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Hydraulic fracturing is a widely used technique for increasing the permeability of coal seams. Its impact on the production capacity of coalbed methane(CBM) wells is significant. This paper makes detailed observations on the exposed coal seams and fracture distribution of 5 CBM wells in Tunlan block of Xishan coalfield, and studies the distribution characteristics of coal structure, microfractures, quartz sand and coal fine by using stereoscopic microscope, scanning electron microscope, and micro CT. The fracturing effect and influencing factors of CBM wells are analysed by combining regional geostress and hydraulic fracturing construction parameters. The research results indicate that the macroscopic fractures morphology produced by hydraulic fracturing are complex and diverse, including horizontal, vertical, X and T-shaped fractures. The degree of coal body fragmentation increases with distance from the CBM well. The coal body closer to the CBM well is mainly composed of fragmented and granular coal, while the coal body further away is mainly composed of undeformed coal with primary fractures. The quartz sands are mainly deposited in horizontal fractures, with only a small amount in T-shaped fractures. Quartz sand rubs, collides, and embeds strongly with the coal fracture surface. The fracturing fluid is then superimposed to fracture the coal body, resulting in a large amount of coal fines. This causes the coal fines to wrap around the quartz sand and block the fractures. The coal fines within the macroscopic fracture are primarily a result of the fracturing of the coal body, the flushing of the coal body by fracturing fluid, and the friction between the quartz sand and the fracture surface during hydraulic fracturing. Coal fine generation in micro-fractures during coal body fracturing. Coal fines and quartz sand accumulate in the fractures, creating enormous resistance to the carrying fluid and preventing the quartz sand from migrating to the far end of the CBM well. The magnitude and direction of the geostress, as well as the strength of the coal seam roof and floor, are important factors that influence the opening and direction of the hydraulic fracturing fractures. The low viscosity of the carrying fluid cannot suspend quartz sand, and the accumulation of quartz sand and coal fine can block the fractures, easy to cause quartz sand and coal fine to aggregate and block fractures.

In Guizhou Province, there are many coal-bearing structural units, many layers of coal, thin thickness, and most of them are low permeability reservoirs, so it is advisable to adopt the process of “staged fracturing and combined layer drainage and production” to develop coalbed methane. Taking Banguan syncline coalbed methane well as an example, this study analyzed the fracturing effect of two vertical Wells on the basis of the scale of fracturing reconstruction, and then studied the influence of fracturing reconstruction effect on the gas and water production of coalbed methane Wells, and finally summarized the reasonable threshold of efficient fracturing construction parameters of Banguan syncline coalbed methane Wells. The results show that hydraulic fracturing is an effective way to increase permeability of low permeability reservoir, and the permeability increase multiple is closely related to the scale of fracturing reconstruction. The differences of sand addition, liquid addition and single-hole displacement per meter of coal seam between the two Wells of Banguan syncline lead to the permeability ratio of 55∶1 after fracturing reconstruction, and the permeability difference after reconstruction is large. Increasing the liquid addition, sand addition and single hole displacement per meter of coal seam is beneficial to improving the transformation effect of coal seam and gas production. It is recommended that the liquid addition per meter of coal seam should not be less than 400 m3, the sand addition per meter of coal seam should not be less than 20 m3, the fracturing construction displacement should not be less than 8 m3/min, and the single hole displacement should be controlled above 0.2 m3/min. According to the permeability of the coal seam after fracturing and reconstruction, it can be appropriate to quickly discharge and depressurize the coalbed methane Wells with good reconstruction effect. Otherwise, the intensity of discharge and production should be controlled to reduce the sensitivity damage of the reservoir caused by fast discharge and depressurization. The research results can provide technical reference for the permeability and energy improvement of low permeability coal seam in this block.

