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The hard roof difficult to collapse easily causes gas accumulation, which threatens the production safety of coal mine. Therefore, roof pre-cracking is required. Although blasting and hydraulic fracturing can also crack the roof, blasting can easily induce rock bursts, whereas hydraulic fracturing needs complex equipment. In contrast, soundless cracking demolition agents (SCDAs) with noise-free, dust-free, and safe characteristics have obvious advantages. The main component of SCDA is calcium oxide, which reacts with water to produce higher expansion pressure. In this paper, focused on the angles of the borehole, the effect of SCDA is analyzed by numerical simulation based on Pingdingshan coal mine. The research results showed that the azimuthal angle α (between borehole projection and the roadway direction) does not significantly affect the efficacy of SCDAs, whereas the influence of borehole elevation angle β is far more significant than that of the azimuthal angle. Therefore, the angle β is a dominant factor influencing the effect of SCDAs. Based on different effects of SCDAs at different angle of boreholes, the weakening unit was established, so the SCDAs could give full play to roof fracturing. Moreover, field tests validated the importance of borehole angle on weakening the hard roofs.

Liquid nitrogen (LN2) cracking is a new waterless cracking technology, and the efficiency of cracking coal from different angles is obviously different. Therefore, the boiling heat transfer of LN2 in contact with media with different stratification angles is studied in this paper. It is found that the longitudinal and transverse wave velocities of ultrasonic waves are positively correlated with the bedding angle of coal samples. With the increase of bedding angle of coal samples, the rate of temperature reduction of coal samples at the initial stage of LN2 quenching first drops and then rises, and the coal temperature decreases fastest in parallel bedding direction. The central temperature of coal samples from different bedding angles has experienced four stages: rapid decline-slow decline-rapid linear decline-slow decline, among which stage 3 lasts the longest and the temperature reduction rate exceeds 50%. The quenching effect parameters are defined, and the quenching effect of four stages is stage 3 > stage 4 > stage 2 > stage 1. Prolonging the time of stage 3 can improve the quenching efficiency of LN2. According to the RPI model, the change of the side wall temperature of coal sample is rapidly decreasing-slowly decreasing, and a low temperature layer with high thermal resistance is formed after rapid decrease, which can inhibit the heat transfer from coal to LN2.

Enhanced coalbed methane (CBM) recovery by injecting liquid CO2 (LCO2) has been a promising research topic for decades. Although the phase-fracturing and blooding effect during injection have been studied, the non-isothermal impact of cryogenic fluids on the coal matrix, especially the effect of thermal cycling generated by the cyclic LCO2 injection process, has not been studied in detail. This paper reports the development of a high-pressure and low-temperature reaction system, which can simulate the effect of thermal cycling on three different coals of different ranks, and the use of an ultrasonic device to study the P-wave velocity (Vp) in different directions before and after the influencing thermal cycling. The results show that all the cores subjected to different thermal cycling fractured to diverse degrees due to their initial texture and rank, and more cycles and lower rank increased the final fracture results. Influenced by thermal cycling, two endpoints of the Vp confidence interval (confidence level of 95%) of the coals decreased, and the velocity range of confidence interval all increased, the change of which might be attribute to the rank and the companying thermal cycling effect, resulting in the changes of crack structure and discrete degree of the velocity. The velocity anisotropy coefficient (k) value of the coal increased as the cycles increased, indicating that thermal cycling alters the coal structure randomly by means of fatigue accumulation and grain bond deterioration. The Vpz/Vpx and Vpz/Vpy value changes indicated that the internal cracks were not generated uniformly in all directions, and the presence of cracks helped the transfer of heat flux, which caused some thermal stress concentration areas to occur, further facilitating crack propagation at the crack tips. Meanwhile, the ΔVp% scatters of lignite, bituminite and anthracite cores were fitted as exponential, DoseResp and linear curves with cycles, respectively (R2 > 0.94). The mean fractal dimension D of the coals all increased with increasing cycles, and were negatively correlated with coal rank and ΔVp%, and positively correlated with Δk. The results provide some useful theoretical guidance for field application.

