Casing drilling is a process that use the casing string as a drilling string. The well is drilled and cased simultaneously in casing while drilling operations after required depth is reached. The casing string would be rotated with top-drive system and drilling fluid would be also circulated same as in conventional drilling. Previously, conventional drilling was deemed uneconomical in anticipating shallow gas kicks that may be encountered. A pilot bit is used to drill this section as the first hole before the hole enlargement process. In this paper, casing drilling will be introduced as a new way to drill in the top hole section. High Equivalent Circulating Density (ECD) generated by casing drilling should not be high -enough that can induce lost'circulation, especially when drilling through a shallow formation. However, to confirm that ECD generated by casing drilling is nearly the same with conventional drilling methods, simulation of behavior of Annular Pressure Loss (APL) in the casing drilling and pilot hole scenario was developed. An integrative program had been established to solve a redundant and trial and error calculation for drilling performance evaluation of casing drilling compared to pilot hole drilling. Also, this paper simulated the flowing behavior in annular flow and its effect on ECD at a critical small annulus in a casing drilling scenario. To maintain the integrity of the well due to narrow clearance of annular volumes means that the effect of APL must be identified. The results of this study show the advantages of casing drilling which can perform good drilling performance, become more efficient, and creates potential valuable savings rather the pilot hole method in mitigating shallow gas drilling risks.
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