The hole sealing technology of solid-liquid materials with three pluggings and two injections for gas extraction hole in the coal mine

The sealing quality of the gas extraction holes determines the extracted gas concentration. Based on this, the paper reveals the basic principle of hole sealing by analyzing the gas leakage mechanism of the borehole. The hole sealing technology of solid-liquid materials with three pluggings and two injections for the gas extraction hole is proposed, and the hole sealing device and material are developed. Through testing the granularity distribution of the solid material, as well as the surface tension and contact angle of the slurry, the hole sealing material that can meet the requirements of accessible, sticky, and anti-deformation is selected. The sealing material enters microcracks and bonds coal rock more easily. First, the solid material is injected for hole sealing. Second, the liquid material can be injected repeatedly to maintain a high concentration for holes with poor sealing and gas concentration attenuation in the late stage of gas extraction. Field tests show that the gas concentration of solid material is 1.3 times that of the conventional material after 30 days of sealing. The liquid material injected after the concentration decline enables the gas extraction concentration to be recovered at 85 %

Pilot test of a fermentation tank for producing coal methane through anaerobic fermentation

The development and utilization of clean energy has long been a focus of research. In the coal bed methane field, most coal bed biogenic methane experiments are small static sample tests in which the initial conditions are set and the process cannot be batch-fed elements and microbial strains, and the gas cannot be collected in batches. Although significant results have been achieved in the coal-to-biogenic methane conversion in China, findings are restricted to the laboratory scale. No successful commercialization of coal bed biogenic methane production has been achieved yet. This study used a large-capacity fermentation tank (5 L) to conduct biogenic methane experiments. Results were compared to those from the traditional laboratory test. The gas production rate and gas concentration were higher when the 250 mL methane test volume was increased to a 5 L fermentation volume, increasing by 20.9% and 2.3%, respectively. The inhibition effect of the liquid phase products was reduced in the large fermentation tank, and the microbial activity was extended by batch feeding trace elements (iron and nickel) and methane strains and by semi-continuous collection of the gas. However, the gas conversion rate can be increased by retaining the H2 and CO2 in the intermediate gas products in the fermentation tank. The gas production rate was increased from 17.9 to 24.6 mL/g, increasing by 37.4%. The simulation pilot test can lay a foundation for the transition from a coal bed biogenic methane laboratory static small sample test to a dynamic pilot test, optimizing the process parameters to improve the reaction efficiency and move forward to commercialization test

The Analysis of Key Issues for Virtual Reservoirs Application in Coalbed Methane Development

Based on flow state of coalbed methane (CBM) migration, coal reservoir broadly is divided into outburst and non-outburst coal. Because of strong water sensitivity and poor hydraulic fracturing effect of outburst coal, hydraulic fracturing measures to increase permeability which used to be applied in non-outburst coal do not suitable for outburst coal. Coal floor is selected as virtual reservoir for roof maintenance when mining coal, Depending on a zero radius drilling and high-energy gas loose blasting technology, the reservoir and the coal floor become transfixion, so that the CBM diffuse to the floor cracks, and then migrate into the wellbore. The "highway" of CBM migration is built through virtual reservoir, which would be expected to break through the forbidden zone of traditional CBM development, and joint two-energy exploitation of coal and CBM can achieve, so that gas disaster and greenhouse gas emission will reduce greatly. Therefore, virtual reservoir has broad application prospect.特集 : 「資源、新エネルギー、環境、防災研究国際セミナー

Theory and technique of permeability enhancement and coal mine gas extraction by fracture network stimulation of surrounding beds and coal beds

AbstractThe existing reservoir stimulating technologies are only applicable to hard coal but helpless for soft coal, which is one of the main factors hindering the CBM industrialization in China. Therefore, it is urgent to develop a universal stimulating technology which can increase the permeability in various coal reservoirs. Theoretical analysis and field tests were used to systematically analyze the mechanical mechanisms causing the formation of various levels and types of fractures, such as radial tensile fractures, peripheral tensile fractures, and shear fractures in hydraulic fracturing, and reveal the mechanism of permeability enhancement by fracture network stimulating in surrounding beds and coal reservoirs. The results show that multi-staged perforation fracturing of horizontal wells, hydraulic-jet staged fracturing, four-variation hydraulic fracturing and some auxiliary measures are effective technical approaches to fracture network stimulation, especially the four-variation hydraulic fracturing can stimulate the fracture network in vertical and cluster wells. It is concluded that the fracture network stimulating technology for surrounding beds has significant advantages, such as safe drilling operation, strong stimulation effect, strong adaptability to stress-sensitive and velocity-sensitive beds, and is suitable for coal reservoirs of any structure. Except for the limitation in extremely water-sensitive and high water-yield surrounding beds, the technology can be universally used in all other beds. The successful industrial tests in surface coal bed methane and underground coal mines gas extraction prove that the theory and technical system of fracture network stimulating in surrounding beds and coal reservoirs, as a universally applicable measure, will play a role in the CBM development in China

