High-rank Coals Research Articles

Characteristics and Origins of the Difference between the Middle and High Rank Coal in Guizhou and Their Implication for the CBM Exploration and Development Strategy: A Case Study from Dahebian and Dafang Block

The coalbed methane (CBM) geology in Guizhou is characterized by a high gas content, pressure and resource abundance, indicating superior CBM resource potential. However, there are also many unfavorable factors, such as complex structure geology, significant regional differences in CBM geology, the widespread development of tectonically deformed coal, and the unclear understanding of the configuration of geological factors for CBM enrichment and high yield, which restrict the increase in CBM production and a large-scale development. Taking the Dahebian Block in Liupanshui coal field and the Dafang Block in Qianbei coal field as examples, this study presented the CBM geological differences between middle- and high-rank coals; their origins were analyzed and the effect of depth on gas content and permeability was discussed. A CBM enrichment and high-yield model was illustrated, and the geologic fitness-related exploration and development methods for Guizhou CBM were finally proposed. The results show that (1) significant differences between the middle- and high-rank coals occur in coal occurrence and distribution, coal qualities, and coal reservoir properties. Compared to Dahebian coal, Dafang coal has a higher coal rank, vitrinite content, and gas content, but a lower number of coal layers and permeability. (2) The sedimentary–tectonic evolution of the Longtan coal-bearing sequence is the fundamental reason for CBM geological differences between the Dadebian Block and Dafang Block, consisting of coal occurrence, qualities, maceral, rank, structure, and their associated reservoir properties. (3) The coordinated variation of gas content and permeability contributes to a greater depth for CBM enrichment and a high yield of the middle-rank coal. It is suggested that the best depths for CBM enrichment and high yield in Guizhou are 600–800 m for the middle-rank coal and 500 m for the high-rank coal, respectively. (4) Considering the bottleneck of inefficient CBM development in Guizhou, we proposed three CBM assessment and development technologies, including the CBM optimization of the classification–hierarchical optimization–analytical hierarchy, multiple coal seams commingling production with the pressure relief of tectonically deformed coal, and surface–underground CBM three-dimensional drainage development. The aim of this study was to provide new insights into the efficient exploration and development of CBM in Guizhou.

Quantitative analysis of heat release during coal oxygen-lean combustion in a O2/CO2/N2 atmosphere by TG-DTG-DSC

Heat release of coal combustion in an oxygen-lean and multi-gas environment is a common phenomenon, coalfield fires caused by it can lead to serious environmental destruction and loss of coal resources. Simultaneous thermal analysis experiments for Bulianta (BLT, high-volatile bituminous coal) and Yuwu coal (YW, anthracite) in 21vol.%O2/79vol.%N2 and 15vol.%O2/5vol.%CO2/80vol.%N2 were carried out to study the law of heat release. Based on the TG-DTG-DSC curves, the combustion characteristic parameters were analyzed. Decreasing O2 concentration caused a significant reduction of local reactivity and further the decreasing maximum heat release rate for low-rank coal, while increasing CO2 concentration caused a significant thermal lag effect and further the increasing maximum heat release rate for high-rank coal. The relationship between the heat release rate and the reaction rate constant was quantitatively analyzed. At the increasing stage of the heat release rate, the heat release rate of the two coals increased conforming to ExpGro1 exponential model. At the decreasing stage of the heat release rate, the heat release rate of YW coal decreased exponentially with the reaction rate constant, while the heat release rate of BLT coal decreased linearly. Regardless of the atmospheres, the conversion rates corresponding to maximum heat release rate of BLT and YW coal were about 0.80 and 0.50, respectively, indicating that the coal rank played a dominant role. The results are helpful to understand the heat release process of coal oxygen-lean combustion in O2/CO2/N2.

Vapour Sorption on Coal: Influence of Polarity and Rank

The surface properties of coal, interactions with gaseous and vapour media, and knowledge of the pore structure are important in terms of preparation, use, and utilisation of coal. This publication combines new unpublished data with analyses included in earlier publications by the research team to expand and systematise information on the sorption of water vapour, methanol vapour, and the saturated and unsaturated hydrocarbons hexane, 1-hexene, heptane, 1-heptene, octane on coals of different ranks. The study showed that the affinity of coal for water and methanol is related to the content of oxygen in the coal rather than the rank of the coal. Water sorption is a multilayer phenomenon, while methanol sorption is a monolayer phenomenon. The water monolayer is greater than that of methanol for low-rank coal, but for the higher-rank coals it is the opposite. The sorption capacity of the applied hydrocarbons depends on the presence or absence of a double bond and the size of the molecule. It increases in the order: n-octane < n-heptane/n-hexane < 1-heptene < 1-hexene. The effect of a double bond is dominant over the influence of the length and shape of the molecule.

