考虑井眼尺寸影响的小井眼坍塌压力计算方法

针对小井眼井壁坍塌问题,基于岩石强度的尺寸效应理论,采用Hoek等人的强度尺寸效应经验公式建立了岩石单轴抗压强度与试样直径之间的关系;并对此经验公式进行了修正,考虑了黏聚力、内聚擦角等岩石强度参数与试样尺寸之间的关系,建立了考虑岩石强度尺寸效应的Mohr-Coulomb强度准则。得到了考虑井眼尺寸影响的坍塌压力计算方法,并提出了两种评价岩石强度尺寸效应的方法。研究结果表明,坍塌压力存在尺寸效应,相对于常规井眼,小井眼的坍塌压力当量密度较低;而不同性质地层岩石的强度尺寸效应是不同的,岩石的非均质性越强,裂缝越发育,小井眼钻井坍塌压力的尺寸效应越强,这为评估小井眼井壁坍塌风险提供了重要参考和依据。

Model for Slimhole Collapse Pressure Calculation Considering the Influence of Borehole Diameter

To solve the wellbore instability problem of slimhole, this paper presents a calculation method of collapse pressure base on the scale effect theory of rock strength. Based on the empirical relation between the uniaxial compressive strength and the specimen diameter proposed by Hoek, a modified empirical relation is studied in which the scale effect of internal frictional angle and cohesion is considered. A modified Mohr-Coulomb failure criterion with the influence of scale effect of rock strength is established. And the collapse pressure which considers the influence of borehole diameter is calculated, Two evaluation methods of size effect of rock strength are presented.The results show that the collapse pressure of slimhole is related to the borehole diameter. Compared with conventional wells, the collapse pressure of slimhole is lower, and this indicates that the slimhole is more stable than the conventional wells. Meanwhile, the scale effect of formation with different properties is also different. The reduction in strength is due to the nonhomogeneity and fracture development of rocks. It is suggested that the scale effect of collapse pressure is getting stronger with the increase of nonhomogeneity and fractures in the rocks. This provides important reference for the evaluation of slimhole collapse risks.