newly formed calcite and aragonite in the aged samples at 120 and 200 °C (Figure 1). The TEM investigations show silica-rich particles, calcite and aragonite as the main phases. The aragonite can be differentiated from the calcite by its habit (platelet for aragonite vs. rhombohedra for calcite) and by electron microdiffraction. The silica-rich particles exhibit morphologies of amorphous granules as in the system MW (Figure 2, E). Their compositions and compositional dispersion are similar to those of the system MW (Table 1). H2S, CO2 and small quantities of H2 (up to 3.5 mol%) and rarely (0.2% of the analyses) of COS have been detected in the residual gas. The amount of mineralized CO2 lies between 60 and 80 mol%, similar to the values observed in the MW system (Figure 3). On the other hand, the amount of mineralized H2S is very low (around 1 mol%) in comparison with the wet systems HW and MW. One type of SFI is characterized under the experimental conditions: it exhibits a monophasic fluid (Figure 2, H as example) composed either of a SCP under the form of either a vapor with H2S, CO2 and traces of H2O (up to 5 mol%) or liquid H2S (H2S(l)) containing dissolved CO2. Steel Corrosion (Results and Discussion Presented in the Supporting Information).

Discussion CSH Composing the Samples. The CSH composing the starting cement (called Tob-1) is a tobermorite with the mean Ca, Si and Al atomic% given in the Table 1. Its calculated formula based on 18 oxygen and 2 hydrogen atoms (of the silicate tetrahedrons and hydroxyl groups) is as follows: Ca5.4Si5.5Al0.4O16(OH)2 · 4H2O (Tob-1). The CSH composing the cement from the HW system (called Tob-2) is also a tobermorite but Ca-depleted, with the following calculated formula: Ca4.8Si5.8Al0.5O16(OH)2 · 4H2O (Tob-2). The amorphous silica-rich particles composing the cement aged in the MW and D systems (called amorphous silica) are interpreted as the starting cement CSH remnants after their drastic decalcification because of the traces of Ca and Al. Their mean calculated formula (based on 2 oxygen atoms) is as follows: amorphous silica: Ca0.05Si0.96Al0.05O2 (amorphous silica). Mineral Assemblages and Crystallinity. In the system Highly Wet, an identical secondary mineral assemblage of Tob-2 plus quartz (Qtz) plus calcite (Cal) is produced at both temperatures from the primary mineral assemblage of Tob-1 plus quartz: Tob-1 + Qtz f Tob-2 + Cal + Qtz* (system Highly Wet at 120 and 200 °C) Qtz*: quartz dissolved within the duration of the 200 °C experiment 286

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Partial dissolution of quartz is only observed at 200 °C and could be due to the combined effect of the high temperature and of the basic pH buffered by the tobermorite. The crystallinity (ratio of platelets to the sum of platelets plus amorphous CSH (C–S–H gel)) of the Tob-2 formed at 200 °C is high while it is lower at 120 °C but it increases with time at this temperature. The overall crystallinity of CSH is nevertheless improved in this system. The sulfidation of the cement ferrites leads to the formation of FeS2 at 200 °C as in Jacquemet et al. (11). In the system Moderately Wet, the initial assemblage of Tob-1 plus quartz is converted to the secondary assemblage of calcite plus amorphous silica plus quartz with no influence of temperature or duration of aging: Tob-1 + Qtz f amorphous silica + Qtz + Cal (system Moderately Wet at 120 and 200 °C) amorphization of CSH occurs. The long-term (60 days) aging has only the effect of lowering the compositional dispersion of the amorphous silica. In the system Dry, the initial assemblage of Tob-1 plus quartz is converted to the secondary assemblage of calcite plus aragonite (Ar) plus quartz plus amorphous silica without influence of temperature: Tob-1 + Qtz f amorphous silica + Qtz + Ar + Cal (system Dry at 120 and 200 °C) amorphization of CSH also occurs. No iron sulfide mineral has been detected in the aged cements from the systems MW and D. The CSH cristallinity changes (improvement in the system HW and amorphization in the systems MW and D), and the development of a heterogeneous mineralogical assemblage could impact cement mechanical resistance (19). However, the features we observed cannot be directly related to effective features of cement as monolith. Further studies involving cement as monolith are necessary to elucidate the mechanical properties of cement. Mineralogical results are identical whether steel is present or not. This observation can be explained by the two following points. First, we suppose the low redox conditions are mainly buffered by the large concentration of H2S and aqueous sulfides rather than surface metallic iron of the steel cylinder. Hence, the presence of steel does not change significantly the redox conditions. Second, the CSH are more sensitive to pH than to redox variations. Carbonation Degree and Contacting Fluid Phase. In our experiments with mixed H2S and CO2 fluids, the formation of calcium carbonates is due to carbonation as in CO2-only experiments on well cement under other (lower) conditions (5–9). H2S has a low effect on cement mineralogy since it reacts (via aqueous HS-) only with the ferrites (minor phases of the cement) by sulfidation to produce S-bearing phases (e.g., pyrite at 200 °C in system HW). In the systems MW and D: (1) the high yield of calcium carbonates; (2) the production of Amorphous Silica by drastic CSH decalcification; (3) the high consumption of CO2 indicate the carbonation is complete, whereas it is only partial in the system HW. The formation of scawtite observed in this system could represent a first stage of carbonation. Indeed, precipitation of this mineral has been already reported by Kapralic et al. (20) in CaO-SiO2-CO2-H2O systems with low concentrations of CO2 for temperatures in excess of 160 °C. The gas/brine ratios in the systems HW and MW correspond to 17 mol of H2S/kg of water and 10 mol of CO2/kg of water in the system HW, 71 mol of H2S/kg of water and 43 mol of CO2/kg of water in the system MW. These amounts guarantee gas excess and coexistence of separate H2S-CO2 SCP/brine phases in these two systems as we observed in

