Geofluids
Volume 2017 (2017), Article ID 3482603, 11 pages
https://doi.org/10.1155/2017/3482603

Research Article

A Geochemical Model of Fluids and Mineral Interactions for Deep Hydrocarbon Reservoirs

Jun Li,¹ Raheel Ahmed,² Qian Zhang,¹ Yongfan Guo,¹ and Xiaochun Li¹

¹State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China
²Dimue Technology Ltd. Co., Wuhan, China

Correspondence should be addressed to Jun Li; lijun2009s@gmail.com

Received 19 February 2017; Accepted 12 June 2017; Published 16 July 2017

Academic Editor: Keyu Liu

Copyright © 2017 Jun Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A mutual solubility model for CO₂-CH₄-brine systems is constructed in this work as a fundamental research for applications of deep hydrocarbon exploration and production. The model is validated to be accurate for wide ranges of temperature (0–250°C), pressure (1–1500 bar), and salinity (NaCl molality from 0 to more than 6 mole/KgW). Combining this model with PHREEQC functionalities, CO₂-CH₄-brine-carbonate-sulfate equilibrium is calculated. From the calculations, we conclude that, for CO₂-CH₄-brine-carbonate systems, at deeper positions, magnesium is more likely to be dissolved in aqueous phase and calcite can be more stable than dolomite and, for CO₂-CH₄-brine-sulfate systems, with a presence of CH₄, sulfate ions are likely to be reduced to S²⁻ and H₂S in gas phase could be released after S²⁻ saturated in the solution. The hydrocarbon “souring” process could be reproduced from geochemical calculations in this work.

1. Introduction

With the exploration and production of middle-shallow oil and gas reservoirs, the main oil/gas fields have come to the late stages of production. More and more intensive exploration work has been done on middle-shallow fields and it is not easy to achieve more breakthroughs. So, researchers are devoting more efforts in deep reservoirs (with depth more than 5000 m). In China, the depositional environment is quite complex and special, so abundant hydrocarbon resources are possible. From the drilling evidence, an effective hydrocarbon reserve was found at more than 7000 m depth in China [1]. More and more research on deep layer hydrocarbon exploration has been carried out in recent years.

For deep hydrocarbon research, fluid-rock interaction is an important topic, as it will influence the fluid composition, physical and chemical properties, and transportation in porous media. The geochemical reactions are more active at locations with both gas and water, such as so-called gas-water transition zones [2, 3]. When gas and water contact, both gas components and mineral will be dissolved in water, and many geochemical reactions could be triggered. In Sichuan basin, H₂S can usually be found from gas reservoirs. The existence of H₂S can be a result of geochemical reactions of dissolved hydrocarbon and sulfates. It is called “souring” process in some literature [2].

Numerical modeling of geochemistry is a useful tool to understand the mechanism of fluid-mineral interactions in deep reservoirs. PHREEQC is one of the most popular geochemistry software packages in hydrological applications [4]. The speciation in water associated with hundreds of chemical reactions can be dealt with. TOUGHREACT is a 3D reactive transportation simulator which is able to calculate geochemical reactions with similar database as PHREEQC [5]. This simulator has been widely used in CO₂ geological storage and geothermal recovery projects. Both of the software programs are powerful for geochemical reaction analysis in porous media. However, for fluid-mineral interactions in deep reservoir, gas-brine phase partitioning and speciation should be carefully considered due to high temperature and pressure. In gas reservoirs, CO₂ usually exists with quite a bit amount of hydrocarbons. So, in this work, we establish a mutual solubility model for CO₂-CH₄-brine systems, which is accurate for a pressure range of 1 bar to 1500 bar, temperature range of 0°C to 250°C, and salinity range of 0 to 6 m. With the solubility calculated by the model, PHREEQC is used to calculate equilibrium of CO₂-CH₄-brine-minerals (carbonates and sulfates).

2. CO₂-CH₄-Brine Mutual Solubility Modeling

We assume that there are two fluid phases (i.e., aqueous phase and nonaqueous phase) existing at given temperature, pressure, and feed composition. CO₂ or CH₄ always dominates nonaqueous phase. Their solubilities in water and H₂O content in nonaqueous phase are desired to be accurately reproduced by a thermodynamic model. In equilibrium state, for each component in the system (e.g., component ), the chemical potential in each phase should be equal. Then we have,

For nonaqueous phase,

where stands for standard chemical potential of component , which is the ideal gas chemical potential at the pressure of 1 bar [6, 7];   is mole fraction of component in nonaqueous phase; is fugacity and is fugacity coefficient; is the gas constant (8.31446 J/K/mol); is temperature in ; and is pressure in bar hereafter.

