Zhang Wei ¹, Li Yilian ¹, Xu Tianfu ^{1, 2}, Qiang Wei ¹, Xiao Shangping ¹

1 School of Environment, China University of Geosciences, Wuhan, 430074, China

2 Earth Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA

 

Abstract:Since industrial revolution, the “greenhouse effect” is one of the most important global environmental issues. Of all the greenhouse gases, CO₂ is responsible for about 64% of the enhanced “greenhouse effect”, making it the target for mitigation, so reducing anthropogenic discharge of carbon dioxide receives more and more attentions. Geological sequestration of CO₂ in deep saline aquifers is one of the most promising options. But because unknown fractures and faults may exist in the caprock layers which can prevent the leakage of CO₂, CO₂ will leak upward into upper potable aquifers, and lead to adverse impacts on the shallow potable aquifer.

In order to assess the potential effect of CO₂ leakage from underground storage reservoirs on fractures and water quality of potable aquifers, this study used the non-isothermal reactive geochemical transport code TOUGHREACT developed by Xu et al to establish a simplified 2-D model of CO₂ underground sequestration system, which includes deep saline aquifers, caprock layers, and shallow potable aquifers, and study and analyze the changes of mineral and aqueous components.

The simulation results indicated that the minerals of deep saline aquifers and fractures should be mainly composed with aluminosilicate and silicate minerals, which not only increase the mass of CO₂ sequestrated by mineral trapping, but also decrease the porosity and permeability of caprock layers and fractures to prevent and reduce the CO₂ leakage. The results in the deep saline aquifer found that the mass of carbon dioxide trapped by minerals and solution phases is limit, the other is remained as a supercritical phase, and so once the caprock aquifers have some unknown fractures, the free carbon dioxide phase may leak from CO₂ geologic sequestration reservoirs by buoyancy.

The simulation results in potable aquifers show that the changes of pH caused by CO₂ injection have significant effects on the water quality in shallow aquifers, especially the increase of trace metal content caused by the dissolution of metal minerals. This conclusion not only helps monitor CO₂ leakage, as a purpose of warning，but also the sudden geochemical changes (such as the decrease of pH and the increase of mineral concentration) in shallow aquifers could be used as indicators of CO₂ leakage from deep geological CO₂ repositories.

Keywords: Geological sequestration; Carbon dioxide; Numerical simulation; Leakage; Fracture; Potable aquifer; Water quality; Heavy metal

1 Introduction

With the development of economy and increase of human activities, global emissions of greenhouse gases (GHG) such as carbon dioxide, methane and nitrous oxides increases rapidly, and leading to the global warming and climate changes. The “greenhouse effect” has already raised the Earth’s average temperature and scientists predict that it will climb gradually by 1–3.58℃ by the year 2100 ^[1]. Of all greenhouse gases, CO₂is responsible for about 64% of the enhanced “greenhouse effect”, making it the target for mitigation of greenhouse gases ^[2]. Atmospheric concentrations of CO₂ have risen from pre-industrial levels of 280 to 358 parts per million by volume (ppmv) in 1994, primarily as a consequence of fossil fuel combustion for energy production ^[1]. Fossil fuels, which nowadays provide 75-85% of the world’s energy ^{[2, 3]}, will still be used as a major component of the world’s energy supply for at least the next century because of their inherent advantages, such as availability, competitive cost, convenience for transport and storage and abundant resources.

Therefore, other ways must be taken to reduce anthropogenic CO₂ emissions. Currently, researches of CO₂ geologic sequestration as an effective way are being carried out extensively. The main storage formations include depleted or depleting oil and gas reservoirs, un-mineable coal seams, and deep saline aquifers ^[4-9]. Injecting CO₂ into the saline aquifer formation is one of the most promising geologic sequestration options for the long term because there are regionally extensive aquifers capable of accepting large volumes of carbon dioxide from power plants without the need for long transport pipelines ^[3].

