During liquid CO2 fracturing treatment in PetroChina Jilin Oilfield, CO2 flows through storage tank, booster pump, blender, fracturing pump, and eventually into wellbore and production zone, generating a very complex phase evolution behavior. morphology control of CO2 is a key in liquid CO2 fracturing, as it greatly affects the stability of fluid and sand supply.

Temperature and pressure sensors are positioned at 12 critical nodes in a liquid CO2 fracturing field test for tight oil to provide insight on CO2 fluid phase evolution behavior. Then an analysis of phase behavior at each process node is done, to investigate its influencing mechanism on operation stability, as well as its major control factors. Based on this, the fracturing technological process is optimized and restructured, aiming at the optimization of fluid performance and construction stability.

Results are as follows. 1) Storage conditions of CO2 are inconsistent between different storage tanks, which may cause vaporization of CO2 during low-pressure fluid-feeding stage, thus greatly affect the construction stability. A buffer vessel should be introduced into the fracturing system, between the tanks and boosters, to improve the fluid-feeding stability. 2) The storage temperature and pressurein Jilin Oilfield is much higher than thatin North America, so the boosters contribute little to phase control here. A heat exchanger is a better alternative. 3) When the pressure difference between in-tank and outlet of blender is less than 0.05 MPa, the sand-feeding process is stable. This can be achieved by adjusting phase behavior of CO2, liquid supplement rate to blender and opening degree of butterfly valve.

Phase evolution during liquid CO2 fracturing has been identified, and the fracturing technological process is optimized and restructured based on this. This helps improve the fluid performance and construction stability, and enhances the success ratio and stimulation result of fracturing.

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