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.
Introduction
Most casing exit systems in the marketplace today are capable of and expected to deliver a one-trip high-quality window; however, as well complexity and depth of the application increases, the operations are becoming increasingly ambitious in both their objectives and risk profile. The casing exit is a critical success factor for any re-entry operation; as this provides the only entrance into the sidetrack for all future operations, therefore it is essential that a window of the correct size and length is created in the desired orientation in one trip.