The purpose of storage CO₂ in a saline aquifer can be achieved through the following four trapping mechanisms ^{[4, 10, 21, 22]}: (1) stratigraphic trapping by low permeability capping structures, (2) solution trapping by dissolution in the formation water, (3) hydrodynamic trapping by the inhibition of dissolved CO₂ due to slow flow of aquifer fluid, and (4) mineral trapping where CO₂ rich reservoir brine reacts with the reservoir rocks to precipitate carbonate minerals. Even though a fraction of the CO₂ may be trapped in mineral phases and dissolved into the surrounding water, the bulk mass of CO₂ injected into the subsurface is expected to remain as a supercritical phase, the free carbon dioxide phase may leak upward into overlaying potable aquifers through imperfect confinement (e.g. fractures in the cap rock or abandoned wells) ^{[11, 12]}. Dissolution of CO₂ in shallow potable aquifers can cause a decrease in pH. Such acidic condition can affect the dissolution and sorption mechanisms of many minerals and groundwater quality by enhancing dissolution and/or desorption of potentially hazardous trace metals ^[12]. For example, galena (PbS) can dissolve and increase significantly Pb concentrations and diminish groundwater quality in acidic condition ^[13].

It was assessed that the potential effect of CO₂ leakage from underground storage reservoirs on fractures (e.g. porosity, permeability and mineral changes) and water quality of potable aquifers (especially the dissolution of minerals containing heavy metals) by numerical simulation in this study. Through this research, the geochemical changes could be used as indicators of CO₂ leakage from deep geological CO₂ repositories. This study not only helps the monitoring of CO₂ leakage, as a purpose of warning, but also provides theoretic foundations for the prevention of CO₂ leakage.

2 Modeling Approach

This study uses the non-isothermal reactive geochemical transport code TOUGHREACT, which developed by introducing reactive chemistry into the framework of the existing multiphase fluid and heat flow code TOUGH2, and can be used for the assessment of mineral alteration in hydrothermal systems, waste disposal sites, acid mine drainage remediation, contaminant transport, and groundwater quality ^{[15, 17, 26]}. Flow and transport in geologic media are modeled based on space discretization by means of integral finite differences. An implicit time-weighting scheme is used for the individual components of the model consisting of flow, transport, and kinetic geochemical reaction. TOUGHREACT uses a sequential iteration approach, which solves the transport and the reaction equations separately. The system of chemical reaction equations is solved by a Newton-Raphson iterative method ^{[14, 15]}.

The model can be applied to one-, two-, or three-dimensional porous and fractured media with physical and chemical heterogeneity. The model can accommodate any number of chemical species present in the liquid, gas, and solid phases. A wide range of subsurface thermo-physical-chemical processes is considered. Major processes for fluid and heat flow are ^{[16, 26]}: (1) fluid flow in both liquid and gas phases under pressure and gravity forces; (2) capillary pressure effect for the liquid phase; and (3) heat flow by conduction, convection and diffusion. Transport of aqueous and gaseous species by advection and molecular diffusion is considered in both liquid and gas phases. Aqueous chemical complexation, acid-base, cation exchange and gas dissolution/exsolution are considered under the local equilibrium assumption. Mineral dissolution and precipitation can be modeled with TOUGHREACT either subject to local equilibrium or kinetic conditions.

3 Problem Setup

As shown in Fig. 1, this study used a simplified 2-D model of CO₂ underground sequestration system, which includes deep saline aquifers, caprock layers, and shallow potable aquifers. On the assumption that the saline aquifer and potable aquifer are sandstone aquifers and they have the same hydrogeological parameters and mineral composition. In the present model, the caprock layer was assumed to be a relatively impermeable shale layer to reduce the impact of caprock layer on this simulation of CO₂ leakage from the fracture. The model assumed that a fracture was not detected in geology survey in the caprock layer, through which injected CO₂ may leak upward into shallow potable aquifer. The depth of saline aquifer was assumed to be 2km, a temperature of 75℃ was used based on a land surface temperature of 15℃ and geothermal gradient of 30℃/km, a pressure of 200bars was used based on pressure gradient of 100bars/km, the caprock layer and potable aquifer were assumed according the same principles. In this reactive transport simulation, hydrogeologic parameters (Table 1), mineral compositions (Table 2) and total dissolved component concentrations (Table 3) were cited from the former researches of Xu et al ^{[13, 15, 17, 18]}, and modified appropriately.

In this 2-D model, the injection well located in the deep saline aquifer and CO₂ is injected through the well at a constant rate of 100kg/s. The injection rate is approximately equivalent to that of generated by a 300 MW coal-fired power plant ^[18]. The CO₂ injection was assumed to continue for a period of 500 years. The fluid flow and geochemical transport simulation was run for a period of 10,000years.

