An improved casing design method and its application in deepwater drilling

Key words:  deepwater drilling  casing design  design coefficient  safe drilling fluid density window  well kick tolerance

Abstract:

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.