Low-temperature Oxidation Stage Research Articles

Pore Morphology and Oxidation Behaviour of Deep-Loaded Granular Coal at High Initial Temperatures

ABSTRACT As coal mining proceeds deeper, fragmented coal is subjected to high geostress and elevated thermal environments, causing changes in its micro-physical properties and spontaneous combustion (SC) risks. Deep mining operations entail significant risks of coal fires. In order to investigate the effects of high stress and high thermal environments on the microstructural and physicochemical damage of fragmented coal, and to elucidate the mechanisms influencing oxidation behavior, we conducted experiments including low-temperature nitrogen adsorption, synchronous thermal analysis, and in-situ diffuse reflectance spectroscopy. These experiments aimed to explore the pore morphology and oxidation characteristics of high initial temperature unloading granular coal (HITU-GC) unloading conditions. The results indicate that with increasing initial temperature, characteristic point temperatures initially rise and then decrease, while both the mass loss and heat release during the low-temperature oxidation stage decrease. After pressure-unloading composite treatment, the oxidation process accelerates with more intense reactions, resulting in an overall increase in heat release. The pore shapes of HITU-GC show no significant difference compared to the original coal, mainly comprising non-rigid aggregates of micropores, platy or layered matrix particles, including numerous slit-shaped pores. The coal surface also exhibits ink bottle-shaped pores and cone-shaped pores. Higher pre-oxidation temperatures correlate with higher specific surface areas. Higher stress levels from pressurized unloading correlate with lower specific surface areas. The specific surface area of sample O60-P16 is reduced by 0.879 m2/g compared to sample O60-P0. Pressurized unloading treatment disrupts mesoporous pores. Following thermal environment and pressure-unloading, the coal’s aromatic hydrocarbon content decreases, while the contents of hydroxyl, aliphatic hydrocarbons, and oxygen-containing functional groups (FG) all increase. Sample O60-P4 shows the highest content of active FG. While thermal environments enhance coal oxidation activity, pressure-unloading treatment increases the extent of coal surface fragmentation. The synergistic effect of both significantly increases the SC risk of coal. This study provides a theoretical basis for controlling thermal hazards associated with high-initial-temperature unloading fragmented coal in deep goaf areas.

Experimental analyses of CO2 sequestration potential in mineralized fly ash and its efficacy in mitigating coal spontaneous ignition risks in underground mine goafs

To effectively utilize fly ash solid waste and realize fly ash resource utilization, carbon sequestration and emission reduction, as well as mine fire prevention and suppression, this paper analyses the components of fly ash by X-ray fluorescence spectrometer (XRF), rheological experiments, the CO2 sequestration experiments by the mineralization of fly ash, thermogravimetric analysis experiments, and flame-retardant property experiments. Additionally, the changes in the physical structure and morphology of fly ash before and after mineralization were analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The carbon sequestration capacity and fluidity of fly ash slurry, as well as the ability of mineralized fly ash slurry to inhibit spontaneous coal combustion are comprehensively investigated. The research results indicate that CO2 is sequestered as calcite on the fly ash surface after mineralization. The optimal CO2 sequestration capacity of fly ash is 65.12 kg/t when 0.5 mol/L NaHCO3 is used as an additive. The fluidity of mineralized fly ash slurry is greater than that of fly ash slurry, and the lower the water-cement ratio is, the worse the fluidity. Mineralized fly ash slurry with a water-cement ratio of 3:1 has a strong carbon sequestration capacity and high fluidity, making it suitable for preventing coal spontaneous ignition in mine goafs. Mineralized fly ash slurry exhibits a regular trend where the better the mineralization effect, the lower the CO release during the low-temperature oxidation stage of coal, and the higher the cross-point temperature. Mineralized fly ash slurry has a stronger ability to inhibit spontaneous coal combustion than does ordinary fly ash slurry.

