Pore Fissure Characteristics And Methane Adsorption Law Of Lean Coal Under Different Stresses

The pore and fissure characteristics and methane adsorption laws of lean coal and its molecular unit cells under different stresses were studied by a combination of laboratory experiments and molecular simulations.The pore and fissure characteristics of lean coal primary structural coal and tectonic coal under different stress were experimentally studied by electro-hydraulic servo press,acoustic emission data acquisition instrument,three-dimensional X-ray imaging Micro-CT system,rock acoustic wave parameter tester,and low-field nuclear magnetic resonance imaging analyzer.The pore and fissure characteristics and methane adsorption law of primary structural coal raw coal pillar,primary structural coal briquette coal pillar and tectonic coal briquette coal pillar with adsorption time were studied by low-field nuclear magnetic resonance analyzer experiments under different methane pressures.Based on proximate analysis,ultimate analysis,nuclear magnetic resonance carbon spectrum analysis,and X-ray photoelectron spectroscopy analysis experiments,the macromolecular structure models of primary structural coal and tectonic coal were constructed.Based on zeo++-0.3 code calculation and Materials Studio software simulation,the pore characteristics and methane adsorption laws of primary structural coal and tectonic coal molecular unit cells under different triaxial stress and uniaxial stress,and the relationship between them were studied.The main conclusions are as follows:（1）The pore and fissure characteristics of lean coal under different stresses are obtained by experimental research.It is obtained that the peak stress of raw coal pillar of primary structure is about 3.7 MPa,the corresponding strain is about 0.8%,and the maximum strain is about 1.5%.The stress is large and the deformation capacity is small.After the raw coal pillar of primary structure reaches the stress peak,the acoustic emission response begins to strengthen,the peak value of ring count is about 5500,and the peak energy is about 40 V·ms.After fracturing,the primary structure coal raw pillar mainly produces crack propagation,which is sudden.The crack process caused by loading is in good agreement with the ring count and energy generated by acoustic emission.Micro-CT scanning experiment shows that when the raw coal block of primary structure coal is in different stress stages,the rime pore diameter on the pore fissure image first decreases and then increases,and cracks occur.The volume and porosity of total pore first decrease and finally increase,and the number,surface area,volume and pore of closed pore first increase,then decrease and finally increase.The order of P-wave velocity from large to small is as follows:primary structure coal raw pillar>primary structure coal raw pillar after fracturing>structural coal briquette pillar>structural coal briquette pillar after fracturing.The damage values of primary structure coal raw pillar and structural coal briquette pillar after fracturing are 0.527 and 0.647 respectively.The pores and fissures of primary structure coal raw pillar are not developed,and the damage value is the small.The nuclear magnetic resonance imaging experiment showed that some cracks occurred after the fracturing of the original coal pillar of the primary structural coal;the area of pore and fracture increased after the fracturing of the tectonic coal briquette.（2）The pore fissure characteristics and methane adsorption law of lean coal under different methane pressure were obtained.The results show that with the increase of methane pressure,the total area and 1#peak area of T₂ spectrum of primary structure coal raw pillar,primary structure coal briquette pillar and structural coal briquette pillar increase according to the Langmuir curve,and the peak position increases to a certain extent;During methane desorption,residual methane is mainly adsorbed in small pore.The porosity occupied by the total methane limit adsorption capacity of primary structure coal raw pillar,primary structure coal briquette pillar and structural coal briquette pillar are 8.156%,111.143%and 142.828%respectively;The porosity occupied by the methane limit adsorption capacity of 1#peak is 3.83%,5.355%and 9.448%respectively;The final area proportion of the 1#peak is 83%,70%and 75%respectively,and the final area proportion of the 2#peak is 9%,23%and 20%respectively;T₁-T₂ spectrum shows that the mobility of methane is low,high and medium respectively.The pores and fissures of primary structure coal are underdeveloped,especially the mesopores and macropores are not developed,the limit adsorption capacity of methane is low,and the methane adsorption capacity of mesopores is also low;The pores and fissures of structural coal are well developed,the mesopores and large pores are developed to a certain extent,and the limit adsorption capacity of methane is large.The methane adsorption capacity of mesopores also increases with the increase of methane pressure,which is prone to damage.（3）Based on proximate analysis,ultimate analysis,nuclear magnetic resonance carbon spectroscopy analysis and X-ray photoelectron spectroscopy analysis,the macromolecular models of primary structure coal and structural coal were constructed,and the pore characteristics and methane adsorption law of coal molecular unit cells under different triaxial and uniaxial stresses were simulated.With the increase of triaxial stress,the pore volume,cavity diameter,accessible surface area,pore size distribution and the proportion of large pores constrained by random ray tracing of primary structural coal and structural coal decrease and the density increases;When the triaxial stress of 0.4 GPa is removed,the cell pore development of primary structural coal is slightly lower than that of structural coal under the same triaxial stress.With the increase of uniaxial stress,the pore volume,cavity diameter,accessible surface area,pore size distribution and the proportion of large pores constrained by random ray tracing first decrease and then increase,and the density first increases and then decreases;Under the same uniaxial stress,the unit cell pore development of primary structural coal is lower than that of structural coal.With the increase of triaxial stress,the ultimate methane adsorption capacity of primary structural coal and structural coal decreases from rapid to slow;When 0.4 GPa triaxial stress is removed,the methane adsorption capacity of primary structural coal is lower than that of structural coal under the same triaxial stress.With the increase of uniaxial stress,the methane limit adsorption capacity of primary structural coal and structural coal first decreases and then increases,and the methane limit adsorption capacity of primary structural coal is less than that of structural coal.The relationship between the pore characteristics of molecular cells of primary structural coal and structural coal and the ultimate adsorption capacity of methane under different triaxial and uniaxial stresses is obtained.The density,pore size distribution,constrained random ray tracing,pore volume occupied by probe molecules and the accessible pore volume variation law of the unit cell model with stress are very similar to that of the ultimate adsorption capacity of methane with stress;The global maximum cavity diameter,maximum cavity diameter,cavity limit diameter and accessible surface area variation law of the unit cell model with stress are consistent with the law of methane limit adsorption with stress;Compared with structural coal,the pore development and methane limit adsorption capacity of primary structural coal are lower.The experimental results of pore fissure development and methane adsorption of primary structural coal and structural coal under stress are consistent with the simulation results.The research conclusion has important theoretical guiding significance for improving gas drainage and disaster control.