摘要
川渝气区的部分井因井内压力大幅度改变,导致水泥环密封完整性失效而发生气窜,引起环空带压。为此,考虑水泥环初始应力状态及井内压力变化特点,建立了水泥环力学模型,论述了页岩气井管柱试压和大型体积压裂、川中高压气井固井后大幅度降低钻井液密度、地下储气库井周期性注采作业等典型工况下水泥环密封完整性可能破坏的形式。研究结果表明:试压和压裂可能导致水泥环周向拉伸破坏,形成径向裂缝;井内压力大幅度降低,使套管壁处水泥环承受的径向拉应力超过抗拉强度,破坏界面胶结,形成微环隙;周期性交变载荷可能引起水泥环疲劳破坏。相应的针对性技术对策为:根据后期作业井内压力变化,选用力学性能匹配的弹性水泥浆或柔性自应力水泥浆;常规套管固井在碰压后立即进行管柱试压;固井后井内压力大幅度下降的井,应用径向预应力固井技术和封隔器防止气窜。
Due to the cement sheath integrity failures by a sharp borehole pressure change,gas channeling easily occurred in some wells of Sichuan and Chongqing oil and gas fields.To this end,considering the initial stress of cement sheath and the characteristics of downhole pressure change,a cement sheath mechanical model was established.And the possible cement sheath integrity failures were also discussed under several typical working conditions,including string pressure test and large fracturing of shale gas wells,precipitous decline of drilling fluid density of high pressure gas wells in eastern Sichuan Basin,periodic injection and production of underground gas storage wells.The study results showed that （1） circumferential tensile fracture may occur on cement sheath caused by pressure test and fracturing,which induces radial cracking; （2） an interface bond may be damaged by a precipitous decline of borehole pressure which makes the radial tensile stress borne by cement sheath at casing exceed tensile strength,inducing microcir-culation; and （3） the cement sheath fatigue failure may appear due to a periodic alternating load.Therefore,countermeasures were presented as follows：to choose elastic cement or flexible selLstressing cement with proper mechanical properties according to bore-hole pressure change at the later stage; to perform the drill stem test （DST） as soon as the common casing cementing is upon the completion of bump-pressure; and to adopt the packer and radial pre-stress cementing technology to prevent gas channeling for such wells with a sharp pressure drop after cementing.
出处
《天然气工业》
EI
CAS
CSCD
北大核心
2014年第4期95-98,共4页
Natural Gas Industry
基金
中国石油天然气集团公司重大现场试验项目(编号:2012f-41)
关键词
水泥环
完整性
页岩气
高压气井
地下储气库
弹性水泥
柔性自应力水泥
cement sheath
integrity
shale gas
high pressure gas well
underground natural gas storage
elastic cement
flexible self-stressing cement
