Geological characteristics and exploration breakthroughs of coal rock gas in Carboniferous Benxi Formation,Ordos Basin,NW China

To explore the geological characteristics and exploration potential of the Carboniferous Benxi Formation coal rock gas in the Ordos Basin,this paper presents a systematic research on the coal rock distribution,coal rock reservoirs,coal rock quality,and coal rock gas features,resources and enrichment.Coal rock gas is a high-quality resource distinct from coalbed methane,and it has unique features in terms of burial depth,gas source,reservoir,gas content,and carbon isotopic composition.The Benxi Formation coal rocks cover an area of 16×104km^(2),with thicknesses ranging from 2 m to 25 m,primarily consisting of bright and semi-bright coals with primitive structures and low volatile and ash contents,indicating a good coal quality.The medium-to-high rank coal rocks have the total organic carbon(TOC)content ranging from 33.49%to 86.11%,averaging75.16%.They have a high degree of thermal evolution(Roof 1.2%-2.8%),and a high gas-generating capacity.They also have high stable carbon isotopic values(δ13C1of-37.6‰to-16‰;δ13C2of-21.7‰to-14.3‰).Deep coal rocks develop matrix pores such as gas bubble pores,organic pores,and inorganic mineral pores,which,together with cleats and fractures,form good reservoir spaces.The coal rock reservoirs exhibit the porosity of 0.54%-10.67%(averaging 5.42%)and the permeability of(0.001-14.600)×10^(-3)μm^(2)(averaging 2.32×10^(-3)μm^(2)).Vertically,there are five types of coal rock gas accumulation and dissipation combinations,among which the coal rock-mudstone gas accumulation combination and the coal rock-limestone gas accumulation combination are the most important,with good sealing conditions and high peak values of total hydrocarbon in gas logging.A model of coal rock gas accumulation has been constructed,which includes widespread distribution of medium-to-high rank coal rocks continually generating gas,matrix pores and cleats/fractures in coal rocks acting as large-scale reservoir spaces,tight cap rocks providing sealing,source-reservoir integration,and five types of efficient enrichment patterns(lateral pinchout complex,lenses,low-amplitude structures,nose-like structures,and lithologically self-sealing).According to the geological characteristics of coal rock gas,the Benxi Formation is divided into 8 plays,and the estimated coal rock gas resources with a buried depth of more than 2000 m are more than 12.33×10^(12)m^(3).The above understandings guide the deployment of risk exploration.Two wells drilled accordingly obtained an industrial gas flow,driving the further deployment of exploratory and appraisal wells.Substantial breakthroughs have been achieved,with the possible reserves over a trillion cubic meters and the proved reserves over a hundred billion cubic meters,which is of great significance for the reserves increase and efficient development of natural gas in China.

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.

Effects of Coal Rank and High Organic Sulfur on the Structure and Optical Properties of Coal-based Graphene Quantum Dots

Coal-based graphene quantum dots(GQDs) were successfully produced via a one-step chemical synthesis from six different coal ranks, from which two superhigh organic sulfur(SHOS) coals were selected as natural S-doped carbon sources for the preparation of S-doped GQDs. The effects of coal properties on coal-based GQDs were analyzed by means of high-resolution transmission electron microscopy(HRTEM), X-ray diffraction(XRD), Fourier transform infrared(FTIR) spectroscopy, X-ray photoelectron spectroscopy(XPS), ultraviolet-visible(UV-Vis) absorption spectroscopy, and fluorescence emission spectra. It was shown that all coal samples can be used to prepare GQDs, which emit bluegreen and blue fluorescence under ultraviolet light. Anthracite-based GQDs have a hexagonal crystal structure without defects, the largest size, and densely arranged carbon rings in their lamellae; the highrank bituminous coal-based GQDs are relatively reduced in size, with their hexagonal crystal structure being only faintly visible; the low-rank bituminous coal-based GQDs are the smallest, with sparse lattice fringes and visible internal defects. As the metamorphism of raw coals increases, the yield decreases and the fluorescence quantum yield(QY) initially increases and then decreases. Additionally, the surface of GQDs that were prepared using high-rank SHOS coal(high-rank bituminous coal) preserves rich sulfur content even after strong oxidation, which effectively adjusts the bandgap and improves the fluorescence QY. Thus, high-rank bituminous coal with SHOS content can be used as a natural S-doped carbon source to prepare S-doped GQDs, extending the clean utilization of low-grade coal.