Effective roof caving plays a significant role in mining safety. A spatiotemporal coupling fracturing method using a non-explosive expansion material (NEEM) is proposed in this paper; this method depends on the high fracturing capacity of the NEEM and a change from vertical charging to inclined charging, aimed at producing new free surfaces step-by-step until the integrity of the entire rock is destroyed from a space-time perspective. The expansion stress and water adsorption of the NEEM were studied to determine the optimum stress increase interval and soak time. With respect to the space aspect, the charging sequence of boreholes was negatively correlated to their incline angles, bringing the NEEM's superiority into full play based on the generation of new free surfaces in steps; with respect to the time aspect, the charging intervals of the inclined boreholes were designed to obtain the best stress compensation and highest fracturing efficiency. A field trial was carried out in Pingdingshan No. 2 mine, the results of which showed that some radial cracks were produced around the charging boreholes and that they propagated to the pilot boreholes. Moreover, four main cracks were formed and were approximately parallel to the direction of the borehole rows in a fracturing group. A fracture network was eventually generated, which verified the expansion capacity of the NEEM and the feasibility of the spatiotemporal coupling fracturing method for field application.

As an unavoidable hazard threatening the mining safety and an important energy resources for human society and environmental protection, coalbed methane (CBM) extraction is proved to be of great importance pre mining. The method of upward borehole extraction is commonly used for underground gas extraction, however, some limitations of the traditional sealing methods and materials always make poor sealing results and extraction efficiency. In this paper, a method of upward borehole self-sealing technology using drill cuttings has been proposed. The sealing failure of the upward boreholes has been investigated and three potential leakage paths for air have also been studied, such as the defects of the sealing material, the channels between materials and the borehole walls, and the cracks around the borehole. An experimental system was been established to simulate upward borehole self-sealing technology, and some critical parameters, like particle size distribution (PSD) and the motion of drill cuttings within the borehole, were investigated. The PSD results indicated that huge amount of small scale particles could be compacted to be dense with low porosity; the motion of particles results shown that the cracks around the boreholes could be occupied by different scales of particles due to the large initial velocity; the angle of repose for accumulated particles increased as the increasing particle volume, blocking the contact interface by some water-coal fines. A field application was carried out in the No. 8 coal mine in Pingdingshan, China, and the high concentration of extracted CBM verified the feasibility of upward borehole self-sealing technology, in which these boreholes could be sealed by cuttings by preventing the three potential leakage paths, and the concentration variances indicated that the effective sealing duration could be recognized. The laboratory and field results all prove that the method of upward borehole self-sealing using drill cuttings could be helpful to improve CBM extraction efficiency and might be a potential and promising technology in the future.

The permeability of coalbed methane (CBM) reservoirs is typically very low and it is challenging and essential to effectively increase coal permeability to maximize CBM recovery. An exploratory study on enhancing coal porosity/permeability using freeze−thaw cycling with liquid nitrogen (LN2) was conducted. The changes of fracture and porosity in coal with the freeze−thaw treatment using LN2 were evaluated using nuclear magnetic resonance (NMR). After freeze−thaw treatment, the coal pore size tended to increase and new pores/fissures were generated. The growth rate of the pore size was positively correlated with the LN2 freezing duration. The effective porosity had a positive correlation with the freezing duration, but the correlation for residual porosity was negative. This means that the volume of irreducible fluid in the coal decreased while the amount of free fluid increased. Scanning electron micrograph studies indicated that the maximum fracture width in the coal samples grew from 5.56 μm at a Tfreezing = 1 min to 100 μm for Tfreezing = 60 min, matching the NMR findings. This study provides a scientific basis and guidance for engineering application of freeze−thaw using liquid nitrogen to enhance coalbed methane extraction.

