Search Results (62)

Novel geothermal power generation systems are being developed that use supercritical CO₂ as the heat transfer medium. In this technology, some CO₂ injected into the underground reacts with surrounding water and rocks to form secondary minerals, such as carbonate minerals and clay minerals; however, the reaction mechanism in the vicinity of the injection well, the subject of this study, has not been clarified. As the first laboratory test, Rishiri Island basalt was reacted with distilled water at 250 °C for 15 days at four different CO₂ concentrations to investigate the difference in reaction depending on the CO₂ concentration. Na, K and Ca increased rapidly until 5 days of the reaction, with higher values at higher CO₂ concentrations; Mg showed characteristic behavior with higher values in the test without CO₂ (using Ar gas). The saturation index of each secondary mineral was calculated, and it was found that carbonate minerals were unsaturated and clay minerals, such as smectite, were supersaturated under all test conditions, which was in agreement with the experimental results. It is concluded that a small amount of clay minerals was formed in this test due to the low pH of the reaction solution caused by the high CO₂ concentration, indicating that dissolution was the main reaction for the rocks in the vicinity of the injection well. Full article

The Mesoproterozoic alkaline Grønnedal-Íka complex (1325 ± 6 Ma) is intruded into old Archean gneissic bedrock between Ikka Fjord and Kangilinnguit (Grønnedal) by Arsuk Fjord in Southwestern Greenland. This 8 × 2.8 km oval-shaped complex constitutes the oldest part of the Gardar Province, representing a failed continental rift across southern Greenland. It comprises outer rings of mainly nepheline syenites with a central plug of Fe- and Ca-rich carbonatites. Here, we present petrological data on the syenites and carbonatites combined with geochemical modelling of groundwater percolating through the Grønnedal-Íka complex and the secondary minerals and fluid chemistry arising from these fluid–rock reactions. The results show that modelling using input data of (1) meteoric water in a closed system with respect to atmospheric CO₂ can (2) dissolve the primary minerals of the syenites and carbonatites and (3) simulate the fluid chemistry of the natural sodium carbonate springs of 3–4 °C and pH 10–11 seeping up through fractures at the bottom of Ikka Fjord, which (4) leads to the deposition of nearly a thousand tufa columns of the cold carbonate mineral ikaite (CaCO₃•6H₂O). Our results thereby support the geochemical relationship between fluid–rock reactions inside the Grønnedal-Íka alkaline complex and the precipitation of ikaite in the shape of submarine tufa columns in Ikka Fjord. The modelling indicates that the groundwater itself can be supersaturated with respect to ikaite and provide the seed crystals that lead to the columnar growth of ikaite up to 20 m tall in the seawater of Ikka Fjord. Full article

The evolution of coal’s pore structure is crucial to the efficient capture of carbon dioxide (CO₂) within coalbeds, as it provides both adsorption sites and seepage space for the adsorbed- and free-phase CO₂, respectively. However, the conventional single fractal method for characterizing pore structure fails to depict the intricacies and variations in coal pores. This study innovatively applies the low-temperature N₂/CO₂ sorption measurement and multifractal theory to investigate the evolution of the microporous structure of coals (e.g., from the Huainan coalfield) during the supercritical CO₂(ScCO₂)–water–rock interaction process. Firstly, we observed that the ScCO2–water–rock interaction does not significantly alter the coal’s pore morphology. Notably, taking the ZJ-8# sample as an example, low-temperature N₂ sorption testing displayed a stable pore volume following the reaction, accompanied by an increase in specific surface area. Within the CO₂ sorption testing range, the ZJ-8# sample’s pore volume remained unchanged, while the specific surface and pore width performed displayed a slight decrease. Secondly, by introducing key parameters from multifractal theory (such as D_q, α(q), τ(q), and f(α)), we assessed the heterogeneity characteristics of the coal’s pore structure before and after the ScCO2–water–rock reaction. The N₂ sorption analysis reveals an increase in pore heterogeneity for the ZJ-8# sample and a decrease for the GQ-13# sample within the sorption testing range. In the context of low-temperature CO₂ sorption analysis, the pore distribution complexity and heterogeneity of the GQ-11# and GQ-13# samples’ pores were escalated after ScCO2–water–rock interaction. The experimental and analysis results elucidated the dual roles of precipitation and dissolution exerted by the ScCO2–water–rock interaction on the micropores of coal reservoirs, underscoring the heterogeneous nature of the reaction’s influence on pore structures. The application of fractal theory offers a novel perspective compared to traditional pore characterization methods, significantly improving the precision and comprehensiveness of pore structure change descriptions. Full article

