大压差条件下水泥环密封完整性分析及展望

为解决目前页岩气井、深井油气勘探开发中广泛存在的环空带压问题,结合理论研究成果及实验室模拟结果,分析得出了目前在"深层、低渗透、非常规"油气资源勘探开发过程中,大压差条件是造成水泥环密封完整性失效主要原因的认识。通过对四川盆地页岩气井、安岳气田和塔里木盆地库车山前等区块高温高压深井在钻井、压裂、生产过程中大压差工况进行梳理和定量计算,运用计算结果解释了上述两个地区气井在大压差条件下水泥环密封完整性失效的原因:加载阶段水泥环周向出现的拉应力、加载—卸载工况使水泥环产生塑性变形、在卸载过程中界面产生拉应力等,造成水泥环拉伸破坏及界面微环隙,从而破坏密封完整性。归纳总结了井筒全生命周期各环节出现密封完整性失效的风险,初步提出了大压差条件下保障水泥环密封完整性的应对措施:①提升水泥石抗变形能力;②提高第一界面及第二界面胶结力;③提高界面接触力。

Analysis and perspective of cement sheath integrity under a high differential pressure

This paper aims to solve the issue of differential pressure in the annulus commonly existed in shale gas and deep oil & gas well drilling. The reasons for cement sheath integrity failures mainly caused by a high differential pressure in the exploration and development process of deep, low-permeability, unconventional oil and gas resources were achieved from theoretical research and experimental simulation. Then, in the case studies of shale gas wells, HTHP wells in the Anyue gasfield, Sichuan Basin, and the Kuche Shanqian block in the Tarim Basin, differential pressures during drilling, fracturing and production process were sorted and quantitative calculation was made to further interpret the above achievement, that is, the cement sheath integrity failure by micro-annulus resulted from tensile stress under gravity-free condition in the solidification period of cementing, plastic deformation in the cement-sheath under loading and unloading working conditions, tensile stress in the interfaces in the process of unloading. Patterns of cement sheath integrity failures under a high differential pressure were analyzed, and the risks of integrity failures during different periods of a wellbore full life cycle were summarized. Preliminary measures and techniques to ensure cement-sheath integrity were proposed as follows: to improve the deformation resistivity of cement stones and to enhance the cementing force of the primary and the second interfaces as well as the force of interface contact.