Permeability Control Mechanism And Potential CBM Recovery Of CO₂-ECBM In Steeply-Dipping Coalbed Methane Reservoirs

Coalbed methane(CBM)resource is rich in Junggar basin,China,accounting for about 26%of the country's CBM reserves.The efficient development of CBM Resources in Xinjiang is of great significance to ensure energy supply.The coalbed methane reservoir in Fukang,jungnan basin of Xinjiang,has tcharacteristics of large dip angle,large thickness and low mechanical strength.The initial parameters such as pressure,permeability,temperature and stiffness of the reservoir tend to change sharply in a limited range.Based on the background of CO₂ Enhanced coalbed methane(CO₂-ECBM)in the large dip and thick coal reservoir in Fukang mining area,the paper studies the mechanism of controlling the permeability of coal seam caused by the change of water content and CO₂ injection in the reservoir by means of literature investigation,laboratory test,theoretical analysis modeling and numerical calculation,and then evaluates the reservoir productivity and CO₂ sealing effect,and provides the implementation of CO₂-ECBM in the future It provides a theoretical basis.The main work and achievements are as follows:(1)The strength deformation,acoustic emission and permeability evolution of coal samples with different bedding inclination angles under uniaxial/triaxial conditions are studied.The relationship between confining pressure and coal sample stiffness is discussed and corresponding mathematical model is established.The spatial distribution characteristics of initial stiffness of coal reservoirs with large dip angle and the influence mechanism of dynamic evolution of reservoir stiffness caused by pressure change on permeability are revealed.(2)The stiffness and additional matrix deformation caused by the change of water content of reservoir are studied experimentally.The theoretical models are established to describe the quantitative relationship between water saturation and elastic modulus and adsorption deformation.The influence of coal stiffness weakening on permeability evolution was analyzed by the theory.Finally,the mechanism of the influence of fluid system evolution on permeability is revealed.(3)Based on the dynamic changes of coal stiffness and additional coal matrix strain caused by the change of reservoir fluid system and stress environment in the process of CO₂-ECBM,the coupling equation of seepage stress heat transfer multi physical field is established.By numerical calculation software COMSOL,production capacity and CO₂ storage in CO₂ flooding and stimulation of large dip and thick coal reservoirs are simulated by multi physics.The evolution characteristics of permeability,reservoir pressure,gas content and water saturation tend to be upward.The sensitivity analysis of geological characteristics(buried depth,dip angle and initial water saturation),production process parameters(injection pressure and injection temperature)is carried out Finally,the well location optimization is carried out,which provides suggestions for the implementation of the field project.There are 81 figures,12 tables and 92 references in this paper.