Influence of mudstone on coal spontaneous combustion characteristics and oxidation kinetics analysis

To explore the spontaneous combustion characteristics and hazards of the low-temperature oxidation (LTO) stage in the process of spontaneous combustion of coal and mudstone, the pore structure, spontaneous combustion characteristic parameters, and exothermic characteristics of coal and mudstone were tested and studied, and the oxidation kinetic parameters were calculated. The results show that mudstone has a larger specific surface area and pore volume than coal. From the fractal characteristics, the pore structure of mudstone is more complex than that of coal. According to the comparison of theoretical and actual gas generation and oxygen consumption rate curves, it is found that there is an interaction between coal and mudstone in the LTO process. With the increase of mudstone mass ratio, gas production, and its oxygen consumption rate increase. Among them, CM-4 (Coal:Mudstone = 1:1) has the highest exothermic intensity and the exothermic factor (A) and fire coefficient (K) increase with the increase of mudstone content. The apparent activation energy of the mudstone sample is lower than that of the raw coal, indicating that the sample after adding mudstone is more likely to have spontaneous combustion in the LTO stage.

Analysis of temperature rise of low temperature adiabatic oxidation of sulfur-containing “coal-like substances”

ABSTRACT Spontaneous combustion of coal is one of the main natural disasters in the process of coal mining, and spontaneous combustion of sulfur-containing coal is an urgent problem to be solved. In order to understand the influence of different sulfur structures and sulfur content on spontaneous combustion tendency of coal, the self-developed synchronous temperature compensation adiabatic oxidation experiment system was used to study the oxidation self-heating performance of a series of sulfur-containing “coal-like materials.” Through the analysis of the heating rate of coal containing specific sulfur structure, it is considered that the time of adiabatic oxidation temperature of different organic sulfur “coal” to rise to 70°C is dimethyl sulfoxide > phenylene sulfide > dibenzothiophene. By analyzing the oxidation temperature rise characteristics of organic sulfur models with different sulfur contents, it is believed that the higher sulfur content, the faster the low temperature oxidation reaction of “coal-like substance,” the faster the temperature rise rate, and the shorter the time to reach the uncontrollable self-heating temperature, so the shorter the spontaneous combustion period. In general, in the stage of low temperature oxidation, active sulfur containing small molecules (dimethyl sulfoxide) promote the oxidation temperature rise, stable structure, larger molecular weight of dibenzothiophene slow down the “coal-like” oxidation temperature rise.

Study on the evolution characteristics of molecular surface active sites of low-rank coal in low-temperature oxidation stage

Compared with other coal types of coal, low - metamorphic coal contains numerous active sites, making it susceptible to spontaneous combustion and posing significant risks during production, transportation, and storage. To further reveal the evolution characteristics and laws of low-rank coal's active sites during the process of low-temperature oxidation of coal (LTOC), X-ray photoelectron spectroscopy (XPS), 13C nuclear magnetic resonance (13C NMR), and Fourier transform infrared (FTIR) experiments were used to determine the basic structural parameters such as the valence state of non-carbon features, types of carbon elements in coal macromolecules, and structure and quantity of functional groups. Combined with molecular modelling software (GaussView 6.0) and macromolecular calculation software (Grimme-XTB), macromolecular models of coal at different temperature points were constructed, and the surface energy field distribution was calculated. The number and energy change information of chemical and physical adsorption sites of low-rank coal during heating were obtained. The results show that during the LTOC, the chemical adsorption sites such as aliphatic hydrocarbons (-CH3/-CH2) have higher reactivity and can be converted into oxygen-containing functional groups (-CHO, –OH, –COOH); the number of physical adsorption sites increased first and then decreased, and reached the maximum at 50 °C. Coal exhibited a higher affinity for O2 at this temperature. The peroxide (-C-O-O-H) undergoes thermal decomposition, leading to H2O and weight loss of coal. The –CO and –COO structures undergo decarboxylation to generate CO, CO2, and other gases. This study comprehensively revealed the evolution of active sites through two key coal oxygen processes, physical and chemical adsorption, based on the gradual temperature elevation of low-rank coal. This holds immense significance for improving and enriching the understanding of the mechanism of coal spontaneous combustion(CSC), preparing more targeted CSC inhibition, and formulating efficient fire prevention and extinguishing measures.

