Shuxun Sang

optimum location of surface wells for remote pressure relief coalbed methane drainage in mining areas

Abstract Based on engineering tests in the Huainan coal mining area, we studied alternative well location to improve the performance of surface wells for remote pressure relief of coalbed methane in mining areas. The key factors, affecting location and well gas production were analyzed by simulation tests for similar material. The exploitation results indicate that wells located in various positions on panels could achieve relatively better gas production in regions with thin Cenozoic layers, low mining heights and slow rate of longwall advancement, but their periods of gas production lasted less than 230 days, as opposed to wells in regions with thick Cenozoic layers, greater mining heights and fast rates of longwall advancement. Wells near panel margins achieved relatively better gas production and lasted longer than centerline wells. The rules of development of mining fractures in strata over panels control gas production of surface wells. Mining fractures located in areas determined by lines of compaction and the effect of mining are well developed and can be maintained for long periods of time. Placing the well at the end of panels and on the updip return airway side of panels, determined by lines of compaction and the effect of mining, would result in surface wells for remote pressure relief CBM obtaining their longest gas production periods and highest cumulative gas production.

Pore characteristics and controlling factors of the Lower Cambrian Hetang Formation shale in Northeast Jiangxi, China

To identify the microscopic pore characteristics and controlling factors of Hetang Formation Shale in the Lower Yangtze Region, the pore types, pore size distribution characteristics, and controlling factors of the Lower Cambrian Hetang Formation (Є1h) marine shale in Northeast Jiangxi were analyzed by using low-temperature liquid nitrogen, X-ray diffraction, scanning electron microscope, mercury intrusion porosimetry, isothermal adsorption experiment, and geochemical indicator test system. The research results show that the pore size distribution curve of Hetang Formation Shale is characterized by “two peaks” and dominated by micropore (2 nm) and mesopore (47–82 nm). The hysteresis loop shows that the open parallel-plate pore and slit pores are the main pore types in shales. The pore volume of Hetang Formation Shale is only positively related to total organic carbon, without obvious correlation with mineral composition and thermal evolution degree. The controlling factors of pore structure characteristics of Hetang Formation Shale are rather complicated. Further analysis shows that diagenesis and excessive thermal evolution are the two main controlling factors restricting the microscopic pore characteristics. Due to great burial depth, organic matter generates numerous micropores during pyrolysis and hydrocarbon generation, and clay minerals generate a lot of micropores and mesopores during conversion from montmorillonite to illite. On the other hand, the development of mesopore and macropore is far better than that of nanoscale pore, because rigid quartz mineral is the dominant composition of shale and the strong compaction resistance of quartz can increase macropore volume with the increase of shale burial depth. It can be inferred that Hetang Formation Shale is a relatively ideal horizon for shale gas development, since the proportion of potential free gas is relatively high and induced cracks are prone to be formed, which is conducive to seepage and desorption of shale gas.

Fine-grained pyrite in some Chinese coals

Fine-grained pyrite in some Chinese coals has been investigated, with analysis by optical microscopy and a scanning electron microscope equipped with energy dispersive X-ray spectrometer illustrating that the interaction between mineral and organic matters results in the irregular pyrite crystal present in coal. When considering the co-existing carbon, the density of fine-grained pyrite in coal is lower than that of mineral pyrite. Organic sulfur and pyritic sulfur in coal can be converted into each other under the favorable conditions. However, the fine-grained pyrite crystal pattern is still detected when the S/Fe ratio increases to 2:1, meaning that the fine-grained pyrite shifts from dissemination to crystallization. Fine-grained pyrite in coal has a strong activity and its surface is susceptible to be oxidized. Oxidized fine-grained pyrite usually forms sulfate (mainly gypsum), and sometimes may be converted to marcasite. Fine-grained pyrite in coal, when their granularity is small enough, may be chemically associated with organic components by non-pyritic Fe–S bond rather than physically embedded in coal matrix. So, it is not suitable to express the mineral using the FeS2 formula.

Characteristics and significance of heterogeneity of sea-land transitional facies shale gas reservoir in North Guizhou, China

In order to identify the characteristics of the longitudinal heterogeneity of the sea–land transitional facies shale gas reservoir in the upper Yangtze region of North Guizhou, studies on the lithological combination, rock and mineral composition, geochemical parameters and reservoir microanisotropy characteristics of Longtan Formation in the study area are conducted on the basis of core observation, testing of geochemistry and reservoir physical property and well logging interpretation. The studies show that the lithological assemblages of the Longtan Formation are diverse and form an amina interbedding of “sand-mud-coal” with obvious cyclicity characteristics. There is a large longitudinal difference in rock and mineral composition and the average mass fraction of the clay mineral is 39.83%, which is obviously higher than that of the marine shale in North America and South China; the longitudinal heterogeneity of the organic matter abundance is high, with an average of 2.17% in the upper part, and 4.51% in the lower part; in accordance with the results observed with the scanning electron microscope and results calculated through pore fractal, the microscopic pore heterogeneity of the reservoir is high. The comparison and analysis of connecting wells with different scales in the study area show that the control effect of the depositional environment on longitudinal macroscopic heterogeneity of Longtan Formation is obvious, and the longitudinal microscopic heterogeneity is controlled through diagenesis. Meanwhile, studies with main coal mining seam as the seam section division method conclude that the heterogeneity of Coal Seam Sections 4 to 5 and Coal Seam Sections 13 to 15 is significantly smaller than that in other seam sections, and the Coal Seam Sections 4 to 5 and Coal Seam Sections 13 to 15 can be considered as a priority key seam section during development of shale gas.

