Pertamina EP with North Kedung Tuban structure is located in East Java Indonesia. The drilling target is to produce gas from "Kujung" carbonate formation. The characteristic of this carbonate are vugular rock which caused total loss circulation, contain high concentration H2S and 20% CO2. Conventional curing loss methods such pump Lost Circulation Material (LCM) with various type, carbonate cementing and blind drilling, were not the good option since after loss circulation occur, gas kick immediately come up.

To encounter this situation, Floating Mud Cap Drilling (FMCD) technique with sandwich system was applied. This method used Rotating Control Device (RCD) as surface barrier and multiple treating line tied to the annulus casing valve to pump down sandwich. While blind drilling performed, water pumped down through drill string and sandwich system fluid pumped through annulus. Sandwich is a combination of water + kill mud + water and so on. Volume, Kill Mud Weight and time delay for pumping were obtained from previous well data and observations until effective parameters were obtained during blind drilling.

Sandwich fluid purposed as hydro static barrier to overcome gas influx come up to surface during blind drilling. To help good hole cleaning, high viscosity mud pumped frequently through drill string. When TD reached, sandwich system continue applied until BHA pulled inside casing. To accommodate tripping inside casing, change of RCD rubber element and RIH completion string, Downhole Casing Valve is attached in previous casing as mechanical barrier. As a result, the well completed with no gas kick occur while blind drilling and completion string placed as purposed accordingly. Furthermore, good communication between all personnel involved are necessary.

This paper contains the Lesson Learned of FMCD application which is effective and efficient proven in NKT-001 and NKT-001TW well, with the drilling time and cost of hole section 8-1/2" 80% lower than conventional methods.

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