Hydrogen Bonds in Coal——The Influence of Coal Rank and the Recognition of a New Hydrogen Bond in Coal

By means of in situ diffuse reflectance FTIR, the IR spectra of 6 coals with different ranks were obtained from room temperature to 230 ℃. A new curve fitting method was used to recognize the different hydrogen bonds in the coals, and the influence of coal ranks on the distribution of hydrogen bonds(HBs) in the coals and their thermal stability were discussed. The results show that there is another new HB(around 2514 cm -1 ) between the -SH in mercaptans or thiophenols and the nitrogen in the pyridine like compounds in the coals, and the evidence for that was provided. The controversial band of the HB between hydroxyl and the nitrogen of the pyridine like compounds was determined in the range of 3028-2984 cm -1 , and the result is consistent with but more specific than that of Painter et al .. It was found that the stability of different HBs in the coals is influenced by both coal rank and temperature. For some HBs, the higher the coal rank, the higher the stability of them. Within the temperature range of our research, the stability of the HB between the hydroxyl and the π bond increases to some extent for some coals at temperatures higher than 110 or 140 ℃.

Effect of the benzene ring of the dispersant on the rheological characteristics of coal-water slurry:Experiments and theoretical calculations

In this study,low-rank coal-water slurry(LCWS)was prepared using polyoxyethylene dodecylphenol ether(PDPE)and polyoxyethylene lauryl ether(PLE),respectively.A combination of experiments and simulations was used to investigate the pulping properties and microscopic mechanism of the LCWS samples prepared using the two agents,so as to explore the influence of benzene ring on the performance of dispersant.The results of the LCWS preparation experiments revealed that the pulp-forming performance of PDPE exceeded that of PLE.When LCWS concentration is 62%,64%,and 66%,the apparent viscosity corresponding to PDPE is 247.80,504.17,and 653.10 mPa·s,and the apparent viscosity corresponding to PLE is 548.10,1470.61,and 1549.98 mPa·s,respectively.The C_(1000)(When the apparent viscosity is 1000 mPa·s,the corresponding concentration of LCWS is defined as C_(1000))values of PDPE and PLE are 67.60%and 62.95%,respectively.In addition to the van der Waals forces and hydrogen bonds between the PDPE and/or PLE molecules and coal,the benzene rings of PDPE presentπ-πstacking effect with the aromatic rings of coal.That could facilitate and strengthen the adsorption of PDPE on coal,which would be conducive to further improving the dispersion of coal particles.The two dispersants have no significant difference in effect on the pyrolysis of LCWS.The simulation results indicated that the times for PDPE and PLE molecules to reach flat adsorption state on coal are approximately 290 and 565 ps,respectively.The self-diffusion coefficient(D)of the PDPE and PLE on coal is 3.16 x 10^(-6)and6.57×10^(-6)m~2/s,respectively,and their interaction energies with coal are 785.71 and 648.60 kcal/mol,respectively.The results of the simulation calculations demonstrated that PDPE adsorbed on coal easier than PLE,and its binding is more stable than that of PLE owing to theπ-πstacking effect,which is conducive to uniform dispersion of coal in solution.The simulation results confirmed the experimental results.

Characteristics of oxygen consumption of coal at programmed temperatures

Oxygen consumption is an important index of coal oxidation.In order to explore the coal-oxygen reaction,we developed an experimental system of coal spontaneous combustion and tested oxygen consumption of differently ranked coals at programmed temperatures.The size of coal samples ranged from 0.18~0.42 mm and the system heat-rate was 0.8℃/min.The results show that, for high ranked coals,oxygen consumption rises with coal temperature as a piecewise non-linear process.The critical coal temperature is about 50℃.Below this temperature,oxygen consumption decreases with rising coal temperatures and reached a minimum at 50℃,approximately.Subsequently,it begins to increase and the rate of growth clearly increased with temperature.For low ranked coals,this characteristic is inconspicuous or even non-existent.The difference in oxygen consumption at the same temperatures varies for differently ranked coals.The results show the difference in oxygen consumption of the coals tested in our study reached 78.6%at 100℃.Based on the theory of coal-oxygen reaction,these phenomena were analyzed from the point of view of physical and chemical characteristics,as well as the appearance of the coal-oxygen complex.From theoretical analyses and our experiments,we conclude that the oxygen consumption at programmed temperatures reflects the oxidation ability of coals perfectly.