This research evaluates the mineral ions and their concentrations in formation water from five well samples of the Bibi Hakimeh oil field (Iran). The analysis reveals the presence of calcium (Ca²⁺), sodium (Na⁺), and magnesium (Mg²⁺) cations, as well as sulfate (SO₄²⁻), bicarbonate (HCO₃⁻), and chloride (Cl⁻) anions, which are soluble in water within the Bibi Hakimeh oil formation. Furthermore, mineral deposits of CaSO₄, CaSO₄.2H₂O, CaCO₃, and MgCO₃ are investigated and predicted using StimCADE 2 software. The findings highlight the significant chemical precipitation of calcium sulfate and calcium carbonate mineral deposits under the operating conditions of the Bibi Hakimeh oil well. The geochemical composition of the formation waters is discussed to understand the equilibrium conditions and possible influence of the physical parameters. Additionally, this study examines the interaction between rock and water of the Bibi Hakimeh formation, revealing a notable correlation between the concentration of calcium and magnesium ions and the water–rock reaction in this field. Full article

The dissolution of dolomite can not only provide the chemical components in hot springs but also provide a high-quality reservoir for geothermal resources. However, there is still debate about the main controlling factors and mechanisms of the dissolution process of dolomite. The Shuijing hot springs in Guizhou Province are rich in SO₄²⁻ and the geothermal reservoir is dolomite, which provides an excellent opportunity to understand the role of SO₄²⁻ in the dissolution process of dolomite. In this paper, water–rock interaction experiments were conducted at different temperatures to study the effects of SO₄²⁻, pH, and CO₂ on the dissolution of dolomite from the Shuijing hot springs geothermal reservoir. The results indicate that temperature is a significant factor affecting the chemical composition of hot springs water, with higher temperatures having a more pronounced effect on the dissolution of dolomite. At lower temperatures of 25 °C and 90 °C, the molar ratio of the released Ca²⁺ and Mg²⁺ during the dissolution of dolomite in the initial reaction stage generally approaches the Ca/Mg molar ratio of dolomite, exhibiting congruent dissolution. However, at elevated temperatures of 150 °C, the released Ca/Mg molar ratio surpasses the Ca/Mg molar ratio of dolomite, demonstrating an incongruent dissolution characteristic with Ca²⁺ being preferentially released over Mg²⁺. Additionally, the relative importance of CO₂, SO₄²⁻ and pH on the dissolution degree of dolomite is CO₂ > SO₄²⁻ > pH = 4 > pH = 7 > pH = 10. The promotion effect of SO₄²⁻ on dolomite dissolution indicates that the greater the SO₄²⁻ concentration, the stronger the dissolution of dolomite, and its dissolution ability is enhanced with the increase in temperature. Furthermore, the effect of CO₂ on the dissolution of dolomite is stronger than that of SO₄²⁻, leading to the oscillating fluctuation trend of the released Ca²⁺ and Mg²⁺. Full article

