Fractal Characteristic Of Sandstone's Strain In True Triaxial Compression Test With The Effect Of CO₂/Water Pore Fluid

In geotechnical engineering,the influence of water,CO₂ and other fluids on human production activities cannot be ignored.Owing to the universality of water in nature,pore water is used as the main research object in the correlation studies of pore flow.In recent years,due to the application of CO₂ flooding and CO₂ geologic sequestration technology in industry,the influence of CO₂ on the mechanical properties of reservoir rock has become a hot research issue.Geotechnical materials are heterogeneous,non-continuous and anisotropic materials,and their mechanical behaviors show randomness.Traditional research methods have certain limitations on this nonlinear system.As a new theory,fractal theory has been active in many fields.Fractal approach to the description of the complex system of real property and state,to quantitatively describing the irregular and complicated things or phenomena in the real world,it provides a new way to solve the problem of rock and soil mechanics.Based on the demand background above,in this paper,the effects of effective confining pressure and pore flow are considered,and the triaxial compression test of sandstone is carried out,the mechanical properties and destructive characteristics of sandstone under different confining pressures and pore flow are studied.Based on the fractal theory,according to the experimental results,the strain fractal rules in the process of rock compression fracture under different pore flow are explored,and the effects of effective confining pressure on the fractal characteristics are analyzed.The main research work and conclusions are as follows:?1?The triaxial compression test of sandstone under CO₂/water pore flow is carried out.Under the condition that dry,saturated CO₂ and saturated water respectively serves as pore flow,the fracture strength and deformation behavior of sandstone are obtained in three kinds of effective confining pressure levels.Then the effects of pore flow and effective confining pressure on the mechanical properties of sandstone are analyzed.The results show that,the pore flow with high pressure reduces the peak strength and elastic modulus of dry sandstone.The effect of water on sandstone is mainly the hydration of water and rock while the effect of CO₂ on sandstone is mainly the adsorption between CO₂ and sandstone.Compared with CO₂,water makes sandstone more vulnerable to destruction than.With the increase of effective confining pressure,the peak strength and elastic modulus of sandstone increase.?2?The influence of CO₂/water pore flow on the sandstone strain fractal characteristics is investigated.In this paper,the strain fractal curves of the compression-controlled failure process of dry,saturated CO₂ and saturated water sandstone under the effective confining pressure of 10MPa are analyzed.The results indicate that under the effective confining pressure of 10MPa,the range and maximum of fractal dimension are sorted as dryness>CO₂>water,and the fractal curves of different pore flow show different characteristics in all stages of sandstone compression and failure.The fractal dimension of sandstone varies with the increase of effective confining pressure.In the low confining pressure state,the fractal dimension peak results are:dryness>CO₂>water,and the fractal dimension peak results in the high confining pressure state are:dryness<CO₂<water.?3?The effect of effective confining pressure on the fractal characteristics of sandstone strain is investigated.Under three groups of confining pressures,the evolution law of the fractal curves of dry,saturated CO₂ and saturated water sandstone is analyzed respectively.Results indicate that:As for dry sandstone,with the increase of effective confining pressure,the fractal calculation results decline.The larger the confining pressure,the smaller the decreasing amplitude of the same interval is and the smoother the curve becomes.For saturated water sandstone,with the increase of confining pressure,the fractal calculation results generally rise up,and the variation amplitude is getting larger and larger.In terms of saturated CO₂ sandstone,with the increase of confining pressure,overall the fractal curve rises up with small amplitude.Under the same confining pressure,the characteristics of the fractal curve of drying and CO₂ are close,and when the confining pressure is increasing,the two approaches coincide on the whole.With the increase of effective confining pressure,the change tendencies of the peak fractal dimension of different sandstones are shown as:the dry sandstone decreases,the saturated CO₂ sandstone rises slightly,and the saturated water sandstone goes up sharply.?4?Comprehensive analysis of the influence of CO₂/water hole flow and effective confining pressure on the fractal characteristics of sandstone strain is conducted.Fitting the correlation curves between the amplitude reduction of peak fractal dimension and effective confining pressures of saturated CO₂/water sandstone,the results show that with the increase of effective confining pressure,relative to the dry sandstone with the same effective confining pressure,the amplitude reduction of the peak fractal dimension of saturated water sandstone decreases linearly and the saturated CO₂sandstone decreases exponentially.Furthermore,based on the analysis of fractal curves in?2?and?3?,the curves of each component were rapidly rising before the sandstone failure.Therefore,in engineering application,the sudden increase of strain fractal dimension can be used as a sign of rock fracture.