Key technologies of microbial mining residual coal and CO2-fly ash co-filling in the impacted geological body of coal mining

The gob of coal mine is an important area for achieving the goal of “dual carbon” in China. The geological body formed by coal mining, which can enrich coalbed methane and provide substrate and space for later microbial activities and mineralization filling is defined as the mining influence body. The proposed technologies include the residual coal extraction through mining influence body microorganisms and the co-mineralization and filling of CO2-fly ash. The broad prospects of the technology in the secondary development of mining, from the perspectives of necessity and feasibility, are elaborated upon regarding the safe storage of CO2 and efficient disposal of fly ash solid waste in coal-fired power plants. The overall concept is to utilize mining influence body as a anaerobic fermentation “factory” and microorganisms as “workers” to process the existing raw materials of the “factories”including residual coal, thin coal seams, dispersed organic matter, and injected CO2. The ultimate goal is to produce methane, thereby achieving the resource utilization of microbial mining for residual coal and CO2. The combination of CO2 and alkaline fly ash simultaneously achieves the mineralization storage of CO2 and the filling of mining influence body. The key scientific issues are involved in this technology encompass the classification of mining influence body and the characteristics of organic matter, elucidating the mechanism of anaerobic fermentation under in-situ conditions specific to mining influence body, investigating the cooperative mineralization mechanism of microbial-CO2-fly ash, as well as undertaking a demonstration project for constructing the key technology of microbial residual coal mining and filling. The laboratory physical simulation of the in-situ conditions of the mining influence body demonstrates that the residual coal and organic-rich mud shale have the capability to generate biomethane, with methane production further enhanced by a small quantity of fly ash. The dynamic experiment of simulated groundwater recharge demonstrates that the nutrient recharge significantly impacts the anaerobic fermentation system. Specifically, the system with a cycle period of 14 days was consistent with the cycle of methanogens, which can ensure the continuous and efficient operation of the anaerobic fermentation system. After a curing period of 28 days, the test specimen containing high calcium fly ash, CO2, and mine water exhibited a compressive strength of 12.31 MPa. Additionally, each ton of fly ash had the potential to store approximately 21.99 m3 of CO2 through mineralization, highlighting the dual benefits of CO2 emission reduction and goaf solidification achieved by utilizing fly ash. The engineering test target area was optimized based on the purpose of microbial coal residue mining and fly ash filling. Also, the groundwater retention area was identified as the optimal location for CO2 mineralization and fly ash filling. The natural trap formed by mining activities and the trap formed by artificial filling were one of the more favorable engineering test targets. The proposed technologies of microbial residual coal mining, CO2 and fly ash co-filling are aimed at providing a novel technical approach for carbon emission reduction and goaf ecological environment management in China

Enhancement of bioconversion of coal to methane by graphene

The research of enhancing biomethanation of coal has been paid much attention, which is an effective measure for increasing coalbed methane production. Adding conductive material to the digestive system can effectively accelerate direct interspecific electron transfer and increase methane production, which has great potential in enhancing the anaerobic digestion of organic matter. In this study, long-flame coal was used as the substrate to construct an anaerobic digestion system. The effect of the addition of graphene on biomethane production was discussed from the aspects of cumulative methane yield, the changes of key intermediates in the liquid phase, the microbial community structure, the methane metabolic pathway, and the changes of surface functional groups in residual coal after anaerobic digestion. The results showed that adding 0.4 g/L of graphene to the anaerobic digestion system based on coal effectively enhanced the entire anaerobic digestion process, not only enhanced methane production, but also brought forward the peak of methane production. At the early stage of digestion, the activities of hydrolytic bacteria (Paraclostridium) and hydrogen-production and aceogenic microflora (Alcaligenes and Sphaerochaeta) were enhanced, and sufficient nutrients were accumulated in the early stage. At the peak of methane production, the abundance of Methanoculleus decreased while the abundance of Methanosarcina significantly increased after the addition of graphene. The β subunit and γδ subunit of acetyl-coa decarbonyase/synthase, as key enzymes in the acetic acid synthesis pathway, increased by 233.54% and 3.32%, respectively. This significantly increased the abundance of Methanosarcina and mainly produced methane in the form of acetic acid nutrition. The abundance of Geobacter and Anaerovorax bacteria that can use ethyl acetate increased, and the Geobacter with high abundance were likely to DIET with Methanosarcina by bioelectric connection assisted by graphene. This electron transport mode accelerated the formation of biomethane to some extent. The carbonyl carbon (C=O) and carboxyl carbon (COO—) on the surface of residual coal decreased by 42.8% and 49.5%, respectively, after the addition of graphene, indicating that graphene effectively promoted the degradation of coal by microflora. The addition of graphene improves the activity and degradation efficiency of microflora, speeds up the process of anaerobic digestion, provides abundant substrate for methanogenic microflora, and improves methane production