Serpentine (Mg₃Si₂O₅(OH)₄), like quartz, dolomite and magnesite minerals, is a versatile mineral group characterized by silica and magnesium silicate contents with multiple polymorphic phases. Among the phases composed of antigorite, lizardite, and chrysotile, lizardite and chrysotile are the most prevalent phases in the serpentinites studied here. The formation process of serpentinites, which arise from the hydrothermal alteration of peridotites, influences the ratio of light rare earth elements (LREE) to heavy rare earth elements (HREE). In serpentinites, the ratio of light rare earth elements (LREE)/heavy rare earth elements (HREE) provides insights into formation conditions, geochemical evolution, and magmatic processes. The depletion of REE compositions in serpentinites indicates high melting extraction for fore-arc/mantle wedge serpentinites. The studied serpentinites show a depletion in REE concentrations compared to chondrite values, with HREE exhibiting a lesser degree of depletion compared to LREE. The high ΣLREE/ΣHREE ratios of the samples are between 0.16 and 4 ppm. While Ce shows a strong negative anomaly (0.1–12), Eu shows a weak positive anomaly (0.1–0.3). This indicates that fluid interacts significantly with rock during serpentinization, and highly incompatible elements (HIEs) gradually become involved in the serpentinization process. While high REE concentrations indicate mantle wedge serpentinites, REE levels are lower in mid-ocean ridge serpentinites. The enrichment of LREE in the analyzed samples reflects melt/rock interaction with depleted mantle and is consistent with rock–water interaction during serpentinization. The gradual increase in highly incompatible elements (HIEs) suggests that they result from fluid integration into the system and a subduction process. The large differential thermal analysis (DTA) peak at 810–830 °C is an important sign of dehydration, transformation reactions and thermal decomposition, and is compatible with H₂O phyllosilicates in the mineral structure losing water at this temperature. In SEM images, chrysotile, which has a fibrous structure, and lizardite, which has a flat appearance, transform into talc as a result of dehydration with increasing temperature. Therefore, the sudden temperature drop observed in DTA graphs is an indicator of crystal form transformation and CO₂ loss. In this study, the mineralogical and structural properties and the formation of serpentinites were examined for the first time using thermo-gravimetric analysis methods. In addition, the mineralogical and physical properties of serpentinites can be recommended for industrial use as additives in polymers or in the adsorption of organic pollutants. As a result, the high refractory nature of examined serpentine suggests that it is well-suited for applications involving high temperatures. This includes industries such as metallurgy and steel production, glass manufacturing, ceramic production, and the chemical industry. Full article

CO₂–water–rock interactions have an important impact on the stability and integrity of the caprock in CO₂ geological storage projects. The injected CO₂ in the reservoir enters the caprock via different mechanisms, leading to either the dissolution or precipitation of minerals. The mineral alterations change the porosity, permeability, and mechanical properties of the caprock, affecting its sealing capability. To evaluate the sealing effectiveness of overlying caprock and identify the influencing factors, numerical simulations and experiments were carried out on the mudstone Dongying Formation in Dezhou, China. Based on high-temperature and high-pressure autoclave experiments, batch reaction simulations were performed to obtain some key kinetic parameters for mineral dissolution/precipitation. Then, they were applied to the following simulation. The simulation results indicate that gaseous CO₂ has migrated 7 m in the caprock, while dissolved CO₂ migrated to the top of the caprock. Calcite is the dominant mineral within 1 m of the bottom of the caprock. The dissolution of calcite increases the porosity from 0.0625 to 0.4, but the overall porosity of the caprock decreases, with a minimum of 0.054, mainly due to the precipitation of montmorillonite and K-feldspar. A sensitivity analysis of the factors affecting the sealing performance of the caprock considered the changes in sealing performance under different reservoir sealing conditions. Sensitivity analysis of the factors affecting the sealing performance of the caprock indicates that the difference in pressure between reservoir and caprock affects the range of CO₂ transport and the degree of mineral reaction, and the sealing of the caprock increases with the difference in pressure. Increasing the initial reservoir gas saturation can weaken the caprock’s self-sealing behavior but shorten the migration distance of CO₂ within the caprock. When the content is lower than 2%, the presence of chlorite improves the sealing performance of the caprock and does not increase with further chlorite content. This study elucidates the factors that affect the sealing ability of the caprock, providing a theoretical basis for the selection and safety evaluation of CO₂ geological storage sites. Full article

