Abstract: Long horizontal section wells have become an increasingly important technology for the development of unconventional oil and gas resources. In the long horizontal section, severe cuttings bed accumulation with extensive distribution poses a high risk of cuttings-induced pipe sticking. To mitigate the risk, it is crucial to understand the mechanism by which cuttings beds generate resistance on the drill string and accurately quantify such resistance. The traditional linear contact friction model has limited applicability, as it relies on empirical friction coefficients, which does not meet the requirements for precise prediction. Therefore, based on the classical Coulomb friction theory while considering the surface contact relationship between drill string and cuttings bed, a new prediction model for cuttings bed friction has been developed. Sixty-six sets of experiments are conducted using a friction torque measuring device for the rotary drill string to validate the distributed force assumptions in the model and assess its prediction accuracy. The experimental and model prediction results indicate that the surface contact friction theory can explain the resistance exerted by the cuttings bed on the drill string motion. In contrast, the traditional linear contact friction model significantly underestimates the frictional resistance of the cuttings bed. The variation in cuttings bed friction can be divided into three stages, depending on the cuttings accumulation degree, and does not follow a simple linear or exponential trend. As the friction coefficient between cuttings bed and drill string increases, the frictional resistance of the cuttings bed also increases. When the top surface of cuttings bed is above the center of drill string, the cuttings bed density has an influence on the frictional resistance, and it increases as the density rises. The hypothesis of a sinusoidal distribution of normal pressure aligns more closely with the actual conditions. Compared with the experimental results, the average error rate of the predicted values from the final model is 7.56 %, which meets the engineering requirements. The findings indicate that the new cuttings bed friction model can provide valuable guidance for predicting and controlling the frictional resistance and torque of pipe string in drilling and completion operations.

Key words: long horizontal section well, cuttings bed, frictional resistance, frictional torque, cuttings-induced pipe sticking

摘要： 长水平段水平井作为开发非常规油气资源的重要技术手段越来越受到重视。长水平段中岩屑床堆积严重、分布范围广,极易发生沉砂卡钻事故。为了降低沉砂卡钻风险,亟需揭示岩屑床对钻柱阻力形成的机理,并准确量化岩屑床阻力,传统线接触阻力理论模型适用性有限,依赖经验式的摩阻系数不能满足精细化预测的需要。基于经典库伦摩擦理论,考虑钻柱与岩屑床之间的面接触关系,建立了岩屑床阻力预测新模型,并利用旋转钻柱摩扭装置开展了66组实验,对模型中分布力假设和预测精度进行了验证。实验和模型预测结果表明:面接触阻力理论能够解释岩屑床对钻柱运动产生阻力的原因,传统线接触阻力模型大大低估了岩屑床所产生的阻力;岩屑床摩擦力的变化随岩屑床堆积程度的不同可分为3个阶段,并非简单的线性或指数关系;随着岩屑床与钻柱摩擦系数增大,岩屑床摩擦力增大;当岩屑床顶面高于钻柱中心时,岩屑床密度才对摩擦力产生影响,岩屑床摩擦力随密度增加而增大;正压力的正弦分布假设更加符合实际情况。通过与实验结果对比,最终模型预测值的平均误差率为7.56 %,满足工程需要。

关键词: 长水平段水平井, 岩屑床, 摩擦阻力, 摩擦扭矩, 沉砂卡钻