Experimental Study On CO₂/Water Injection To Stimulate Coal Shear Dilatancy And Coalbeds Infiltration

Coalbeds are generally characterized by low permeability in China,which seriously restricts the result of coalbed methane extraction.Therefore,enchancing the permeability of low permeability coal seam is the key to improve the the efficiency of coalbed methane extraction.In this thesis,CO₂/water injection to stimulate coal shear dilatancy and coalbeds infiltration was considered as the object of study.Using a combination of theoretical analysis and laboratory experiments to analyze the mechanism and characteristics of CO₂/water injection to stimulate coal shear dilatancy and the influence of different factors such as stress state and fluid migration on coal rock deformation and acoustic emission characteristics.The failure criterion of CO₂/water injection to stimulate coal rock shear dilatancy was studied by theoretical analysis and laboratory experiments.The self-developed servo-controlled seepage equipment for thermal-hydrological-mechanical coupling of coal was employed to perform an experimental study on three stress paths of axial compression,simultaneous unloading of axial stress and confining stress,and fluid injection to stimulate coal rock shear dilatancy under conventional triaxial stress conditions.The superior capacity dilatancy characteristics of coal under the condition of CO₂/water injection.Using the variation law of volume strain and dilatancy factor of raw coal specimens during the experiment to analyze the effect of stress state on the fluid-stimulated shear dilatancy of coal.Through acoustic emission monitoring and CT scanning,the effects of fluid migration on fluid-stimulated shear dilatancy of coal are analyzed.The research results are shown as follows:(1)Based on Mohr Coulomb's linear shear failure theory and effective stress law,the shear slip criterion of coalbeds natural fracture considering pore fluid pressure is obtained through elastic mechanics analysis.By analyzing the criterion of shear slip,we can find that the force of fluid injection to stimulate the shear dilatancy of coal is deviatoric stress,and the fluid pressure is only an inducing factor.According to theoretical analysis and experimental verification,it is confirmed that the Mohr-Coulomb criterion can better describe the failure law of coal under the condition of fluid injection.However,the process of fluid injection into the coalbeds is very complicated,which may lead to changes in effective stress coefficient,capillary pressure,and coal strength.Therefore,it is difficult to calculate accurate fluid-induced shear dilatancy failure criteria for coal.(2)The dilatancy capacity of coal depends on the effective confining pressure at the time of failure.Quantitative analysis of volume dilatancy characteristics of raw coal under three stress conditions using volume strain and dilatancy factor found that compared with the axial compression,the actual confining pressure when the raw coal under the Unloading stress and fluid injection stress failure is smaller,it shows stronger dilatancy characteristics.Therefore,in the on-site fluid-stimulated coalbeds,due to the injection of high-pressure fluid,the effective confining pressure of the coalbeds will be reduced,and the yield deformation of the raw coal will be expanded.A large number of pores and fissures within the raw coal will develop,and a high permeability zone will be formed in the coalbeds.In addition,the two stress paths of unloading stress and fluid injection are relatively similar.But the change of effective stress coefficient,the capillary action between fluid coal matrix,adsorption and pore fluid pressure under fluid injection conditions directly act on the fracture surface,and the large difference in mechanical unloading stress results in a large degree of difference in the deformation and dilatancy characteristics of the raw coal specimens,which cannot be completely equivalent.(3)The effective deviatoric stress is the key factor for the fluid to stimulate the shear dilatancy of coal.When the deviatoric stress is not enough to cause shear failure of the coal,even during the fluid injection process,even if the volume strain and dilatancy factor of the coal continue to develop toward the expansion trend with the increase of fluid pressure.But in the end,the coal still shows the deformation characteristics of compression,which will not be conducive to the increase of coalbeds permeability,and may even reduce the coalbeds permeability.When the deviatoric stress can cause shear failure of the coal body,increasing or decreasing the deviatoric stress will not have a significant impact on the dilatancy capacity of the coal,and the coal always maintains a good capacity dilatancy characteristics.Under different confining pressure conditions,CO₂/water injection can stimulate the coal to produce a more obvious shear dilatancy phenomenon,and the small difference in effective confining pressure leads to little difference in coal dilatancy capacity under the fluid injection stress path.(4)The failure of coal caused by CO₂/water injection is a fluid-solid coupling process,and its influence on the stress state of the coal depends on the distribution of pore stress in the coal.Therefore,the fluid migration in the coal has a very obvious effect on its dilatancy characteristics.The CO₂/water injection rate and the permeability of the experiment specimens determine the speed of fluid migration.A faster fluid injection rate or a lower raw coal permeability may cause uneven fluid pressure distribution within the coal.Shear deformation is mainly concentrated in the upper part of coal,causing local destruction of the coal,which will be detrimental to the penetration effect of the entire coal.The property of the injected fluid also affects the velocity of fluid migration.Under the same conditions,the shear failure of coal is more evenly distributed than that of water injection when CO₂ is injected.