the fluid inclusions. No phase equilibrium data relevant to our system and P–T conditions have been found in the literature. In the systems HW and MW, the brine is thus supposed to be saturated with respect to H2S and CO2. In the system D, the small amount of water detected in the SCP could have been released by CSH dehydration associated with their decalcification and amorphization. The various measurements of S concentrations (from 0.1 to 5.6 mol/L) in brine of inclusions are attributed to a heterogeneous trapping (in time) of brine within the inclusions (17). If we assume the higher value to represent equilibrium between SCP and brine at the experimental conditions, the H2S solubility in the system H2S-CO2-H2O-NaCl-cement-steel at 500 bar-120 and 200 °C must be around 5.6 mol/L. This value is close to the results of Selleck et al. (21) for the system H2S-H2O at 207 bar-138 and 171 °C (6.1 and 5.7 molal). The solubility of CO2 was not measured in the study and no literature data are available to prove and quantify the effect of H2S on CO2 solubility for our conditions. The lack of aqueous C noticed in the inclusions of the system HW is attributed also to the heterogeneous trapping of fluid by the inclusions. The observed C-free brines could have been trapped just after C consumption by mineral reactions and before recharge in C by CO2 dissolution. The reductive and basic character of the brine in the system HW is confirmed by the occurrence of HS- and H2S(aq) in order of abundance. The observation of both basic and more neutral brines in the inclusions of the MW system is again related to heterogeneous trapping by the inclusions. We can assume the basic solutions are trapped in the early stages of the experiments when CSH are not yet destroyed and act as a basic pH buffer while the neutral liquids are trapped later when CSH are destroyed and no longer play this role. Only the brine fraction of the two-phase fluid is trapped by the inclusions in the system HW attesting that the quartz lamella and consequently cement (as it surrounds the lamella) contact this phase. In the system MW, the characterization of the three types of inclusions (monophasic aqueous liquid, SCP, and two-phase liquid) indicates that the two-phase fluid contacts the cement. The SFI results from the system D show the particles of cement are surrounded by monophasic H2S-CO2 SCP without occurrence of liquid water under experimental conditions. The SFI investigations show the degree of carbonation is actually related to the contacting fluid phase of the minerals: partial carbonation occurs when CSH contact only brine (system HW); complete carbonation occurs when CSH contact at least H2S-CO2 SCP (systems MW and D). Thus, we demonstrated that no liquid water is needed for CSH to react with CO2 (i.e., solid-state reaction); this is much more effective at carbonation than liquid watermediated reaction and aragonite precipitation is specifically related to this solid-state reaction. On the other hand, Regnault et al. (22) had shown that CH was totally carbonated to form calcite and aragonite in anhydrous CO2 experiments. The moderate carbonation occurring in the system HW could be due to the presence of the liquid water surrounding the CSH which acts as an intermediate agent between the CO2 and the minerals; it lowers the access of CO2 to mineral surface by diffusion barrier effect and availability limitation (solubility constraint). The inhibition of carbonation by high water content has been already reported in several studies (8, 23–25) but it is explained rather by diffusion limitation of gaseous CO2 by liquid water within porous space than by chemical process limitations. Despite the presence of liquid water in the system MW, the solid-state carbonation via SCP seems to be the predominant process. Contrary to the system HW, the quartz is not dissolved in the systems MW and D where the SCP contacts the minerals. Implications for Well Cement Durability in Acid Gas Storage. This study implies that carbonation of well cement

sheath would be complete in zones along the wellbore where liquid water is suspected to be negligible or absent (by vaporization into the supercritical phase) as in the injection zone or at the wellbore-caprock contact where the “bubble” of supercritical phase could accumulate. Exposure of cement sheath to brine with dissolved H2S and CO2 (in water-rich zones of the storage) would lead to a partial carbonation. CSH crystallinity would be improved by partial carbonation, and CSH would be amorphized by complete carbonation. This could have an impact on the mechanical behavior of cement. However, the features observed in this study cannot be directly related to effective features of cement as monoliths. Further studies involving cement as a monolith are necessary to ascertain the textural, petrophysical, and mechanical evolution of cement and to allow more solid conclusions on well cement durability in such a context.

Acknowledgments This work was supported by TOTAL through its Residual Gas Management (RGM) research program. We thank the leader and the members of the RGM program for permitting the submission of this publication. We also thank Jean-Paul Emeraux and Jafar Ghanbaja from Université Nancy 1 for the XRD TEM analyses respectively, and Thérèse Lhomme from G2R laboratory for the Raman analyses.

Supporting Information Available XRD and TEM analysis details, summary of XRD analyses results, Al/Ca vs. Si/Ca plot of CSH analyses by TEM, results and discussion concerning steel corrosion. This material is available free of charge via the Internet at http://pubs.acs.org.

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