川西气田长水平段井壁失稳机理及对策

Mechanisms on wellbore instability in long horizontal intervals of West Sichuan gasfield and countermeasures

四川盆地川西气田中三叠统雷口坡组气藏为潮坪相沉积,埋藏深、储层薄、井壁稳定性差,前期完钻11口水平井井壁失稳频发,影响开发效益。为了确保川西气田后续长水平井打成、打好、打快,有效降低井壁失稳复杂率,以川西海相水平段为研究对象,分析其井壁失稳机理,以及裂缝渗流对井壁稳定性的影响,制定对策措施。研究结果表明:井区井壁失稳主控因素为应力、裂缝、岩性;易塌层分布在最小水平应力钻进方向、裂缝发育区,掉块岩性为石灰岩或灰质白云岩;水基钻井液条件下,岩石强度下降了11.4%~17.5%,坍塌压力增加0.11MPa/100 m;油基钻井液条件下,不考虑裂缝渗流,岩石强度不会因油基钻井液浸泡及其浸泡时间而降低,考虑裂缝渗流,裂缝摩擦系数因油基钻井液浸泡下降12.8%,坍塌压力增加0.05~0.08 MPa/100 m。结论认为,按研究成果精细预测的钻井液密度窗口、水平段轨迹优化,确保了6口长水平井顺利完钻,井壁失稳复杂率降低43%,完井测试获天然气产量172×10^(4)~325×10^(4)m^(3)/d。

Featured by deep burial depth,thin thickness and poor wellbore stability,most gas reservoirs of Leikoupo For-mation in west Sichuan gasfield belong to tidal flat facies deposition.There existed frequent wellbore instability in 11 hori-zontal wells,which impacted on development benefits.So,in order to ensure the smooth,fast and successful drilling in sub-sequent long horizontal wells in this field and diminish instability complexities with effect,several marine horizontal inter-vals in western Sichuan Basin were taken as objectives to figure out instability mechanisms and the influence of fracture seepage on wellbore stability as well as to make countermeasures.Results show that(i)the main influential factors on well-bore instability in this block are stress,fracture and lithology;(ii)easy collapse is commonly occurred in certain areas with well-developed fractures along drilling direction of minimum horizontal stress,and collapse rocks are limestone or limy do-lomite;(iii)when adopting water-based drilling fluid,the rock strength decreases by 11.4%–17.5%and the collapse pres-sure increases by 0.11 MPa/100 m,respectively;and(iv)when employing oil-based drilling fluid,its immersion and soak-ing time may not lead to strength decrease if fracture seepage is not taken into account.If considering this seepage,howev-er,the fracture friction coefficient may decline by 12.8%,and the collapse pressure raises by 0.05–0.08 MPa/100 m due to this immersion.In conclusion,both drilling-fluid density window and trajectory of horizontal intervals,which may be ob-tained from precise prediction,ensure a smooth drilling in 6 long horizontal wells,drop complexities of wellbore instability by 43%,and achieve gas production of(172–325)×10^(4) m^(3)/d in the completion test.