修正设计系数的套管层次与下入深度设计方法

传统井身结构设计过程中用以确定安全钻井液密度窗口所采用的各类设计系数均采用经验或统计估算的方法,且在全井段各设计系数取值相同。对原有安全钻井液密度窗口确定准则中的部分公式和系数进行改进和修正,并提出一种修正设计系数的井身结构设计方法,新方法中各类设计系数将根据实际井条件精细计算,避免过度浪费安全窗口空间,使井身结构设计结果更为合理高效。以西非深水区块B-1井为例进行实例计算。结果表明,不同井段、不同井眼尺寸对各类设计系数具有明显影响,尺寸差距越大,设计系数的差异性也越大,其对井身结构设计结果的影响也越大。
In conventional casing designs for oil and gas well drilling, all the design coefficients used for the determination of safe drilling fluid density window are defined by empirical evaluation or statistical methods, and a consistent value of each coefficient is taken for all well sections. For drilling under complicated conditions, especially in deepwater drilling and at high temperature and high pressure (HTHP) conditions or for ultra-deep wells, the safe drilling fluid density window required is much narrower, and the conventional casing design method is not appropriate. When the values of the design coefficients are taken too low, it may lead to less safe drilling conditions, but when they are too high, it can waste some safe drilling fluid window space, resulting unnecessary casing and drilling cost. In this paper, an improved casing design method was proposed, in which each design coefficient needed to be calculated and determined based on the well structure in order to optimize the safe drilling fluid density window. The new method has been applied for a case study of a deepwater well in west of Africa, and the results show that the wellbore size, depth and the drilling string size have significant influence on the determination of the design coefficients values