Experimental Study on the Mechanical Response Mechanism and Injectivity with Supercritical CO2 Injection in Anthracite

Author:Niu Qing He

Supervisor:cao li wen


Degree Year:2019





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.