Experimental Evaluation of Energy Enhancement and Fracturing Fluid Flowback Effect for CO2 Pre-Fracturing in Shale Reservoirs

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Abstract
Liquid CO2 pre-fracturing is a promising reservoir stimulation technology due to its advantages of increasing reservoir energy, low reservoir damage, and high fracturing fluid flowback rate. The on-site flowback scheme currently relies mainly on experience and requires further optimization of the energy enhancement effect and the flowback system. This paper presented a study on the changing law of liquid CO2 pre-fracturing energy-enhancing effect and fracturing fluid flowback rate in shale reservoirs was carried out by applying the experiments of drive-off and pressure-reducing flowback. The results indicate that, the injection of CO2 followed by linear gel fracturing fluid increased the energy (pressure) of the core system by approximately 322.8% compared to the injection mode of CO2 fracturing alone, and increased the energy (pressure) of the core system by approximately 24.1% compared to the injection mode of linear gel fracturing fluid. The energizing effect of the core system showed a trend of increasing and then decreasing with the increase of CO2 injection, and the best effect was achieved when the injection ratio of linear gel fracturing fluid to CO2 was 1:1. The flowback rate of the fracturing fluid and oil recovery both increased as the flowback differential pressure increased. Extending the soak time can increase the oil recovery, but the increase gradually decreases with the extension of the soak time. Additionally, the increase in soak time can improve the flowback rate of the core with relatively high permeability, but it has little effect on the flowback rate of the core with very low permeability.

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