Low-Field NMR Experimental Study on the Effect of Confining Pressure on the Porous Structure and Connectivity of High-Rank Coal.

To study the influence of different confining pressures on the pore structure and connectivity of high-rank coal, the high-rank raw coal of the Shanxi Xinjing Mine No. 9 coal seam was studied. A low-field nuclear magnetic resonance (LNMR) test system and a vacuum pressurized water saturation system were used to analyze the samples. The T2 spectra of samples, saturated with water under different confining pressures and containing residual water after centrifugation, were tested. The coal sample pore size distributions, permeabilities, free fluid values, bound fluid values, and other parameters were obtained, and a calculation model of the coal pore connectivity ratio was established. The results were as follows. When the saturated pressures were 5, 10, 15, 20, 25, and 30 MPa, the pore diameters of the coal samples were mainly concentrated in the ranges of 0.00023–0.069 and 1.29–24.09 μm. Among them, micropores (<10 nm) and small pores (10 < 100 nm) account for the main part, mesopores (100 < 1000 nm) were underdeveloped, and relatively few macropores (>1000 nm) and fissures developed. As the confining pressure increased, the coal porosity and connectivity showed a trend of decreasing, then increasing, and finally remaining basically unchanged. The total pore connectivity rates of the coal samples were 37.0–62.6%. The interconnection rates of the micropores, small holes, mesopores, and macropores are 2.90–34.55, 89.09–99.03, 97.09–100, and 100%, respectively. The total pore connectivity followed an exponential functional relationship with permeability, and the critical confining pressure of high-rank coal was 25 MPa. These results provide a scientific basis for the high-pressure water injection of high-rank coal seams.

Multifractal Investigation on Multi-scale Pore Structure Heterogeneity of High Rank Coal Reservoirs

Multifractal Investigation on Multi-scale Pore Structure Heterogeneity of High Rank Coal Reservoirs

Hydrogen storage potential of coals as a function of pressure, temperature, and rank

We conducted measurements of hydrogen adsorption on three coal samples of varying ranks at high pressure (0 to 102 bar) and elevated temperatures (303 K to 333 K) to assess their hydrogen storage potential. The excess adsorption capacity increased with increasing pressure but decreased with increasing temperature irrespective of coal rank. The highest hydrogen adsorption recorded was 0.721 mol/kg for the anthracite coal at 303 K and 102 bar. Furthermore, the hydrogen adsorption capacity correlated positively with coal vitrinite and fixed carbon contents (i.e. the high-rank coal exhibited greater adsorption), while all samples depicted predominantly type-I adsorption behavior for the entire pressure range. Micropore analysis and Fourier-transform infrared spectroscopy measurements were conducted to explore the microstructural and surface chemistry associated with these adsorption trends. The micropore content of the three samples followed the order: anthracite > sub-bituminous > bituminous, while H2 adsorption followed the trend: anthracite > bituminous > sub-bituminous – i.e., no direct correlation between coal micropore content and its H2 adsorption capacity – attributable to high clay content of bituminous coal which lowered its micropore content. Moreover, bituminous, and sub-bituminous samples exhibited an abundance of oxygen-containing functional groups, while anthracite coal depicted notable aromatic content – suggesting that the H2 adsorption capacity is a complex function of coal surface chemistry and micropore content. Overall, high-rank coal seams at high pressure and temperature showed the largest hydrogen adsorption i.e., analogous to CO2 adsorption potential albeitat lower absolute values. These results, therefore, provide preliminary data on the hydrogen storage potential of coal seams and the associated scientific understanding of the mechanisms causing hydrogen adsorption.

