Wellbore stability model in shale formation under the synergistic effect of stress unloading-hydration  

作　　者：DING Yi LIU Xiangjun LIANG Lixi XIONG Jian LI Wei WEI Xiaochen DUAN Xi HOU Lianlang 

机构地区：[1]State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu 610500,China [2]School of Geoscience and Technology,Southwest Petroleum University,Chengdu 610500,China [3]School of Science,Southwest Petroleum University,Chengdu 610500,China

出　　处：《Petroleum Exploration and Development》2023年第6期1478-1486,共9页石油勘探与开发（英文版）

基　　金：Supported by the National Natural Science Foundation of China(42202194);Science and Technology Cooperation Project of the CNPC-SWPU Innovation Alliance(2020CX040102).

摘　　要：According to the transversely isotropic theory and weak plane criterion, and considering the mechanical damages due to stress unloading and hydration during drilling, a shale wellbore stability model with the influence of stress unloading and hydration was established using triaxial test and shear test. Then, factors influencing the wellbore stability in shale were analyzed. The results indicate that stress unloading occurs during drilling in shale. The larger the confining pressure and axial stress, the more remarkable weakening of shale strength caused by stress unloading. The stress unloading range is positively correlated with the weakening degree of shale strength. Shale with a higher development degree of bedding is more prone to damage along bedding. In this case, during stress unloading, the synergistic effect of weak structural plane and stress unloading happens, leading to a higher weakening degree of shale strength and poorer mechanical stability, which brings a higher risk of wellbore instability. Fluid tends to invade shale through bedding, promoting the shale hydration. Hydration also can weaken shale mechanical stability, causing the decline of wellbore stability. Influence of stress unloading on collapse pressure of shale mainly occurs at the early stage of drilling, while the influence of hydration on wellbore stability mainly happens at the late stage of drilling. Bedding, stress unloading and hydration jointly affect the wellbore stability in shale. The presented shale wellbore stability model with the influence of stress unloading and hydration considers the influences of the three factors. Field application demonstrates that the prediction results of the model agree with the actual drilling results, verifying the reliability of the model.

关 键 词：SHALE DRILLING BEDDING stress unloading HYDRATION shale strength wellbore stability 

分 类 号：TE24[石油与天然气工程—油气井工程]