During the mining of coal seams with outburst hazard, abnormal gas emissions in front of the coal mining working face (CMWF) may induce gas overrun. To address this technical problem, this study analyzed the permeability variation of coal in front of the CMWF at different stress paths through physical experiments, numerical simulation and on-site tests. The spatial-temporal evolution law of the unloading area of the working face under dual stress disturbance caused by hydraulic punching (HP) and coal seam mining was explored; next, a secondary enhanced extraction technology was proposed and applied in the Shoushan No. 1 Coal Mine, Henan Province, China. The results reveal the following: (1) the coal permeability decreases linearly with increasing confining pressure (CP) and axial pressure (AP) under Stress Paths 1 and 2 (that is, fixed AP and CP). (2) The coal permeability is negatively related to the distance from the stress peak point under Stress Paths 3 and 4 (that is, AP and CP are, respectively, the vertical stress and horizontal stress before the stress peak). (3) As the distance from the peak stress declines, the reduction amplitude of coal permeability in the test area first decreases, and then increases, under Stress Paths 5 and 6 (that is, the vertical stress as CP and the horizontal stress as AP). The plastic damage range of coal around the HP cavities expands due to the dual impact of HP and coal seam mining, which can realize both regional unloading and provide channels for gas extraction within 60 m in front of the CMWF. According to the gas extraction concentration of boreholes, the coal body in front of the CMWF is divided into three zones: efficient, effective and original extraction zones. The efficient extraction zone is within 20 m in front of the CMWF, with an average gas extraction concentration of over 30%. In the effective extraction zone, the gas extraction concentration falls with the increase in the distance from the CMWF. The original extraction zone is beyond 50–60 m, and the borehole gas concentration stabilizes below 10%. The number of extraction boreholes in the stress disturbance area of the middle-floor gas extraction roadway accounts for 5–10% of the total number of boreholes, but its maximum monthly extraction volume can reach 38.5% of the total volume.

Understanding the pore wetting process is of great significance for determining fluid wetting behavior in porous media such as coal, and for the development and utilization of natural gas reservoirs, however, most studies on the process simplify porous media as glass capillaries, which may lack practical application. To better understand and quantitatively characterize the actual pore wetting process of coals, a new, low-field nuclear magnetic resonance (LF-NMR) experimental characterization method was developed. The experiment used equal grain sizes of glass beads and coals to yield porous media with variable pores of specific size ranges. LF-NMR was then used to characterize the pore wetting process of a given amount of water in the porous media at different times. Six groups of high-rank coals were also characterized by contact angle experiments to analyze the role of the surface contact angle on pore wetting. The results show that the pore wetting process can be divided into three ideal physical phases: a first touch state, a semi-free water state and an inter-granular diffusion or quasi-static state. The wetting pore size distribution (W-PSD) spectrum obtained by measurement of the transverse relaxation time and the transformation coefficient reflected change in pore size during wetting, with the location of the W-PSD quantified by the equivalent wetting pore size. The equivalent wetting pore size obtained at a final time of 24 h was used to characterize the pore wettability, it was significantly correlated with the surface contact angle with a linearity of up to 0.99. However, the W-PSD or the equivalent wetting pore size is dependent on the surface relaxivity. Overall, the experimental characterization method used in this study is an effective way to study the “black box problem” of the wetting process in porous media, as it is simple and has low sample requirements.

Anhydrous fracturing methods which increase permeability during the mining of coalbed methane (CBM), shale gas, etc. are promising techniques for future development. One such technique, ultrasonic fracturing, has several advantages over other methods. These advantages include a wide range of application, low consumption of energy, concentrated application of energy, high efficiency, and lack of pollution generation. Ultrasonic waves mainly fracture coal masses through cavitation and heating processes but there are numerous factors influencing the effect of the ultrasonic excitation. Therefore, it is important to investigate these other effects. Of particular interest is the effect of moisture content. For this study, we carried out physical experiments using ultrasonic fracturing technology to investigate this factor. Coal masses with different moisture contents were subjected to ultrasonication and the fracturing and thermal effects were investigated using nuclear magnetic resonance (T2 spectra) and infrared thermal imaging. Moreover, the effect of the moisture content on the ultrasonic fracturing was studied by employing the concept of fractal dimension. The results show that the moisture content has a significant effect on the ultrasonic fracturing of a coal mass. The increase in the number of pores in the coal mass is significantly greater when the moisture content is 8% compared to those in coal masses with lower moisture contents (samples with 6%, 4%, 2% and 0% were also tested). Samples with higher moisture contents exhibit larger left shifts in the extreme T2 values and greater changes in fractal dimension. On the other hand, the temperature of the coal gradually decreases along the axis of the specimen (from the end where the ultrasound is generated to the other end). The higher the moisture content is, the smaller the temperature increase due to ultrasonic excitation. The ultrasonic waves accelerate the vaporization of moisture inside the coal mass, and this vapor promotes the development of pores and overflows. As a result, a virtuous circle is formed: the porosity and permeability of the fracturing coal mass are increased, which further promotes the desorption of the CBM. This, to some extent, also increases the connectivity between the pores in the coal mass.

