Experimental Study On The Mechanical Response Mechanism And Injectivity With Supercritical CO₂ Injection In Anthracite

Injecting supercritical CO₂ into deep unmineable coal seams can not only reduce CO₂ emissions,but also realize the enhanced recovery of CH₄,which possesses dual benefits of environment and energy.Taking the anthracite reservoir in southern Qinshui Basin as the research object,the self-developed“experimental simulation system for the CO₂ injection and enhanced coalbed methane recovery”as the research platform,based on the field investigation,theoretical analysis and experimental simulation,this paper clarified the evolution characteristics of adsorption capacity-swelling strain-swelling stress,the response mechanism of mechanical properties and the permeability dynamic change of coal-rock,established the permeability model of coal-rock under multi-factor influences and the evaluation model of supercritical CO₂ injectivity for coal reservoir,revealed the laboratory evidences of enhanced injectivity methods of intermittent CO₂ injection,N₂ displacing CO₂ and pre-fracturing.?1?The evolution and change mechanisms of volumetric swelling strain and swelling stress of coal-rock after being adsorbed by CO₂ were studied and revealed,the anisotropic characteristics of swelling strain of coal-rock were found and quantitatively characterized.The adsorption swelling of coal-rock is caused by CO₂ adsorption,the volumetric swelling strain and adsorption capacity show the stage relationship of“slow swelling-rapid swelling-stable swelling”.The adsorption swelling strain of coal-rock exhibits anisotropic characteristics,i.e.,swelling capacity in the vertical bedding plane direction>swelling capacity in the parallel face cleat direction>swelling capacity in the parallel butt cleat direction.The anisotropic swelling strain can be quantitatively characterized by the anisotropic index,which shows a positive relationship with the increasing of CO₂ adsorption pressure.Under the constrained conditions,CO₂ injection can lead to the concomitant swelling stress in coal-rock,which increases with the increase of adsorption time.Moreover,the CO₂ injection pressure promotes the swelling stress,the confining pressure and temperature possess the obvious restraint effect on swelling stress.?2?The evolution characteristics of mechanical properties of coal-rock was analyzed,the failure modes of coal-rock after being injected by CO₂ was divided,and the softening mechanism of coal-rock was clarified.CO₂ injection alters the mechanical property of coal-rock,which mainly manifests in the decrease of peak strength and elastic modulus and the increase of Poisson's ratio.And the changes of these mechanical parameters are affected by the CO₂ injection time,the phase state of CO₂ and moisture content.The failure patterns of coal-rock after being injected by subcritical CO₂,supercritical CO₂ and supercritical CO₂+water change from brittle shear failure,transitional shear failure to plastic failure.During the CO₂ injection process,the softening of coal-rock is related to the alterations of its macromolecular structure,surface free energy,softening temperature,pore-fracture system and structural damage.?3?The dynamic evolution law of permeability was obtained,the main controlling factors and anisotropic characteristics of permeability were ascertained,and the dynamic permeability model of coal-rock was established.The permeability of coal reservoir is integrated controlled by CO₂ adsorption,effective stress,moisture content and temperature,CO₂ adsorption and effective stress are the main influencing factors.The permeability of coal-rock shows the anisotropic characteristic,which obeys the following order:permeability in the parallel face cleat direction>permeability in the parallel butt cleat direction>permeability in the vertical bedding plane direction.The bedding plane seepage is controlled by the effective stress,contrarily,the cleat seepage is controlled by the adsorption of CO₂.?4?The evaluation model of CO₂ injectability was constructed,the Engineering effect and scientific basis of enhanced injectivity methods of intermittent CO₂ injection,N₂ displacing CO₂ and pre-fracturing were provided.This paper established the permeability model of coal-rock and the supercritical CO₂ evaluation model of CO₂injectivity,and the reliability and validity of this evaluation model was corroborated by laboratory experiments.Based on the fact that the injectability attenuation of coal reservoir is caused by the direct CO₂ injection,this paper reveraled the effectiveness and scientific basis of the enhanced injectivity methods of intermittent CO₂ injection,N₂ displacing CO₂ and pre-fracturing,results show that that the pre-fracturing exhibits the best effect,followed by intermittent CO₂ injection and N₂ displacing CO₂.This dissertation contains 136 figures,34 tables,and 290 references.