Study on the oxidized gas production characteristics and spontaneous combustion tendency of pre-oxidized water-soaked coal in lean-oxygen environments.

The oxidation characteristics and spontaneous combustion (SC) tendency of raw long-flame coal (RC), water-soaked 200-day coal (S200), pre-oxidized water-soaked coal at 200°C (O200S200), and pre-oxidized soaked coal at 300°C (O300S200) in an oxygen-poor environment were investigated using a programmed warming system. The results show that pre-oxidation water-soaked treatment (PWT) promotes the coal-oxygen complex reaction and increases the rate of coal oxygen consumption (OCR) and the rate of carbon and oxygen compound production. The rate of CO and CO2 production of the water-soaked (WS) coal increased by 0.329mol·(cm3·s)-1 and 0.922mol·(cm3·s)-1, respectively, compared with that of the original coal sample. PWT reduces the activation energy of coal in the low-temperature oxidation stage (the maximum difference can be up to 110.99kJ/mol) and enhances the oxidizing and heat-releasing capacity. There was a synergistic effect between the pre-oxidation (PO) and WS treatment, and the lowest comprehensive determination index of the SC propensity of coal in O200S200 samples was 831.92 which was 4.72 lower than that of RC samples, presenting a more SC tendency. Low oxygen concentration has an inhibitory effect on the oxidation characteristic parameters of coal, and the apparent activation energy of the low-temperature oxidation stage of pre-oxidized water-soaked coal (PWC) increased to 206.418kJ/mol at 3% oxygen concentration. The lower the oxygen concentration of the anoxic environment, the lower the risk of SC of the coal samples. The results of the study can provide theoretical guidance for the identification and prevention of SC disasters in coal seams with shallow burial and close spacing.

Effect of water soaking and air drying on the thermal effect and heat transfer characteristics of coal oxidation at the low-temperature oxidation stage

The microstructure, thermal behaviour (30 °C-300 °C) and thermophysical properties (30 °C-300 °C) of raw and water-soaked air-dried coal samples were analysed through scanning electron microscopy, differential scanning calorimetry and laser thermal conductivity meter to study the effect of water soaking and air drying on the thermal effect and heat transfer characteristics of coal at the low-temperature oxidation stage. Experimental results show that the action of water soaking and air drying changes the apparent morphology of coal, increasing the density of surface pore distribution and number of all pore types to different degrees. Under the same temperature conditions, water-soaked air-dried coal has lower heat absorption and heat release than raw coal. Moreover, the time required by water-soaked air-dried coal to reach thermal equilibrium is 32.4 s faster than that needed by raw coal. The activation energy of water-soaked air-dried coal calculated by using the KAS and FWO methods is reduced to about 83% of that of raw coal. This result indicates that water soaking and air drying increase the tendency of coal to self-ignite. The thermal diffusivity of water-soaked air-dried coal is higher than that of raw coal and shows a maximum difference of 0.005 mm2/g at 60 °C. The specific heat capacity and thermal conductivity of water-soaked air-dried coal are lower than those of raw coal and show the maximum differences of 0.19 J g/K and 0.18 W/m·K at 210 °C and 300 °C, respectively. Although the spontaneous combustion of water-soaked air-dried coal spreads at a faster rate than that of raw coal, the hazards following the spontaneous combustion is less than that of raw coal. This research contributes to understanding the effect of water soaking and air drying behaviour on coal spontaneous combustion characteristics.

Study on the kinetics of chemical structure reaction in coal catalyzed by OH free radicals

This study investigated the influence of OH free radicals on the reactivity of aromatic rings and their side chain active groups. Quantum chemistry theory and in situ diffuse reflectance infrared Fourier transform spectroscopy were applied to perform simulations and experiments. Density functional theory was used to calculate the electrophilic substitution of aromatic hydrocarbons of six model structures by OH free radicals. The reactivities of the key elementary reaction of the side chain active group before and after the reaction were compared. The results showed that the reaction energy barriers of the addition and elimination of OH radicals of aromatic hydrocarbons were lower than 40 kJ/mol, indicating that the reactions can occur at room temperature and emit large amounts of heat. The electronic effect of phenolic hydroxyl and benzene rings further improved the reactivity of the aromatic ring C–H. Combining these results with the change rule of the phenolic hydroxyl structure in coal under isothermal oxidation conditions, we conclude that OH radicals participate in the formation of the phenolic hydroxyl structure and further catalyze the oxidation reaction of coal at low temperatures. This finding is significant for the heat storage stage of low-temperature oxidation in coal.

