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；增加一些从句的使用，文章会更棒；连接词过少，建议增加衔接词汇的积累。

With the exploration of Changning-Weiyuan Shale Gas National Demonstration Zone in Sichuan basin of China in 2013, there were some wellbore integrity problems in several shale gas wells. For examples, casing string could hardly be set into the well hole, and cement sheath could not seal completely. Even worse, the casing in horizontal section collapsed during multi-stage hydro-fracturing operation. If this happened, the bridge plugs could not be seated at the right sections or drilled by grind tool which was set into the hole through the coiled tube. What’s more, some fracturing sections would have to be abandoned. All of these troubles have seriously affected the production of each shale gas well and restricted the exploration of shale gas plays. (Tian, Z. et al. 2015, Last, N.C. et al. 2006, Lian, Z. et al. 2015) From now on, the wellbore integrity have raised among many scholars. As discovered by those authors (Goodwin, K.J. and Crook, R.J., 1992), exposure of steel casing to excessive temperature increases or internal test pressures caused diametrical and circumferential casing expansion. This circumferential force created a shearing force at the cement/casing interface, causing failure at the cement/casing interface or radial fracturing of the cement sheath from the inner casing surface to the outer casing (or borehole) surface. Through the finite element studies (Berger, A. et al. 2004), the author presented the presence of voids in the formation or cement would result in a reduction of the collapse resistance of casing up to 60%. But the eccentricity had a minor effect on the collapse resistance of casing.