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Description
Slickwater is one of the most commonly used fracturing fluids, however it consumes amount of water, induces reservoir damage and flows back difficultly. CO2 is friendly to tight reservoir, which can enhance oil recovery and facilitate fracturing fluids backflow by interacting with oil. Furthermore, the injected CO2 will be adsorbed and permanently stored in the formation, alleviating the greenhouse effect [1-2]. Nevertheless, researches on the enhancement recovery potential of CO2 as pre-flush injecting fluid before fracturing are quite few [3-4]. Therefore, we quantitatively investigated the stimulation potential of CO2 prepad for tight reservoirs by experimental and numerical methods.
First, the influence of CO2 on the volume of crude oil was studied through Constant Composition Expansion. Meanwhile, the displacement efficiency of CO2 was calculated by laboratory experiments. Then, comparison experiments with slickwater flooding were also conducted. Next, a conceptual model was established by using CMG commercial software, considering CO2 dissolving, and the resulted miscible process and oil swelling. We injected CO2 followed by water in the model to simulate CO2 pre-pad. As well as, the effect of CO2 on stimulation and fracturing fluid flowback was quantitatively evaluated. Finally, CO2 injection rate, CO2 injection volume and soak time for fracturing operation was obtained by Response Surface Method, providing a vital guidance for oil-field.
The results show that: (1) When 25% mole fraction of CO2 injected into crude oil, the volume factor and swelling factor of crude oil increased by 19% and 33%, respectively. That indicated CO2 can make crude oil fully swelling and remarkably increase its elastic energy; (2) The displacement experiment displayed that the displacement efficiency of CO2 is four times higher than slickwater, and the oil breakthrough time of CO2 flooding is just one quarter slickwater. Combined with the result of Nuclear Magnetic Resonance experiment, it is showed that the displacement efficiency of slickwater is confined by the pore size distribution of core. While the displacement efficiency of CO2 is not affected by the pore size distribution of core because of small molecule and easy diffusion ability of CO2. Thus, the oil recovery by injecting CO2 into reservoir is notable and widely applicable; (3) The numerical simulation showed that the recovery factor of CO2 as prepad is 9% higher than slick water. The flowback rate of the fracturing fluid when injecting CO2 prepad is 1.4 times greater than injecting slickwater, because fracturing fluid is easier to flowback after the full interaction between CO2 and crude oil during the soak stage. That indicates CO2 is an effective and promising to enhance recovery for tight oil. (4) At last, Response Surface Method was applied to optimize multiple operating parameters. It turned out that best CO2 injection rate is 4m3/min, optimal CO2 injection volume is 500m3, and proper soak time is 9 days . According to the research, the effect of CO2 in enhancing oil recovery and improving fracturing fluid backflow outperforms slickwater. CO2 as prepad injecting fluid before fracturing has a wide prospect to stimulate tight reservoirs.
References
[1]Li S, Zhang D. How effective is carbon dioxide as an alternative fracturing fluid?[J]. SPE Journal, 2018.
[2]Fang C, Chen W, Armor M. Simulation study of hydraulic fracturing using super critical CO2 in tight[C]//Abu Dhabi International Petroleum Exhibition and Conference. Society of Petroleum Engineers, 2014.
[3]Tamayo H C, Lee K J, Taylor R S. Enhanced aqueous fracturing fluid recovery from tight gas formations: foamed CO2 pre-pad fracturing fluid and more effective surfactant systems[J]. Journal of Canadian Petroleum Technology, 2008, 47(10).
[4]Wang F, Wang Y, Zhu Y, et al. Application of Liquid CO2 Fracturing in Tight Oil Reservoir[C]//SPE Asia Pacific Oil & Gas Conference and Exhibition. Society of Petroleum Engineers, 2016.
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