Enhanced coalbed methane (ECBM) achieved by injecting liquid carbon dioxide (LCO2) has been proposed and applied in industrial production for decades and has been demonstrated to be an applicable method to boost CBM production. Most of the studies have concentrated on the gas bursting and flooding effect and have rarely focused on the accompanying “freeze–thaw” phenomenon, and the temperature effect of cyclic LCO2 injection on the pore variation of different coals has been partly investigated. In this paper, the influence of cycle parameters, such as cycle number and cycle time, on the pore variation was studied. Infrared thermal imagery (ITI) and low-field nuclear magnetic resonance (NMR) were used to measure the temperature and pore size distribution (PSD) change, respectively. The results show the following: (1) The gas pressure displayed square cyclicity with different cycle time, the temperature of gasified CO2 was almost 248.15 K, and the end and lateral surface temperatures of a core were in the range from 259.35 to 261.85 K, which could cause the water within the pores to freeze with a 9% volume increase, and the fracturing formula was deduced; (2) The relaxation time spectra obtained by different cycle parameters expressed changeable PSD of cores with increasing cycle parameters, and the magnified proportion of bulk water and capillary water, as well the diminished proportion of adsorbed water, all indicated that the increased number of macropores and mesopores formed a larger free volume; (3) The increased total porosity φt and the decreased T2cutoff of six cores with the increasing cycle parameters meant that the larger cycle number could enhance the porosity due to amount of damage accumulation, and the larger cycle time might make the water freeze completely with larger ice swelling stress; (4) There is a polynomial fitting between relative increase ratio Rφ and cycle time, and the fitting coefficients were all higher than 0.99, and the larger the cycle time was, the greater the Rφ(e/t) increment and Rφ(r/t) decrement were. The interval increase ratio Iφe was positively correlated to cycle time without obvious increase behavior; however, the Iφr variation expressed that the greater the cycle number was, the lesser the Iφr with the increasing cycle time was, which indicates that the increasing cycle parameters might help the proportion of connected pores to increase and provide more pathways for permeable fluid; (5) The NMR permeability kSDR of a core increased as the cycle number increased, and the longer cycle time was superior in terms of permeability enhancement.

A non-explosive expansion method (NEM) is proposed to apply in the field of reservoir fracturing for the extraction of coalbed methane. Specimens with mechanical properties similar to primary coal were prepared, and their fracture behavior was investigated under different side stresses. Acoustic emission (AE) and infrared thermal imagery were utilized to record the parameters (energy and amplitude) of the AE events and the surface temperature distributions, respectively. The results showed that specimens were all destroyed using NEM with different crack morphologies under diverse side stresses, and the fracture evolutions were greatly dependent on the structure and side stresses, which caused complex crack numbers and crack distributions. Several periodic energy concentrations and the stair increasing pattern of cumulative energy all indicated that the fracturing process of NEM did not finish instantaneously but continued for a long time. A quantitative analysis of crack number and distribution by box counting showed that the larger the number of fractures, the greater the fractal dimension and the greater the complexity, and the difference in fracture density positively correlated to the fractal dimension was influenced by side stresses. Fractures might be determined by the coupled effect of swelling force and released heat according to the chemical reaction of agents. The relationship between swelling force and tension stress was deduced based on the thick-wall cylinder theory. A semi-submersion test was carried out to evaluate the effect of released heat on crack generation preliminarily in consideration of material volume and influence time. The greater material volume and longer influence time created a number of blocks and produce complex crack faces, largely related to the released heat inducing a complete temperature gradient.