Cation exchange for mercury and cadmium of xanthated,sulfonated,activated and non-treated subbituminous coal,commercial activated carbon and commercial synthetic resin:effect of pre-oxidation on xanthation of subbituminous coal

A subbituminous coal was oxidized with air at 150℃on a fixed bed for 4 h and xanthated with carbondisulfide in a basic solution,at 30 or 5-10℃.This xanthated coal was evaluated for the removal of Hg^(2+)and Cd^(2+)from 7,000 mg/L aqueous solutions;metal concentrations were detemmined by atomic absorption spectrometry.The ionexchange of the xanthated coal was compared against those of the original subbituminous coal,a sulfonated subbituminouscoal,activated carbon,commercial activated carbon,and commercial synthetic resin.The commercial synthetic resinshowed the highest exchange capacity(concentration factor 98%)followed by the xanthated coal(concentration factor 96%).The retention of cadmium on the sulfonated subbituminous coal was lower(exchange capacity 0.56 meq/g)thanthat of xanthated coals(1.85±0.09 meq/g).Our xanthated coal showed a better Cd^(2+)removal(8l%o against 15%)than anon preoxidized 40-h-xanthated coal,which shows that oxidation of coal increased the amount of oxygenated groups whichenhanced xanthation.

Investigation of polycyclic aromatic hydrocarbons from coal gasification

The hazardous organic pollutants generated from coal gasification, such as polycyclic aromatic hydrocarbons(PAHs), are highly mutagenic and carcinogenic. More researchers have paid particular attention to them. Using air and steam as gasification medium, the experiments of three kinds of coals were carried out in a bench scale atmospheric fluidized bed gasifier. The contents of the 16 PAHs specified by US EPA in raw coal, slag, bag house coke, cyclone coke and gas were measured by HPLC to study the contents of PAHs in raw coal and the effects of the inherent characters of coals on the formation and release of PAHs in coal gasification. The experimental results showed that the distributions of PAHs in the gasified products are similar to raw coals and the total\|PAHs content in coal gasification is higher than in raw coal(except Coal C). The total\|PAHs contents increase and then decrease with the rise of fixed carbon and sulfur of coal while there has an opposite variation when volatile matters content increase. The quantities of PAHs reduce with the increase of ash content or the drop of heating value during coal gasification.

Volume and Surface Distribution Heterogeneity of Nano-pore in Coal Samples by CO2 and N2 Adsorption Experiments

In this paper,the heterogeneity of adsorption pores in middle and high rank coal samples were analyzed by using low temperature N2 and CO2 adsorption technology and fractal theory.The following results were achieved.1)According to the results of volume and surface fractal dimension,meso-pores can be classified into Mep-1,Mep-2,and Mep-3,respectively.Micro-pore can be classified into Mip-1,Mip-2,and Mip-3,respectively.2)Pore types play an important role in affecting the heterogeneity of meso-pores.The volume heterogeneity(VHY)of Mep-1 is simpler than that of Mep-2 and Mep-3 in type A samples.However,the VHY of Mep-1 becomes gradually larger than that of Mep-2 and Mep-3 from type A to type B and C.The VHY of open pore in the same diameter is higher than that of semi-open or closed pore.Meanwhile,the surface heterogeneity(SHY)of types A and B samples is significantly larger than that of type C,the SHY of semi-open or closed pores is more complicated than that of open pores.3)Coal rank mainly affects the heterogeneity of micro-pores.The heterogeneity of type A is always smaller than that of type B and C.The VHY of Mip-1 is more complicated than that of Mip-2 and Mip-3 in the same samples,and the sensitivity of the VHY of Mip-1 and Mip-2 to the degree of coal rank is smaller than that of Mip-3.Meanwhile,the SHY of Mip-1 and Mip-2 is simpler than that of Mip-3 in the same sample,the SHY of micro-pores remains stable as the pore size decreases,and the affect of coalification level on SHY decreases with the decrease in pore diameter.Full-scale fractal characterization has enabled quantitative characterization of adsorption pore properties and provided useful information with regards to the similarity of pore features in different coal reservoirs.

Coalbed methane desorption characteristics controlled by coalification and its implication on gas co-production from multiple coal seams

In this work,CH4 isothermal adsorption measurements were carried out on 64 coal samples collected from western Guizhou Province of China,and the coalbed methane(CBM)desorption processes were quantitatively analyzed.The results show that the Langmuir volume and the Langmuir pressure are controlled by coalification,and tend to increase as the vitrinite reflectance changes from 0.98% to 4.3%.Based on a division method of CBM desorption stages,the CBM desorption process were divided into four stages(inefficient,slow,fast and sensitive desorption stages)by three key pressure nodes(the initial,turning and sensitive pressures).The fast and sensitive desorption stages with high desorption efficiency are the key for achieving high gas production.A theoretical chart of the critical desorption pressure(P_(cd))and its relationship with different pressure nodes was established.The higher-rank coals have the higher initial,turning and sensitive pressures,with larger difference between pressure nodes.Most CBM wells only undergo partial desorption stages due to the differences in P_(cd) caused by the present-gas content.Under the same gas content conditions,the higher the coal rank,the less desorption stages that CBM needs to go through.During coalbed methane co-production from multiple coal seams within vertically superposed pressure systems,the reservoir pressure,the P_(cd),the initial working liquid level(WLL)height,and coal depth are key factors for evaluating whether coal seams can produce CBM simultaneously.It must be ensured that each production layer enters at least the fast desorption stage prior to that the WLL was lower than the depth of each layer.Only on this basis can all layers achieve the maximum gas production.

