The Western Build-up area of the Karachaganak field (operated by Karachaganak Petroleum Operating b.v. - KPO b.v.) brought such new challenges to the project as drilling subhorizontal in narrow mud weight window, unmanageable mud losses, and formation fractures. With reference to the loss of circulation on offset vertical wells drilled in the Former Soviet Union (FSU) in this area, it is anticipated that the loss zone may not be cured by conventional remedial actions such as pumping LCM, cement or gunk.

The uncontrollable lost circulation in the target reservoir induced to implement the special techniques for further development of the field. The pressurized mud-cap drilling, also known as Closed Hole Circulation Drilling (CHCD) is not new at the Karachaganak filed. Several years ago, "drilling without returns" was successfully introduced in the field, but a rotating control device (RCD) was employed to drill blind without directional control of the well. For KPO b.v. it was very important to deliver the horizontal well to maximize reservoir contact, optimizing potential for high rate production while minimizing drawdown.

For a directional drilling service provider, it was critical to be sure that the technologies would adapt for drilling in such environment. Aggressive attention and composed approach were required for planning wells with RSS (Rotary Steerable System), MWD (Measurements-While-Drilling) and LWD (Logging- While-Drilling) equipment as the performance of those tools in a CHCD environment was influenced by different challenges in terms of data transmission (mud pulse telemetry), steerability, hydraulics and maintenance.

To improve drilling efficiency, operator together with service companies worked closely to implement significant BHA and Bit design changes. Using an integrated drill bit design platform and dynamic system analysis, engineering team simulated the downhole environment and evaluated the possible behavior of the Bit, Hole Opener and bottom hole assembly (BHA) in hard formation. At the preparation stage, close attention was given also to the bending stress analysis due to expected DLS 4-8deg/30m as prevention action from possible damage and fatigue of the drill string components. First time worldwide the underreaming while drilling from the 6 ¾ to the 8-inch size, in a build-up section in a CHCD mode, was successfully delivered due to significant adjustments to the drilling practices and technologies modification.

The proposed paper will review the recent results of the first directional hole enlargement job in a CHCD mode, special approaches that can be used in CHCD environments and share best practices in well planning and execution of similar wells.

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