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.


Two-phase flow is the flow phenomenon of two different fluid phases flowing simultaneously through a conduit. Generally, liquid and gas phases are the components of this commonly encountered flow type. Since 1950's, the flow problem of two-phase fluids has been the subject of research in many different engineering practices.

In petroleum industry, the applications of two-phase flow start from drilling and continue till the refining process. In depleted reservoirs, underbalanced drilling techniques are required in order to prevent any possible formation damages. This enhanced technology diminishes the risks of contaminating the reservoir. Thus, in order to determine the design parameters accurately, the flow behavior of aerated fluids should be well known. Another important usage of two-phase flow takes place during the transportation of the produced oil and gas via the pipelines. Since the oil and gas fields are mainly in remote onshore areas or in offshore, the pipeline systems are of great importance. Reliable engineering calculations should be carried out as the overall distances of these pipeline systems are considered.

With the improving technology, the innovative methods demand for the better understanding of two-phase flow systems. As in case of CT drilling combined with underbalanced techniques in extended reach wells, the flow problem becomes more complicated when compared with single-phase flow of drilling mud in conventional drilling.

When this wide range of application of two-phase flow in petroleum engineering is considered, the appropriate determination of flow parameters of two-phase fluid systems becomes highly important. The focus of this study is the flow of two-phase fluids through concentric annuli, i.e., horizontal and highly inclined sections of wellbores.

Literature review

Through the investigation of two-phase flow phenomenon, extensive theoretical and experimental studies have been carried out. The proposed models can be grouped into two categories; namely general models and mechanistic models. The early models developed for two-phase fluid systems were flow pattern independent. These general models ignored the complex flow configurations, named as flow patterns, and treated the two-phase flow as a single-phase fluid flow or as a flow of two separated fluids. The most important models are proposed by Wallis [1], Lockhart and Martinelli [2], and Duns and Ros [3]. These general models are the starting points through the progress of modeling two-phase fluid flow. The later studies focused on the determination of flow patterns. The flow mechanism of two-phase fluid systems was examined independently for each flow pattern. Then, governing flow equations were proposed for a given flow pattern. These models were called mechanistic models. As the knowledge of flow behavior of two-phase fluid systems has improved, comprehensive and unified models were developed.

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