The experimental study on integrated hydraulic fracturing of coal measures gas reservoirs

Coal measures are rich in coalbed methane, shale gas and tight sandstone gas, and the three gases co-exploration has become a potentially promising technology in China. During the process of integrated hydraulic fracturing of three gas formations in coal measures, interlayer contradiction occurs due to the formation heterogeneity, which is the key to restricting the application of this technology. In this paper, an experimental device has been designed to study the influencing factors for integrated hydraulic fracturing in coal measure gas reservoir. The results of experimental test and theoretical analysis indicate that the formation permeability and breakdown pressure are the main factors affecting hydraulic fracturing process, and the formations with large difference in these two factors always induce an uneven distribution of fracturing fluid and poor effect of integrated hydraulic fracturing. High flow rate pumping injection rate is conducive to the even distribution of fracturing fluid in various formations. Meanwhile, the proper ball injection is used to limit fluid flowrate to ensure the fluid pressure in high permeability formation is higher than it in low permeability formation, fracturing fluid can flow to low permeability formation through perforation hole. In addition, interlayer crossflow occurs during the migration of fracturing fluid and causes fluid flow to low permeability formation, which is also conducive to the even fracturing. Therefore, high flow rate pumping injection, ball injection and interlayer crossflow control are effective measures for achieving the integrated hydraulic fracturing of coal measure gas reservoirs

The diversity of hydrogen-producing bacteria and methanogens within an in situ coal seam

Abstract Background Biogenic and biogenic-thermogenic coalbed methane (CBM) are important energy reserves for unconventional natural gas. Thus, to investigate biogenic gas formation mechanisms, a series of fresh coal samples from several representative areas of China were analyzed to detect hydrogen-producing bacteria and methanogens in an in situ coal seam. Complete microbial DNA sequences were extracted from enrichment cultures grown on coal using the Miseq high-throughput sequencing technique to study the diversity of microbial communities. The species present and differences between the dominant hydrogen-producing bacteria and methanogens in the coal seam are then considered based on environmental factors. Results Sequences in the Archaea domain were classified into four phyla and included members from Euryarchaeota, Thaumarchaeota, Woesearchaeota, and Pacearchaeota. The Bacteria domain included members of the phyla: Firmicutes, Proteobacteria, Bacteroidetes, Actinobacteria, Acidobacteria, Verrucomicrobia, Planctomycetes, Chloroflexi, and Nitrospirae. The hydrogen-producing bacteria was dominated by the genera: Clostridium, Enterobacter, Klebsiella, Citrobacter, and Bacillus; the methanogens included the genera: Methanorix, Methanosarcina, Methanoculleus, Methanobrevibacter, Methanobacterium, Methanofollis, and Methanomassiliicoccus. Conclusion Traces of hydrogen-producing bacteria and methanogens were detected in both biogenic and non-biogenic CBM areas. The diversity and abundance of bacteria in the biogenic CBM areas are relatively higher than in the areas without biogenic CBM. The community structure and distribution characteristics depend on coal rank, trace metal elements, temperature, depth and groundwater dynamic conditions. Biogenic gas was mainly composed of hydrogen and methane, the difference and diversity were caused by microbe-specific fermentation of substrates; as well as by the environmental conditions. This discovery is a significant contribution to extreme microbiology, and thus lays the foundation for research on biogenic CBM

Numerical investigation of the potential contamination of a shallow aquifer in producing coalbed methane

This paper presents a study on the impact of well integrity failure on coalbed methane (also known as coal seam gas) production and potential shallow water contamination using numerical simulations with a finite-difference method. Two connection types and 12 cases were simulated: Type A - leakage through cement sheath and Type B - impaired zonal isolation at the aquifer interval. The effect of the distance between the aquifer and the coal seam, drainage area and desorption time on gas and water production was also inspected. Results show that both connection types have strong effects on the gas and water production; the cumulative water and gas production increases with increasing drainage radius; the distance between aquifer and the coal seam has a negative effect on the water production but a negligible effect on the gas production; desorption time, ranging from 5 to 30 days, has a negligible effect on the water and gas production. Connection Type A yields a potential water contamination but connection Type B does not. Gas concentration in the shallow aquifer decreases sharply with an increase of distance away from the producer and the unsafe area are within an area with a radius ranging from approximately 50 m to 90 m away from the producer in this study