Coal-Gas Interaction And Its Influence On Gas Migration Under Multi-field Conditions

With the depletion of shallow resources,more and more coal mines in our country turn to deep mining and face the problem of gas treatment under complex multi-field conditions(high ground temperature,high gas pressure,high ground pressure),so it is difficult to ensure safe production.Taking different ash coal samples as the research object,through laboratory experiments,theoretical analysis,and numerical simulation,this paper studies the coal-gas interaction and its influence on gas migration under multi-field conditions as follows:A micro observation device for coal gas adsorption and expansion deformation is developed.The device can study the adsorption expansion deformation characteristics of coal rock gas under multi-field coupling conditions.According to the form characteristics of the output data of the device,the device is based on digital image processing technology.With the help of MATLAB platform,the technology of extracting the information of adsorption expansion is developed.The combination of the two methods can effectively avoid the drawbacks of the existing strain measurement methods(such as the need for skilled manual bonding of strain gauges and bonding of glue resulting in surface cracks being fixed or even blocked by glue and strain gauges).It is helpful to understand the characteristics of adsorption-induced dilatation deformation under multi-physical field coupling conditions.This graphics processing technology also has a broad prospect of engineering application in other fields.The influence of different gas pressure,temperature and displacement constraints on gas adsorption-induced strain in different ash-bearing coal is studied by using microscopic observation device of coal-rock gas adsorption-swelling deformation.The adsorption expansion strain of coal sample under the same gas pressure is divided into 3 stages: rapid growth stage,slow growth stage,and equilibrium stage.The adsorption-swelling strain data of coal sample under different gas pressure can be fitted with Langmuir adsorption isotherm.The same swelling volume corresponds to different expansion space,which shows the anisotropy of the adsorption-induced strain.The adsorption-induced strain of different temperature showed a slight downward trend with temperature rise.In contrast to the adsorption expansion experiments with displacement constraints,some of the outward expansion which is originally in some direction is transformed into the extruding fissure pore space in the coal matrix,until the closed,the excess energy is not completely dissipated and work in the direction of the free surface.Under the condition of low pressure,the expansion is less than the free expansion,the gas pressure is increased,and the deformation results are close.The relationship between pore fissure and permeability in the existing equivalent model is analyzed.It is shown that the change of permeability is represented by the variation of pipe diameter and porosity of the connected pipeline and combined with the analysis of pore characteristics of coal with different ash content.The following fine particles of different ash coal samples are observed under high power microscope.The results show that the two have the same appearance,most of them are smooth and fine ellipsoidal particles and spherical particles.Based on the above experimental results,the models of equal diameter sphere without overlap random stacking model,random diameter sphere without overlap accumulation model and sphere random overlapping packing model are compared and realized.It is concluded that the process method has the advantage of good connectivity to simple porous media,but the pore fracture structure of coal is complex,and the particle accumulation model can not well characterize the migration channel of coal bed methane body.An improved stochastic growth method based on four-parameter growth method is used to establish a model for the characterization of pore fractures in coal and rock.In this paper,the geometric representation model of pore fracture of coal and rock established by MATLAB and the finite element software COMSOL with multi-field coupling is used to realize the data transfer and interaction,and the seepage simulation experiment is carried out under the condition of multi-field coupling.The flow path and flow process of solid-heat flow two-phase fluid on pore-scale are simulated,and the flow path and flow process are reproduced on micro-scale.It is concluded that due to displacement constraints,the coal matrix expands inwardly,compresses the pore space of fracture,the velocity of fluid in the channel decreases and the permeability changes.The permeability is controlled by a small number of pore fissure channels with large pore size,and the velocity of fluid migration in small pore channels is very low.