水合物储层安全钻井液密度下限确定方法

DETERMINING METHOD OF THE SAFE LOWER LIMIT OF THE DRILLING FLUID DENSITY IN HYDRATE-BEARING FORMATIONS

水合物储层钻井过程中的水合物分解会导致井眼的坍塌和失稳。利用人造泥质细粉砂水合物岩样三轴力学实验结果得到了改进的Mohr-Coulomb强度准则,并得到了水合物储层坍塌压力的确定方法,分析了水合物地层钻井过程中坍塌压力的温度敏感性。结果表明:相同的井身结构条件下,利用改进的Mohr-Coulomb强度准则计算得到的钻井液密度下限较大,并且水合物地层的钻井液安全密度下限存在温度敏感性;当钻井液温度低于288. 14 K时,不同井眼轨迹条件下的钻井液密度下限不受温度影响,但随着温度的升高,钻井液密度下限的温度敏感性逐渐显现。

Hydrate dissociation during the drilling operation in the hydrate formation can result in the potential risk of the wellbore collapse. Based on the triaxial mechanical test results of the artificial muddy fine-silt hydrate samples,Mohr-Coulomb strength criterion was modified and improved. And moreover the determining method of the collapse pressure of the hydrate formation was obtained; the temperature sensitivity of the collapse pressure was analyzed in the process of the hydrate formation drilling. The achievements show that under the condition of the same borehole structure,the lower limit of the drilling fluid density calculated by the modified Mohr-Coulomb strength criterion is more larger,and moreover the safe lower limit of the drilling fluid density in the hydrate formation is sensitive to the temperature; when the drilling fluid temperature is lower than 288. 14 K,the lower limits of the drilling fluid density for different wellbore trajectories are not affected by the temperature,while with the rise of the temperature,the temperature sensitivity of the lower limit of the drilling fluid density gradually becomes apparent.