Study on the Inhibition of Coal Spontaneous Combustion by Superabsorbent Antioxidant Inhibitors

ABSTRACT Aiming at the problems of short inhibition life and low efficiency of physical inhibitors for the prevention and control of coal spontaneous combustion, this paper utilizes super water-absorbent (SAR) resin with tea polyphenol (TP) and vitamin C (VC) to prepare an inhibitor that can continuously and efficiently inhibit coal spontaneous combustion. Through the procedure of heating, thermogravimetric experiments are performed to characterize the inhibition performance of super-water-absorbent antioxidant-type inhibitor, and the change of active functional groups in the original coal and inhibited coal samples. The experimental results show that the super-water-absorbent antioxidant retardant can inhibit the coal spontaneous combustion in the low-temperature oxidation stage by absorbing heat through water evaporation and lowering the temperature to isolate the oxygen, and in the super-temperature stage by reducing the number of active functional groups in the coal. Compared with that of the original coal, and the water retention of the highly absorbent resin played the role of cooling and oxygen barrier for the coal; the apparent activation energy of the resisted coal samples in the spontaneous combustion in the three stages of oxygen uptake and weight gain, pyrolysis, and combustion increased by 41.02 kJ/mol, 92.58 kJ/mol, and 133.7 kJ/mol. In the process of temperature increase, the content of -OH, -CH2, -COOH, C=C, and C-H functional groups in the resisted coal samples was lower than that of the original coal, and -OH and -CH2 in the resisted coal samples showed a tendency to decrease, which reduced the reactive functional groups. Therefore, the superabsorbent antioxidant inhibitors can inhibit coal spontaneous combustion reactions by reducing the activity of coal molecules and delaying the process of coal spontaneous combustion reactions through releasing water, absorbing heat, and covering oxygen. The results of this study can be used as a reference for the field of inhibition of coal spontaneous combustion by chemical inhibitors.

Study on the Influence of Coal Structure and Oxidation Performance by Endogenous Bacterium

In order to solve the defects of traditional coal spontaneous combustion prevention technology in a closed goaf, a strain of aerobic endogenous bacteria was isolated from coal and used as a blocking raw material. Based on the metabolic and reproductive characteristics of microorganisms, the experimental study on the inhibition of coal spontaneous combustion by microorganisms was carried out. The colonies were isolated and purified by the dilution concentration plate method and the scribing plate method. The growth morphology of microorganisms was analyzed, and the growth curve was determined. The strains were identified by seamless cloning technology for high-throughput sequencing. The surface morphology of coal was analyzed by SEM, the differences of oxidation characteristic temperature points were analyzed by TG–DTG–DSC images, a programmed heating experiment was used to analyze the concentration of the indicator gas CO, and the changes in microscopic groups before and after microbial action were analyzed by FTIR and XPS spectra. Therefore, the inhibition of coal oxidation by endogenous bacteria was verified from macroscopic and microscopic perspectives. The results show that the coal bacteria isolated from the coal is Lysinibacilus sp. After the culture of Lysinibacilus sp., the surface of the coal demonstrated less detritus, and was relatively smooth. In the early stage of low temperature oxidation of coal spontaneous combustion, the characteristic temperature point of coal oxidation and the reaction between coal and O2 could be delayed by Lysinibacilus sp., and the total heat release was reduced in the combustion process. Not only that, Lysinibacilus sp. could also reduce the CO concentration during coal heating. After the coal was decomposed by Lysinibacilus sp., the C=C thick ring skeleton structure had little effect; however, the aromatic substitution pattern changed. This bacterium had an effect on the C-O bond, reducing the percentage of -CH2- and increasing the percentage of -CH3. It might also use the crystalline water in coal for life activities. The carboxyl carbon in coal changed the most, with a decrease of 12.03%, so it might become the carbon source required for microbial growth. The reproductive metabolism of microorganisms also affected the form of nitrogen, and the percentage of pyridine nitrogen in coal was reduced. The ratio of single-bond carbon to double-bond carbon in raw coal was about 3:2, but after this bacterial action, the ratio of the two was about 1:1. The analytical conclusions of XPS and FTIR spectra were consistent, and the results supported each other.

