Deformation Characteristics Of Coal Matrix And Energy Evolution Mechanism Of Reservoir During CBM Drainage

In the process of coalbed methane(CBM)drainage,reservoir energy is the driving force of fluid production and is affected by many parameters.Revealing the dynamic change law and mechanism of reservoir energy in the process of drainage is of great significance to the optimization of drainage system of CBM wells.This thesis took the coal reservoir in Yuwang Block of eastern Yunnan as the research object.According to the drainage stages of CBM wells,different drainage stages were simulated by displacement experiments on the basis of clarifying the geological background and reservoir physical properties of the study area.The relationship between deformation of coal matrix at different stages and time,flooding pressure,flow rate,permeability and fluid content was systematically studied.The mathematical model of gas elastic energy was improved.Based on the stress analysis of coal sample,a mathematical model of elastic energy transformation of coal matrix in the process of drainage was established.The gas elastic energy,elastic energy and swelling deformation energy of coal matrix in the process of drainage were calculated,and the variation law between them and each parameter was studied.The linkage relationship between coal reservoir energy,drainage control parameters and internal parameters of coal reservoir was revealed.It is found that flooding pressure plays a dual role.It can promote fluid migration and inhibit fluid desorption.The change of coal matrix deformation rate with time conforms to Langmuir's law under the same flooding pressure.Within the experimental range,the deformation rate of coal matrix increases linearly with the increasing flooding pressure.When the fluid content is within a certain range,the relationship between flooding pressure and deformation rate of coal matrix is not obvious.Since then,the rate of deformation rate of coal matrix increases with increasing flooding pressure during nitrogen flooding,and it is opposite during water flooding.In the process of nitrogen flooding,the deformation rate of coal matrix in each stage is similar,and permeability increases with displacement pressure.In the process of water flooding,the deformation rate of coal matrix is absolutely dominant in the initial stage of water flooding,and permeability decreases first and then increases with flooding pressure.In the process of water flooding gas and gas flooding water,the deformation rate of coal matrix and the change rule of parameters are consistent with that of flooding fluid alone,and the correlation with the change rule of displaced fluid is relatively weak.According to the deformation law of coal matrix under flooding pressure,a mathematical model of gas pressure and water pressure in reservoir pressure was proposed preliminarily when gas and water coexist.Gas elastic energy is mainly affected by flooding pressure in the process of CBM drainage.Free,adsorbed and total gas elastic energy increase with increasing flooding pressure.Adsorbed gas elastic energy is much higher than free gas elastic energy.The release of gas elastic energy can promote the fluid migration in pore and fissure,and the swelling deformation of coal matrix can inhibit the fluid migration.The swelling deformation energy of coal matrix is one of the elastic energies of coal matrix.They have opposite trend with time,water saturation,coal matrix deformation rate and flooding pressure.Permeability increases with increasing elastic energy of coal matrix and decreasing swelling deformation energy of coal matrix under the same flooding pressure.Permeability is significantly affected by displacement pressure under different flooding pressures.The change of internal parameters of coal reservoir is affected by the bottom hole pressure difference.The change of internal parameters of coal reservoir drives the constant change of energy of coal reservoir,and then affects the output of gas and water,which in turn affects the internal parameters of coal reservoir.In the process of drainage,reservoir energy and reservoir parameters are interrelated and inseparable,and are regulated by bottom hole pressure difference as a whole.Based on the above understanding,a linkage model between coal reservoir energy,internal parameters of coal reservoir and control parameters of drainage was constructed.There are 94 figures,12 tables and 176 references in this thesis.