 

 

 

 

Fig.1 Schematic diagram for the 2-D model simulation

Table.1 Hydrogeological parameters for 2-D CO₂ injection problem

Table.2 List of initial mineral volume fractions and secondary mineral phases for sandstoneand shale (fault) layers

Table.3 Initial total dissolved component concentrations (mol/kg H₂O) for reactivetransport simulations in the underground sequestration system

Note: Iron is the sum of Fe²⁺, Fe³⁺ and their related complexes. Carbon is the sum of CO₂ (aq), CH₄ (aq), and their related species such as HCO₃^- and acetic acid (aq). Sulfur is the sum of sulfate and sulfide species.

4 Results and Discussion

    4.1 Deep saline aquifers

Although the aim of this study is to analyze and research leakage problems, the mechanisms for the storage of CO₂ in geological formations must be made clear primarily.

The simulation results in the deep saline aquifer (Fig.3) shows oligoclase dissolves completely in both plume and background regions, but the rate of dissolution in plume region (x<100m) is faster than that in background region, otherwise the dissolution of chlorite is more significant in plume region than that in background region. The dissolution of primary aluminosilicate minerals such as oligoclase, and chlorite provide Ca²⁺, Na⁺, Mg²⁺, and Fe²⁺ for the precipitation of secondary carbonate minerals such as ankerite, and dawsonite in plume region (Fig.4). The changes of secondary carbonate minerals are consistent with the changes of CO₂ mineral-trapping capacity during a period of 10,000 years in the deep saline aquifer (Fig.2). So the mass of CO₂ trapped by minerals depends on the dissolution of primary silicate minerals such as oligoclase, and chlorite and the precipitation of secondary carbonate minerals such as ankerite, and dawsonite.

Fig.2 Cumulative CO₂ sequestration by carbonate precipitation for different times.

As Eq. (1) shown, the injected CO₂ can be stored as bicarbonate relative to the simple solubility of CO₂ and is an example of the solution trapping of CO₂ ^[19]. Similar reactions can be written for the dissolution of other carbonates.

    (1)

Oligoclase (CaNa₄Al₆Si₁₄O₄₀)

Chlorite (Mg_2.5Fe_2.5Al₂Si₃O₁₀(OH)₈)

Fig.3 Change of abundance (volume fraction) of primary aluminosilicate minerals. Positive values express precipitation and negative values express dissolution.

Ankerite (CaMg_0.3Fe_0.7(CO₃)₂)

Dawsonite (NaAlCO₃(OH)₂)

Fig.4 Change of abundance (volume fraction) of secondary carbonate minerals. Positive value means precipitation and negative value means dissolution.

The changes of carbonate (Fig.5) and calcite (Fig.6) are not consistent with Eq. (1) completely, although the concentration of carbonate after CO₂ injection is much higher than that in the initial dissolved component (4.32×10^-2 mol/kgH₂O), the concentration of carbonate does not continue increasing over time, the reason may be that carbonate in this model is the sum of CO₂ (aq), CH₄ (aq), and related species such as HCO₃^- and acetic acid (aq).

Fig.5 Concentration of carbonate over times in deep saline aquifers.

In this study, the simulation results can prove that a fraction of CO₂ can be trapped in mineral and solution phases, but the bulk mass of CO₂ injected into the subsurface is expected to remain as a supercritical phase, the free carbon dioxide phase may leak from CO₂ geologic sequestration formations by buoyancy, so the evaluation of security is very important to CO₂ underground sequestration.

Fig.6 Change of abundance of calcite over time in deep saline aquifers.

4.2 Fracture

As mentioned above, the supercritical carbon dioxide is pushed up to the bottom of caprock by buoyancy, but the action of capillary force makes carbon dioxide difficult enter into the caprock ^[19]. But once there are some unknown fractures or faults of high permeability in caprock layers. CO₂ will leak upward from the "fast-track" to upper aquifers or ground surface, which may pose potential risks on the environment.

In this study, one of the most concerns is the effect of CO₂ leakage on the change trend of porosity and permeability in fracture of high permeability. The relevant research findings can also be used to infer the impact of CO₂ injected on the caprock of low permeability.