For aqueous phase,

where is the standard chemical potential of species in an ideal aqueous solution with a hypothetical unit molality [8]; is the molality (in mole/Kg water, molal for short hereafter) of salt in the aqueous phase; is the mole number of 1 kg water (55.508); is the mole fraction of species dissolved in the aqueous phase; is activity of component in aqueous phase; and is activity coefficient of component .

With (1) to (3), we have

Here, equilibrium constant is defined as , so we have

The mutual solubility model of CO₂-CH₄-brine system is established based on the above principle. Equilibrium constants (), fugacity coefficients (), and activity coefficients () should be properly carefully obtained.

2.1. Equilibrium Constants

For equilibrium constant of H₂O, we follow the work of Li et al. [7] with an empirical equation revised from Spycher et al. (2003):

The parameters ( to ) in (6) are all from Li et al. [7], listed in Table 1.

Table 1: Parameters of H₂O equilibrium constant in (6).

Parameters
 K
 K

For equilibrium constants of CO₂ and CH₄, we follow the form of Mao et al. (2013):

where = CO₂, CH₄.    are parameters. The parameters are determined from experimental data of gas solubility in aqueous phase. See Table 2 for more details.

Table 2: Parameters of CO₂ and CH₄ equilibrium constants in (7).

b ₆

CO₂

CH₄

2.2. Fugacity Coefficients

Peng-Robinson equation of state (PR-EOS for short [9]) is used from CO₂, CH₄, and H₂O fugacity coefficients in the nonaqueous phase. PR-EOS is a classic two-parameter equation of state of cubic form. The related parameters of pure component systems can be found from the original work of Peng and Robinson [9]. For gas mixtures, mixing rule is used for the two parameters.

where and are binary interaction parameters of species and . Binary interaction parameters for CO₂, CH₄, and H₂O can be found in Table 3 according to Søreide and Whitson [10].

Table 3: Binary interaction parameter in PR-EOS.

H₂O
CO₂
CH₄

H₂O
—
0.19014
0.485

CO₂
0.19014
—
0.1196

CH₄
0.485
0.1196
—

2.3. Activity Coefficients

For activity coefficients, Pitzer model [11] was successfully used in gas-water-mineral modeling for high salinities in previous works [12–15]. Cations, anions, and interaction between particle pairs are considered to influence the component activity behaviors in aqueous phase. The activity coefficient equations are as follows:

where is cation molality, is anion molality, and , , and are parameters that are functions of temperature and pressure. , , and are known as Pitzer parameters and they are usually estimated from gas solubility data from aqueous solutions with dissolved salts. In this work, Pitzer parameters are usually calibrated from gas solubility from NaCl solutions. is assumed to be 0. As the approximation in Duan and Sun [14] and Duan et al. [16], and are estimated as and . All ternary parameters are estimated as . Pitzer parameters are listed in Table 4.

Table 4: Pitzer parameters for activity coefficients.

Parameters
Equations

2.4. Model Validation

The model performance is evaluated from comparison of model results and related experimental data of CO₂-CH₄-brine systems (including the subsystems).

For CO₂-H₂O-NaCl systems, the experimental studies [17–19] are sufficient, which cover temperature from 0°C to more than 250°C and pressure from 1 bar to more than 1500 bar. From our comparison, the average absolute derivations for most of the data points are less than 10%. Figures 1(a) and 1(b) show a comparison of CO₂ solubilities in pure water and NaCl solutions calculated from this model and related experimental data. We can find that the model solutions agree with the experimental data in the wide ranges of temperature, pressure, and salinity. Figure 1(c) shows the H₂O solubility in nonaqueous (CO₂-rich) phase of the model solutions and experimental data. From the figure, the model can well reproduce H₂O solubility in nonaqueous phase.

(a)

(b)

(c)

Figure 1: Mutual solubilities of CO₂-brine systems. Lines are calculated results from this model, and dots are from experimental data. (a) CO₂ solubility in pure water; (b) CO₂ solubility in NaCl solutions; (c) H₂O solubility in CO₂-rich phase.

