基于裂缝诱导应力场的套管应力影响因素分析

Analysis on Influential Factors of Casing Stress Based on Fracture Induced Stress Field

水平井分段压裂形成人工裂缝,产生的裂缝诱导应力场影响套管应力分布。为研究在裂缝诱导应力场作用下套管应力影响因素,基于理论公式和有限元分析方法,建立套管应力力学模型和裂缝-地层-水泥环-套管三维有限元模型,借助力学模型验证有限元模型的可行性,模拟岩石弹性模量、套管内壁压力、水泥环弹性模量、地应力变化和施工排量对套管应力的影响,并针对徐深气田实际压裂段进行实例分析。研究结果表明:当岩石弹性模量低于15 GPa时,套管发生塑性形变,套管内壁无压力加剧了套管塑性变形程度;当套管内壁压力为100 MPa时,水泥环软硬程度对套管应力基本无影响;套管应力随着水泥环弹性模量的增大呈现增大的趋势,水泥环泊松比和软硬地层不改变套管应力变化趋势;随着地应力差值的增大,套管应力呈现减小的趋势;施工排量为16 m^(3)/min时,套管存在塑性变形,达到极限施工排量18 m^(3)/min时,套管完全失效。研究成果对压裂段套管的损坏防治具有一定的指导意义。

The staged fracturing of horizontal well creates artificial fractures, and the resulting fracture induced stress field affects the distribution of casing stress.In order to identify the influential factors of casing stress under the action of fracture induced stress field, based on the theoretical formula and finite element analysis method, the mechanical model of casing stress and the 3D finite element model of fracture, formation, cement sheath and casing were built.With the help of the mechanical model, the feasibility of the finite element model was verified.The influences of rock elastic modulus, casing inner wall pressure, cement sheath elastic modulus, in-situ stress change and pumping rate on casing stress were simulated.Finally, case analysis was carried out for the actual fracturing section of Xushen Gas Field.The study results show that when the elastic modulus of rock is lower than 15 GPa, plastic deformation occurs in casing, and the absence of pressure on the inner wall of the casing aggravates the plastic deformation of the casing.When the inner wall pressure of the casing is 100 MPa, the hardness of the cement sheath basically has no effect on the casing stress.The casing stress increases with the increase of the elastic modulus of the cement sheath, and the Poisson's ratio of the cement sheath and the soft and hard strata do not change the variation trend of casing stress.With the increase of in-situ stress difference, the casing stress shows a decreasing trend.When the pumping rate is 16 m^(3)/min, the casing has plastic deformation;when the pumping rate reaches the extreme of 18 m^(3)/min, the casing completely fails.The study results have certain guiding significance for the prevention and control of casing damage in fracturing section.