The use of ScCO2 fracturing may be a good way to avoid these issues , , , . Compared with hydraulic fracturing, the fractures produced are relatively straight (Fig. 4) so water retention will not cause pore throats to become plugged. In addition, ScCO2 fracturing of shale improves the speed and rate of fracturing fluid return significantly and shortens the drainage period. It is also suitable for the exploitation of continental shale gas, which has a high clay content . There are also significant safety benefits. ScCO2 fracturing can achieve the same fracturing effect at a threshold pressure of only 50%-60% that required of hydraulic fracturing (Fig. 5), thus reducing the engineering difficulties involved in its safe implementation .
Fig. 5. Comparison of the rock-breaking capacity of water and ScCO2.
Challenges for ScCO2 fracturing technology and directions for future research
At present, there are still many flaws in this technology. Due to the low viscosity of ScCO2 fluid, its sand carrying capacity is insufficient, and it is possible that cracks will become blocked by sand plugs, resulting in implementation failure . During the production process, the induced fractures may cause the pressure to drop rapidly, and the loss radius of the stimulated well may be less than the predicted radius . The phase changes of CO2 are very complex, and a complicated temperature gradient environment will aggravate this. The changes to the temperature and pressure of CO2 fracturing fluid with time under different injection and displacement conditions have been explored by some domestic scholars, leading to some useful conclusions , . Other problems, such as the costs of CO2 preparation, capture, compression and transportation processes, the possibility of gas leakage with the massive use of CO2, and the phenomenon of mineralization caused by CO2 dissolved in formation water still need to be addressed in the future , , .
Conclusion and prospect
(1) ScCO2 fluid has unique properties, such as near-liquid density, low viscosity, strong diffusivity and zero tension, which gives it unparalleled economic and environmental advantages in the process of reservoir reconstruction and extremely positive prospects for shale gas development.
(2) The specific molecular properties of CH4 and CO2 give the latter a good potential to displace the former, especially where ink bottle pores are present. The reaction rate of CH4 proceeds through three stages: a transient increase, a rapid decrease, and a stable stage. ScCO2 fluid action primarily affects high-porosity calcium feldspar and montmorillonite, as can be seen through SEM chromaticity differences before and after reaction at the locations of the two minerals.
(3) ScCO2 fluid fracturing technology has numerous benefits over hydraulic fracturing. It not only solves technical problems that beset hydraulic fracturing, but it also alleviates the environmental problems associated with traditional fracturing methods.
(4) At present, as a new type of fracturing method, ScCO2 technology has great prospects for shale gas development. It does face several problems, such as a need for further theoretical research and practical production testing. Moreover, there is still a need for improvement in some theoretical and technical areas and a need for further investigation into development costs.
The authors gratefully acknowledge the support of the Foundation Research Project of Jiangsu province (Youth Fund Project) of China (No. BK20150179) and the Fundamental Research Funds for the Central Universities (No. 2015XKZD07) of China, and the Postdoctoral Science Foundation of Jiangsu Province.
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Corresponding author at: School of Resources and Earth Science, China
University of Mining & Technology, Xuzhou 221116, China.
E-mail address: [email protected] (M. Wang).
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