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

作者词汇基本功很好，高级词汇表达也比较恰当；作者应增加文中长句和从句和使用；文中基本没有过渡词和衔接词，建议增加衔接词汇的积累。

3.1. Basic parameters setting According to the theory of elastic mechanics, stress concentration occurs near the circular hole in the plate. But if the boundary size exceeds 5-6 times of the wellbore diameter, the effect of stress concentration is too small to be ignored (Li, J. et al. 2005). Therefore, in order to avoid the stress concentration, the model boundary size is 10 times of the wellbore parameter. So the parameters of this FEM is as shown in Table.1. Table.1 Basic parameters of the FEM Wellbore diameter, mm 215.9 Casing outer diameter, mm 139.7 Casing wall thickness, mm 9.17 Casing Elastic Modulus, GPa 210 Casing Poison’s Ratio 0.3 Casing thermal expansion coefficient, /℃ 1.06×10-5 Cement Elastic Modulus, GPa 10 Cement Poison’s Ratio 0.17 Cement thermal expansion coefficient, /℃ 1.0×10-5 Formation Elastic Modulus, GPa 22 Formation Poison’s Ratio 0.23 Formation thermal expansion coefficient, /℃ 1.02×10-5 3.2. Far-field stress setting According to the explanation of far-field stress in X shale well, the maximum field stress is 48MPa, and the minimum is 29MPa. The overburden pressure gradient is 0.023MP/m. So at the horizontal section which the vertical depth is about 1500m, the overburden pressure is about 35MPa. Far-field stress can be set by the Keywords *Field in the software. The mesh in the model is set as the CPE4R plane strain element. Mesh elements are as shown in Fig.4. 4.1. The influence of cement channeling There are many possibilities for poor cementing quality, and the most typical is the cement channeling. In order to simulate this situation, the models of different cementing ring defection angle and defection phase angle are established respectively. The defection angles are 30 ° / 45 ° / 60 ° / 75 ° / 90 ° / 120 ° / 180 ° / 270 ° respectively and the defection phase angles are 0°/15°/ 30°/45°/ 60°/75°/ 90° respectively. The simulation results are as shown in Fig.5 flowing. From Fig.5, it shows that the casing stress is greatly affected by the defection of the cement ring under the non-uniform stress condition. When the angle of the defection angle increases, the casing Von Mises stress increases rapidly, then decreases slowly, and finally keeps at a certain level. But the stress is still great. In the same defection angle, the casing Von Mises stress increases with the increase of the defection phase angle, but the increment is not great. When the phase angle is greater than 60°, the casing stress is at the high level. The reason for the above can be explained as follows. When the defection angle of the cement ring is small, the casing can hardly deform, so the stress will be very great for stress concentration. But with the increase of defection angle, the casing can deform freely, which alleviate the stress concentration effect of the casing and make its stress decrease. Even though, the stress is still much larger than that of good cement sheath. It can be concluded that good cement sheath is very important in horizontal shale gas wells. 4.2. The influence of temperature