Study on Mechanism of Improving Recovery of Shale Oil by Preposed CO2 Fracturing:Taking Changqing Oilfield as an Example
The oil production characteristics of CO2fracturing flowback fluid in nano-micron pore were studied through a series of exper-iments,and the differences and mechanisms of crude oil utilization in different layers were compared and analyzed.The results show that the average displacement efficiency of shale oil is about 60%during CO2 flowback.The contribution of the crude oil in large pores to the displacement efficiency is more than 70%,and the residual oil is mainly distributed in small pores.In the process of CO2 flowback,the reserve recovery degree of Chang 8 and Chang 7 layers is similar,both better than that of Chang 6 layer.The energy enhancement coeffi-cient of Chang 8 and Chang 7 layers is 9.5 and 3.8 times higher than that of Chang 6 layer,indicating better energy enhancement effect.The low-carbon components in crude oil can be extracted in the flowback process of the preposed CO2 fracturing liquid of hydrau-lic fracturing,with an increase of 6.2%below C13 and a decrease of 7.5%between C13 and C33.CO2 not only changes the wettability of rocks,but also promotes the formation of a continuous phase of crude oil in the nano-micron pores,thereby reducing the interfacial mi-gration resistance between gas and liquid phases.CO2 flowback fluid flooding can effectively reduce the lower limit of pore throat crude oil utilization,with a minimum of 149 nm.
CO2 fracturingenhanced oil recoveryeffect mechanismshale oil reservoir
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