Carbon Dioxide Prevents Oxygen Adsorption at Low-Temperature Oxidation Stage of Low-Rank Coal: Laboratory Study and Molecular Simulation

Carbon dioxide (CO2) is widely used in the prevention and control of spontaneous coal combustion. In this manuscript, three low-rank coals with different metamorphic degrees were selected as the research objects. The temperature-programmed experiments, in situ infrared cooling experiments, simulation of the competitive adsorption of CO2 and oxygen (O2) in coal pores, and simulation study of the CO2 inhibition of the coal oxygen composite reaction were used to obtain the role and effect of CO2 in preventing oxygen adsorption in coal at the low-temperature oxidation stage. It was concluded that CO2 can displace the O2 near the pore wall to physically prevent the adsorption of O2. Through the changing law of heating rate and a kinetics analysis, it was found that CO2 can increase its activation energy by 5.3–108.3% during the slow heating stage of coal and reduce its heat rate. At around 120 °C, coal loses the protective effect of CO2. From the changes in functional groups, it can be seen that when coal was cooled in the CO2 atmosphere, mainly pyrolysis and condensation reactions occurred due to the lack of O2. In addition, CO2 can also inhibit the chain reaction of the chemical adsorption of oxygen in coal. This work provides a theoretical basis for CO2 prevention and the control of spontaneous coal combustion.

Oxidation Kinetics Analysis of Crude Oils with Different Viscosities

In order to compare the oxidation kinetics parameters of crude oils with different properties in the process of crude oil oxidation, six different crude oil samples were selected to analyze the oxidation characteristics of crude oils with different properties. In order to study the oxidation of crude oil, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) synchronous analyzer were used for crude oil in the oxygen environment between 25 °C and 900 °C at a heating rate of 20 °C/min. The experimental results were based on crude oil oxidation using TGA and DTG experimental data to evaluate the oxidation mechanism of different crude oils, so as to better understand the situation in the oxidation reaction process. At the same time, the oxidation stage of crude oil was divided according to DSC data. Arrhenius method was used to analyze the oxidation kinetic parameters of crude oils with different properties, and the activation energies and pre-exponential factors of different crude oils were calculated. The experimental results show that the oxidation stage of crude oil can be divided into three stages: low-temperature oxidation, fuel deposition, and high-temperature oxidation. The low-temperature oxidation reaction begins at 280 °C, and the high-temperature oxidation reaction occurs at 400 °C. The low-temperature oxidation activation energy of an oil sample is 39.73–77.74 kJ/mol. The activation energy of the high-temperature oxidation is in the range of 106.27–264.47 kJ/mol. The activation energy of crude oil in the low-temperature oxidation stage increases with the increase of crude oil viscosity and decreases with the increase of crude oil viscosity in the high-temperature oxidation stage. Therefore, during the high-temperature oxidation stage, high-viscosity crude oil is more prone to reactions.

Investigation of the effect of different distilled water, rainwater and seawater mass ratios on coal spontaneous combustion characteristics