The liquid nitrogen (LN2) fracturing technology is a promising and effective means for coalbed methane stimulation. The flow and heat transfers in coal with LN2 injections directly influence the characteristics of fracture initiation and propagation which are being considered as determining factors of methane recovery efficiency of coal seams. The aim of this study is to probe the heat and mass transfer behaviors and fracturing mechanisms of LN2 stimulation. Using combined infrared thermal imaging, ultrasonic detection, and acoustic emission techniques, experimental work was carried out on simulated coal samples to study the heat and mass transfer behaviors under triaxial stress condition. Compared with a single injection, cyclic LN2 injections resulted in denser fracture pattern in the samples. And it was also found that temperature decreased and rose more quickly during the freezing and thawing processes for cyclic injections. With the same injection duration, the cooling range of the cyclic injection was much larger than that of the single injection. In the single injection for 10,000 s, the whole sample was mainly shrunk due to freezing, whereas “freezing shrinkage–frost heave–freezing shrinkage” occurs in the whole sample after 6 times of the cyclic LN2 injections for 7000 s. The maximum acoustic emission energy signal appeared in the 6th cycle, proving that fractures in the samples connected to form a network, which coincided with a decrease in the temperature gradient in the infrared thermal image. Based on analysis of the infrared temperature results, this study established a model for the temperature of the samples with LN2 injection time and demonstrated the changes in the cooling radius and temperature gradient during LN2 injections.

Liquid CO2 enhanced coalbed methane recovery has been proposed for decades, and the accompanying effects of phase-transition and adsorption of liquid CO2 slightly influenced the coal structure strength. Studying the coal strength variation under the effects of liquid CO2 is of great significance for evaluating the selected liquid CO2 injection parameters, CO2 injectability, and coal cracking initiation capacity. However, the coupling effects of the temperature gradient and adsorption of cyclical liquid CO2 on the coal strength have not been systematically studied. In this study, based on the cyclical liquid CO2 fracturing experimental system, the low-rank coals were processed under the effects of liquid CO2 with different cyclical parameters. Subsequently, a uniaxial compression test was carried out to investigate the mechanical responses of the processed coals under the sole and coupled effect of liquid CO2 temperature and adsorption, respectively, using acoustic emission and strain gauge. The results showed that coals exhibited different destruction behaviours and fracture morphologies; for example, coals influenced by the sole effect showed an “axial separation” failure pattern, while liquid CO2 coupling affected coals showed a “separation and spallation” destruction form. Compared to raw coals, the affected coals contained a larger ring count and accumulated energy J, indicating that thermal cycling induced by uneven temperature distribution enhanced the generation of new cracks and expansion of the original cracks, and the enlarged crack volume provided sufficient adsorption sites for CO2 molecules to decrease the Gibbs free energy. The negative relationship between σc and cyclical parameters showed that the alternative temperature shock significantly degraded the cohesive strength among the grains with amounts of the damage accumulation, leading to tensile failure. The Je/Jt and Jr/Jt scatters positively and negatively correlated with the corresponding cyclical (cycles and time) and mechanical parameters (E and σc), respectively, indicating that the more liquid CO2 affected, the greater the damage accumulation, and the larger the destructive plastic region and lower the strain energy storage capacity, because of the synergy of the cold shock and gas adsorption. The research results provided theoretical guidance for optimizing liquid CO2 injection parameters for balancing the investment and output benefit of coalbed methane.

Liquid CO2 fracturing technology has been introduced to enhance coalbed methane (CBM) recovery for the decades, under its multiple-effects of phase transition and flooding, by complicating the crack distribution and improving the reservoir permeability. When huge amounts of cryogenic fluids were cyclically injected into the reservoirs, the accompanying effects of temperature shock and adsorption might have some potential impacts on the fracturing results, and their influence degree and the related mechanical deterioration were required to systematically studied. Three different rank coals were chosen and cyclically treated by liquid CO2, and the uniaxial compression behaviors of these affected coals were studied to investigate the destruction patterns, the evolutions of mechanical parameters, and the mechanism of damage failure. Compared to the raw coals, the ones affected by temperature-adsorption coupling effects of liquid CO2 displayed a more complex “separation + spallation” destruction pattern, while the ones treated by the sole effect had a destruction pattern of “spallation” and “separation” respectively. The more complex the destruction pattern, the larger the fractal dimension, and the greater the structure degradation. The affected coals exhibited diverse mechanical responses, for example, the compression strength σc and elastic modulus E had negative correlations with the increasing cyclical parameters, while Poisson’s ratio μ and damage variable Dv positively correlated with the increasing affecting parameters, respectively, which manifested that the accompanying cyclical temperature shock could facilitate the crack growth and the CO2 adsorption aggravated the strength deterioration, finally destroying the coals with the smaller yield strength. A new damage model was established by jointly considering the grain anisotropy from the microscopic aspect and the diversity of mechanical responses from the macroscopic perspective, and the induced unrecoverable deformations greatly weakened the cohesive stress among the grains or matrix skeleton, finally destroying the coal structure by producing amounts of new cracks. Furthermore, the produced new-raw cracks provided amounts of adsorption sites for CO2 molecules, which further decreased the surface energy and accelerated shear failure. The results were helpful to the injection parameters optimization, the low-cost of CO2 injection invests and the potential profit maximization of CBM utilization.

