Coiled tubing (CT) is widely associated with underbalanced drilling technologies. Especially in depleted reservoirs, drilling need for underbalanced and extended reach wells is increased where CT is widely used. In this work, optimization of volumetric requirements for liquid and gas phases is investigated in long horizontal and inclined sections of CT applications for underbalanced drilling. A mathematical model is introduced in order to predict the flow characteristics of multiphase flow through an annulus. Flow patterns and frictional pressure losses are evaluated using the experimental data of a wide range of liquid and gas flow rates recorded at a field-scale annular flow loop with common CT drilling dimensions as well as circular pipes. Practical curves are developed for determining the optimum flow rate combinations for CT applications using the developed model. A sensitivity analysis is also conducted on underbalanced and CT drilling parameters on pressure drop and flow patterns.

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