Mapping of Stress Sensitivity Affected by Water Variation to Microscopic Pore Distributions in Medium- and High-Rank Coals

Mapping of Stress Sensitivity Affected by Water Variation to Microscopic Pore Distributions in Medium- and High-Rank Coals

Prediction of Critical Desorption Pressure of Coalbed Methane in Multi-coal Seams Reservoir of Medium and High Coal Rank: A Case Study of Eastern Yunnan and Western Guizhou, China

Prediction of Critical Desorption Pressure of Coalbed Methane in Multi-coal Seams Reservoir of Medium and High Coal Rank: A Case Study of Eastern Yunnan and Western Guizhou, China

High Rank Coal Pore Fracture Structure and its Impact on Reservoir Characteristics in the Southern Qinshui Basin

Although the southern Qinshui Basin is the most successful area for coalbed methane (CBM) development in China, the production of CBM wells in different blocks in the area is significantly different. One of the key reasons is the difference in pore structure in various-ranked coal. In this study, No. 3 coal seam of Sihe and Zhaozhuang blocks in southern Qinshui Basin was selected as the research object to investigate the high rank coal pore fracture structure and its impact on reservoir characteristics. Mercury intrusion porosimetry (MIP), low-temperature liquid nitrogen adsorption (LTN2A), scanning electron microscopy (SEM), and isothermal adsorption tests were conducted. The results show that the Sihe No.3 coal seam was mainly composed of open cylindrical and flat pores with a high proportion of transition pores (10–100 nm), large specific surface area, good connectivity, strong adsorption capacity, high gas content, and reservoir energy. Zhaozhuang No.3 coal had high proportion of mesopores (100–1,000 nm), small specific surface area, poor pores connectivity, weak adsorption capacity, poor gas content, low reservoir energy, and critical desorption pressure. The proportion of cylindrical pores, parallel plate pores, and wedge-shaped pores closed at one end was high. The anomalies in pore morphology and pore structure characteristics of coal reservoir were the main factors that caused variation in gas production of No.3 coal seam in Sihe and Zhaozhuang blocks.

Critical pore size for micropore filling in coal samples with different rank coals.

The objectives of this study were to explore the occurrence and migration of coalbed methane in coals of different ranks and reveal the microscopic reservoir space and the mechanism of coalbed methane. To meet these objectives, this study selected six coal samples of different coal ranks for low-pressure N2 adsorption experiments, explored the critical pore filling characteristics of packed N2 molecules in the coals, and analyzed the low-pressure N2 adsorption/desorption experimental isotherms using the DFT method and DA equation based on the micropore filling theory. Finally, the critical filling pressure and pore size range for micropore filling were determined, and the analysis results were verified by combining the Langmuir, DA, and BET equations. The results showed that, from low to high coal rank, the N2 adsorption/desorption isotherms of the coal samples transition from type Ⅱ to type Ⅰ. The proportion of N2 molecules in low-rank coals in the form of micropore filling and monolayer adsorption was higher than that in high-rank coals. The critical pressure and critical pore size for micropore filling exhibited U-shaped correlations with the coal rank. Low-rank coals (lignite and long flame coal) were gradually filled in the relative pressure range P/P0 ≈ 1E-4–0.03, and medium- and high-rank coals (gas coal, 1/3 coking coal, lean coal, and anthracite) were filled in the relative pressure range P/P0 ≈ 1E-4–0.01; the corresponding critical pore size ranges were 1.7–2.19 and 1.61–2.00 nm, respectively.

Experimental study on preparation of nanoparticle-surfactant nanofluids and their effects on coal surface wettability

To improve the efficiency of coal seam water injection, the influence of nanofluids on coal surface wettability was studied based on the nano drag reduction and injection enhancement technology in the field of tertiary oil recovery. The composition optimization and performance evaluation of nanofluids with nano-silica and sodium lauryl sulfate as the main components were carried out, and the effects of the nanofluid with the optimal ratio on coal wettability were studied through spontaneous upward imbibition experiments. The results show that the composite nanofluid has a lower surface tension, and the lowest value of the interfacial tension is 15.79 mN/m. Therefore, the composite nanofluid can enhance the wettability of coal. However, its effects on coal samples with different metamorphic degrees is different, that is, low rank coal is the largest, middle rank coal is the second, and high rank coal is the least. In addition, a functional relationship between time and imbibition height is found for pulverized coal with different particle sizes. When the particle size of pulverized coal is 60–80 mesh, the wettability of nanofluid to coal is best. The findings in this paper provide a new perspective for improving the water injection efficiency for coal seams with low permeability.