Prediction is the first step to prevent and control coal outburst geological disasters. Generally, both limits and disadvantages are generated for the traditional prediction methods of drill cuttings from a Φ42 mm borehole with increased mining depths. To investigate the discharge law of drill cuttings and improve the prediction index, briquettes were drilled with different borehole diameters under various side stresses. All the briquettes were loaded by a tri-axial experimental system to simulate the side stress of coal rock, and acoustic emission (AE) was used to monitor the AE events and record their characteristics during the separate drilling processes. The results showed that a larger borehole diameter and higher surrounding rock stress caused an increase in the quantity of drill cuttings (S). A power function relationship between S and the borehole diameter was found under the same side stress, and S was positively correlated to the side stress of a certain borehole diameter. Incremental drill cutting quantity (ΔS) was proposed to be the prediction index and the relationship between ΔS and the borehole diameter was fitted as a power function with a fitting coefficient of more than 0.99. The coupled values of ΔS and AE energies measured from a Φ85 mm borehole were more sensitive than those of a Φ35 mm borehole. In addition, the peak stress area using the larger drill bit was delayed by approximately 40 mm compared to that of the Φ35 mm drill bit. Larger diameter boreholes are preferable for larger regions of stress-relief and outburst removal; as a result, the use of larger diameter boreholes provides technological support to improve mine safety and increase production efficiency.

The aims of this research are to quantitatively evaluate the complexity of the pore structure in coal frozen with liquid nitrogen (LN2) and then study the influence of the modified pore system on coalbed methane (CBM) extraction. To do this, nuclear magnetic resonance (NMR) and fractal dimension theory were used to determine the properties of the coal's pore system after samples of low rank coal had been frozen and then thawed. The fractal dimensions of pores in frozen-thawed coal samples were divided into five types according to pore size and the state of the fluid in the coal pores. The results showed that the fractal dimension DA of adsorption pores was less than two, indicating that these pores did not exhibit fractal characteristics. The fractal dimensions Dir and DT representing closed pores and total pores presented low fitting precision, so the closed pores showed insignificant fractal characteristics. However, the fractal dimensions DF and DS representing open pores and seepage pores had high fitting precision, suggesting that open and gas seepage pores exhibited a favorable fractal characteristic. Correlation analysis revealed that DF and Ds were negatively correlated with LN2 freezing time and the number of freeze-thaw cycles. After being frozen and thawed, coal porosity and permeability showed a strong negative correlation with fractal dimension and this relationship allowed predictive models for permeability and fractal dimensions (DF and DS) to be constructed. The models showed that the smaller the fractal dimension, the more uniformly the pores were distributed and the higher their degree of connection. These properties favor the production of CBM. This study also showed that compared with single LN2 freezing events, repeated cyclic freezing with LN2 followed by thawing is more favorable for CBM production.

Fracturing technology through liquid nitrogen (LN2) injection alters the physical properties of coal and rock masses because of the thermal and pressure effects. In order to investigate heat transfer and fracture propagation behaviors under in situ geological condition with LN2 injections, experimental work was conducted to study the LN2 induced rock/coal failures under true triaxial stress conditions. During the experiments, the temperature, ultrasonic waves, and acoustic emission location detection were monitored to determine the intensity and complexity of specimen failure under singular or cyclic LN2 injections. The results show that a single LN2 injection mainly transfers heat through the solid skeleton and only damages areas adjacent to the injection tube. However, cyclic injections formed a propagated fracture network and the heat can sequentially transfer further along the induced fractures. Moreover, plastic deformation occurred in the entire volume of the sample and the main fractures coalesced until the sample failed. Based on the spatial locations of the acoustic emission sources, the dynamic fracturing within the sample progressed after LN2 injection was clarified. The experimental results can provide evidences for the proposed crack propagation model for the coal masses. The research revealed that high-pressure nitrogen gas transferring liquid water to the tips of new fractures is essential for cyclic LN2 injection to form effective frost-heaving forces and fracturing. Therefore, the fracturing efficiency of the cyclic LN2 injection is far higher than that of singular LN2 injections.

