Effects Of Supercritical CO₂ On Shale Micro-characteristics And Tensile Mechanical Behaviour

The efficient development of supercritical CO₂ enhanced shale gas can not only improve shale gas recovery,but also achieve CO₂ geological storage and reduce carbon emission pressure.CO₂ injection into the shale reservoir will induce the dissolution and precipitation of mineral components,affect the micro-characteristics and mechanical behaviour of the shale,especially the change in tensile mechanical behaviour,and that will affect the initiation and propagation of fracturing cracks,which directly affects the shale reservoir reconstruction effect.However,at present,the change law and effect mechanism of shale tensile mechanical properties after supercritical CO₂ are unclear.Therefore,it is of great engineering significance to carry out research on the changing law and effect mechanism of shale tensile mechanical properties after supercritical CO₂for the efficient development of supercritical CO₂ enhanced shale gasThis article takes the Longmaxi Formation shale in Fuling area as the research object,through the self-developed high-temperature and high-pressure shale soaking device to conduct supercritical CO₂ soaking shale preprocessing,and sets Ar soaking control test.XRD,SEM,NMR and L-PGA were used to test the shale micro-characteristics.The Brazilian split test of shale was performed by an electronic precision material testing machine,and the failure mode of the shale was monitored by an ultra-high speed imaging and data processing system.Comprehensive analysis of the micro-characteristics and tensile characteristics of shale at different treatment conditions,revealing the effect of supercritical CO₂ on the tensile characteristics of shale.The main research results obtained are as follows:(1)The change law of shale micro-characteristics after supercritical CO₂ soaking was obtained.After the treatment of supercritical CO₂,the tight cementation between the mineral particles of the shale were destroyed,the porosity increased,the aperture distribution increases overall,and the average aperture increased.Calcite is the main mineral that reacts between supercritical CO₂ and shale.(2)The deterioration law of shale tensile mechanical parameters after supercritical CO₂ soaking was clarified.After the treatment of supercritical CO₂,the tensile strength,split modulus,and cumulative energy of shale were significantly reduced..After supercritical CO₂ soaking,the shale is more likely to be destroyed,and the deterioration tendency of the tensile strength of the bedding surface is more serious than that of the matrix.(3)The change characteristic of shale failure mode after supercritical CO₂ soaking was discovered.Supercritical CO₂ causes complex failure modes of shale specimen.In the process of failure,the number of secondary cracks,the propagation distance and the communication between them have been significantly improved,it is easier to form secondary cracks along the bedding plane,and the secondary cracks have the ability to continue to propagate after through the existing cracks,complex crack networks can be formed in local areas and even the entire shale specimen.(4)The effect mechanism of supercritical CO₂ on the shale tensile mechanical behaviour was revealed.Calcite dissolution results in the destruction of carbonate cementation,reducing the cementation strength of shale.The effect of plasticized organic polymer and anisotropic expansion of the matrix causes local failure of shale.Changes in shale pore structure cause the shale skeleton to be damaged,reducing its load bearing capacity.The reduction of the existing micro-crack surface energy results in the reduction of the tensile stress required for crack tip propagation,causes cracks to propagate more easily.The combined effect leads to the deterioration of the tensile properties of shale after supercritical CO₂ soaking.The study results show that supercritical CO2 enhanced shale gas efficient development has a positive impact on reducing fracturing energy consumption and forming complex fracture networks and improving shale gas recovery.