油气井井周失稳区域研究现状
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。
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]
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