• Energy Research

In order to explore the influences of effective stress change on gas adsorption–desorption behaviors, primary undeformed coal (PUC) and tectonically deformed coal (TDC) from the same coal seam were used for adsorption–desorption experiments under different effective stress conditions. Experimental results showed that gas adsorption and desorption behaviors were controlled by the coal core structure and the pore-fissure connectivity under effective stress. The coal matrixes and fissures were compressed together under effective stress to reduce connectivity, and it was difficult for gas to absorb and desorb as the stress increased in primary undeformed coal. The loose structure of tectonically deformed coal cores can help gas to fully contact with the coal matrix, resulting in higher adsorption gas volumes. The support of coal particles in tectonically deformed coal cores weakens the compression of intergranular pores when effective stress increases, which in this study manifested in the fact that while the volumetric strain of the coal matrix change rapidly under low effective stress, but the adsorbed gas volume did not decrease significantly. The reduction in effective stress induced the rapid elastic recovery of the coal matrix and the expansion of cracks, and increased desorption gas volumes. The stress reduction significantly increased the initial gas volume of the tectonically deformed coal, while promoting slow and continuous gas desorption in primary undeformed coal. Therefore, the promotion effect of the reservoir pressure reduction on gas desorption and coal connectivity enhancement can help to improve coalbed methane recovery in primary undeformed coal and tectonically deformed coal reservoirs.

In order to explore the influences of effective stress change on gas adsorption–desorption behaviors, primary undeformed coal (PUC) and tectonically deformed coal (TDC) from the same coal seam were used for adsorption–desorption experiments under different effective stress conditions. Experimental results showed that gas adsorption and desorption behaviors were controlled by the coal core structure and the pore-fissure connectivity under effective stress. The coal matrixes and fissures were compressed together under effective stress to reduce connectivity, and it was difficult for gas to absorb and desorb as the stress increased in primary undeformed coal. The loose structure of tectonically deformed coal cores can help gas to fully contact with the coal matrix, resulting in higher adsorption gas volumes. The support of coal particles in tectonically deformed coal cores weakens the compression of intergranular pores when effective stress increases, which in this study manifested in the fact that while the volumetric strain of the coal matrix change rapidly under low effective stress, but the adsorbed gas volume did not decrease significantly. The reduction in effective stress induced the rapid elastic recovery of the coal matrix and the expansion of cracks, and increased desorption gas volumes. The stress reduction significantly increased the initial gas volume of the tectonically deformed coal, while promoting slow and continuous gas desorption in primary undeformed coal. Therefore, the promotion effect of the reservoir pressure reduction on gas desorption and coal connectivity enhancement can help to improve coalbed methane recovery in primary undeformed coal and tectonically deformed coal reservoirs.

Reservoir pressure relief is a practical method to enhance permeability for coalbed methane (CBM) extraction in tectonically deformed coal (TDC) reservoirs. To explore the coal permeability response to stress changes, the primary undeformed coal (PUC) and TDC from the same coal seam were sampled for the pore-fissure structure analysis, mechanical property test, and permeability experiments under different stress loading-unloading methods in this study. The experimental results demonstrated that the coal permeability is more sensitive to the changes in confining pressure (perpendicular to airflow) than axial stress (parallel to airflow). Coal permeability decreases negatively exponentially as the confining pressure increases, and its change process with increased axial pressure can be divided into five stages in this study. The pore structures and mechanical properties of coal samples affected their permeability response to stress changes. Under the stress loading condition, the coal matrix and fractures of PUC samples were compressed simultaneously, and the permeability was regulated by the pore-fissure structures in the coal matrix. Due to the deformation and displacement of coal particles, the permeability of the TDC sample is predominantly dependent on changes in intergranular pores. At the initial stress unloading stage, the fissure recovery and expansion lead to a rapid increase in permeability, but the permeability cannot rereach the original value when the stress is fully released. Furthermore, the influencing factors of coal permeability in response to stress loading-unloading also include confining pressure conditions and coal matrix adsorption swelling. Research on the permeability response characteristics of the stress loading-unloading process can provide some clarifications for the reservoir depressurization and permeability enhancement of CBM extraction in the TDC reservoir.

Reservoir pressure relief is a practical method to enhance permeability for coalbed methane (CBM) extraction in tectonically deformed coal (TDC) reservoirs. To explore the coal permeability response to stress changes, the primary undeformed coal (PUC) and TDC from the same coal seam were sampled for the pore-fissure structure analysis, mechanical property test, and permeability experiments under different stress loading-unloading methods in this study. The experimental results demonstrated that the coal permeability is more sensitive to the changes in confining pressure (perpendicular to airflow) than axial stress (parallel to airflow). Coal permeability decreases negatively exponentially as the confining pressure increases, and its change process with increased axial pressure can be divided into five stages in this study. The pore structures and mechanical properties of coal samples affected their permeability response to stress changes. Under the stress loading condition, the coal matrix and fractures of PUC samples were compressed simultaneously, and the permeability was regulated by the pore-fissure structures in the coal matrix. Due to the deformation and displacement of coal particles, the permeability of the TDC sample is predominantly dependent on changes in intergranular pores. At the initial stress unloading stage, the fissure recovery and expansion lead to a rapid increase in permeability, but the permeability cannot rereach the original value when the stress is fully released. Furthermore, the influencing factors of coal permeability in response to stress loading-unloading also include confining pressure conditions and coal matrix adsorption swelling. Research on the permeability response characteristics of the stress loading-unloading process can provide some clarifications for the reservoir depressurization and permeability enhancement of CBM extraction in the TDC reservoir.

