| Nowadays,the theoretical research and exploration and development for medium-high rank coal have been very mature in China.Although there is a considerable gas reserves of low rank coal in China,its exploration and development is still in the research stage.Low rank coal is different from the medium-high rank coal in physical property,chemical structure and pore-fracture properties,and also has the characteristics of high water content,resulting in the significant differences with medium-high rank coal in rock mechanics properties and gas-solid interaction.Taking the coal reservoir of low rank coal(Ro,max=0.65%)in Yuanzigou mine as the research background,and aiming at the characteristics of "high gas pressure and low gas content" and high inherent moisture content of 2# coal seam,the paper mainly studied the effects of moisture on the gas adsorption and desorption properties,and the characteristics of mechanics damage and permeability evolution,then analyzed the influence of inherent moisture on coal seam gas drainage.The main conclusions are as follows:1)The low rank coal has more oxygen-containing functional group and developed micro-pores system,which leads to the water-holding capacity of the low rank coal in Yaozigou mine was significantly higher than that of the tested medium-high rank coal samples,and the highest moisture content reaches 9.91%.Moisture has significant effect on adsorption capacity of Low rank coal,so that the Langmuir volume falls from 39.97cm3/g in dry coal to 12.51cm3/g in the sample with moisture content of 6.90% at a rate of 68.7%,which is related to the strong water-holding capacity of low rank coal.Compared to the traditional linear equation,the relationships between wet coal adsorption capacity and water content can better described by negative exponential.The change of diffusion coefficient of the low-rank coal with moisture content exhibits the law of ascending and descending alternations,which is controlled by the blocking effect from moisture on gas migration,and the reduction of gas adsorption amount.2)In the conventional triaxial compression test,without gas,the strength of briquette coal and raw coal show exponential and nearly linear decrease trend,respectively;the cohesion of raw coal is 3.49~ 7.93 MPa,and it is about 6 times than these of briquette coal.With increasing of moisture content,the change of internal friction angle is not obvious,and the cohesion shows significant decreasing trend,therefore,the reduction in cohesion is the mean reason of the reduction of coal strength.In the gas,water coexistence system,the adsorption of gas and moisture in coal caused the reduction of coal surface energy,resulting in the reduction of coal strength.When gas pressure is constant,the increase of moisture weakens the influence of gas adsorption effect on coal strength,resulting in the softening coefficient of coal due to gas adsorption effect shows increase trend.3)Based on the Drucker-Prager strength criterion and the distribution of the Weibull function,a mesoscopic statistical damage model of coal containing gas is established.This model can retrieve the full stress-strain relations well,and reflect chanracteristics of the induction and promotion of coal damage due to the water and gas.Water has more obvious inhibitory effect on the acoustic emission(AE)than gas;compare to dry coal,the AE accumulative count and AE energy value decreased by 14.32 times and 23.42 times in water-saturation coal than these in dry coal.And with the increase of water content,the rupture of coal shows that the change from a sudden rupture type to stable rupture type and the trend of weakened brittle and enhanced plastic failure feature.4)The evolution of coal permeability is controlled by the swelling effect of coal matrix due to the adsorption of gas and moisture,as well as effective stress.Under the equal confining pressure of the hydrostatic pressure,the permeability of coal with high inherent moisture content shows a gradually increasing trend with the increasing of gas pressure,which is related to the gas adsorption swelling effects of coal becomes weak due to the adsorption of moisture.Then the effective stress is control factor.In the complete stress-strain process,acoustic emission and damage variable of coal containing gas and water is conformity with the permeability evolution.In compression and elastic stage,AE activity calms and damage variable is 0,and the permeability decreased by severals times;in the post-peak stage,AE activity is quite lively,and permeability of dry and wet coal surge 2~3 orders of magnitude,and the damage variable is increasing rapidly,and close to 1.5)Based on the assumption of uniaxial strain,an improved SD model is proposed to characterize the evolution of coal permeability in the elastic stage.The calculation of the SD model shows that the ultimate expansion deformation of low rank coal is negative exponential relationship with water content;combined with the relationships between adsorption,coal deformation,coal permeability and moisture content,the improved SD model can be used to retrieve the permeability data under hydrostatic pressure,and can reflect the dynamic evolution law of coal permeability under different water content.In the complete stress-strain process,combined with the mesoscopic statistical damage model and the SD improved model,and bringing the damgage variable and the surge coefficient of permeability,under the stress perturbation effect,the permeability model and considering the moisture effect is constructed.6)Based on the elastic medium characteristics of dual-porosity and single permeability for coal body,and combined with the diffusion equation,fracture seepage equation,improved SD model and deformation equation,the coal seam gas migration model under coal,gas and water coupling effect is developed.The COMSOL software calculation results show that,when the initial gas content is fixed,the influence of adsorption capacity of coal and the initial permeability on gas migration in coal seam and the residual coal seam gas content is the maximum.Therefore,for in-situ low rank coal reservoir,the initial permeability is certain,and inherent moisture content is so high that the adsorption capacity of coal is low,and the proportion of free gas content increase and gas pressure is large,which has positive effect on coal seam gas drainage. |
PDF Full Text Request