Experimental Simulation On Stress And Strain Effects As Supercritical CO₂ Being Injected Into Deep Anthracite Reservoirs

The deep anthracite reservoir, a potential geological body for CO₂ storage, is widly developed in Qinshui basin, China. CO₂ will be in supercritical state in deep coal reservoir because of high reservoir pressure and high tempreture. Coal reservoir will occour stress-strain effect during supercritical CO₂ being injected into coal seam, which affects the injectivity of coal reservoir. Laying on the independent development experimental devices to simulate CO₂ injection and enhancing CH4 recovery as fundamental innovation platforms, and considering anthracite reservoir basic geological model under conditions of temperature and pressure in the deep of Qinshui basin as study background, this paper conducted an experimental simulation on the stress-strain effect during supercritical CO₂ being injected into deep anthracite reservoirs, including adsorption and swelling, permeability variation, mechanical properties weakening, injectivity variation, and the following conclusions are obtained,（1） According to anthracite reservoir geological characteristics revealed by coal mine and CBM exploration wells, the experimental simulating conditions of stress and strain effects as supercritical CO₂ being injected into deep anthracite reservoirs are determined.（2） Desiged and developed experimental devices to simulate CO₂ injection and enhancing CH4 recovery as fundamental innovation platforms, and developed software for calculating supercritical CO₂ adsorption volume; An innovative method was designed to test volume strain during coal adsorpting; Formed the methodology of experimental stimulating stress-strain effect during supercritical CO₂ being injected into deep anthracite reservoirs.（3） The relationship between CO₂ adsorption volume and adsorption equilibrium pressure cannot be fitted by Langmuir equation, and the similar conclusion to the relationship between volumetric strain and equilibrium pressure. However, CO₂ density instead of adsorption equilibrium pressure can solve the problem that supercritical CO₂ not exsist saturated vapor pressure. The relationship between CO₂ adsorption volume and CO₂ density can be fitted by the equation which similar to Langmuir equation. In addition, based on the thermodynamic theory, a full coupling mathematical model of adsorption- stress-strain was derived.（4） The experimental simulation were conducted on mechanical properties of anthracite response to supercritical CO₂ injection, and the results revealed that the elastic modulus and strength decreased, but the poisson’s ratio increased because of supercritical CO₂ injection. Both adsorpting and pore pressure played soften role on the strength of coal body. Futhurmore, based on the Moore-coulomb failure criteria, equilibrium adsorption equation as well as Griffith crack tensile strength criteria, mechanical models were built and explained the mechanism of adsorpting and pore pressure weaken on the strength of coal body.（5） The relationship between permeability of anthracite core and temperature, confining stress, volume strain present negative exponent variation. The comprehensive affect of temperature, confining stress, and volume strain are consisered for the model of permeability variation. The injection of supercritical CO₂ induce few micro-pore was damaged and pore volume expand.（6） The injectivity of coal seam will decline when the coal seam depth increase, and the the injectivity of coal seam present negative exponential decline relate to cumulative injection time. The injectivty varation model was built according to the injectivty model, adsorpting- stress-strain model and the permeability variation model, then revealed the mechanism of adsorpting- volume strain, mechanical properties, pore pressure, confining stress effect on injectivty varation. In addition, the method is proposed that injecting N2 to enhance the injectivity of supercritical CO₂ into deep coal seam, the mechanism of this method was discussed, and the experimental simulation data show that this method is feasible.