Status of research on biogenic coalbed gas generation mechanisms

Biogenic coalbed gas,how it is generated and the geochemical characteristics of the gas are gaining global attention.The ways coalbed gas is generated,the status of research on the generation mechanism and the methods of differentiating between biogenic gasses are discussed.The generation of biogenic coalbed methane is consistent with anaerobic fermentation theory.Commercial biogenic coalbed gas reservoirs are mainly generated by the process of CO2 reduction.The substrates used by the microbes living in the coal include organic compounds,CO2,H2 and acetate.The production ratio and quantity of biogenic coalbed methane depend on the exposed surface area,the solubility and permeability of the coal and the microbial concentration in the coal seam.It is generally believed that biogenic coalbed gas has a value for δ13C1<-5.5%,C1/C1+>0.95.The H isotope ratio is controlled by both the environment and the generation mechanism:typically δD1<-20%.Biogenic methane formed by CO2 reduction has more δD1 than that formed by acetate fermentation.

New Approaches and Markers for Identifying Secondary Biogenic Coalbed Gas

According to the adsorption-desorption characteristics of coalbed gas and analysis of various experimental data, this paper proposes that the generation of secondary biogenic gas （SBG） and its mixing of with the residual thermogenic gas at an early stage inevitably lead to secondary changes of the thermogenic gas and various geochemical additive effects. Experimental results also show that the fractionation of the carbon isotope of methane of coal core desorption gas changes very little; the δ13C1 value of the mixed gas of biogenic and thermogenic gases is between the δ13C1 values of the two ＂original＂ gases, and the value is determined by the carbon isotopic compositions and mixing proportions of the two ＂original＂ methanes. Therefore this paper proposes that the study on the secondary changes of the thermogenic gas and various additive effects is a new effective way to study and identify SBG. Herein, a systematic example of research on the coalbed gas （Huainan coalbed gas） is further conducted, revealing a series of secondary changes and additive effects, the main characteristics and markers of which are： （1） the contents of CO2 and heavy-hydrocarbons decrease significantly; （2） the content of CH4 increases and the gas becomes drier; （3） the δ13C and δD values of methane decrease significantly and tend to have biogenetic characteristics; and （4） the values of 513C2 and δ13Cc02 grow higher. These isotopic values also change with the degradation degrees by microbes and mixing proportions of the two kinds of gases in different locations. There exists a negative correlation between the △13C1 It＇S δ13Cco2 values. The δ13Cc2-c1 values obviously become higher. The distributions of the △δ^13Cco2-C1 values are within certain limits and show regularity. There exist a positive correlation between the N2 versus Ar contents, and a negative correlation between the N2 versus CH4 contents, indicating the down forward infiltration of the surface water containing air. These are important markers of the generation and existence of SBG .

Prediction of multi-borehole undermine coalbed gas drainage

By analyzing the flow character of a single drainage borehole in its effectingtime and the correlative theory introduced,the reason for 'inflexion' appearance in theflow character curve of the single draining borehole in a multi-borehole was studied.Takingthe theory of permeation fluid mechanics and so on as basis,the coalbed gas flowmodel was set up,and the numerical simulation analyzer was built for undermine gasproducts.With the results from the analyzer,the gas capacity could be calculated underdifferent conditions and comparisons made with the site measurement data.

The First Huge Coalbed Gas Field in China Developed

AccordingtosourcesfromCNPC,ahugecoalbedgasfield,coveringagas-bearingareaof1768km2andpossessingresourcesofover400billioncubicmeters,hastakenitsshapeinQinshuiofShanxiProvince.Thepresentprovengasreservesstandat47.4billioncubicmetersandtheprobablegasrese...