The effects of CO2 on organic groups in bituminous coal and high-rank coal via Fourier transform infrared spectroscopy

The interactions between supercritical CO2 and coal and their effects on changes in the coal pore structure and organic groups play a critical role in the CO2 geological storage-enhanced coalbed methane recovery. To investigate the effects of supercritical CO2 on organic groups in coals of different ranks and its mechanisms under different temperature and pressure conditions, CO2 sequestration processes in bituminous coals and high-rank coals were replicated using a high-pressure reactor. Four coal samples of different ranks were exposed to supercritical CO2 and water under three temperatures and pressures for 240 h. Fourier transform infrared spectroscopy was used to provide semiquantitative ratios and Fourier transform infrared spectra of coal samples before and after the supercritical CO2–H2O treatment. The results show that interactions between supercritical CO2 and coal were controlled by the coal macromolecular structure, and semianthracite is the inflection point of interaction characteristics for coal samples of different ranks. Bituminous coal, including high- and low-volatility bituminous coal, has a low degree of condensation of its aromatic structure, and its aromatic nuclei can facilitate addition reactions. Swellings primarily break cross-links between aromatic nuclei in the same aromatic layer. These characteristics favor the polymerization addition of aliphatic side chains of aromatic nuclei, causing an increase in the degree of condensation of the aromatic structures in bituminous coal. High-rank coals including semianthracite and anthracite have a high degree of condensation of their aromatic structures, and the aromatic nuclei favor substitution reactions. Swellings primarily break cross-links connecting different aromatic layers, and bond dissociation reactions and sulfuration reactions are more significant for high-rank coal. These characteristics cause a decrease in the degree of condensation of the aromatic structure in high-rank coal. Temperature and pressure have a great impact on interactions between supercritical CO2 and coal and are controlled by the reaction types of the organic groups. With the increase in experimental temperature and pressure, the changes in the organic group content can be classified as the descending type, the rising type, the lower opening parabola type, and the upper opening parabola type. 45.0°C and 10 MPa is the inflection point of the changes in the organic group content. Descending- and rising-type changes favor addition, bond dissociation, and sulfuration reactions, which are endothermic. The reaction rate of supercritical CO2 and the organic groups increases, and the effects caused by temperature and pressure decrease as the temperature and pressure increase. Lower opening parabola- and upper opening parabola-type changes favor substitution, oxidation, and addition polymerization reactions, which are exothermic. These changes were significantly affected by a variety of reactions and were suppressed by high temperature and pressure. When the temperature is ≤45.0°C and the pressure is ≤10 MPa, supercritical CO2 has remarkable effects on alkyl and hydroxy groups and has a stronger effect on bituminous coal. When the temperature is >45.0°C and the pressure is >10 MPa, supercritical CO2 has remarkable effects on oxygen- and sulfur-containing groups and has a greater effect on high-rank coals.

Experimental study on the velocity sensitivity of coal reservoir during coalbed methane drainage in southern Qinshui Basin

Determination of the velocity sensitivity in coal reservoirs during the different production stages of coalbed methane wells is fundamentally crucial to adopt appropriate drainage technologies. To address this need, simulation experiments of coal samples from southern Qinshui Basin in China were conducted to test the variation of coal permeability with fluid flow. The pore structures were tested before and after the simulation experiment by using mercury injections, and the pore shape was observed using scanning electron microscope (SEM). The results show that formation water with fast flow may remove solid particles and that there is no velocity sensitivity under the experimental conditions of different coal samples and formation waters during the water production and depressurization stages of the coalbed methane well. There is a trend of the velocity sensitivity in the coalbed methane reservoir showing high concentration of solid particles during the stages of water production and depressurization. Coal permeability decreases with the increase of the fluid flow, there are different levels of velocity sensitivity in the coalbed methane reservoir during gas production of the coalbed methane well. The critical drainage flow should be within 11.26 m3/d during gas production of the coalbed methane well. The generation of the velocity sensitivity will make the pore structure of the coalbed methane reservoir poorly. During the stage of gas production, the formation water produces poorly, and the solid particles adhered to the surface of coal easily fall off and are deposited in the transition pore and micropore, which further results in the decrease of coal permeability.

Study on geochemistry discriminate method of gas emission in goaf

In order to effectively distinguish the source of gas gush in the goaf of the coal mine, Sihe coal mine was chosen as the studied object. Carbon isotope of methane, ethane, carbon dioxide and hydrogen isotope of methane in gas from the desorbed gas of the working coal seam and the near coal seam, and gas in the goaf were tested, by systemic sampling coal samples and gas samples of the goaf. Combined with gas component from the goaf, the working coal seam and the near coal seam, the proportion of gas gush in the goaf were determined by the numerical calculation on separate source of isotopes. The results show that the difference of gas component, stable carbon and hydrogen isotopes of gas from the different coal seams is obvious. And the methane concentration in gas increase along with the increase of coal seam depth, carbon and hydrogen isotopes in gas become heavier along with the deepen of burial depth of coal seam depth. The results of the numerical calculation on separate source indicate that the proportion of the gushed gas in the lagged traverse No.1 from the desorbed gas of coal seam No.3~No.8-2 is 38%, 37%, 12%, 8%, 5%, the proportion of the gushed gas in the lagged traverse No.2 from the desorbed gas of coal seam No.3~No.8-2 is 22%, 36%, 24%, 12%, 6%.