Formation Mechanism Of Solid Plugging In Synergistic Stimulation Of Liquid CO₂ And Hydraulic Fracturing

Based on the characteristics of liquid CO₂ and water-based fracturing fluids,the synergistic fracturing method using liquid CO₂ combined with hydraulic fracturing can significantly improve fracturing efficiency and yield increasing effect.However,during the synergistic reforming of dry CO₂ fracturing and hydraulic fracturing,since the temperature of the liquid CO₂ injected earlier is low?-25°C?,solid plugging may occur in the ground pipeline when the subsequent water-based fracturing fluid is injected,which seriously affects the safe operation of fracturing.In this paper,the structure and formation of plug in pipeline were,studied and a kind of chemical additive to block the generation of plugging was prepared.Through the experimental analysis and simulation calculation,the method of restrain plugging is obtained,reducing technical risks during the synergistic reforming of liquid CO₂ dry-fracturing and hydraulic fracturing to ensure the safe operation of fracturing.Based on the field operation parameters and equipment conditions,a visual evaluation device for fluid flow safety of pipeline was designed to simulate the flow of on-site surface pipeline.The device can withstand pressures up to 40MPa and the temperature can be as low as-30°C.Plugs were obtained through laboratory simulation experiments and the causes of blockages were analyzed.It is mainly due to the extremely low wall temperature after the liquid CO₂ injection is completed,so the water-based fracturing fluid will freeze when it passes through.At the same time,based on Joule-Thomson effect and filed condition of 3MPa and-25°C,the phase state of liquid CO₂ was calculated before and after throttling with HYSYS software.Based on this,using the solution freezing point as an indicator,the formulation of the blockage inhibitor system was optimized?55%ethylene glycol+20%Rd+1.5%KCl+25%water?.The isolation liquid freezing point is below-40°C and will not freeze below-25°C when it is mixed with water-based fracturing fluid at a ratio of 1:2,which shows good compatibility with liquid CO₂ and water-based fracturing fluid.The influence of formation rock permeability is less than 5%.Numerical simulation was used to study the change of tube wall temperature during the process of injecting the barrier fluid and the minimum amount of barrier fluid that can be injected into the water-based fracturing fluid.The result shows that the minimum separation solution is 0.53m³ for each pipeline,ensuring that the pipe temperature is greater than 0°C.The pilot test shows that the isolation liquid and the on-site process method can effectively solve the pipeline blockage problem during the combination of dry-type liquid CO₂ fracturing and hydraulic fracturing.The entire fracturing process is continuous and uninterrupted,which ensures the safety of fracturing construction and improves the fracturing efficiency,so it has important application value.