Fig.7 shows the decrease of porosity and permeability over time in the fracture. This phenomenon may be caused by the precipitation of carbonate minerals, which is consistent with CO₂ mineral sequestration shown in Fig.8. The similar result was obtained in the deep aquifer reservoir.

As shown in Fig.8, the mass of CO₂ sequestrated by carbonate mineral precipitation in fracture is far less than that in deep saline aquifers. Through the comparison of secondary mineral abundance changes in Fig.4and 9, it was found that the differences between the fracture and deep saline aquifer are consistent with that in Fig.8. The study above in deep saline aquifers found a close relation between primary mineral precipitation and secondary mineral dissolution, precipitation of dawsonite (NaAlCO₃(OH)₂), requires Na⁺ provided by oligoclase (CaNa₄Al₆Si₁₄O₄₀) dissolution, and precipitation of ankerite (CaMg_0.3Fe_0.7(CO₃)₂) requires Ca²⁺provided by oligoclase dissolution mostly and some by calcite dissolution, Fe²⁺ and Mg²⁺ supplied by chlorite (Mg_2.5Fe_2.5Al₂Si₃O₁₀(OH)₈) reduction. The initial mineral volume fraction of oligoclse and chlorite for sandstone layers (deep saline aquifers) is far more than that for shale layers (fractures) (Table 2). So in the model, the CO₂ mineral trapping capacity mainly depends on the volume fraction of oligoclase and chlorite.

It has also been reported that protons (H⁺) in aqueous solution will interact with aluminosilicate minerals (e.g. feldspars, zeolites and clay minerals) releasing ions such as Ca, Mg, and Fe which can permanently fix CO₂ as the carbonate minerals, and Fyfe et al found that silicate minerals such as olivine could be sources of Ca and Mg ^{[1, 20]}. This is the reason why siliciclastic aquifers are considered better candidates for CO_2­sequestration than carbonate aquifers which will dissolve with CO₂ injected ^[1]. This simulation results also confirmed the above conclusions, which provide a theoretical basis for the selection of CO₂ underground reservoirs. Not only the reservoirs, but also the mineral components of upper caprock should be mainly composed with aluminosilicate and silicate minerals, which can enhance the mass of CO₂ sequestrated by minerals and reduce the possibility of CO₂ leakage from caprock by the decrease of porosity and permeability to ensure the safety of underground disposal projects of CO₂.

Permeability

Porosity

Fig.7 Permeability and porosity vs. time in the fracture.

 

Fig.8 Cumulative CO₂ sequestrated by carbonate mineral precipitation in deep saline aquifer and fracture.

4.3 Potable aquifers

During the process of CO₂ geological disposal, risk assessment is very important. One of the most concerns is the potential influences of CO₂ leakage on the geochemical characteristics and quality of upper potable groundwater including pH, metal minerals, and so on.

Dawsonite

Ankerite

Fig.9 Change of mineral abundance of dawsonite and ankerite in deep saline aquifer and fracture.

In this study, to study the effects of CO₂ leakage on groundwater quality, the simulation assumed Pb and Zn bearing minerals, galena and sphalerite, which present in the rock with 1% volume fraction each (Table2 and 3).

As Eq. (2), (3), (4) and (5) shown ^[23], with the leakage of injected CO₂ through fracture in the upper caprock, CO₂ is dissolved in water, and forms H₂CO₃ (aq), HCO₃^-, and CO₃^2-, which will result in a decrease of pH ^[12]. In the simulation results (Fig.10), the lowest pH is about 5, and with minerals dissolved, pH can be buffered to about 6, which is below the value of pH in “Water Quality Standard for Drinking Water” (GB5749-85) promulgated by China. However, the pH of shallow potable aquifer before CO₂ leakage conform to the standard (Table 3), so the leakage of injected CO₂ has a adverse influence on the changes of pH in potable aquifers.

        (2)

        (3)

        (4)

        (5)

Fig.11 and 12 illustrate the changes of Zn and Pb concentration respectively, solid line in Fig.11 and 12 is the primary standard of “Water Quality Standard for Groundwater” (GB/T14848-1993), and the concentration changes have the same trend. Pb poses a significant public health threat through long term internal accumulation that can result in damages to the blood system, nervous system, digestive system and kidney, especially to young children and pregnant women ^{[12, 24]}, so this paper studied and analyzed the changes of Pb concentration in order to determine the effect of CO₂ leakage on the concentration of heavy metal ions in potable aquifers. This study mainly considers the potential impact of dissolved galena on Pb concentration, because galena is one of the most important base metal sulfide minerals and one of the main minerals controlling the mobility of lead in the subsurface ^[12].

