Research On The Cooling Process Of Proppant For CO₂ Fracturing

At present,the most commonly used reservoir reconstruction technology at reservoir is hydraulic fracturing technology.However,there are many disadvantages in hydraulic fracturing,such as a large amount of waste of water,swelling of clay and the damage of the residue of fracturing fluid,the incomplete return of the discharge of water and the high cost of sewage treatment.For strong water sensitivity,strong water lock layer,hydraulic fracturing has great limitations.The CO2 fracturing technology was born,and CO₂ fracturing technology is an anhydrous fracturing technology instead of conventional water base fracturing fluid with liquid CO2.Because liquid CO2 storage environment is low temperature and high pressure environment,so the whole CO₂fracturing equipment working process must strictly control the pressure and temperature.Proppant is stored at room temperature,and the proppant must be cooled before fracturing fluid mixing.The proppant cryogenic tank proposed in this project is the cooling operation for the proppant.The equipment uses the liquid CO₂ to cool the proppant,and the proppant after the cooling is sent into the mixer to carry out the mixing operation.The device can effectively and quickly reduce the temperature of proppant,so as to ensure the demand for proppant in fracturing operation.The cooling process of the proppant can be divided into two stages:In the first stage,the whole tank is in a superheated high temperature state,and the injected liquid CO₂ is in the supercooled low temperature state.The huge temperature difference leads to the phase transition of the liquid CO₂ endothermic and absorbs a large amount of heat,thus reducing the temperature of the proppant sharply.During this process,the low temperature gas generated by liquid CO₂ gasification continues to heat convection with proppant,and continues to reduce proppant temperature.In the second stage,the whole tank is in the low temperature state of CO2 saturation temperature.In this stage,there is only a convection heat transfer between the liquid CO₂ and the proppant and gas inside the tank.The temperature difference is small,the convection heat transfer is weak,and the temperature change in the tank is small.On the basis of analyzing the theoretical model of mass transfer and heat transfer in the cooling process of the proppant,this paper studies the cooling process of the proppant by two methods of thermodynamics and simulation.The specific contents and conclusions include:1.The mathematical model of the heat transfer and mass transfer during the cooling of the proppant is discussed.The quantitative analysis of the change of the CO₂phase transition and its physical property is carried out,and the formula of the CO₂property under different phase states is determined.2.The results of Fluent simulation and thermodynamic calculation are compared and analyzed.It is found that the results of simulation simulation are close to the results of theoretical calculation,which proves the feasibility and reliability of the simulation simulation.3.The cooling process of proppant under different technological conditions is compared and analyzed.It is found that increasing the injection volume of liquid CO₂can shorten the cooling time to a certain extent.,but after a certain value?15L/s?,the cooling rate is not changed much,but the pressure loss will result,and the entrance structure of the double side symmetry arrangement can be good.Improving the pressure peak at the inlet pipe,reducing the pressure loss and reducing the final cooling temperature;reducing the outlet pressure of the outlet can shorten the cooling time of the proppant,but it can not reduce the outlet pressure without restriction due to the engineering requirements before and after the fracturing process;by reducing the filling volume of the proppant,the pressure can be reduced.It can improve its cooling efficiency to a certain extent.