页岩气井套管内压周期变化对水泥环密封失效的实验研究

压裂、回注、储气库注采等作业将造成套管内压的循环变化,最终导致水泥环的密封失效.针对前人套管内压循环变化造成水泥环密封失效破坏的实验研究并结合最新的实验结果,分析了是否考虑地层岩石约束两种井眼系统工况条件下的水泥环失效破坏方式.分析结果显示:未考虑地层岩石约束的实验易导致水泥环的周向拉伸破坏而产生径向裂纹;套管-水泥环界面过渡区的存在能降低水泥环的径向压应力和周向拉应力,从而降低水泥环产生拉伸破坏的风险;考虑地层岩石的约束时,水泥环内外界面受到套管和地层岩石的径向和周向约束,阻止了已经达到拉伸强度极限的水泥环发生拉伸破坏而产生径向裂纹的趋势;套管内压增大时,井下工况中水泥环主要发生塑性屈服破坏产生微环隙,基本不会发生周向拉伸破坏.研究成果可为固井优化设计提供参考.

Experimental analysis and Discussion on cement sheath failure caused by the varied casing internal pressure in shale gas wells

Fracturing or gas storage and other operations will cause the cyclic change of casing internal pressure, which eventually lead to the sealing failure of cement sheath. According to the test results of cement sheath failure caused by cyclic loading of casing internal pressure in previous work, the failure types of cement sheath under two conditions on whether formation rock is considered or not are analyzed. The analytical results show that the radial compressive stress and the circumferential tensile stress on the cement sheath can be reduced by the existence of the interface transition zone between the casing and the cement sheath. The risk of tensile failure of the cement sheath can also be reduced. When the constraints of the formation rock is considered, the radial and circumferential deformation of internal and external wall of the cement sheath are constrained by the casing and formation rock, which can prevents the cement sheath reaching the tensile strength limit and avoid tensile failure. With the increase of the internal pressure in casing, plastic failure of cement sheath occurs and the micro annulus is generated on interfaces. The results of this paper can provide reference for cementing optimization design.