Simulation of biogenic coalbed gas from anthracite in the south of Qinshui Basin, China

High-rank coal, such as anthracite, has been considered difficult to generate biogas because of the high coalification degree. Selecting anthracite from Sihe Coal Mine, Qinshui Basin, China, as substrate, this study carried out a simulation experiment of biogas generation for 80 days, the purpose of which was to verify whether anthracite could be biodegraded to produce biogas under laboratory conditions. The results showed that the selected anthracite can be utilized by methanogenic bacteria to produce biogas and the approximate production field was 1.79 mL/g, which was less than that of the lower rank coal of other published studies. The generation process can be divided into a rapid growth stage (0–30 days) and a slow descent stage (30–80 days). CO2 and CH4 are the main components of biogas, although some heavy hydrocarbons were also tested. The CO2 concentrations were low (< 30%), and the δ13C-CH4 values were positive (−39.9‰ to −45.8‰), which suggested that the main biogas generation pathway was acetic fermentation. But at the same time, the concentrations of CH4 and CO2 were mutually increasing and decreasing with the passage of experiment time, and δ13C-CH4 tends to be lighten in the later stage(40–80 days), suggesting that parts of biogenic CH4 were generated by way of CO2 reduction.

Relationship between the Geological Origins of Pore-Fracture and Methane Adsorption Behaviors in High-Rank Coal.

Coal is characterized by a complex pore-fracture network and functional groups, which are derived from various geological origins and which further affect methane adsorption. To explore the relationship between the geological origins of pore-fractures and methane adsorption behaviors, we conducted pore structure tests and adsorption isotherms on six Qinshui high-rank coals. The pores and fractures were observed using an optical microscope (OM), a field emission scanning electron microscope (FESEM), and a high-resolution transmission electron microscope (HRTEM), and the pore structure parameters were determined using mercury intrusion and low-pressure N2 and CO2 adsorption. High-pressure CH4 adsorption isotherms were obtained at 30 °C using the manometric method. Results show that the Qinshui high-rank coals develop five stages of pore size distribution, consisting of the smaller micropore stage (0.3–1 nm), the larger micropore and smaller mesopore stage (1–10 nm), the mesopore and smaller macropore stage (10–110 nm), the microfracture stage (0.11–40 μm), and the larger macropore stage (>40 μm). The micropores dominate the total pore volume (PV) and specific surface area (SSA). Pores and fractures of various morphologies and sizes have different geological origins, which are related to coalification and stress field evolution. Methane adsorption on coals mainly occurs in the micropores as a form of volume filling. The maximum pore size for complete gas filling (MPSCGF) ranges from 0.60 to 0.88 nm in Qinshui high-rank coals. The coal-forming geological processes, such as coalification and stress field evolution, contribute to various pores and fractures, which show different pore sizes and functional groups. The geological origins of pores and fractures control the methane adsorption behaviors in coals by way of the pore size and functional groups. Surface coverage-related methane adsorption behavior occurs in fractures, primary pores, and large-scale secondary pores, while micropore filling is the methane adsorption behavior in macromolecular pores and small-scale secondary pores. The aim of this study is to provide a new insight into the methane adsorption on coals from the geological process of the formation and modification of pores and fractures.

Experimental research on the spontaneous combustion of coal with different metamorphic degrees induced by pyrite and its oxidation products

To study the effect of pyrite on the spontaneous combustion of coal, three types of low-sulfur coal samples with different metamorphic degrees, namely HQL (lignite), DR (bituminous), and YQ (anthracite), were selected to impregnate FeS2 and its oxidation products. The results of thermal analysis experiments showed that Fe3+, Fe2+, and H+ had evident catalytic effects on the low-temperature oxidation of the three samples, with the catalytic order being Fe3+ > Fe2+ > H+. A self-heating experiment on coal showed that Fe3+ reduced the shortest ignition time of the above samples by 55.4%, 49.8%, and 45.3%, respectively, and its catalytic ability was the strongest. With the increase in the degree of metamorphism, the promotion effect of Fe2+ gradually decreased, and the promotion effect of H+ on the low-temperature oxidation of DR was the greatest. Through an FTIR analysis, it was found that Fe3+, which is a strong oxidant and free radical, could catalyze the oxidation reaction of the active structures –CH2- and –OH. Fe2+ promoted O2 and H+ in low-rank coals to generate H2O2 and made them to further generate hydroxyl radicals to oxidize the structure in the coal. In high-rank coals, Fe2+ catalyzed the oxidation reaction in the coal mainly through its oxidizability with decreasing H+ concentration. H+ promoted the low-temperature oxidation of coal mainly by adsorbing oxygen and removing ash to increase the specific surface area. Combining the oxidation reaction of pyrite under dry and wet conditions, pyrite could catalyze the spontaneous combustion of low-rank coals under dry conditions. Under humid conditions, pyrite could better catalyze the spontaneous combustion of high-rank coals.

