Study On The Mechanism Of Proppant Transport In Fractures During Supercritical CO₂ Fracturing

Supercritical CO₂ fracturing has great potential for developing unconventional oil and gas reservoirs but the difficulty of efficiently pumping proppant into artificial fractures during fracturing has been found in field practice,limiting the application of this technology.To address this challenge,this paper uses a combination of laboratory experiments,theoretical modeling and numerical simulations to investigate the mechanism of proppant transport within the fracture and the influence of parameters in supercritical CO₂ fracturing,and the main findings are as follows.An experimental set-up is designed and constructed for supercritical CO₂ transport proppant in fractures,and the transport and placement characteristics of supercritical CO₂ carrying proppant are experimentally investigated.The results show that the flow patterns of supercritical CO₂ transporting proppant in the planar fracture include scouring,throwing and fluidization,and the supercritical CO₂ slurry has obvious stratified flow characteristics.With constant mass flow rate,increasing injection temperature,lowering injection pressure or reducing proppant concentration will increase the dune placement distance,and the small size proppant（70/140 mesh）has strong transportability in low viscosity supercritical CO₂.A significant correlation is shown between the equilibrium height of the dune and the fluid Reynolds number.A two-phase flow model of supercritical CO₂ slurry considering fluid flow and heat transfer in the fracture is developed using computational fluid dynamics（CFD）coupled with discrete element method（DEM）.The mechanism of proppant transport in planar fractures is revealed by particle-level analysis.The results indicate that the process of supercritical CO₂ transport proppant in planar fractures can be divided into the flat laying stage,the proppant dune development stage and the proppant bank development stage;Viscous drag transport,lifting transport,and fluidization transport are three proppant transport mechanisms for supercritical CO₂ slurry flow,among which the fluidization transport plays a more dominant role.Rough fracture models with surface characteristics of supercritical CO₂ fractured rocks are established in the form of laser morphological scanning reconstruction and spectral synthesis method of numerical generation,and the transport characteristics of the proppant within the rough fracture and the influence of parameters are investigated.The results indicate that the trajectory of proppant transport within the rough fractures is fluctuating and jumping in the lateral and vertical directions,and the morphology of dune placement shows a non-uniform distribution in the form of waves or even clusters.When the ratio of particle diameter to the average fracture width is not greater than 0.4,the proppant can achieve better transport in rough fractures.The transport distance of the proppant within the rough fracture decreases with increasing fractal dimension and increases with increasing root mean square（RMS）of asperity height value.A complex fracture network model with rough walls is constructed,and the steering transport characteristics of supercritical CO₂-carrying proppant in the rough fracture network and the flat fracture network are compared.The effects of key pumping parameters and injection methods on the proppant distribution in the rough fracture network are obtained.Results found that the steering transport mechanisms of proppant into branching fractures in rough fracture networks are gravitational slip,high-velocity fluid suspension,and fracture structure induction.Viscosification of supercritical CO₂ requires more than 20 times（about 0.94 m Pa·s）to significantly improve dune placement.Increasing the injection velocity,reducing the particle size,lowering the proppant concentration and density,and combining the proppant size in a"large first and then small"way can enhance the support effect on secondary fractures.The research results reveal the flow characteristics of proppant slurry and proppant mechanism of proppant transport within the fracture in supercritical CO₂ fracturing and the influence law of key parameters,which is expected to provide theoretical basis and reference for the optimized design of pumping parameters.