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

作者词汇基础扎实，拼写也很棒；错误句子很少，但从句使用不熟练；行文流畅，但衔接词比较少。

In this paper, the actual casing damage of shale gas plays in Sichuan is investigated, and the cementing quality of fracturing section is statistically analyzed. The results show that there is a great correlation between poor cementing quality and casing damage. This is the same with other scholars' conclusions (De Andrade, J. et al, 2014, Peng, Q. et al, 2015, Yu H. et al. 2015, Fan, M. et al. 2016). The main manifestation of poor quality of wells is the cement ring channeling. By statistically analyzing the cementing quality of X shale well in Sichuan shale gas plays, the cementing quality at casing collapse section is obtained, as shown in Fig.1, 2. Fig. 2. SLG(Solid-Liquid-Gas) imaging of X shale well (Brown representing Cementing Ring, Green representing Liquid, Red representing Gas). From the chart above, the cementing quality in some fracturing sections is poor. Even worse, there have be serious casing damage problems in these sections of X shale well. From this situation, it can be concluded that there should be a great relationship between poor cementing quality and casing collapse. 2.2. Fracturing operation analyzing During fracturing operation, high pressure fracturing fluids are pumped into the shale formation, and the temperature near wellbore dropped sharply. Then, great stress along casing string will appear. All of these stresses will have very great influence on wellbore integrity. Therefore it is necessary to investigate how these factors such as the cement channeling, temperature changing and fracturing pressure, affect casing stress, even the wellbore integrity. After setting the cement slurry into the well hole, the confining wall, cement ring and casing string will be consolidated as a whole. It is assumed that the contact area between them is well cemented and no slip appears. For the sake of convenience of study, assuming that the assembly is homogeneous isotropic and elasticity (Li, J. et al. 2004). Since the axial dimension of CCF is much larger than the radial dimension, the assembly is simplified to the plane strain model according to the theory of rock mechanics and elastoplastic mechanics (Yu, H. et al. 2014). For this reason, there is only strain in the radial direction and no strain in the axial direction. Therefore, the plane FEM of the combination of CCF is established. As shown in Fig.3. Using this FEM, the influence of cement defection, temperature change and fracturing pressure on casing stress is studied.