Experimental data of CH₄-H₂O-NaCl system are also sufficient with temperature from 0 to more than 250°C and pressure from 1 bar to more than 1500 bar [20]. Figures 2(a) and 2(b) show the comparison of CH₄ solubilities in water and NaCl solutions of experimental data and this model. Figure 2(c) shows the experimental data of H₂O in nonaqueous (CH₄-rich) phase and the related model solutions. From the comparisons, the experimental data can be well reproduced by the model.

(a)

(b)

(c)

Figure 2: Mutual solubilities of CH₄-brine systems. Lines are calculated results from this model, and dots are from experimental data. (a) CH₄ solubility in pure water; (b) CH₄ solubility in NaCl solutions; (c) H₂O solubility in CH₄-rich phase.

Compared with single gas (CO₂ or CH₄)-brine systems, gas mixture (CO₂ and CH₄ existing at the same time)-brine systems have less experimental data. The existing data are also not systematic. Qin et al. [21] have studied phase equilibria for CO₂-CH₄-H₂O system at 325 K and 376 K and with pressure from 100 bar to 500 bar. 21 data points were generated in the work. We compared their results with our model. From the comparison (see Figure 3), we can conclude that the model can predict mutual solubilities for CO₂-CH₄-H₂O system.

(a)

(b)

(c)

(d)

Figure 3: CO₂/CH₄ solubilities in water in CO₂-CH₄-H₂O systems at different temperature and pressure. Dots are from Qin et al.’s [21] experimental data, and lines are from this model.

In summary, the comparison of the model solutions with existing experimental data shows that the model can well reproduce and predict mutual solubility data of CO₂-CH₄-brine systems in wide ranges of temperature, pressure, and salinity. The model is reliable to be used in gas-water-mineral equilibrium analysis.

3. CO₂/CH₄-Water-Mineral Interactions in Deep Environments

In Sichuan basin, carbonates (such as dolomite or calcite) are the dominant minerals in some natural gas reservoirs; meanwhile sulfates (such as gypsum or anhydrite) and clay minerals are also commonly found [22, 23]. In Sichuan natural gas reservoirs, CH₄ is always accompanied with other components such as CO₂, N₂, or H₂S [24]. PHREEQC is a famous software package for water-mineral interaction calculations. Pressure effects can be considered using its third version [4]. With an accurate mutual solubility model of CO₂-CH₄-brine systems, geochemical reactions in CO₂-CH₄-water-mineral systems can be calculated by combining this model and the PHREEQC functionality. Through this research, we aim to find out

(i)the influences on geochemical reactions in depth (i.e., temperature and pressure increase or decrease);(ii)sensitivity of gas components (i.e., CO₂ or CH₄) to water composition, mineral dissolution, or precipitation.

In this work, the calculations are based on Sichuan basin background. The hydrostatic pressure is assumed to be 100 bar/Km, and geothermal gradient is assumed as 25°C/Km according to a previous work [25] with surface temperature set as 25°C. The depth range of the research is from 3000 m to 6000 m. Relationships of depth, temperature, and pressure are shown in Figure 4. To clarify the influences from gas components, sodium chlorite is considered as the only salt that is dissolved in water as an initial solution. Geochemistry equilibrium of CO₂-CH₄-brine-dolomite, CO₂-CH₄-brine-calcite, and CO₂-CH₄-brine-gypsum/anhydrite systems is studied. Two gas compositions are considered, pure CH₄ or 10% CO₂ + 90% CH₄, to evaluate CO₂ influences.

Figure 4: Temperature, pressure, and depth relationships.

Table 5 lists the species of ions, minerals, and gases which get involved in geochemical reactions in CO₂-CH₄-brine-carbonate systems and CO₂-CH₄-brine-sulfate systems.