Possible deterioration of a rock’s structure and mechanical properties due to chemical reactions between the host rock, formation water, and CO₂ requires due attention. In this study, cylindrical sandstone specimens obtained from the Hanna Formation, Wyoming, were prepared under three treatment conditions: dry, submerged in water, and treated with water + CO₂ for one week at a pressure of 5 MPa and room temperature. Specimens were subjected to three effective confining pressures of 5, 15, and 25 MPa. The mechanical test results show that water + CO₂ treatment, on average, decreases the peak strength and elastic modulus of the specimens by 36% and 20%, respectively, compared to dry specimens. For all three effective confining pressures, the dry specimens exhibited higher compressive strengths, larger Young’s moduli, and more brittle behavior. CO₂-treated specimens showed significantly lower calcite contents. Full article

The integrity of caprock sealing is a crucial factor in guaranteeing the safety and long-term feasibility of CO₂ saline aquifer storage. In this study, we identified three principal mechanisms that give rise to topseal failure: (1) gradual CO₂ seepage through the upper cap, (2) capillary seal failure resulting from the pressure increment due to CO₂ injection, and (3) localized overpressure causing cap rupture. Through the integration of numerical simulation and geomechanics, this study offers a sealing assessment for the caprock. The thorough analysis of the sealing performance of the Guantao formation reveals that after 2000 years of CO₂ injection, the caprock would undergo intrusion by 35 m without any leakage risk. Moreover, investigations into CO₂–water–rock interactions suggest that precipitation reactions outweigh dissolution reactions, leading to a decreased permeability and an enhanced sealing performance. The most likely fracture mode identified is shear fracture with a critical caprock fracture pressure of 36.48 MPa. In addition to these discoveries, it is significant to consider ongoing research aimed at enhancing our ability to predict and manage potential risks associated with carbon capture and storage technologies. Full article

A cushion gas is an indispensable and the most expensive part of underground natural gas storage. Using CO₂ injection to provide a cushion gas, not only can the investment in natural gas storage construction be reduced but the greenhouse effect can also be reduced. Currently, the related research about the mechanism and laws of CO₂ as a cushion gas in gas storage is not sufficient. Consequently, the difference in the physical properties of CO₂ and CH₄, and the mixing factors between CO₂ and natural gas, including the geological conditions and injection–production parameters, are comprehensively discussed. Additionally, the impact of CO₂ as a cushion gas on the reservoir stability and gas storage capacity is also analyzed by comparing the current research findings. The difference in the viscosity, density, and compressibility factor between CO₂ and CH₄ ensures a low degree of mixing between CO₂ and natural gas underground, thereby improving the recovery of CH₄ in the operation process of gas storage. In the pressure range of 5 MPa–13 MPa and temperature range of 303.15 K–323.15 K, the density of CO₂ increases five to eight times, while the density of natural gas only increases two to three times, and the viscosity of CO₂ is more than 10 times that of CH₄. The operation temperature and pressure in gas storage should be higher than the temperature and pressure in the supercritical conditions of CO₂ because the diffusion ability between the gas molecules is increased in these conditions. However, the temperature and pressure have little effect on the mixing degree of CO₂ and CH₄ when the pressure is over the limited pressure of supercritical CO₂. The CO₂, with higher compressibility, can quickly replenish the energy of the gas storage facility and provide sufficient elastic energy during the natural gas production process. In addition, the physical properties of the reservoir also have a significant impact on the mixing and production of gases in gas storage facilities. The higher porosity reduces the migration speed of CO₂ and CH₄. However, the higher permeability promotes diffusion between gases, resulting in a higher degree of gas mixing. For a large inclination angle or thick reservoir structure, the mixed zone width of CO₂ and CH₄ is small under the action of gravity. An increase in the injection–production rate intensifies the mixing of CO₂ and CH₄. The injection of CO₂ into reservoirs also induces the CO₂–water–rock reactions, which improves the porosity and is beneficial in increasing the storage capacity of natural gas. Full article

