页岩气井水泥环完整性研究

摘要

四川盆地内的几个国家级页岩气示范区气井环空带压、套管变形等问题较为突出.为此,通过分析页岩气钻完井过程井筒内压力变化的条件,建立了计算井筒压力变化全过程的应力—应变状态下的套管—水泥环—围岩体系弹塑性力学模型,应用该模型分析了变内压下第一界面、第二界面微环隙的产生和发展,进而结合实际现场数据进行计算.结果表明:①加载过程不会产生微环隙,但加载内压的大小决定了水泥环塑性变形的程度;②卸载时内压的降低将导致界面受拉,从而产生微环隙,破坏水泥环的完整性;③可以通过优选水泥浆配方,提高水泥环力学性能,结合施工工艺措施优化来保障页岩气井水泥环完整性.13口井的现场试验应用结果表明,水平段固井质量平均优质率达92.67％,其中已投产的11口井均未出现环空带压,有效地改善了页岩气示范区的气井环空带压问题.该项研究成果可为同类区块固井水泥环完整性管理提供帮助.%Sustained annulus pressure and casing deformation of gas wells are common in several shale gas demonstration areas in the Sichuan Basin.In view of this,the conditions for pressure change inside the wellbore in the process of drilling were analyzed,and then an elastic-plastic mechanical model of casing-cement sheath-surrounding rock system for calculating the whole change process of wellbore pressure in the stress-strain state was established.By virtue of this model,the generation and development of micro-annulus at the first and second interfaces under various internal pressure conditions were analyzed.Then,combined with the actual field data,calculation was performed.Results show that no micro-annulus is generated in the process of loading,but the loaded internal pressure determines the plastic deformation degree of cement sheath.Besides,the decrease of internal pressure during the unloading exerts tension on the interface,leading to the generation of micro-annulus,and consequently the integrity of cement sheath is damaged.Finally,the mechanical performance of cement sheath can be improved by optimizing the slurry formula,and operation technologies are used to guarantee the integrity of cement sheath in shale gas wells.This model was applied in 13 wells for field tests.It is indicated that the average good quality rate of horizontal section cementing is 92.67％,and 11 wells which have been put into production are not subjected to sustained annulus pressure.It is demonstrated that the sustained annulus pressure of gas wells in shale gas demonstration areas is improved effectively.This research achievement can be used as a reference for integrity management of cement sheath in the similar blocks.