Study On The Adsorption-permeability Mechanism Of Coal And Rock Under The Action Of Temperature And Stress

Coalbed methane(CBM)is a co-product of coal formation,and it is also a kind of clean energy.Strengthening the development of CBM has important strategic value and economic significance.The adsorption/desorption amount and permeability are important parameters reflecting the output capacity and extraction efficiency of coalbed methane wells,which have certain reference significance for judging the difficulty degree of coalbed methane migration.With the normalization of deep resource mining,problems such as high reservoir pressure,high temperature,and high in-situ stress have exacerbated the complexity of adsorption and seepage evolution mechanisms.In addition,in order to further improve the efficiency of CBM development,coal seam heat injection has gradually become the key research direction of stimulation technology.Therefore,studying the adsorption and permeability characteristics of coal under the combined effects of temperature and stress is of great significance for enhancing the recovery of CBM.In this paper,the coal from Songzao mine in Chongqing and Sihe mine of the Jincheng coal factory in Shanxi were taken as the research objects.The isothermal adsorption test was carried out by using the isothermal adsorption equipment,and the seepage test under different temperature and stress conditions was carried out by using the triaxial servo-controlled seepage equipment.On this basis,the adsorption model and thermodynamic potentials equation considering the effect of temperature were established,and the expression of coal adsorption/desorption deformation and deformation under the action of stress and temperature was further derived.Based on the different boundary conditions,the coal adsorption-deformation-permeability model considering the influence of temperature and stress was established.The main research results are as follows:1)The isothermal adsorption test of coal at different temperatures was carried out to analyze the adsorption characteristics of coal under the influence of temperature and pressure.Based on the boundary conditions of constant external stress,constant pore pressure and constant effective stress,the seepage test of coal at different temperatures was carried out,and the relationship between temperature,effective stress and pore pressure and permeability was discussed,and the influence law of temperature and stress on adsorption and seepage characteristics of coal was revealed.2)Considering the influence of temperature and real gas behavior on coal adsorption capacity,the dual-site Langmuir model and L-F model were modified and compared to optimize the prediction performance of the models.On this basis,the thermodynamic potential equation of coal and gas system was further established,and the interaction between coal and gas under the action of temperature and pressure was expounded.The equivalent adsorption enthalpy was used to predict the adsorption capacity of coal,which further revealed the mutual feedback relationship between adsorption and thermodynamic properties.3)Based on the newly built adsorption model,the expression of coal deformation under the influence of temperature and stress was derived.Combined with the condition of constant external stress,the permeability model considering temperature,pore pressure and slippage effect,and the modified S&D permeability model were constructed respectively.The applicability of the permeability model was verified by the relevant tests,and the evolution law and catastrophe mechanism of coal deformation and slippage effect restricting permeability under the action of temperature and pressure were revealed.4)Based on the interaction between coal matrix and fracture,the effective deformation factor was introduced to characterize the contribution of the change of coal matrix width to the change of fracture width under the action of temperature and stress.The permeability model of coal suitable for the different boundary conditions was constructed,and verified by tests,revealing the evolution law of permeability of coal under the joint influence of temperature,stress and matrix-fracture interaction.Based on the research results,the field application of heat injection technology was discussed,which provided the theoretical basis for increasing production and optimizing benefits of CBM.