For deep underground coal mining ecosystems, research on microbial communities and geochemical characteristics of sediments in different functional zones is lacking, resulting in the knowledge of zone-level mine water pollution prevention and control being narrow. In this study, we surveyed the geochemical distinctions and microbial communities of five typical functional zones in a representative North China coalfield, Xinjulong coal mine. The data indicated that the geochemical compounds and microbial communities of sediments showed distinguishing features in each zone. The microbial community richness and diversity were ranked as follows: surface water > rock roadways > sumps > coal roadways ≥ goafs. Canonical Correlation Analysis (CCA), Spearman correlation and co-occurrence network analysis demonstrated that microbial communities were sensitive and closely related to hydrochemical processes. The microbial community distribution in the underground mine was closely related not only to nutrient elements (i.e., C, S, P and N), but also to redox-sensitive substances (i.e., Fe and As). When it comes to mine water pollution prevention and control, the central zones are goafs. With the increase in goaf closure time, total nitrogen (TN), total organic carbon (TOC) and total sulfur (TS) decreased, but As, Fe and total phosphorus (TP) gradually increased, and the characteristic pollutant SO₄²⁻ concentration in water samples decreased. Additionally, the sulfate-reducing bacteria (SRB) had relatively higher proportions in goafs, suggesting goafs were able to purify themselves. In practical engineering, in situ nitrogen injection technology used to expel oxygen and create an anaerobic environment can be implemented to enhance SRB reducing sulfate in goafs. Meanwhile, because coal mine pollution discharge generally only discharges mine water and leaves sediment underground, the pollutants can be transferred to the sediment by strengthening the relevant reactions including the heavy metal solidification and stabilization function of bacteria. Full article

CO₂ is being considered as an effective alternative working fluid for geothermal applications due to its superior fluid dynamics and heat transfer properties compared to water. Utilizing sedimentary rocks for geothermal energy recovery through a CO₂-plume geothermal system, especially in carbonate reservoirs, has been shown to be a practical approach that eliminates the need for hydraulic fracturing. However, uncertainties remain regarding the thermal and hydraulic behavior, particularly the chemical interactions between CO₂ and carbonate rocks. This study develops a comprehensive wellbore–reservoir coupling reactive transport model based on specific information obtained from the Ordovician limestone geothermal reservoir in Shandong, China. The model aims to assess the feasibility of heat extraction in carbonate reservoirs by evaluating the heat extraction performance and fluid–rock interaction. The results indicate a rapid temperature drop after CO₂ breakthrough due to the Joule–Thomson effect. Simultaneously, the fluid transitions into and maintains a two-phase state throughout the operation. Chemical reactions within the reservoir are not aggressive since complete mixing between unsaturated water and CO₂ only occurs in the vicinity of the production well, highlighting the potential of utilizing carbonate reservoirs for efficient heat extraction in geothermal systems. Further research is needed to optimize the performance of CO₂-based geothermal systems in carbonate reservoirs. Full article

The key features of the interaction between peridotites of the continental lithospheric mantle and reduced hydrocarbon-rich fluids have been studied in experiments conducted at 5.5 GPa and 1200 °C. Under this interaction, the original harzburgite undergoes recrystallization while the composition of the fluid changes from CH₄-H₂O to H₂O-rich with a small amount of CO₂. The oxygen fugacity in the experiments varied from the iron-wustite (IW) to enstatite-magnesite-olivine-graphite/diamond (EMOG) buffers. Olivines recrystallized in the interaction between harzburgite and a fluid generated by the decomposition of stearic acid contain inclusions composed of graphite and methane with traces of ethane and hydrogen. The water content of such olivines slightly exceeds that of the original harzburgite. Redox metasomatism, which involves the oxidation of hydrocarbons in the fluid by reaction with magnesite-bearing peridotite, leads to the appearance of additional OH absorption bands in the infrared spectra of olivines. The water content of olivine in this case increases by approximately two times, reaching 160–180 wt. ppm. When hydrocarbons are oxidized by interaction with hematite-bearing peridotite, olivine captures Ca-Mg-Fe carbonates, which are products of carbonate melt quenching. This oxidative metasomatism is characterized by the appearance of specific OH absorption bands and a significant increase in the total water content in olivine of up to 500–600 wt. ppm. These findings contribute to the development of criteria for reconstructing metasomatic transformations in mantle rocks based on the infrared spectra and water content of olivines. Full article

