The Optimisation Design of Buffer Vessel Based on Dynamic Balance for Liquid CO2 Fracturing

Abstract In liquid CO2 fracturing, liquid CO2 is injected into reservoirs to crack open the formation, providing highly conductive paths for hydrocarbons. It's superior to water-based fracturing, because water is excluded during the whole process, resulting in formations free from water damage. Many field tests have been conducted in Jilin oilfield, with some problems exposed. The uncontrollability of fluid feeding due to the pressure difference among storage tanks, the liquid CO2 vaporization in low pressure fluid feeding stage, and the slow-dissolving of additives are three main issues. To solve the above problems, a pressure-bearing inclosed tank is designed as the buffer vessel, which is placed between the liquid CO2 storage tanks and the boost pumps. Meanwhile, the additive adding system, which used to be located at the discharge connection of blender, is moved forward to the place near the discharge connection of the buffer vessel. A heat exchanger is added to the process too, to eliminate the vaporization. At last, a static mixer is installed after the blender. During construction, liquid CO2 of different storage tanks is firstly driven into the buffer vessel, discharged into the boost pumps, mixed with the proppants in blenders, and at last subcooling using the heat exchanger before driven into the pumps. In this case, the pressure of low-pressure fluid feeding system become stable, the residual volume of liquid CO2 in a single tank is reduced, and the vaporization is eliminated. Meanwhile, the one-time storage of liquid CO2 is somewhat increased. After discharged from the buffer vessel, liquid CO2 is mixed with the additives immediately, which greatly prolongs the mixing time. The mixer also aids to the dissolving of additives. The additives dissolve in liquid CO2 earlier, thus the proppant-carrying capacity and friction property of fracturing fluid are greatly improved both in the surface pipeline and in the wellbore segment. The technological process of liquid CO2 fracturing is adjusted, which realizes the optimization of fluid feeding system and additive adding system, improves the construction stability, and greatly enhances the stimulation effect.