The influence of closed pores and stacked coal grains on gas transport in CO2 injection enhanced CH4 recovery process

CO2 injection enhanced coalbed methane (CH4) recovery technology can achieve the enhancement of CH4 production and the effective geological storage of carbon dioxide in coal seams. A coal seam is composed of matrix and fractures. Matrix has open pores and closed pores and fractures may have stacked coal grains. However, the influences of closed pores in matrix and stacked coal grains in fractures on CO2 injection enhanced CH4 recovery have rarely been studied. This paper develops a numerical simulation model to explore the hydraulic-mechanical coupling responses in CO2 injection enhanced coalbed methane (CO2-ECBM) recovery process. This model considers the binary gas displacement and transport in ternary pores, the deformation of coal with different porosity, and fractal stacked coal grains in fractures. After verification with the data available from literature, this model is used to simulate a field trial in the Qinshui Basin. The simulation results show that the contribution of gas in closed pores to the total CH4 production enhanced by CO2 injection is more than 60% in high-rank coal and is proportional to the volume fraction of closed pores. The stacked coal grains in fractures affect the effective roughness and can be expressed by three structural parameters. Higher effective roughness induces lower cumulative CO2 injection and lower cumulative CH4 production and the conical bottom diameter of cone-like element has the largest influence. These findings can promote our understanding for the influence of closed pores and stacked coal grains on gas transport in coalbed gas reservoirs from a multi-scale perspective.

The Influence Mechanism of Pore Structure of Tectonically Deformed Coal on the Adsorption and Desorption Hysteresis

Pore is the main adsorption and desorption space of coalbed methane (CBM). Pore size configuration and connectivity affect the adsorption/desorption hysteresis effect. Using tectonically deformed coal (TDC) and original structure coal of medium- and high-rank coal as the research objects, through the N2/CO2 adsorption experiment to analyze the pore size distribution and connectivity of different scales. We investigate the control mechanism of heterogeneous evolution in the key pore scales against adsorption/desorption hysteresis characteristics during coal metamorphism and deformation by combining the CH4 isothermal adsorption/desorption experiment under 30°C equilibrium moisture. The findings indicate that the super micropores (&amp;lt;2 nm) are mainly combination ink bottle-shaped pores and have worse connectivity as the degree of metamorphism and deformation increases. The super micropores occupy the vast majority of pore volume and specific surface area; its pore size distribution curve change presents an “M” bimodal type and is mainly concentrated in two pore segments of 0.45–0.70 nm and 0.70–0.90 nm. The effect of ductile deformation exerts a significantly greater effect on super micropores than brittle deformation. The exhibited adsorption–desorption characteristics are the result of the combined effect of the unique pore structure of the TDCs and different moisture contents. The presence of a large number of super micropores is the most important factor influencing the degree of gas desorption hysteresis. The “ink-bottle effect” is the primary cause of gas desorption hysteresis. For CBM development, some novel methods to increase desorption and diffusion rate at the super micropores scale should be considered.

Swelling Characteristics and Interaction Mechanism of High-Rank Coal during CO2 Injection: A Molecular Simulation Study.

In CO2-enhanced coalbed methane (CO2-ECBM) engineering, accurate knowledge of the interaction mechanism of CO2 and coal matrix is crucial for improving the recovery of CH4 and contributing to the geological sequestration of CO2. This study is performed to prove the accuracy of molecular simulation and calculate the variation characteristics of pore structure, volumetric strain, mechanical properties, Fourier transform infrared (FT-IR) spectra, and the system free energy by molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods. According to the obtained results, a relationship between pore structure, swelling strain, mechanical properties, chemical structure, and surface free energy was established. Then, the correlation of various coal change characteristics was analyzed to elucidate the interaction mechanism between CO2 and coal. The results showed that (1) the molecular simulation method was able to estimate the swelling mechanism of CO2 and coal. However, because the adsorption capacity of the molecular simulate is greater than that of the experiment and the raw coal is softer than the macromolecular structure, the molecular results are slightly better than the experimental results. (2) As pressure increased from 0 to 4 MPa, the intramolecular pores and sorption-induced strain changed significantly, whereas when the pressure increased from 4 to 8 MPa (especially at 6–8 Mpa), there was an increase of the intermolecular pores and mechanical properties and transition from elastic to plastic. In addition, when the pressure was >8 MPa, the coal matrix changed slightly. ScCO2 with a higher adsorption capacity results in greater damage and causes larger alterations of coal mechanical properties. (3) The change of the coal matrix is essentially controlled by the surface free energy of the molecular system. Evalence affects the aromatic structure and changes the volume of the intramolecular pores, thus affecting the sorption-induced strain change rate. Enon affects the length of side chains and the disorder degree of coal molecules and changes the volume of the intramolecular pores, thus affecting the mechanical property change rate. Our findings shed light on the dynamic process of coal swelling and provide a theoretical basis for CO2 enhancing the recovery of CH4 gas in coal.