When water comes into contact with coal, the risk of coal spontaneous combustion should be reassessed. In order to analyze the effect of distilled water, rainwater and seawater on the coal self-heating, thermogravimetric analysis (TGA) was applied to investigate the differences between the macroscopic oxidation properties of raw coal and water-immersed coal. The risk of coal spontaneous combustion increases after water immersion, but different types of water have different degrees of influence on the spontaneous combustion of coal. The microscopic pore structure and elemental changes on the surface of coal samples before and after water immersion were studied by Scanning Electron Microscope (SEM), low-pressure nitrogen gas adsorption (LP-N2GA) and Energy Dispersive Spectroscopy (EDS) experiments. Fourier infrared spectroscopy (FTIR) was used to investigate the change of active groups. The results show that the pore structure of coal samples immersed in water is much more developed than that of raw coal. In the low-temperature oxidation stage, moisture evaporation consumes much oxidation heat and inhibits the coal self-heating. After the stage, it promotes the coal spontaneous combustion. The content of the hydroxyl group increases, and the content of carbonyl and carboxyl decreases. The alkali metal elements can act as catalysts and active carriers of oxygen, enhancing the oxidation activity of coal. The results are helpful to understand the mechanism of different distilled water, rainwater and seawater mass ratios on coal spontaneous combustion and avoid potential self-heating after immersion.

Oxidation behaviors of Hongqian heavy crude oil characterized by TG-DSC-FTIR-MS within full temperature regions

To better understand in-situ combustion (ISC) theory and reveal the oxidation behavior and mechanism of Hongqian (HQ) heavy crude oil, the thermo-oxidative characteristics and kinetics of HQ heavy oil were studied under air conditions using TG-DSC-FTIR-MS technique. According to the TG/DTG-DSC curves, the oxidation reactions of HQ heavy oil can be divided into low temperature oxidation (LTO), fuel deposition (FD) and high temperature combustion (HTC) stages. The reaction mechanism for each reaction interval was comprehensively analyzed by online FTIR and MS analysis. In FTIR spectra, H2O, hydrocarbon, CO2, CO, –COOH, and aromatic ring were observed during the oxidation process, which are used to explain the occurrence of the possible decomposition/isomerization of hydroperoxides in the later stage of LTO and the important role of light hydrocarbons evaporation, thermal oxidative cracking and polycondensation for coke formation at FD stage, coke combustion at HTC stage, as well as the possible hysteresis of dehydrogenation and aromatization at FD and HTC stages. The strong variation of activation energy with conversion degree confirms the complex oxidation reactions. MS analysis of H2, CH4, H2O, CO, O2, and CO2 produced during oxidation process helps to verify these reaction mechanisms; and provide more information about the in-situ H2 generation and the conditions where H2 generation may occur as well as the generation of CO and its possible reaction mechanism in LTO reactions. These obtained data aid in a deeper insight into crude oil oxidation mechanism and provide reference for establishing accurate reaction scheme and reaction kinetic model for the precise simulation of ISC processes.

Experimental Study on the Influence of Pore Structure and Group Evolution on Spontaneous Combustion Characteristics of Coal Samples of Different Sizes During Immersion.

To study the influence of water immersion on the evolution of the groups and spontaneous combustion characteristics of coal samples with different sizes, raw coal from the Fengshuigou Coal Mine operated by Pingzhuang Coal Company in Inner Mongolia was studied. The infrared structural parameters, combustion characteristic parameters, and oxidation reaction kinetics parameters of D1-D5 water immersion coal samples were tested, and the mechanism of spontaneous combustion during the oxidation of submerged crushed coal was investigated. The results were as follows. The water immersion process promoted the re-development of coal pore structure, and the micropore volume and average pore diameter were 1.87-2.58 and 1.02-1.13 times those of raw coal, respectively. The smaller the coal sample sizes, the more significant the change. At the same time, the water immersion process increased the contact point between the active group and oxygen in the coal, and the C=O, C-O, and -CH3/-CH2- groups in coal were further promoted to react with oxygen to generate -OH functional groups and improve the reactivity of coal. The characteristic temperature of water immersion coal was affected by the temperature rise rate, coal sample size, coal voidage, and other factors. Compared with the raw coal, the average activation energy of the water immersion coal with different sizes decreased by 12.4-19.7%, and the apparent activation energy of the coal sample with a size of 60-120 mesh was the lowest on the whole. In addition, the apparent activation energy in the low-temperature oxidation stage was significantly different.

