Experimental Study Of Interaction Of CO₂-Water-Shale Under Different CO₂ Phase State

In recent years,the demand for energy is increasing in china.As a kind of unconventional natural gas,shale gas is abundant in China and can effectively alleviate the problem of energy shortage.At present,hydraulic fracturing shale technology has the problem of low shale gas output due to easy expansion of clay minerals in water and high water consumption.Supercritical carbon dioxide enhanced shale mining technology appears in our field of vision.The application of supercritical carbon dioxide enhanced shale mining technology can not only improve shale gas recovery rate,but also complete carbon dioxide storage and guarantee the dual benefits of economy and environment.When carbon dioxide is used as conventional fracturing fluid to fracture shale,the interaction between carbon dioxide,water and shale in reservoir conditions will affect the microstructure and macroscopic mechanical properties of shale,and further affect the initiation and expansion mechanism of fracturing fractures.Therefore,the study on the influence of carbon dioxide-water-shale interaction of different phase states on the microstructure and mechanical properties of shale is the premise for the follow-up study on the mechanism of supercritical carbon dioxide fracturing and the mechanism of fluid seepage in the process of carbon dioxide enhanced shale gas exploitation.In this paper,the Ordovician wu feng formation shale in Si chuan basin was selected as the experimental sample.Using testing methods of Low-temperature nitrogen adsorption,nuclear magnetic resonance(NMR),XRF and XRD analysis,scanning electron microscopy of SEM,MTS mechanics test system and acoustic emission(AE)acquisition system characterization to research shale mineral composition,element content,the changing rule of the microscopic pore structure and mechanical properties parameters under the action of different phase of carbon dioxide-water-shale.The main research achievements are as follows:(1)Through XRD and XRF characterization tests,it is found that the mineral components of shale are composed of quartz,carbonate,such as calcite and dolomite,clay minerals,as well as minerals such as pyrite and plagioclase,The main elements are O and Si.After the interaction of different phases of carbon dioxide-water-shale,the clay minerals and carbonate in shale will be eroded and precipitated,resulting in the increase of mineral content such as quartz and dolomite,and the decrease of content such as calcite and clay minerals.At the same time,the content of O and Si in shale increased,while the content of metal elements such as Ca and Fe decreased.(2)Through Low-temperature nitrogen adsorption and nuclear magnetic resonance testing and analysis,it is found that the specific surface area and pore volume of shale show a decreasing trend,the average pore diameter increases to different degrees,the proportion of micropores and mesopores in shale decreased,the number of macropores increased after the interaction of different phases of carbon dioxide-water-shale.The change of the number of micropores determines the reduction of the specific surface area of shale.(3)Uniaxial compressive strength test was carried out on the shale specimen,and it was found that compared with the original specimen,uniaxial compressive strength and elastic modulus of the shale specimen decreased to the interaction of different phases of carbon dioxide-water-shale.(4)Through the shale under uniaxial compression of the acoustic emission test analysis,compared with the original specimen,found that shale damage acoustic emission signal obviously increased,the cumulative ae counts,destruction of energy intensity is high after the interaction of different phases of carbon dioxide-water-shale.It shows that the uniaxial compressive strength of shale is abate,the compressive strength of internal crack increased and well versed in,shale has higher compression process damage after the interaction of different phases of carbon dioxide-water-shale.