GRE写作Issue模拟题:The Importance of Science and Art

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At the same time, the casing has been consolidated with the formation and cement, which can hardly deform. Then there will be thermal stress in CCF assembly (Liu, K. et al, 2016). Assuming that the thermal expansion coefficient is α/℃and the temperature change is ΔT, then the thermal stress will be σT. The calculation formula is as follows. (1) For simulating the influence of temperature difference on the casing stress, the temperature changes in the model are set as 0℃/20℃/40℃/60℃/80℃/100℃ respectively. The results are shown as Fig.6. Fig.6 Influence of different temperature on casing stress As shown in Fig.6, when the defection angle is less than 30°, the maximum Von Mises stress of casing will increase with the increase of the temperature difference. On the other hand, when the angle is greater than 30°, the casing stress will decrease with the increase of the temperature difference. For the 30° defection angle, the maximum stress is almost the same with different temperature changes. This can be explained by the fact that when the defection angle is small, the casing can hardly deform and a large thermal stress occurs for different temperatures. On the contrary, when the defection angle is greater, the casing can deform freely, which alleviates the thermal effect of the casing and make its stress decrease. When the defection angle is greater than 180°, the maximum Von Mises of casing will be almost the same for different temperatures. 4.3. The influence of fracturing pressure Good cement sheath Fig.7 Influence of different fracturing pressure on casing stress for good cement sheath It is shown in Fig.7 that the influence of fracturing pressure and temperature difference for good cement sheath. It can be concluded that when the temperature difference is small, the maximum Von Mises stress of casing decreases and then increases with the increasing of pressure. When the fracturing pressure is about 40MPa, the maximum stress of the casing can be minimized. On the other hand, when the temperature difference is high, stress increases with increasing pressure. Poor cement quality Fig.8 Influence of different fracturing pressure on casing stress at 30° defection angle of cement sheath Fig.9 Influence of different fracturing pressure on casing stress at 60° defection angle of cement sheath Fig.10 Influence of different fracturing pressure on casing stress at 90° defection angle of cement sheath The figures above show the influence of different fracturing pressures and temperature changes on the casing Mises stress for three groups of the defection angle. From the curves, it can be seen that the maximum Mises stress of the casing decreases and then increases with the increase of the fracturing pressure when the angle and the temperature difference are both small. In addition, when the defection angle is small and the temperature difference is large or the defection angle is large, the maximum Mises stress of the casing increases with the increase of the fracturing pressure. The maximum Mises stress of the casing is also the largest when the temperature difference is small, which is consistent with the conclusion in the previous section. 5. CONCLUSION Cement sheath is expected to provide mechanical zonal isolation and wellbore integrity during multi-stage hydro-fracturing operation. Cement defection and temperature change may result in abnormal casing stress, even casing collapse. A FEM of CCF assembly using completion and fracturing operation data is applied to calculate stress distribution of casing during fracturing operation. Some useful conclusion can be get from the simulation. For the poor cementing quality, the maximum Mises stress of casing increases and later decreases with the increase of the cement channel angle. Considering the temperature, when the angle of cement channel is less than 30°，the maximum stress of the casing increases with the increase of the temperature difference. At the same time, the stress decreases when the angle is more than 30°. For the different fracturing pressures, when the angle of cement channel and the temperature difference are small, the maximum Mises stress of the casing firstly decreases and then increases with the increase of the fracturing pressure. On the other hand, when the angle is small and the temperature change is large, or the angle is large, the maximum Mises stress increases with the increase of pressure. It can provide some theoretical directions for multi-stage fracturing operation from the above results. Good cementing quality is very necessary for casing integrity. During fracturing, pumping pressure and fracturing fluid temperature should be set at a reasonable range for the safety of wellbore. Future work aims at developing a 3D model simulating underground conditions more precisely. And since behavior of cement and formation follows the poroelastic theory (Bois, A.P. et al 2011), further studies considering the poroelaticity of the cement and formation are needed.