High pressure air injection in ultra-low permeability reservoirs: Effects of physical and chemical reactions on oil recovery

This research aims to investigate the effects of physical and chemical reactions on oil recovery during high-pressure air injection (HPAI) in ultra-low permeability reservoirs. The oxidation reaction characteristics of dead oil were investigated using thermal analysis experiments and oxidation tube experiments. Additionally, pressure-volume-temperature (PVT) analysis was employed to simulate the physical and chemical reactions between air and live oil under formation conditions. The results indicated that the fuel deposition (FD) stage exhibited less prominence in light oil due to its lower content of resins and asphaltenes. Additionally, the presence of cuttings impeded the volatilization of lighter components during low-temperature oxidation (LTO) stage, effectively storing fuel required for subsequent high-temperature oxidation (HTO) stage, consequently facilitating a direct transition from LTO to HTO. Notably, the debris derived from the ultra-low permeability reservoirs possesses an enlarged specific surface area, facilitating enhanced contact between the oil and oxygen. This heightened contact effectively reduces the activation energy required for the LTO stage by an appreciable 22.9 kJ/mol. Furthermore, the PVT experiments reveals a quadratic relationship between pressure and viscosity, while a cubic relationship exists between pressure and the expansion rate of crude oil. Moreover, the thermal effects of oxidation reactions and the dissolution of the gas phase have been found to exert a more substantial influence on reducing oil viscosity, in contrast to the oxygen addition reaction and the extraction of lighter components, which have a minor influence on increasing oil viscosity. Lastly, in the context of developing HPAI techniques for ultra-low permeability reservoirs, optimizing the gas injection rate emerges as a viable strategy to effectively enhance oil recovery, provided that the occurrence of gas channeling is effectively mitigated.

Effects of water immersion on the pore structure and thermodynamic properties of coal gangue

A large amount of coal gangue (CG) produced in coal mining accumulates to form CG hills, and the combustion properties change under the effect of leaching, increasing the risk of spontaneous combustion. The study was mainly carried out by nitrogen adsorption, infrared spectroscopy and thermogravimetry on the physicochemical properties of the CG under different immersion times (30, 60, 90 days). And the activation energy of the immersion CG at different oxidation or combustion states was calculated. The results show that the pyrite contained in gangue is oxidized after immersion, and the heat released by oxidation and the acid solution formed have an obvious effect on the pore expansion of gangue. The mesoporous and macroporous increase by 2.61% and 0.95% respectively, which strengthens the physical and chemical adsorption of oxygen for CG. The immersion process also destroys the structure of the gangue, transforming stable functional groups into active ones, especially the most significant increase in hydroxyl and oxygen-containing functional groups. The characteristic temperature changed under the action of water immersion, the cracking temperature increased, and other characteristic temperatures decreased, among which the ignition temperature decreased by about 20℃. In addition, the water immersion process inhibits the low-temperature oxidation stage of CG and promotes the high-temperature combustion stage, and the risk of spontaneous combustion is the strongest after 60 days of immersion in water. The results can provide guidance for the prevention of spontaneous combustion in drenched CG hills.

Analysis of the wetting and exothermic properties of preoxidized coal and the microscopic mechanism

In the processes of coal mining and transportation, low-temperature oxidation occurs continuously, and it is accompanied by multiple wetting. The effect of water on coal spontaneous combustion (CSC) cannot be ignored. To study the wetting and exothermic properties of preoxidized coal and the mechanism of its effect on CSC, molecular dynamics simulations, microcalorimetry, scanning electron microscopy (SEM) and fourier transform infrared spectrometer (FTIR) are used to analyze the molecular dynamics mechanism, heat of wetting (Hw), surface structure and microscopic group changes. Experimental results show that the surface potential distribution of coal is uneven, which leads to the adsorption of polar water molecules; the total energy of the system continues to decrease during the adsorption process. The preoxidation treatment further develops pore fissures, increases the percentage of hydrophilic groups and evaporates the original water. Therefore, in the coal low-temperature oxidation stage, the higher the degree of preoxidation is, the greater the Hw value. After preoxidation and wetting, the loose and porous characteristics of coal are further reflected; the oxygen percolation and adsorption are enhanced, which accelerates the system heat accumulation and thus intensifies the risk of CSC. The experimental results provide an important theoretical basis for the prevention of CSC.