Coalbed methane well sand (pulverized coal) problem restricts the production life of coalbed methane well, resulting in high maintenance costs, reduce productivity, and because of the large contact area between thin interbed coal seam and surrounding rock, the problem of sand production (pulverized coal) is prominent. In the Surat Basin of Australia, the coal seam cementation is loose and the coal/mud/sandstone interlayer is complex. In the statistical wells, 100% of the production wells have different degrees of sand production problems, and 33.5% of which have serious sand production, frequent workover operations, and high production costs. Based on the systematic analysis of the reservoir characteristics, sand production characteristics and production rules in Surat Basin, the comprehensive sand discharge prevention technology of “expansion packer + blind tube” and “screw pump + slurry rotor + automatic shunt valve + drilling through casing shoes” is adopted according to the research on the particle settlement law, combined with the prediction of sand production level and the optimization of sand control technology. The problem of sand production in coal measure strata in Surat Basin has been solved in stages. The field application results show that the average continuous production time of a single well increases from 275 days to nearly 400 days, and the workover frequency is reduced by nearly 50%.

A large amount of pulverized coal is easy to be produced in the process of hydraulic fracturing of coalbed methane reservoir. The accumulation and settlement of pulverized coal will cause serious blockage to the fractures formed by fracturing, and reduce the permeability and conductivity of fractures and affect the effect of fracturing stimulation of coalbed methane wells. Therefore, a new type of coal powder dispersant MFFS-2 was developed through a large number of laboratory experiments and combined with the reservoir characteristics of the target CBM block, and its comprehensive performance was evaluated. The experimental results show that when the mass concentration of MFFS-2 reaches 0.3%, the surface tension of water solution can be reduced to less than 25 mN/m, and the contact angle of coal surface can be reduced from 115.3° to less than 60° with good surface activity and wettability; when the suspension of coal powder is standing for 10 hours, the suspension rate of coal powder can reach more than 30% with the addition of 0.3% MFFS-2. When 0.3% MFFS-2 solution is used to displace for 2 hours, the yield of pulverized coal can reach more than 60%, which indicates that MFFS-2 has good suspension dispersion and carrying effect on pulverized coal; in addition, the damage rate of MFFS-2 to the matrix permeability of block coal core in the target area is less than 10%, and the damage degree of fracturing fluid system to the permeability of coal core is reduced after adding MFFS-2 into the active hydraulic fracturing fluid. The field fracturing operation results of coalbed methane well show that the fracturing operation of S-1 well is smooth after adding new coal powder dispersant MFFS-2, a large amount of coal powder is carried out in the backflow fluid, and the gas production is high, which achieves the expected fracturing effect. Moreover, the construction effect of the new active hydraulic fracturing fluid in the same block is significantly better than that of the conventional active hydraulic fracturing fluid.

The scientific understanding of the fluid flow and mass transfer mechanism in pores and fractures of coal seam are the fundamental strategy to achieve the efficient production of coalbed methane (CBM). During CBM development, the occurrence, migration, and production characteristics of coalbed water not only determine the drainage and depressurization effect of CBM well, but also significantly affect the desorption, migration, and production process of CH4, which is crucial to the efficient drainage and production control of CBM well. Taking the Fanzhuang block in the southern Qinshui Basin as an example, based on the occurrence form, production process, and flow form of coalbed water, the research on the mechanism of phase change and mass transfer process of coalbed water has been carried out. The threshold pore diameter of phase change and the mass transfer process of coalbed water in anthracite reservoir under the engineering background have been quantified, and its engineering implications for CBM production have been discussed. The results show that the occurrence state of coalbed water can be changed dynamically, and the fluid pressure difference is the main mechanism of its dynamic change. In the single-phase flow stage of water, the migration of movable water is mainly affected by the surface tension. Under the engineering background, the migration and production of movable water in pores and fractures with macropore (pore diameter > 50 nm) is the precondition of the overall release of coal seam pressure, and lays the foundation for a high and stable production of CBM well. In the gas-water two-phase flow stage, the transformation of bound water (capillary water) to movable water is greatly affected by the wettability of coal, and the interface property and interaction between gas and water. The pressure condition requirements of the transformation of bound water (capillary water) to movable water increase due to the capillary effect. In the bubbly flow and plug flow stage, CH4 desorption promotes the transformation of bound water (capillary water) to movable water in some pores and fractures with mesopore (2−50 nm) scale, which determines the overall release of coal seam pressure. This also indicates that after the CBM well has casing pressure, the overall release of the coal seam pressure is the result of the joint action of the gas and water, and the slow transition from the water production stage to the depressurization and gas production stage is necessary for the high production of the CBM well. The continuous casing pressure holding may lead to early formation of slug flow in pores and fractures with mesopore scale, which is not conducive to the high and stable production of CBM well. Moreover, with the decrease of coal seam pressure, the liquid water in pores and fractures with the equivalent pore size > 40 nm can be vaporized and separated to the gaseous free water, which forms competitive adsorption/desorption with the adsorbed CH4. However the water in pores and fractures with the equivalent pore size < 40 nm is preserve with liquid or transported by wetting migration, and has little influence on the transformation of bound water (capillary water) to movable water in the gas-water two-phase flow stage.