Coal Seam Correlation in Terrestrial Basins by Sequence Stratigraphy and Its Implications for Paleoclimate and Paleoenvironment Evolution

Coal correlation cannot only guide coal and coalbed methane exploitation,but may also provide valuable information for the evolution of the paleoclimate and paleoenvironment during coalaccumulation periods.It is difficult to undertake coal correlation in a terrestrial basin due to multiple superimposed coal seams and a lack of effective marker beds.In this study,based on the analyses of coal-bearing sedimentary environments and the observations of field outcrops and drilling cores,three key sequence surfaces of the Middle Jurassic Xishanyao Formation were identified in the middle part of the southern Junggar Basin.Three third-order sequences(S.Ⅲ1,S.Ⅲ2 and S.Ⅲ3)and ten fourth-order sequences were divided within the Xishanyao Formation.Regional coal correlation was finally identified,combined with the layered vertical pattern coal and its interval thickness,coal maceral,and coal quality data.In terms of the coal correlation within a specific sequence set,the interval of the B2–B3 coals was approximately three times thicker than that of the B3–B5 coals,which was the main marker for regional coal correlation in the Lower Xishanyao Formation.Furthermore,the phenomena of steadily developed B4 coal seam(2–3 m in thickness)across the Liuhuanggou coal mining area(eastern study region)and the large-small-large variation for intervals between the B8 and B9,B9 and B10,and B10 and B11 coal seams in the Manasi coal mining(western study region)were also an effective means for local coal correlation.Increasing vitrinite and sulphur content and decreasing inertinite content in a third-order sequence were found,which were likely to be closely related to lake level fluctuations.Based on the variation of coal macerals,the wildfire frequency gradually decreased from the bottom to the top of the Xishanyao Formation.The predicted O2level of the S.Ⅲ1,S.Ⅲ2,and S.Ⅲ3 of the Xishanyao Formation was 26.36%,24.22%,and 22.51%,respectively.Except for global paleoclimate effects,the provenance direction changes caused by the Bogda Mountains uplifts would result in a regular variation of coal maceral and coal quality across the study region.

Coalbed methane enrichment model of low-rank coals in multi-coals superimposed regions: a case study in the middle section of southern Junggar Basin

The Middle Jurassic Xishanyao Formation in the central section of the southern Junggar Basin has substantial amounts of low-ranked coalbed methane(CBM)recourses and is typically characterized by multi superimposed coal seams.To establish the CBM enrichment model,a series of experimental and testing methods were adopted,including coal maceral observation,proximate analysis,low temperature nitrogen adsorption(LTNA),methane carbon isotope determination,porosity/permeability simulation caused by overburden,and gas content testing.The controlling effect of sedimentary environment,geological tectonic,and hydrogeological condition on gas content was analyzed in detail.The results demonstrate that the areas with higher gas content(an average of 8.57 m3/t)are mainly located in the Urumqi River-Santun River(eastern study area),whereas gas content(an average of 3.92 m3/t)in the Manasi River-Taxi River(western study area)is relatively low.Because of the combined effects of strata temperature and pressure,the gas content in coal seam first increases and then decreases with increasing buried depth,and the critical depth of the inflection point ranges from 600 m to 850 m.Affected by the changes in topography and water head height,the direction of groundwater migration is predicted from south to north and from west to east.Based on the gas content variation,the lower and middle parts of the Xishanyao Formation can be divided into three independent coalbearing gas systems.Within a single gas-bearing system,there is a positive correlation between gas content and strata pressure,and the key mudstone layers separating each gas-bearing system are usually developed at the end of each highstand system tract.The new CBM accumulation model of the multi-coals mixed genetic gas shows that both biological and thermal origins are found in a buried depth interval between 600 m and 850 m,suggesting that the coals with those depths are the CBM enrichment horizons and favorable exploration regions in the middle section of the southern Junggar Basin.An in-depth discussion of the low-rank CBM enrichment model with multi-coal seams in the study region can provide a basis for the optimization of CBM well locations and favorable exploration horizons.