Table 5

(a) Ions, minerals, and gases involved in CO₂-CH₄-brine-carbonate systems

Cations
Anions
Neutral ions
Minerals and gases

H⁺
OH⁻
H₂O
Aragonite

Ca²⁺

CH₄
Calcite

CO₂
Dolomite

CaOH⁺
Cl⁻
(CO₂)₂
Halite

Mg²⁺

CaCO₃
CH₄ (g)

H₂
CO₂ (g)

MgOH⁺

MgCO₃
H₂ (g)

Na⁺

NaHCO₃
H₂O (g)

NaOH
O₂ (g)

O₂

(b) Ions, minerals, and gases involved in CO₂-CH₄-brine-sulfate systems

Cations
Anions
Neutral ions
Minerals and gases

H⁺
OH⁻
H₂O
Anhydrite

Ba²⁺

BaSO₄
Aragonite

BaCO₃
Barite

BaOH⁺

NaHCO₃
Calcite

Cl⁻
CaCO₃
Dolomite

(CO₂)₂
Gypsum

Ca²⁺
HS⁻
MgCO₃
Halite

S²⁻
CaSO₄
Sulfur

CaOH⁺

MgSO₄
Witherite

Mg²⁺

NaOH
H₂ (g)

MgOH⁺

O₂
H₂O (g)

Na⁺

H₂S
H₂S (g)

H₂
O₂ (g)

CH₄
CH₄ (g)

CO₂
CO₂ (g)

3.1. CO₂-CH₄-Brine-Carbonate Systems

For CO₂-CH₄-brine-carbonate systems, cases of fluid equilibrium with calcite and dolomite are studied, respectively. Figure 5 shows the molality of carbon (including , CO₂, , CaCO₃, , , and MgCO₃) dissolved in aqueous phase with different depths, gas compositions, and salinities. Figure 6 shows the molality of calcium (including Ca²⁺, CaCO₃, , and CaOH⁺) and magnesium (including Mg²⁺, MgOH⁺, MgCO₃, and ) that is dissolved in aqueous phase. From Figure 6, it is shown that CO₂ in the gas phase will promote calcite or dolomite dissolution. From the calculations, we find that, with CO₂ existing in the system, carbon concentration in aqueous phase increases with depth. From 3000 m to 6000 m, the carbon molality is almost doubled in Figure 5 at different salinities. However, compared with calcium, magnesium is more solvable and increases with depth. From our calculation, in fluid-dolomite systems, with an increase in temperature and pressure, more calcite precipitates. We can conclude that, in deep carbonate environments, calcium is more likely to precipitate and magnesium ion is more likely to be rich in aqueous phase and transport to shallower areas due to diffusion gradient. So, in general, calcite approaches being existing in deeper environments and dolomite is more likely to be existing in shallower environments.

Figure 5: Molality of total carbon dissolved in water varying with depth at different salinities and gas compositions. Dashed yellow line represents the case of pure CH₄ of gas in the system. Blue lines represent results from fluid-calcite systems (with gas composition CO₂ : CH₄ = 1 : 9 in mole). Red lines with dots represent the result from fluid-dolomite systems (with gas composition CO₂ : CH₄ = 1 : 9 in mole).

Figure 6: Molality of elements Ca and Mg dissolved in water varying with depth (with sodium chlorite molality 0). Yellow line represents the case of pure CH₄ of gas in the system. Red lines with dots represent results from fluid-dolomite systems (with gas composition CO₂ : CH₄ = 1 : 9 in mole). Blue line represents results from fluid-calcite systems (with gas composition CO₂ : CH₄ = 1 : 9 in mole).

3.2. CO₂-CH₄-Brine-Sulfate Systems

The element sulfur can have different chemical valences such as −2, 0, +4, and +6 in nature. When sulfates are dissolved in water, sulfur is usually in +6 valence state. It could be reduced to other valence states when reducer exists in the solution. In deep gas reservoirs in Sichuan basin, sulfates commonly exist. Different fluid compositions may trigger different redox geochemical reactions and lead to different forms of sulfur or even reservoir properties.

In this work, we perform several numerical experiments to evaluate the influence of gas composition and depth on fluid-mineral equilibrium. For gas composition, we considered three cases: pure CH₄, 10% CO₂ + 90% CH₄, and pure CO₂. The calculations covered depth from 3000 m to 6000 m. Figure 7 presents S(−2) (i.e., sulfur dissolved in water with chemical valence −2, which can be S²⁻, HS⁻, and H₂S as ions) and S(+6) (i.e., sulfur dissolved in water in chemical valence +6, which can be , , CaSO₄, and ) concentration in equilibrium of gas-water-gypsum. From Figure 7, we can find the following:

(1)With pure CO₂ in gas, S(−2) in water is extremely low, and more CH₄ is dissolved in water leading to higher S(−2) concentration.(2)Higher CO₂ mole fraction in gas phase will lead to higher S(+6) concentration in water phase.(3)With higher depth, higher S(+6) concentration can be found, but depth influence on S(−2) concentration is not clear.