Shale oil resources are abundant, but reservoirs exhibit strong heterogeneity with extremely low porosity and permeability, and their development is challenging. Carbon dioxide (CO₂) injection technology is crucial for efficient shale oil development. When CO₂ is dissolved in reservoir formation water, it undergoes a series of physical and chemical reactions with various rock minerals present in the reservoir. These reactions not only modify the reservoir environment but also lead to precipitation that impacts the development of the oil reservoir. In this paper, the effects of water–rock interaction on core porosity and permeability during CO₂ displacement are investigated by combining static and dynamic tests. The results reveal that the injection of CO₂ into the core leads to reactions between CO₂ and rock minerals upon dissolution in formation water. These reactions result in the formation of new minerals and the obstruction of clastic particles, thereby reducing core permeability. However, the generation of fine fractures through carbonic acid corrosion yields an increase in core permeability. The CO₂–water–rock reaction is significantly influenced by the PV number, pressure, and temperature. As the injected PV number increases, the degree of pore throat plugging gradually increases. As the pressure increases, the volume of larger pore spaces gradually decreases, resulting in an increase in the degree of pore blockage. However, when the pressure exceeds 20 MPa, the degree of carbonic acid dissolution will be enhanced, resulting in the formation of small cracks and an increase in the volume of small pores. As the temperature reaches the critical point, the degree of blockage of macropores gradually increases, and the blockage of small pores also occurs, which eventually leads to a decrease in core porosity. Full article

Geological storage is one of the most important measures to reduce carbon emissions. The newly developed oilfield A in the Pearl River Mouth Basin of the South China Sea is associated with a large amount of CO₂ with a purity of up to 95%. Two weakly consolidated sandstone saline aquifers located above the oil reservoir can be used for CO₂ storage, but the CO₂ geochemical reaction characteristics in the aquifers should be investigated clearly, which may cause significant damage to the physical properties of the reservoirs and caprocks of the aquifers. In this paper, static CO₂ geochemical reaction experiments and rock thin section identifications were carried out using drill cuttings and sidewall cores, respectively. A numerical simulation was conducted according to the reactor conditions to explore the equilibrium state of the CO₂ geochemical reaction. Through these studies, the characteristics of the geochemical reaction, its impact on the physical properties of the formation, and the CO₂ storage potential by mineral trapping in the target aquifers were revealed. The results show that the two saline aquifers have similar physical properties. The reservoirs are mostly made up of fine-to-medium-grained sandstones as quartz arenite with a considerable amount of feldspar, which can provide favorable pore space for CO₂ storage, while the caprocks are fine-grained felsic sedimentary rocks that can have a good sealing effect. However, both the reservoirs and caprocks contain a certain amount of carbonate and clay minerals. Mineral dissolution dominates in the CO₂ geochemical reaction process, and more Ca²⁺ and Mg²⁺ is released into the formation water. The theoretical maximum CO₂ mineral trapping capacity in the aquifers is 0.023–0.0538 mol/100 g rock, but due to the dynamic equilibrium of the geochemical reaction, the amount of mineralized CO₂ in most of the rock samples is negative, and the average utilization factor is only −55.43%. As a result, the contribution of mineral trapping to the CO₂ storage capacity takes −0.32%, which can be ignored. In the future, it is necessary to conduct detailed research to reveal the effect of a CO₂ geochemical reaction on storage safety, especially in offshore weakly consolidated sandstone saline aquifers, which could be important sites for large-scale CO₂ storage in China. Full article