As one of the crucial factors contributing to coal spontaneous combustion, the oxidation of pyrite is a complex process involving multiple reactions, particularly in the presence of oxidants (Fe3+ and O2) and bacteria. However, experimental results based on mineral-pyrite are not entirely applicable to coal-pyrite due to their differences in formation environments and compositions. This study selected two types of coal-pyrite and one type of mineral-pyrite as research to conduct oxidation experiments with the participation of oxidant (Fe3+) and bacteria (Acidithiobacillus ferrooxidans), respectively, to obtain the following conclusions. Under natural conditions, the chemical oxidation rate of pyrite is slow, but the addition of oxidant Fe3+ and bacteria can significantly accelerate the oxidation rate. The promotion effect of oxidant Fe3+ on the oxidation reaction is stronger than that of bacteria. Under the same conditions, the oxidation rate of coal-pyrite samples is slightly higher than that of mineral-pyrite, due to the relatively higher impurities content, poorer crystal structure, and humic acid in the coal seams. Additionally, different compositions of coal-pyrite samples can lead to various oxidation degrees under different conditions. Therefore, the oxidation process and mechanism of pyrite in coal seams are complex and affected by many factors, which need further study to prevent coal spontaneous combustion accurately and effectively.

As one of the crucial factors contributing to coal spontaneous combustion, the oxidation of pyrite is a complex process involving multiple reactions, particularly in the presence of oxidants (Fe3+ and O2) and bacteria. However, experimental results based on mineral-pyrite are not entirely applicable to coal-pyrite due to their differences in formation environments and compositions. This study selected two types of coal-pyrite and one type of mineral-pyrite as research to conduct oxidation experiments with the participation of oxidant (Fe3+) and bacteria (Acidithiobacillus ferrooxidans), respectively, to obtain the following conclusions. Under natural conditions, the chemical oxidation rate of pyrite is slow, but the addition of oxidant Fe3+ and bacteria can significantly accelerate the oxidation rate. The promotion effect of oxidant Fe3+ on the oxidation reaction is stronger than that of bacteria. Under the same conditions, the oxidation rate of coal-pyrite samples is slightly higher than that of mineral-pyrite, due to the relatively higher impurities content, poorer crystal structure, and humic acid in the coal seams. Additionally, different compositions of coal-pyrite samples can lead to various oxidation degrees under different conditions. Therefore, the oxidation process and mechanism of pyrite in coal seams are complex and affected by many factors, which need further study to prevent coal spontaneous combustion accurately and effectively.

Abstract We conducted simulation experiments on supercritical carbon dioxide (ScCO2)-H2O-rock interactions for caprock samples obtained from the Qinshui Basin, China, to explore the influence of carbon dioxide (CO2) injection on the sealing capability of deep unminable coal seam caprocks as it relates to CO2 geological storage. This research focused on the changes in mineral composition, pore-fissure structure, permeability, and mechanical properties of the caprocks after ScCO2-H2O treatment. These results revealed that ScCO2-H2O-rock interactions lead to the formation of dissolution pores on the contact surface and a significant increase in macropores volumes and permeability, while the precipitation of secondary minerals can prevent CO2 from entering the rock in the later reaction stage. The sealing capacity of intact caprock initially decreases and then changes slightly after being affected by ScCO2-H2O-rock interactions. The caprock integrity determines the security of the CO2 geological storage. However, both natural and induced fractures provide channels for CO2 leakage. The compressive strength of the rock was reduced by 36 %, and the toughness increased after 60 days of ScCO2-H2O treatment. Thus, the formation of new fractures and the reopening of pre-existing faults in caprock may occur before damage under stress changes and ScCO2-H2O-rock interactions. Fracturing and shedding of fault gouge and particles inside the fractures by high-pressure ScCO2-H2O fluid can expand the fractures in caprock significantly, and hence increase the risk of CO2 leakage. Therefore, deep coal seams with thick and intact caprock should be selected to increase the safety of CO2 geological storage. Furthermore, the CO2 injection pressure must be considered to prevent the formation of additional fractures and occurrence of rock failure resulting from the softening effect of long-term ScCO2-H2O-interaction.

Abstract We conducted simulation experiments on supercritical carbon dioxide (ScCO2)-H2O-rock interactions for caprock samples obtained from the Qinshui Basin, China, to explore the influence of carbon dioxide (CO2) injection on the sealing capability of deep unminable coal seam caprocks as it relates to CO2 geological storage. This research focused on the changes in mineral composition, pore-fissure structure, permeability, and mechanical properties of the caprocks after ScCO2-H2O treatment. These results revealed that ScCO2-H2O-rock interactions lead to the formation of dissolution pores on the contact surface and a significant increase in macropores volumes and permeability, while the precipitation of secondary minerals can prevent CO2 from entering the rock in the later reaction stage. The sealing capacity of intact caprock initially decreases and then changes slightly after being affected by ScCO2-H2O-rock interactions. The caprock integrity determines the security of the CO2 geological storage. However, both natural and induced fractures provide channels for CO2 leakage. The compressive strength of the rock was reduced by 36 %, and the toughness increased after 60 days of ScCO2-H2O treatment. Thus, the formation of new fractures and the reopening of pre-existing faults in caprock may occur before damage under stress changes and ScCO2-H2O-rock interactions. Fracturing and shedding of fault gouge and particles inside the fractures by high-pressure ScCO2-H2O fluid can expand the fractures in caprock significantly, and hence increase the risk of CO2 leakage. Therefore, deep coal seams with thick and intact caprock should be selected to increase the safety of CO2 geological storage. Furthermore, the CO2 injection pressure must be considered to prevent the formation of additional fractures and occurrence of rock failure resulting from the softening effect of long-term ScCO2-H2O-interaction.