Mechanical criterion for coal and gas outburst:a perspective from multiphysics coupling

Although a series of hypotheses have been proposed,the mechanism underlying coal and gas outburst remains unclear.Given the low-index outbursts encountered in mining practice,we attempt to explore this mechanism using a multiphysics coupling model considering the effects of coal strength and gas mass transfer on failure.Based on force analysis of coal ahead of the heading face,a risk identification index C_(m)and a critical criterion(C_(m)≥1)of coal instability are proposed.According to this criterion,the driving force of an outburst consists of stress and gas pressure gradients along the heading direction of the roadway,whereas resistance depends on the shear and tensile strengths of the coal.The results show that outburst risk decreases slightly,followed by a rapid increase,with increasing vertical stress,whereas it decreases with increasing coal strength and increases with gas pressure monotonically.Using the response surface method,a coupled multi-factor model for the risk identification index is developed.The results indicate strong interactions among the controlling factors.Moreover,the critical values of the factors corresponding to outburst change depending on the environment of the coal seams,rather than being constants.As the buried depth of a coal seam increases,the critical values of gas pressure and coal strength decrease slightly,followed by a rapid increase.According to its controlling factors,outburst can be divided into stress-dominated,coal-strength-dominated,gas-pressure-dominated,and multi-factor compound types.Based on this classification,a classified control method is proposed to enable more targeted outburst prevention.

Pore size distribution of high volatile bituminous coal of the southern Junggar Basin: a full-scale characterization applying multiple methods

Studying on the pore size distribution of coal is vital for determining reasonable coalbed methane development strategies.The coalbed methane project is in progress in the southern Junggar Basin of northwestern China,where high volatile bituminous coal is reserved.In this study,with the purpose of accurately characterizing the full-scale pore size distribution of the high volatile bituminous coal of the southern Junggar Basin,two grouped coal samples were applied for mercury intrusion porosimetry,low-temperature nitrogen adsorption,low-field nuclear magnetic resonance,rate-controlled mercury penetration,scanning electron microscopy,and nano-CT measurements.A comprehensive pore size distribution was proposed by combining the corrected mercury intrusion porosimetry data and low-temperature nitrogen adsorption data.The relationship between transverse relaxation time(T2,ms)and the pore diameter was determined by comparing the T2 spectrum with the comprehensive pore size distribution.The macro-pore and throat size distributions derived from nano-CT and rate-controlled mercury penetration were distinguishingly analyzed.The results showed that:1)comprehensive pore size distribution analysis can be regarded as an accurate method to characterize the pore size distribution of high volatile bituminous coal;2)for the high volatile bituminous coal of the southern Junggar Basin,the meso-pore volume was the greatest,followed by the transition pore volume or macro-pore volume,and the micro-pore volume was the lowest;3)the relationship between T2 and the pore diameter varied for different samples,even for samples with close maturities;4)the throat size distribution derived from nano-CT was close to that derived from rate-controlled mercury penetration,while the macro-pore size distributions derived from those two methods were very different.This work can deepen the knowledge of the pore size distribution characterization techniques of coal and provide new insight for accurate pore size distribution characterization of high volatile bituminous coal.

Experimental and numerical study of coal mechanical properties during coalification jumps

The mechanical characteristics of coal reservoirs are important parameters in the hydraulic fracturing of coal.In this study,coal samples of different ranks were collected from 12 coal mines located in Xinjiang and Shanxi,China.The coal ranks were identified with by the increased Maximum vitrine reflectance(Ro,max)value.The triaxial compression experiments were performed to determine the confining pressure effect on the mechanical properties of coal samples of different ranks.The numerical approaches,including the power function,arctangent,and exponential function models,were used to find the correlation between coal elastic modulus and the confining pressure.The fitting equations of compressive strength and elastic modulus of coal ranks were constructed under different confining pressures.The results showed that the coal compressive strength of different ranks has a positive linear correlation with the confining pressure.The coal elastic modulus and confining pressure showed an exponential function.Poisson’s ratio of coal and confining pressure show negative logarithmic function.The stress sensitivity of the coal elastic modulus decreases with the increase of confining pressure.The coalification jump identifies that the compressive strength,elastic modulus,and stress sensitivity coefficient of coal have a polynomial relationship with the increase of coal ranks.The inflection points in coalification at Ro,max=0.70%,1.30%,and 2.40%,are the first,second,and third coalification jumps.These findings provide significant support to coal fracturing during CBM production.

Fractal classification and natural classification of coal pore structure based on migration of coal bed methane

According to the data of 146 coal samples measured by mercury penetration, coal pores are classified into two levels of 【65 nm diffusion pore and 】65 nm seeping pore by fractal method based on the characteristics of diffusion, seepage of coal bed methane(CBM) and on the research results of specific pore volume and pore structure. The diffusion pores are further divided into three categories: 【8 nm micropore, 8-20 nm transitional pore, and 20-65 nm mini-pore based on the relationship between increment of specific surface area and diameter of pores, while seepage pores are further divided into three categories: 65-325 nm mesopore, 325-1000 nm transitional pore, and 】1000 nm macropore based on the abrupt change in the increment of specific pore volume.

Multi-Layer Superposed Coalbed Methane System in Southern Qinshui Basin, Shanxi Province, China

Multi-coalbed developed in Carboniferous–Permian coal-bearing strata of southern Qinshui Basin, and different coal-bearing segments have different coalbed methane（CBM） reservoiring characteristics. Analysis of previous studies suggests that the essence of an unattached CBM system is to possess a unified fluid pressure system, which includes four key elements, namely, gas-bearing coal-rock mass, formation fluid, independent hydrodynamic system and capping layer condition. Based on the exploration and exploitation data of CBM, it is discovered that the gas content of coal seams in southern Qinshui Basin presents a change rule of non-monotonic function with the seam dipping, and a turning point of the change appears nearby coal seam No. 9, and coal seams of the upper and the lower belong to different CBM systems respectively; well test reservoir pressure shows that the gradient of coal seam No. 15 of the Taiyuan Formation is significantly higher than that of coal seam No. 3 of the Shanxi Formation; the equivalent reservoir pressure gradient of coal seam No. 15 ＂jumps＂ obviously compared with the reservoir pressure gradient of coal seam No. 3 in the same vertical well, that is, the relation between reservoir pressure and burial depth takes on a characteristic of nonlinearity; meanwhile, the vertical hydraulic connection among the aquifers of Shanxi Formation and Taiyuan Formation is weak, constituting several relatively independent fluid pressure systems. The characteristics discussed above reveal that the main coal seams of southern Qinshui Basin respectively belong to relatively independent CBM systems, the formation of which are jointly controlled by sedimentary, hydrogeological and structural conditions.

A review on transport of coal seam gas and its impact on coalbed methane recovery

This paper presents a summary review on mass transport of coal seam gas(CSG)in coal associated with the coalbed methane(CBM)and CO_(2) geo-sequestration enhanced CBM(CO_(2)-ECBM)recovery and current research advances in order to provide general knowledge and fundamental understanding of the CBM/ECBM processes for improved CBM recovery.It will discuss the major aspects of theory and technology for evaluation and development of CBM resources,including the gas storage andflow mechanism in CBM reservoirs in terms of their differences with conventional natural gas reservoirs,and their impact on CBM production behavior.The paper summarizes the evaluation procedure and methodologies used for CBM exploration and exploitation with some recommendations.