Today's reserves are often stranded in deep, complex reservoirs and, due to economic or environmental constraints, sometimes have to be connected from a single drill site, resulting in wellbore construction methods such as extended-reach drilling (ERD) and multilaterals. Casing exits often play a crucial part in these types of wellbore construction methods. Milling a long window with low dogleg severity is the key to success since every subsequent run into the wellbore – rotary steerable system, liner and completion systems – will have to pass through the window unobstructed. The successful execution of a casing exit based on surface parameters alone becomes more and more challenging as depth and deviation of the application increases.

A system has been developed that measures all the physical parameters downhole at the window-milling assembly itself, and transmits them to surface for rig-site and remote viewing, enabling real-time control of casing exit operations. The system captures and processes all the milling parameters downhole and then uses measurement-while-drilling mud pulse telemetry or stiff-line / wired pipe to send selected and critical information to surface. The window-milling process can be enhanced by monitoring dynamic behavior such as mill vibration, weight on bit and bending moment to make real-time decisions and reduce risk and nonproductive time (NPT).

This paper describes the downhole performance sub technology and closed-loop control system, and demonstrates by a number of case histories the risk-reduction value of monitoring the downhole parameters in real time. The case histories presented includeboth drillpipe-deployed as well as coiled-tubing-deployed casing exit systems.

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