Study On The Proppant-carrying Laws In The Fracture Of Supercritical Carbon Dioxide Fracturing

Supercritical carbon dioxide fracturing technology combines the energy development and greenhouse gas utilization,which is a new environment-friendly fracturing technology with a broad prospect in the efficient development of unconventional oil and gas resources such as shale gas.However,the low proppant-carrying capacity of supercritical carbon dioxide is one of the difficulties limiting the development of this technology.It is of great significance to study the proppant-carrying laws of supercritical carbon dioxide in the fractures to promote the development of supercritical carbon dioxide fracturing technology.Therefore,numerical simulations,theoretical analyses and experimental methods were used to study the issues of flows and proppant-carrying of supercritical carbon dioxide in this paper,which could promote the development of relevant theories and technologies.Firstly,the flow and heat transfer calculation model of supercritical carbon dioxide is built based on the finite volume method,which considers the effect of heat and mass transfer in the fracture.The distribution laws of temperature field,density field and viscosity field of supercritical carbon dioxide in the fracture are analyzed,and the influence laws of the injection temperature,injection displacement and the porosity of formation rocks on the properties of carbon dioxide in the fracture are studied.The results indicate that the temperature on the top wall of the fracture is higher than the bottom wall,while the density and viscosity are lower.The density and viscosity of CO₂in the fracture increase with the decrease of injection temperature and the increase of injection displacement.The density and viscosity of CO₂are relatively higher in the low-porosity formations,which are more favorable for proppant-carrying.Secondly,the settling model of particles in supercritical carbon dioxide is established,and a visual experimental device is developed for the particle settling experiments in carbon dioxide under high temperature and pressure environment.The experiments indicate that the settling model has good calculation accuracy.The results show that the effective gravity and drag force of CO₂are the dominant forces in the process of particle settling.The particle moves with variable acceleration and reaches stability until increasing to the terminal velocity.The terminal velocity of particle settling is positively correlated with the temperature,particle size and density,while negatively correlated with the pressure.The terminal velocity in supercritical carbon dioxide is 1.5-2.5 times than that in distilled water which demonstrates that CO₂has lower proppant-carrying capacity.The terminal velocity of the nonspherical particle is lower than that of the spherical particle with the same equivalent diameter.Increasing CO₂pressure,reducing CO₂temperature and decreasing particle size and density are beneficial to improve the proppant-carrying capacity.Furthermore,the proppant-carrying model of CO₂in the fracture is built based Euler-Euler method.Relevant experiments on proppant-carrying of CO₂are conducted,which verify the accuracy of the model.The results indicate that the flow of proppant-bed in the fracture could be divided into a static layer,a rolling layer,a jumping layer,a suspended layer and a pure flow layer.After the proppant bed is formed at the fracture entrance,it accumulates and expands in height and length with the continuous injection of proppants until reaching the equilibrium height.The equilibrium height is positively correlated with injection temperature,proppant volume concentration,density,diameter and negatively correlated with injection pressure and displacement.The equilibrium time is positively correlated with the injection pressure and negatively correlated with other parameters in this section.Compared with clean water,the proppant-bed formed by supercritical carbon dioxide is higher and shorter,and the proppant placement is more uniform under small-size fracture inlet conditions.Finally,a method for the evaluation of supercritical carbon dioxide tackifiers is presented.The relationship between Reynolds number and Archimedes number is deduced ranging from1000 to 25000 based on the particle settling experiments.The experimental evaluation method of CO₂tackifiers is established,which has good measurement accuracy.Two different tackifiers of CO₂are evaluated with this method,which indicate that this method can be used to evaluate the effects of the CO₂tackifiers of different types and concentrations.Aiming to study the issue of the proppant-carrying flow of supercritical carbon dioxide,the laws of carbon dioxide flow,particle settling and proppant-carrying in the fracture are systematically studied with theoretical analyses,numerical simulations and experimental researches.The influence factors of proppant-carrying in the fracture are investigated and an effective evaluation method of CO₂tackifiers is established.The results obtained in this paper provide a basis for the design of supercritical carbon dioxide fracturing and the development of CO₂tackifiers.