Abstract:

Due to the small casing diameter and the insufficient gas tightness of casing coupling threads，the old well cannot be converted into an injection-production well for the underground gas storage. Therefore，the old wellbore must be plugged and reconstructed to ensure effective plugging. An old well of the Ye County gas storage in Henan Province was taken as an example，and the well section to be milled and plugged was determined according to the formation lithology and well diameter. A three-dimensional finite element model of the cement plug-formation system based on Cohesive elements was built for the milling-plugging old well section，the peeling failure laws of the cement plug-formation interface under the constant-pressure condition were clarified，and the limit peeling length of the interface was computed. The results show that for old wells without perforated sections，it is necessary to set cement plug barriers in the cap rock and salt sections，respectively，to prevent natural gas leakage in the vertical and lateral directions. Under the gas storage condition with a constant pressure of 20 MPa，the bottom of the cement plug presents slight deformation after being compressed，resulting in the shear deformation of the cement plug-formation bonding interface，and peeling occurs，as such deformation accumulates to a certain extent. With the increasing simulation time，the length of the peeling interface gradually increases and approaches the equilibrium，and the ultimate peeling failure length is 18.4 m. Moreover，the peeling length decreases with the growing elastic modulus of the cement plug. It is recommended to use the high-elastic-modulus cement slurry system for well plugging. The findings of this research help to decide whether the milled-plugged section length of the old well in the field is sufficient，and provide theoretical guidance for ensuring the long-term safe operation of the gas storage.

Key words: gas storage, plugged section, cement plug-formation interface, peeling failure, limit peeling length, numerical simulation

摘要：

受限于套管直径小、套管连接螺纹不具备气密封性等因素，老井无法直接转为储气库注采气井，因此老井井筒必须进行封堵改造，并保障封堵的有效性。以河南叶县储气库某老井为例，根据地层岩性、井径情况，确定了老井段铣封堵层位，建立了基于内聚力模型单元（Cohesive）方法的盐穴储气库段铣封堵老井水泥塞-地层系统三维有限元模型，明确了储气库定压条件下水泥塞-地层界面剥离失效规律，得到了界面的极限剥离长度。研究表明：对于无射孔段老井，为防止天然气在纵向、横向上的泄漏，需要在盖层段和盐层段分别建立水泥塞屏障体；储气库压力恒定20 MPa条件下，水泥塞底部受压后会发生一定程度微小形变，致使水泥塞-地层胶结界面产生剪切方向变形，累计到一定程度后即发生剥离；随着模拟时间增加，剥离段长度逐渐增加并趋于稳定，极限剥离失效长度为18.4 m；水泥塞-地层界面剥离失效长度随水泥塞弹性模量的增大而降低，建议使用高弹模水泥浆体系进行井筒封堵作业。研究结果有助于判断现场老井段铣封堵长度是否足够，为保障储气库长久安全运行提供理论指导。

关键词:

储气库, 封堵层位, 水泥塞-地层界面, 剥离失效, 极限剥离长度, 数值模拟