Comparison of coalbed gas generation between Huaibei-Huainan coalfields and Qinshui coal basin based on the tectono-thermal modeling

The geothermal history and the tectonic subsidence history of the Huaibei-Huainan coalfields were reconstructed by using the vitrinite reflectance data, and their correlative restriction on coalbed gas generation of Huaibei-Huainan coalfields and Qinshui coal basin was discussed. The burial, thermal, and maturity histories of are similar between Huaibei coalfield and Huainan coalfield, obviously different from those of Qinshui coal basin. Based on the tectono-thermal evolution characters of Huaibei-Huainan coalfields and Qinshui basin, the process of coalbed gas generation can be divided into three stages： （1） Dur- ing Early Mesozoic, both in Huaibei-Huainan and Qinshui, the buried depth of Permian coal seams increased rapidly, which resulted in strong metamorphism and high burial temperature of coal seams. At this stage, the coal rank was mainly fat coal, and locally reached coking coal. These created an environment favoring the generation of thermogenic gas. （2） From Late Ju- rassic to Cretaceous, in the areas of Huaibei-Hualnan, the strata suffered from erosion and the crust became thinning, and the Permian coal-bearing strata were uplifted to surface. At this stage, the thermogenic gas mostly escaped. Conversely, in Qinshui basin, the cover strata of coal seams kept intact during this stage, and the thermogenic gas were mostly preserved. Furthermore, with the interaction of magmatism, the burial temperature of coal seams reached higher peak value, and it was suitable for the secondary generation of thermogenic gas. （3） From Paleogene onward, in area of Huainan-Huaibei, the maturity of coal and burial temperature were propitious to the generation of secondary biogenic gases. However, in Qinshui basin, the maturity of coal went against genesis of second biogenic gas or thermogenic gas. By comparison, Huaibei-Huainan coalfields are dominated by thermogenic gas with a significant biogenic gas and hydrodynamic overprint, whereas Qinshui basin is dominated mainly by thermogenic gas.

Multi-scale pore fractal characteristics of differently ranked coal and its impact on gas adsorption

Well-developed pores and cracks in coal reservoirs are the main venues for gas storage and migration.To investigate the multi-scale pore fractal characteristics,six coal samples of different rankings were studied using high-pressure mercury injection(HPMI),low-pressure nitrogen adsorption(LPGA-N2),and scanning electron microscopy(SEM)test methods.Based on the Frankel,Halsey and Hill(FHH)fractal theory,the Menger sponge model,Pores and Cracks Analysis System(PCAS),pore volume complexity(D_(v)),coal surface irregularity(Ds)and pore distribution heterogeneity(D_(p))were studied and evaluated,respectively.The effect of three fractal dimensions on the gas adsorption ability was also analyzed with high-pressure isothermal gas adsorption experiments.Results show that pore structures within these coal samples have obvious fractal characteristics.A noticeable segmentation effect appears in the Dv1and Dv2fitting process,with the boundary size ranging from 36.00 to 182.95 nm,which helps differentiate diffusion pores and seepage fractures.The D values show an asymmetric U-shaped trend as the coal metamorphism increases,demonstrating that coalification greatly affects the pore fractal dimensions.The three fractal dimensions can characterize the difference in coal microstructure and reflect their influence on gas adsorption ability.Langmuir volume(V_(L))has an evident and positive correlation with Dsvalues,whereas Langmuir pressure(P_(L))is mainly affected by the combined action of Dvand Dp.This study will provide valuable knowledge for the appraisal of coal seam gas reservoirs of differently ranked coals.

Seepage law and permeability calculation of coal gas based on Klinkenberg effect

Focused on the Klinkenberg effect on gas seepage, the independently developed triaxial experimental system of gas seepage was applied to conduct research on the seepage characteristics of coal seam gas. By means of experimental data analysis and theoretical derivation, a calculation method of coal seam gas permeability was proposed, which synthesized the respective influences of gas dynamic viscosity, compressibility factor and Klinkenberg effect. The study results show that the Klinkenberg effect has a significant influence on the coal seam gas seepage, the permeability estimated with the method considering the Klinkenberg effect is correct, and this permeability can fully reflect the true seepage state of the gas. For the gas around the standard conditions, the influences of dynamic viscosity and compressibility factor on the permeability may be ignored. For the gas deviating far away from the standard conditions, the influences of dynamic viscosity and compressibility factor on the permeability must be considered. The research results have certain guiding significance in forming a correct understanding of the Klinkenberg effect and selecting a more accurate calculation method for the permeability of coal containing gas.

Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams

Multiple coal seams widely develop in the deep Chinese coal-bearing strata. Ground in situ stress and coal seam gas pressure increase continuously with the increase of the mining depth, and coal and gas outburst disasters become increasingly severe. When the coal is very deep, the gas content and pressure will elevate and thus coal seams tends to outburst-prone seams. The safety and economics of exploited firstmined coal seams are tremendously restricted. Meanwhile, the multiple seams occurrence conditions resulted in different methane pressure systems in the coal-bearing strata, which made the reservoir reconstruction of coal difficult. Given the characteristics of low saturation, low permeability, strong anisotropy and soft coal of Chinese coal seams, a single hydraulic fracturing surface well for reservoir reconstruction to pre-drain the coalbed methane(CBM) of multiple seams concurrently under the different gas pressure systems has not yet gained any breakthroughs. Based on analyses of the main features of deep CBM reservoirs in China, current gas control methods and the existing challenges in deep and multiple seams, we proposed a new technology for deep CBM reservoir reconstruction to realize simultaneous high-efficiency coal mining and gas extraction. In particular, we determined the first-mined seam according to the principles of effectiveness and economics, and used hydraulic fracturing surface well to reconstruct the first-mined seam which enlarges the selection range of the first-mined seam. During the process of mining first-mined seam, adjacent coal seams could be reconstructed under the mining effect which promoted high-efficiency pressure relief gas extraction by using spatial and comprehensive gas drainage methods(combination of underground and ground CBM extraction methods). A typical integrated reservoir reconstruction technology, ‘‘One well for triple use", was detailed introduced and successfully applied in the Luling coal mine. The application showed that the proposed technology could effectively promote coal mining safety and simultaneously high-efficiency gas extraction.

Preferential adsorption behaviour of CH_4 and CO_2 on high-rank coal from Qinshui Basin,China

In order to better understand the prevailing mechanism of CO2 storage in coal and estimate CO2 sequestration capacity of a coal seam and enhanced coalbed methane recovery （ECBM） with CO2 injection into coal, we investigated the preferential adsorption of CH4 and CO2 on coals. Adsorption of pure CO2, CH4 and their binary mixtures on high-rank coals from Qinshui Basin in China were employed to study the preferential adsorption behaviour. Multiple regression equations were presented to predict CH4 equi- librium concentration from equilibrium pressure and its initial-composition in feed gas. The results show that preferential adsorption of CO2 on coals over the entire pressure range under competitive sorption conditions was observed, however, preferential adsorption of CH4 over CO2 on low-volatile bituminous coal from higher CH4-compostion in source gas was found at up to 1O MPa pressure. Preferential adsorp- tion of CO2 increases with increase of CH4 concentration in source gas, and decreases with increasing pressure. Although there was no systematic investigation of the effect of coal rank on preferential adsorp- tion, there are obvious differences in preferential adsorption of gas between low-volatile bituminous coal and anthracite. The obtained preferential adsorption gives rise to the assumption that CO2 sequestration in coal beds with subsequent CO2-ECBM might be an ootion in Qinshui Basins, China.

Abnormal Concentration and Origin of Heavy Hydrocarbon in Upper Permian Coal Seams from Enhong Syncline,Yunnan,China

Coalbed gas （CBG） in Enhong syncline, eastern Yunnan China, is character- ized by high concentration of heavy hydrocarbon with the highest content of ethane, which is more than 30%. Some previous investigators paid much attention to the abnormal concentration of heavy hydrocar- bon in the CBG of Enhong, but few have researched on its origin. This article describes the characteristics of abnormal high concentration of heavy hydrocarbon in Enhong syncline and analyzes its reason from the aspects of origin and evolution of heavy hydrocarbon by carbon isotope, coal petrography, and coal rank. Features of gas carbon isotope composition display that there is no inorganic gas or off components in the CBG, which is classified to thermogenetic gas produced by humic material, with characteristic of secondary biogenic gas in shallow coal seam. The concentration of heavy hydrocarbon in Enhong syncline increases with the increase of vitrinite, vitrinite/inertinite ratio, and hydrogen/carbon ratio and decreases with the increase of inertinite, so hydrogen-rich vitrinite may be a very important factor resulting in the abnormal concentration of heavy hydrocarbon. It also increases with the increase of degree of coalifica-tion of coking to lean coals during which the peak of heavy hydrocarbon generation is reached. Therefore, we think that high concentration of heavy hydrocarbon originated from the coupling effect of higher content of the hydrogen-rich vitrinite in the coal and the coal rank of coking to lean coals.