Thermo-oxidative behavior and kinetics analysis of light and heavy oils based on TG, DSC, and FTIR

In-situ combustion (ISC), as a time-honored replacement technology for heavy reservoir development, needs to improve its application effect urgently. The oxidation exothermic behavior and reaction activity of heavy oil directly determine the EOR effect of ISC technology during air injection. In this work, the oxidation heat effect and combustion activity of light and heavy crude oils, and the physical properties of their oxidized oils were studied and compared through thermogravimetry (TG), differential scanning calorimetry (DSC), static non-isothermal oxidation and Fourier transform infrared spectroscopy (FTIR). The results indicate that the main reaction pathway of MH light oil is distillation and volatilization under 200 °C, the mass loss is obvious and the heat release is more than that at the high temperature oxidation (HTO) stage. After the full reaction of FC heavy oil at 350 °C, there is still about 30% low temperature oxidation (LTO) left. After the LTO and fuel deposition (FD) stages, heavy oil can produce more coke. MH light oil and FC heavy oil show an obvious mass loss at the stage of LTO and HTO, which is 68.8% and 27.3% (10 °C/min) respectively, the corresponding enthalpies are 3.02 kJ/g and 15.668 kJ/g as well. Between 470 and 500 °C, the endothermic reaction of heavy oil becomes weak, which may relate to the condensation of heavy components such as resins that release heat. The LTO activation energy of light oil decreases with the conversion, and temperature increase can promote the oxidation reaction of active components. The LTO activation energy of heavy oil increases with the conversion, while that in FD and HTO stage increases obviously, which indicates that coke combustion activity is low. The content of C1–C6 in heavy oil decreases from 5.81% to 3.57% with heating rates. The oxidation reaction in the full temperature range can improve the quality of heavy crude oil. The vibration peak at 1707 cm-1 of the MH oxidized oil gets weak, the vibration peak at 1539 cm-1 disappears, and the number of carboxyls, ether groups, aromatic rings, and miscellaneous molecules in the oxidized oil declines. The vibration peak at 3460 cm-1 of the oxidized oil enhances significantly, and the number of carboxyls and hydroxyl molecules increased.

A numerical simulation investigation on low permeability reservoirs air flooding: Oxidation reaction models and factors

A precise oxidation reaction model determines the reliability of numerical simulation results and the prediction of field performance of air injection-based enhanced oil recovery (EOR). By reviewing the classical oxidation reaction model and combining with geological characteristics of a low permeability reservoir, this work creatively divides pseudo-components and temperature regions and further proposes the improved low temperature oxidation (LTO), middle temperature oxidation (MTO), and high temperature oxidation (HTO) numerical models with chemical reaction coefficients. The EOR effects of reservoir temperature, permeability, air flow rate and bottom hole pressure during LTO and full temperature regions are determined on the basis of a 3D reservoir simulation model via CMG STARS. The results show that peak oil production rates of N2 flooding and LTO are 17.70 and 29.97 m3/d, respectively. The cumulative oil production in LTO stage is about 35%, which is higher than that in N2 flooding. The oxygen addition reaction in LTO stage can release much heat and increase sweep efficiency from thermal expansion and viscosity reduction. The peak oil production rates of LTO and full temperature oxidation (FTO) reactions are 42.16 and 44.34 m3/d, respectively, and the cumulative oil production of the FTO model was about 17% higher than that of LTO. The FTO model could more truly reflect the production performance during air flooding of low permeability oil reservoir. Strong oxygen consumption capacity and significant heat release effects can be observed under higher initial temperature, permeability, air flow rate, and lower bottom hole pressure. Thermal flooding and flue gas flooding are the main EOR mechanisms in FTO process. Considering the inescapable field problem of gas channeling, thorough contact between oil and air can be promoted by artificially preheating oil reservoir and adjusting air injection rate and pressure. Thus, oil recovery can be further improved under the premise of ensuring the safety of field application of air flooding technique.