Accurate classification of drainage and extraction reservoir types of coalbed methane wells can lay a foundation for the study of pressure propagation in drainage process. Taking 3# coal seam of Changcun Mine Field as the research object, the main parameters affecting the conductivity of coal reservoir and the recharge capacity of surrounding rock in the process of drainage and extraction are analyzed and selected. The method of obtaining these parameters is proposed, and the critical value is determined. The study area is divided into four types of drainage reservoirs. The water production and gas production of CBM wells in the field verify the reliability of the theory. The significance of classification of drainage and extraction reservoir types was discussed. The results show that the study area can be divided into high-conductivity weak supply reservoir, high-conductivity strong supply reservoir, low-conductivity weak supply reservoir and low-conductivity strong supply reservoir. Among them, the high conductivity and weak recharge reservoir is most conducive to reduce coal reservoir pressure, showing the characteristics of small water production and short casing pressure time. The low conductivity and strong recharge reservoir shows the characteristics of large water production and long casing pressure time.

Coalbed methane (CBM) development in coal mining area has good safety, economic and energy benefits, but methane emission is common in surface well sites. In view of this, taking the CBM wells in Qinshui County of Shanxi Province as an example, the methane emission location and emission volume of CBM wells are obtained, and the prediction model of methane emission volume is established through field investigation and multiple regression analysis. The results show that 81% of CBM wells have methane emission. Wellhead and drainage outlet are the two main positions of methane emission. The proportion of wells with emission volume between 0-10 m3/d is 59%. On the whole, the emission accounts for 1.53% of the gas well production. There is almost no methane emission in the single liquid phase flow stage. The methane emission in the gas-liquid two-phase flow stage is positively correlated with the bottom hole flow pressure, times of stroke and gas production, and negatively correlated with the pump embedment and water production. The bottom hole flow pressure and the pump embedment have the greatest impact on the methane emission. In the single-phase gas flow stage, methane emission is positively correlated with gas production and casing pressure, and negatively correlated with stroke frequency and water production.

Aiming at the high gas content at the wellhead and drainage outlet of coalbed methane well site, which affects the normal safety production and is a serious waste of resources. The shortages of the existing wellhead sealing device and water pipeline gas separation device are analyzed, the gas collection technology for the coalbed methane well site is proposed. The self-adjusting deflected wellhead sealing device and the water pipeline gas separation and collection device are introduced, and the process flow of gas separation and collection in gas- containing water pipeline of coalbed methane well is designed. The simulation test shows that the simulation test shows that the technology can separate and pressurize the gas in the gas-liquid mixture up to 0.2 MPa, which verifies the feasibility of this technology.

In order to solve problems of existing pressure and temperature data collecting modes of coalbed methane well such as information island caused by manual collection, inability to collect data consecutively, water leakage of pressure sensor and temperature sensor and so on, a pressure and temperature monitoring system of coalbed methane well was proposed. System structure was given as well as compositions of hardware and software. Design scheme of a new type of sensor was focused on, which adopted MEMS technology, vacuum encapsulation and resistive load modulation mode to send collected data. Testing and trail running results show that the system has better reliability, real-time performance and operability, which can reduce field personnel and accidents effectively.