页岩气井体积压裂条件下的水泥环界面裂缝扩展

射孔会对井筒完整性造成局部损伤,尤其是导致水泥环本体破坏以及第一界面(套管-水泥环界面)、第二界面(水泥环-围岩界面)的胶结脱离、微环隙等,在后期页岩气水平井体积压裂过程中,由于高压流体的注入,界面微环隙将会加剧甚至发生裂缝扩展,影响封固性能。针对这一问题,建立了界面裂缝扩展数学模型,通过流体连续性方程、泊肃叶定律和净压力与缝宽弹性关系以及边界条件联立得到非线性偏微分方程组,利用对控制方程无因次化的方法求得自相似解。使用该模型对现场算例进行了计算,并分析了各参数对界面裂缝长度的影响。研究结果表明:适当提高水泥环弹性模量,适当降低井口压力和压裂时间有利于减小界面裂缝扩展长度,有利于保障水泥环长期有效封固。研究结果能够对页岩气井体积压裂条件下的水泥环封固性能进行评价和预测,同时在满足油气开采要求的前提下,对相邻射孔段间距的优选,也具有一定的指导意义。

Cement ring interface crack propagation under volume fracturing in shale gas well

Perforating can result in local damage to wellbore integrity, especially the destruction of cement ring as well as the debonding or micro-annulus of the first interface(between casing and cement ring) and the second interface(between cement ring and surrounding rocks). In the later volume-fracturing process in shale gas horizontal well, due to the injection of high-pressure fluid, interface micro-annulus will become aggravated and even lead to fracturing propagation, which will influence the sealing performance. To solve this problem, a mathematical model of interface crack propagation was established. Nonlinear partial differential equations were obtained by the fluid continuity equation, Poiseuille law, the elastic relationship between net pressure and seam width as well as the boundary conditions. Meanwhile, the self-similar solution was solved by governing equation dimensionless method. This model was used to calculate field cases and analyze the influences of each parameter on interface crack length. The study results show that proper increase of cement ring's elastic modulus and reduction of wellhead pressure and fracturing time is beneficial to reduce the interface crack propagation length and guarantee long-term effective seal. The study can be used to evaluate and predict the cement ring sealing performance under volume fracturing in shale gas wells, and provide guidance for the optimization of adjacent perforation spacing on the premise of satisfying hydrocarbon production requirements.