油气井井周失稳区域研究现状

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本文摘要：（由ai生成）

井壁稳定性是钻井工程的关键，Zoback建议崩落角度范围。钻井液密度、地应力场和井眼轨迹影响井壁失稳。Haimson发现井壁在水平最小地应力方向坍塌。层理或裂缝地层需考虑各向异性。多场耦合效应导致井壁坍塌失稳。井壁稳定性研究通过本构模型、有效应力分布和失效准则确定钻井液密度上下限及失稳区域，为钻井提供科学依据。

现场钻井过程中发现，在可控范围内的井壁崩落不会导致井壁的坍塌失稳，Zoback^[160]等定义了被工程广泛接受的崩落建议范围，即直井允许崩落角度为90°，水平井允许崩落角度为30°；若能允许井壁一定角度的崩落控制井周失稳区域而不至于导致井壁的坍塌，即可达到降低钻井液密度的目的，进而提升钻速，减小抽汲和激动压力，提高钻井液流变性等等，为油气资源开采带来巨大的经济效益^[161]^-^[167]。对井周失稳区域的研究目前尚较为少见，魏凯^[168]等认为井壁失稳区域的面积是反映井壁稳定性的重要指标，直接反映了井壁坍塌掉块体积的多少，表征了井壁坍塌风险的程度，并分析了钻井液密度、井眼轨迹对井壁失稳区域特性的影响，研究表明钻井液密度、地应力场类型和井眼轨迹均对井壁失稳区域类型、面积及其严重程度影响显著；Haimson^[169]借助光学显微镜和扫描电子显微镜，研究了不同地应力状态下的花岗岩、石灰岩和砂岩地层中的直井井壁微观力学失稳机理，结果表明不同岩性地层，井壁均在水平最小地应力方向坍塌，形成“月牙状”井眼；根据岩石类型的不同，这些微裂缝可以是拉伸或剪切开口，在颗粒间或颗粒内延伸。井壁在水平最小地应力方向的坍塌是一种非膨胀的微观机制的结果，这种机制包括岩石局部的晶粒脱粘和重新堆积，并由应力集中形成孔隙度降低的压实带，并在循环钻井液的帮助下，去除在压实带形成过程中脱落的松散颗粒和岩石碎块，逐渐形成椭圆形井眼。Zoback^[160]对上千口实钻直井的研究证实了井壁崩落发生在水平最小地应力方位，如图1-13，在加拿大Lac do Bonnet花岗岩地层钻进的直井和水平井井眼成像图均清晰地说明了在最小地应力方向，井壁发生明显垮塌。

Hashemi^[170]等采用厚壁圆筒实验研究了疏松砂岩地层井壁失稳机理，实验结果分析还发现选择合适的失稳判别准则对于预测井壁坍塌行为具有重要意义，当井眼内无压力作用时，Mohr-Coulomb准则不适用于预测井眼破坏，而Mogi-Coulomb准则更能反映不同含水率疏松砂岩的井壁失稳机理；丁立钦等^[171]发现MC准则预测井壁崩落区域比MGC准则预测的大，而范翔宇等^[31]研究了MC、MGC和EMGC三种岩石强度准则对井周剪切崩落区域的影响，研究发现当井底压力低于坍塌压力时，MC准则预测井眼形状为椭圆形，而E-MGC准则预测井眼形状近似四边形，且预测井周坍塌失稳区域面积更大；Meier等^[172]研究发现，随着井眼直径的增加，成核破裂所需的临界压力显著降低，坍塌失稳区域随井眼直径的增大逐渐减小。

以上研究与结论在常规储层中取得了较好的预测和应用效果，但在层理或裂缝发育地层，需考虑强度和弹性各向异性因素的影响，重新研究井周失稳区域的分布特征^[173]^-^[177]。Lee等^[161]考虑了页岩弱面结构，研究了弱面对井眼坍塌区域形状的影响，结果表明井眼轨迹与裂缝面的相对位置及原地应力机制对井眼坍塌区域形状影响显著；Li等^[178]、Wang等^[179]、马天寿等^[180]、Meier等^[181]均考虑了页岩层理的弹性各向异性、强度各向异性对井壁稳定的影响，建立了页岩地层井周失稳区域模型，研究发现考虑裂缝影响计算出的失稳区域的形状不是传统的“月牙”形状，而是近似长方形，如图1-14所示，考虑到岩石的弹性和强度各向异性，井壁的坍塌方位并不指向前人认为的水平最小地应力方向^[182]^-^[190]，因此，在基于成像测井资料观测到的井壁垮塌方位预测地应力方位时，应同时考虑岩石的弹性和强度各向异性。

图1-13 在Lacdo Bonnet花岗岩地层钻进的井眼成像图^[169]

图1-14 层理页岩地层井周失稳区域分布示意图^[179]

在考虑多场耦合对井周失稳区域影响的研究中，大量成果证明地层钻开后，井壁坍塌失稳首先发生在地层内部，且坍塌呈现一定时间效应；钻井液与井周围岩的接触，会引起岩石水化，水化带内岩石含水量随井周半径和时间而变化，岩石力学和强度特征也会随含水量而变化，所以含水带内的岩石变成了变含水、变模量和变强度的复杂岩体介质^[192]^-^[197]。除多场耦合作用外，真实地层中岩石可能随机发育多条裂缝，裂缝发育地层井周失稳规律仍有待研究^[198]。综上所述，井壁稳定性研究的基本方法是通过建立描述地层力学特点的本构模型，得到井周有效应力分布，再结合失效准则计算支撑井壁稳定的钻井液密度上下限，可进一步确定井壁失稳区域，同时地层力学参数还受到层理、弱面、多场耦合作用的影响，且具有时间效应，且对井周失稳区域的研究可为钻井工程设计、井眼轨迹优化提供了更加全面的指导意见。

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来源：现代石油人