井壁失稳判断准则及应用分析

为了研究现场实际情况下适用的井壁失稳判断准则 ,根据井壁不稳定性理论和机理的分析 ,综合评价了库仑 -摩尔准则、Lade准则和 Drucker- Prager准则及其修正形式 ,以及每个准则的应用特点 .采用常规三轴压缩 Drucker- Prager破坏准则 ,进行了在水平各向同性地层和水平各向异性地层条件下 ,对保持井壁稳定所需的临界钻井液密度和临界钻井液含盐量与井眼方位角和井斜角之间的关系进行了分析和计算 .结果表明 :在水平应力各向同性的应力场 ,钻井液密度相对于井眼方位角显示了明显的对称性 ,但是 ,随着井斜角的增加而增加 ;在水平应力各向异性的应力场 ,钻井液密度相对于井眼方位角等于 90°处具有明显的对称性 ,随着井斜角的增加而增加 ,但井斜角等于60°～ 1 2 0°范围内例外 ;对于不同的井眼方位角和井斜角 ,为保持井壁稳定 ,钻井液应具有一定高的含盐量 .

Study and Application of Sidewall Instability Criteria

In order to provide practical sidewall instability criteria for oil fields, three criteria in common use and their modified formulae are evaluated based on the mechanism and causes of sidewall instability, and their characteristics are introduced. The relationships between the critical density and critical salt content of drilling fluid and hole azimuth and deviation angle in horizontally isotropic and anisotropic formation respectively, are studied. The results show that, (1) in horizontally isotropic earth stress field, the relationship between the critical density of drilling fluid and hole azimuth has symmetrical characteristic, and the critical density increases with the increase of deviation angle; (2) in horizontally anisotropic earth stress field, the relationship is symmetrical relatively to 90°hole azimuth, and the critical density also increases with the increase of deviation angle not including the range of 90°~120°; (3) when hole azimuth and deviation angle are given, the critical salt content of drilling fluid can be determined according to the result of the study in the paper.