Liquid CO 2 (LCO 2 ) enhanced coalbed methane recovery had been studied in laboratory experiments and field applications, supporting many improvements and achievements. Previous studies primarily investigated the gas bursting, flooding effect and adsorption effect; however, the freezing-thawing phenomenon (drikold formation and gasification) that commonly occurs during the LCO 2 injection process was insufficiently studied. The freezing-thawing phenomenon might enhance the pore volume and change the permeability evolution of the coalbed; thus, cyclical LCO 2 injection was proposed to exploit the phenomenon, and the influence of multiple freezing-thawing cycles on the coal pores was investigated in this paper. Nuclear magnetic resonance (NMR) and infrared thermal imagery (ITI) were used to monitor the pore variation and surface temperature distribution, respectively. Low temperatures could make the saturated water in the pores freeze and undergo a 9% volume increase. The three coals used in this experiment displayed different crack intensities and forms with ITI. After cyclical LCO 2 injection, the NMR amplitude increased, and the T 2 range was widened under a saturation condition, while the cores under a centrifuge state had lower amplitudes and a narrower T 2 range; this difference indicated that the pore structure could be altered by multiple freezing-thawing cycles of LCO 2 . The more freezing-thawing cycles the cores experienced, the greater the change in pore structure was. The total porosity φ t and effective porosity φ e increased while the residual porosity φ r and T 2cutoff values decreased with more freezing-thawing cycles. However, the variations with coal rank were observed; with higher coal ranking, φ t and φ e increased less, and the φ r and T 2cutoff values decreased less, which suggests that lower ranking coals could be most easily affected by the LCO 2 enhanced recovery method and have the most improved pore connectivity. Moreover, the enhancement ratio of φ t and φ e increased for all three coals tested, which could be fit with quadratic functions with fit coefficients greater than 0.99. The increasing relative ratio D e / t of anthracite was fit with a linear function, while the lignite and bitumite were fit with quadratic functions. These functions all indicate that the multiple freezing-thawing cycles of LCO 2 injection had a positive impact on the enhancement efficiency of pore porosity. Finally, a potential field application of cyclical LCO 2 injection was also discussed to improve the fracturing effect.

Liquid carbon dioxide (LCO 2 ) fracturing technology has been applied in the enhanced coalbed methane recovery (ECBM), and it has made some progress in the physical experiments and field applications. However, the freeze phenomenon during the injection process might induce coal matrix shrinkage, hindering the fracturing efficiency. A multiple cycle LCO 2 fracturing technology is proposed, and the feasibility of the cryogenic effect from LCO 2 on the crack evolution of five different coal cores under the loading state was investigated by using an innovative cryogenic loading experimental system. Nuclear magnetic resonance (NMR) and infrared thermal imaging (ITI) were used to measure the pore changes and temperature distribution, respectively. After 25 injection cycles, some cracks on the side and lateral surfaces of five cores were generated, and a low temperature distribution was formed. The temperature values were almost less than −18 °C, which could cause the saturated water to freeze into ice with a 9% volume increase; thus, the stress analysis diagram during one cycle injection was analyzed, and the initiation criterion was deduced. The T 2 spectra variation showed that the various pore sizes changed with the increased number of cycles. The peaks increased in amplitude and shifted to the right under saturated conditions, while they decreased and shifted to the left under centrifuged conditions, causing the amplitude increment ΔA in the post-test stage to be greater than that in the pre-test stage, which indicated that the cryogenic effect of LCO 2 could significantly improve the connectivity of pores. The total porosity φ t and effective porosity φ e of all five cores increased with the number of cycles. A quadratic function described the relationship between incremental ratio of φ e ( D c ) and cycle number, the fitting coefficients for which all exceeded 0.99, which indicated that the cryogenic effect of LCO 2 could improve the permeability observably.