瞬态力-热耦合作用下水泥环完整性分析

Analysis on cement sheath integrity under transient thermo-mechanical coupling effect

页岩气井工程实践表明,套管压裂易导致水泥环完整性发生破坏出现环空带压。基于套管压裂工程实际,建立井筒温度场模型和套管-水泥环-地层组合体有限元模型,采用解析法和数值法结合方式,计算页岩气井压裂过程中瞬态力-热耦合对水泥环应力大小、分布影响规律。结果表明:压裂过程中水泥环内外壁温差先增大后减小,压裂初近内壁处存在陡峭温度梯度,易导致内壁应力显著提高;瞬态力-热耦合作用导致水泥环内壁应力大幅提升,加剧了水泥环完整性失效的风险,压裂初期为水泥环易发生损坏的"风险段";水泥环内壁最大应力随着时间变化,易产生"多裂纹"形态,加剧环空带压。研究结果可为页岩气井压裂过程中水泥环完整性设计控制提供参考。

The practical shale gas well engineering shows that casing fracturing can easily destroy the integrity of cement sheath,leading to sustained annular pressure(SAP).In this paper,the wellbore temperature field calculation model and the finite element model of casing-cement sheath-formation assembly were established based on the actual casing fracturing engineering.The effect laws of transient thermo-mechanical coupling on the magnitude and distribution of cement sheath stress were calculated by using analytical method and numerical method comprehensively.It is shown that the temperature difference between inside and outside the wall of cement sheath increases firstly and then decreases in the process of fracturing.Steep temperature gradient occurs near the inside wall at the initial fracturing and it tends to result in obvious increase of stress on the inside wall.Due to the effect of transient thermo-mechanical coupling,the stress on the inside wall of cement sheath is increased significantly,and the risk of cement sheath integrity failure is aggravated.The initial fracturing stage is the“risk period”when cement sheath failure tends to happen easily.As time goes,the maximum stress on the inside wall of cement sheath results in multiple cracks easily and makes the SAP more serious.The research results can provide the reference for the design and control of cement sheath integrity during the fracturing of shale gas well.