页岩气井压裂交变载荷水泥环密封能力研究

Study on Sealing Ability of Cement Sheath Under Cyclic Loading in Shale Gas Well Fracturing

页岩气井压裂后的环空带压问题严重影响页岩气安全高效开发,且低弹性模量水泥浆体系及环空加压固井工艺有效降低页岩气井环空带压力的力学机理尚未明确。为此,针对页岩气井水平段和垂直段井筒结构差异,考虑水泥石残余应变,建立了环空加压固井提高水泥环界面径向应力与密封能力的计算方法,分析了环空加压压力和残余应变对界面径向应力的影响规律。研究结果表明:页岩气井压裂过程中套管内压周期变化将导致水泥环内外界面产生较大的径向循环载荷,进而引起水泥环残余应变和界面微环隙,最终造成水泥环密封完整性失效;针对不同水泥环残余应变值需控制环空加压压力下限以满足界面密封能力要求,环空加压固井增强垂直段水泥环密封能力的效果显著大于水平段的效果。现场应用效果显示,提供的模型计算结果与现场实际相符,可为该技术工程推广及应用提供理论支持。

The problem of annular pressure buildup(APB)after shale gas well fracturing seriously affects the safe and efficient development of shale gas,and the mechanical mechanism of the low elastic modulus cement slurry system and annular pressurization cementing process effectively reducing the annular pressure of shale gas wells is not yet clear.Therefore,aimed at wellbore structure difference between horizontal and vertical sections of shale gas wells,considering the residual strain of hardened cement,a calculation method for improving the radial stress and sealing ability of the cement sheath interface through annular pressurization cementing was established,and the influence of annular pressurization pressure and residual strain on the radial stress of the interface was analyzed.The study results show that the cyclic variation of casing internal pressure during shale gas well fracturing leads to significant radial cyclic loading at the inner and outer interfaces of the cement sheath,then leads to residual strain and interfacial micro-annulus of the cement sheath,and ultimately results in the failure of sealing integrity of the cement sheath;for different residual strain values of cement sheath,the lower limit of annular pressurization pressure needs to be controlled to meet the requirements of interfacial sealing ability;the effect of annular pressurization cementing on enhancing the sealing ability of cement sheath in the vertical section is significantly greater than that in the horizontal section.The field application effect shows that the model calculation results are consistent with the actual field situation,providing a theoretical support for the promotion and application of this technology in the shale gas fields.