永川南区深层页岩气水平井小井眼井壁稳定性研究

Wellbore stability of slimhole in deep shale-gas horizontal wells,Yongchuan southern area

四川盆地南部(以下简称川南)深层下志留统龙马溪组页岩地层受高应力和钻井液水化影响,部分水平井发生井壁失稳。为了探讨深层页岩气水平井6.5 in小井眼建井的可行性,以川南永川南区深层龙马溪组页岩地层为研究对象,开展了XRD、SEM以及不同工作液条件下的膨胀率、岩石力学参数等基础理化测试,并结合力学—化学耦合、流体—固体耦合作用的井壁失稳模型研究,明确了永川南区龙马溪组储层页岩在钻井液浸泡前后、不同井眼尺寸和不同岩石尺寸条件下的井壁稳定性变化规律。研究结果表明:①永川南区龙马溪组页岩整体致密,矿物定向分布,石英、方解石含量高,岩性硬而脆;②黏土含量较少,以伊利石、绿泥石为主,含少量伊蒙混层,且水基、油基钻井液条件下膨胀应变普遍低于5‰,吸水膨胀能力弱;③井下压力环境下页岩具有较高的力学强度,油基钻井液对页岩力学强度的影响变化总体在10%以内。结论认为:①钻井液浸泡前后,不同井眼尺寸对井壁稳定性影响不大;②6.5 in小井眼井壁稳定性较8.5 in井眼更优,能够满足小井眼建井井壁稳定性需求。

Impacted by high stress and drilling-fluid hydration,some horizontal wells in southern Sichuan Basin suffer wellbore instability in deep shale of Longmaxi Formation.So,the deep shale of Longmaxi Formation in Yongchuan south⁃ern area was taken as an example to probe the feasibility of slimhole shaft building with 6.5-in in these deep wells.And XRD and SEM,along with basic physical-chemical tests of expansion ratio and rock mechanical parameters were conducted under different working-fluid conditions.Additionally,change laws of wellbore stability under different sizes of hole and rock before and after being soaked by drilling fluid were pointed out through combining with instability models under the action of mechanical-chemical coupling and fluid-solid coupling.Results show that(i)the shale in this formation is over⁃all tight whose minerals emerge as a kind of oriented extension with rich quartz and calcite,and it is hard and brittle;(ii)there exists a little clay in the shale,being dominated by illite and chlorite with a little illite and smectite interlayer.The swelling strain is generally lower than 5‰with the poor water-swelling capacity under conditions of water-and oil-based drilling fluid;and(iii)in the environment of downhole pressure,the mechanical strength is stronger,and the influential change of oil-based drilling fluid is within 10%globally.In conclusion,various hole sizes before and after drilling-fluid soaking exert less influence on the wellbore stability.What's more,superior to 8.5-in one,6.5-in slimhole can meet re⁃quirements for the wellbore stability of slimhole shaft building.