(a)

(b)

Figure 7: Concentration of S(+6) and S(−2) in aqueous phase varying with depth in equilibrium of gas (pure CH₄, 10% CO₂ + 90% CH₄, or pure CO₂), water, and gypsum.

It is clear that CH₄ is the key component for S(+6) to be reduced to S(−2) species in water. The related redox geochemical reaction is

When CH₄ and are dissolved in water, the above reaction is triggered, and CH₄ is oxidized from C(−4) to C(+4). In the meantime, is reduced to S²⁻.

Figure 8(a) shows the amount of calcite precipitation for different cases of geochemical equilibrium. Referring to Figure 8(a), in case of pure CO₂ in gas phase, there is no calcite precipitation; with higher CH₄ mole fraction in gas phase, more calcite can be precipitated; in deeper environments, more calcite can be precipitated. This phenomenon is also connected with sulfur reduction. With CH₄ dissolved in water, more sulfate is consumed and more calcium ions are dissolved in water. In this process, carbanions are generated because of the redox reaction. With more and more calcium and carbanion in the solution, calcite becomes saturated and precipitates. Another product is H₂S in gas phase. With more and more S(−2) generated in water, S²⁻ and H⁺ approach combining with one another, and H₂S becomes saturated and is released in gas phase. As shown in Figure 8(b), with more CH₄ in gas phase, more H₂S will be generated in gas phase at equilibrium states. From this study, we can find that CH₄-water-sulfate redox reaction could be a mechanism of H₂S origin in gas reservoirs [2]. From the figure, we can also find that, at higher depth, more H₂S can be generated. This result agrees with the statement from Li et al. [2].

(a)

(b)

Figure 8: (a) Mole number of calcite precipitated and (b) mole number of H₂S released to gas with 1 KG water in equilibrium with gas (pure CH₄, 10% CO₂ + 90% CH₄, or pure CO₂) and gypsum at different depths.

4. Conclusions

In this work, an accurate mutual solubility model is constructed with “fugacity-activity” method for CO₂-CH₄-brine systems. This model has a wide application range of pressure, temperature, and salinity, which can be used for fluid phase equilibrium in deep hydrocarbon reservoirs.

Combined with the mutual solubility model and PHREEQC, the equilibrium CO₂-CH₄-brine-mineral systems under deep reservoir conditions can be calculated. The mutual solubility model can be used to calculate the mole numbers of CO₂/CH₄ dissolved in brine at given temperature, pressure, and salinity. With the dissolved mole numbers of CO₂/CH₄, PHREEQC is used to calculate the speciation between aqueous phase and mineral.

CO₂/CH₄-brine-carbonate (i.e., dolomite or calcite) and CO₂/CH₄-brine-sulfate (i.e., gypsum or anhydrite) equilibria were studied with the above methodology. From the study, we find the following:

(1)For CO₂/CH₄-brine-carbonate (calcite or dolomite) systems, with an increase in depth, calcium is more likely to precipitate as calcite and magnesium is more likely dissolved in aqueous phase. In other words, dolomite could be rich in shallower position and calcite may approach being existing at deeper locations.(2)With CH₄ present in the CO₂/CH₄-brine-sulfate (gypsum or anhydrite) systems, redox reaction is triggered and S(+6) is reduced to S(−2). H₂S will be released when S(−2) becomes saturated in aqueous phase. This process could be one of the origins for H₂S in gas reservoirs in Sichuan basin, China.

This work is an attempt to do preliminary fluid-mineral interaction calculations with a new established accurate mutual solubility model of CO₂-CH₄-brine systems combined with PHREEQC, version 3. The geochemical reaction parameters are still needed to be validated for high temperature and pressure. Also, more systematic research work of gas-water-minerals is still required in the future according to real depositional environments.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work is supported by National Natural Science Foundation of China (Grant no. 41502246). Dimue Tech. Ltd. Co. provided technical support during the research. The authors also acknowledge the sponsorship from National Key R&D Program of China (2016YFE0102500).

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