Fracture variation in high-rank coal induced by hydraulic fracturing using X-ray computer tomography and digital volume correlation

Hydraulic fracturing is a key technology for improving the permeability of coal reservoirs. Understanding the variation of hydraulically induced fractures is crucial for improving coalbed methane production. In this study, we conducted a laboratory-simulation of hydraulic fracturing in coal and applied X-ray computer tomography (CT) and digital volume correlation (DVC) to quantify the spatial distribution, structural variation, and propagation of fractures with an aperture greater than 28.4 μm. Hydraulic fracturing increased the aperture, volume (by 5.3, 32.2, 2.2 and 2.8 times) and surface area (by 1.1, 9.9, 1.8 and 0.8 times), and simplified fracture morphology in the four tested samples. Moreover, the significant influence range in the axial direction of hydraulic fracturing on fracture is 4.2 cm, 4.4 cm, 1.9 cm and 2.9 cm, respectively, with fracture connectivity reaching 61.8%, 99.3%, 77.7%, and 91.1%. A low in-situ stress differential resulted in the formation of a complex network of many fractures with a small volume. A high in-situ stress differential resulted in the formation of large fractures with a simple morphology. The X-ray CT images also showed that new fractures originated in and propagated along the mineral–maceral interface. DVC shows that high volume displacement and strain occur in the fracture area induced by hydraulic fracturing, and it has a good application prospect in the investigation of the microevolution and microdamage of fractures in coal.

Characteristics of methane adsorption/desorption heat and energy with respect to coal rank

Changes of heat and energy during methane adsorption and desorption are essential factors affecting the production of coalbed methane. To characterize adsorption and desorption behaviors, low-temperature nitrogen adsorption experiments and methane isothermal adsorption/desorption experiments were carried out on coals with different rank. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) of the coal samples studied range from 0.29 m2 g−1 to 2.31 m2 g−1, and Barrett–Joyner–Halenda (BJH) pore volume ranges from 0.95 × 10−3 cm3 g−1 to 6.14 × 10−3 cm3 g−1. It is observed that the BET SSA and BJH pore volume decrease first and then increase with the increase of coal rank. The isometric heat in the adsorption process is less than that in the desorption process. There is an energy difference between adsorption and desorption, which makes the sorption process irreversible. The larger the Langmuir volume, the higher the adsorption capacity, making the external energy required for the adsorption process smaller and the limit isosteric heat more minor. At different temperatures, the Gibbs free energy of samples BD-1, LL-1 and HC-1 changes between 2.26–2.98 kJ mol−1, 2.05–3.24 kJ mol−1 and 1.91–3.08 kJ mol−1, respectively, indicating that the process of methane desorption on coal is a spontaneous reaction. The cumulative reduction of surface free energy (Δγ) and the reduction of surface free energy at each pressure point (Δγp) show that the adsorption capacity of the low- and medium-rank coal are more easily affected by temperature, while that of the high-rank coal is less affected by temperature. Moreover, the smaller the BET SSA, the greater Δγ and Δγp.

Investigation on effective stress coefficients and stress sensitivity of different water-saturated coals using the response surface method

The relative magnitude of confining pressure and pore pressure, a key parameter for stress sensitivity of coal permeability, can be characterized by the effective stress coefficient (ESC). In this study, the stress sensitivity experiments of different water-saturated coals were carried out according to the dewatering of coalbed methane (CBM) wells. The ESC was calculated based on the response surface method (RSM), revealing the effect of water saturation on the ESC and stress sensitivity of high-rank coal. The results showed that the stress sensitivity coefficient of these high-rank coals ranged from 0.8136 to 1.5631, indicating a strong stress sensitivity. With growing net pressure and water saturation, the gas permeability of coals changed in a negative power exponential form. Compared with the binary linear regression (BLR), the effective stress calculated by the ESC using the RSM could better reflect the stress sensitivity of permeability. In addition, the ESC increased significantly along with water saturation. Finally, a mathematical model considering ESC and reservoir pressure was established to calculate the “turning point” of dynamic permeability, where the controlling factors of permeability transform from effective stress to matrix shrinkage.