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.
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 [
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 [
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 [
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
For aqueous phase,
With (
The mutual solubility model of CO2-CH4-brine system is established based on the above principle. Equilibrium constants (
For equilibrium constant of H2O, we follow the work of Li et al. [
The parameters (
Parameters of H2O equilibrium constant in (
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For equilibrium constants of CO2 and CH4, we follow the form of Mao et al. (2013):
Parameters of CO2 and CH4 equilibrium constants in (
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CO2 |
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CH4 |
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Peng-Robinson equation of state (PR-EOS for short [
Binary interaction parameter in PR-EOS.
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H2O | CO2 | CH4 |
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H2O | — | 0.19014 | 0.485 |
CO2 | 0.19014 | — | 0.1196 |
CH4 | 0.485 | 0.1196 | — |
For activity coefficients, Pitzer model [
Pitzer parameters for activity coefficients.
Parameters | Equations |
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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 [
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 [
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. [
CO2/CH4 solubilities in water in CO2-CH4-H2O systems at different temperature and pressure. Dots are from Qin et al.’s [
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.
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 [ the influences on geochemical reactions in depth (i.e., temperature and pressure increase or decrease); 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 [
Temperature, pressure, and depth relationships.
Table
Ions, minerals, and gases involved in CO2-CH4-brine-carbonate systems
Cations | Anions | Neutral ions | Minerals and gases |
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H+ | OH− | H2O | Aragonite |
Ca2+ |
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CH4 | Calcite |
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CO2 | Dolomite |
CaOH+ | Cl− | (CO2)2 | Halite |
Mg2+ |
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CaCO3 | CH4 (g) |
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H2 | CO2 (g) | |
MgOH+ | MgCO3 | H2 (g) | |
Na+ | NaHCO3 | H2O (g) | |
NaOH | O2 (g) | ||
O2 |
Ions, minerals, and gases involved in CO2-CH4-brine-sulfate systems
Cations | Anions | Neutral ions | Minerals and gases |
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H+ | OH− | H2O | Anhydrite |
Ba2+ |
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BaSO4 | Aragonite |
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BaCO3 | Barite |
BaOH+ |
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NaHCO3 | Calcite |
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Cl− | CaCO3 | Dolomite |
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(CO2)2 | Gypsum |
Ca2+ | HS− | MgCO3 | Halite |
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S2− | CaSO4 | Sulfur |
CaOH+ |
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MgSO4 | Witherite |
Mg2+ |
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NaOH | H2 (g) |
MgOH+ | O2 | H2O (g) | |
Na+ | H2S | H2S (g) | |
H2 | O2 (g) | ||
CH4 | CH4 (g) | ||
CO2 | CO2 (g) |
For CO2-CH4-brine-carbonate systems, cases of fluid equilibrium with calcite and dolomite are studied, respectively. Figure
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).
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).
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 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. Higher CO2 mole fraction in gas phase will lead to higher S(+6) concentration in water phase. With higher depth, higher S(+6) concentration can be found, but depth influence on S(−2) concentration is not clear.
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
Figure
(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.
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: 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. 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.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
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).