Equivalent circulation density (ECD) at bottom hole is an important parameter affecting the safety of drilling operations. Accurate prediction of ECD is the prerequisite for safe drilling operations. Presently in China the prediction of ECD is often to calculate with traditional hydraulic models. Calculation of ECD with traditional hydraulic models gives results that roughly reflect the change pattern of ECD, but the precision of the model used and the fuzziness of input parameters for the model make it difficult to get accurate ECD; the calculated ECD is quite different with the actual ECD, and there exist uncertainties in the prediction of ECD. It is therefore necessary to perform uncertainty analysis on the predicted ECD on the bottom of the hole. Based on the comprehensive analyses of models for quantitative calculation of ECD, the sources from which the uncertainty of ECD is produced are discussed first in this paper, then a formula for ECD uncertainty calculation is derived on the basis of the Uncertainty Theory, and a case history of ECD uncertainty calculation is presented. The discussions made in this paper have presented a new clue for scientifically describing ECD, and provided a technical support of safe drilling operations.%井底当量循环钻井液密度(ECD)是影响钻井安全的重要参数,准确预测ECD是保障钻井安全有效进行的前提.目前,中国钻井作业大多数仍采用传统的水力学计算模型对ECD进行预测.根据传统的钻井水力学计算模型得到的结果能够大致反映ECD的变化趋势,但是由于其模型的精度以及计算模型中输入参数的模糊性等问题,使得准确预测ECD难度大,计算值与实际值之间必然存在误差,存在一定的不确定性.因此,需要对钻井过程中ECD的预测结果进行不确定性分析.笔者在综合分析ECD定量计算模型的基础上,首先讨论了ECD不确定性的来源,然后基于不确定度理论推导了ECD不确定度计算公式,并进行了实例分析,为ECD的科学描述提供了一种新思路,为保障钻井安全提供技术支持.
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