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

复杂句使用不错，句法规范；文章词汇表达丰富，高级词汇积累做的也很棒；采用了简单的衔接手法，行文流畅。

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 adopting 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.