As Fig.11 (a) shown, the study found that the concentrations of Pb don’t exceed the primary standard of “Water Quality Standard for Groundwater” (GB/T14848-1993)(0.005mg/L≈2.413×10^-8mol/kgH₂O) and the action level (7.843×10^-12mol/kgH₂O) of Pb under the Safe Drinking Water Act in 1991 in America. ^[12]. But comparing with the concentration of Pb before CO₂ leakage (2.413×10^-8mol/kgH₂O, Table 3) and the changes of Pb concentration in background region (Fig.11 (b)), the concentration of Pb in plume region has increased by the leakage of CO₂ injected. The reason why the changes of Pb concentration are not insignificant is the effect of pH on the dissolution of metal minerals, Fig.10 and 11 (a) shown that when injected CO₂ leak into potable aquifer, the pH decreases to about 5 and the concentration of Pb increases significantly; but the buffer role of minerals dissolved on pH will reduce the concentration of Pb. Xu ^[13] and Wang et al ^[12] found in their simulation research that when pH less than 5, galena can be dissolved significantly, the concentration of Pb increases remarkably and exceeds the water quality standards; but when pH is higher than 5, the dissolved Pb concentration will be lower, which are similar with our simulation results.

CO₂ used for geological disposal usually comes from the combustion of fossil fuel, in which contains a few of acid gases such as H₂S and SO₂ ^{[25, 27]}. Although CO₂ should be captured by physical and chemical methods before CO₂ injection, but it can not be removed completely; once the acid gases are injected with CO₂ into underground reservoirs, it will inevitably cause the marked decrease of pH and the dissolution of metal mineral, and have adverse impacts on the water quality of potable aquifers. The future simulations and researches should be focus on the injection of acid gases such as H₂S and SO₂.

 

 Fig.10 Changes of pH over times in shallow potable aquifers

 

Pb

(a)

  

(b)

Fig.11 Concentration of Pb in shallow potable aquifers. (a) Solid line is the primary standard of “Water Quality Standard for Groundwater” (GB/T14848-1993)（0.005mg/L ≈ 2.413×10^-8mol/kgH₂O）；（b）The distance exceed200m is the background region in which the leakage of CO₂ from deep saline aquifers does not exist.

Zn

(a)

(b)

Fig.12 Concentration of Zn in shallow potable aquifers.

5 Conclusions

In this study, the non-isothermal reactive geochemical transport code TOUGHREACT was used to establish a 2-D model and research the impacts of CO₂ leakage from underground reservoir on fractures and potable aquifers.

The simulation results indicated that the minerals of deep saline aquifers and fractures should be mainly composed of aluminosilicate and silicate minerals, which not only increase the mass of CO₂ sequestrated by mineral trapping, but also decrease the porosity and permeability of caprock layers and fractures to prevent or reduce the probability of CO₂ leakage.

The other emphasis of study is to evaluate the latent influences of CO₂ leakage on geochemical changes and water quality in potable aquifers, especially the dissolution of metal mineral. The simulation results reveal that the concentrations of Pb in potable aquifer are lower than corresponding water quality standard, but the leakage of injected CO₂ can lead to the dissolution of galena and the increase of Pb concentration. Through the researches and analyses of the results above and other simulation results (Xu, and Wang et al), it was found that the changes of pH caused by the leakage of injected CO₂ play a vital role in the dissolution of metal minerals and the increase of metal concentrations. So the geochemical changes in shallow aquifers (such as the decrease of pH and the increase of mineral concentration) suddenly could be used as indicators of CO₂leakage from deep geological CO₂ repositories.

In this paper, as a result of the lack of relevant dynamics data and the limit simulation capability of code TOUGHREACT, current 2D model established in this study didn’t consider the aqueous complexation and oxidation, and the rock adsorption. The future simulation studies would supplement and improve current TOUGHREAR model through laboratory test and related data collection. The numerical simulation software modified will be used to accurately simulate the relevant problems of geological sequestration of carbon dioxide.

Acknowledgements

We thank Wu ChenXi for reviews of the manuscript. This work was supported by NSFC project (No. 40472122).

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