Underbalanced coiled tubing drilling (UBCTD) has been utilized more frequently in recent years and can provide significant economic benefits when applied in the proper field setting. UBCTD requires a comprehensive evaluation of the proposed application, a commitment of resources by the operator and service providers, and a clear understanding of, (and tolerance to), the learning curve. As established coiled tubing drilling operations continue to push the technical limit forward, advances are assured.

The main scope of this paper is to document a template or flowchart for putting together a UBCTD project, covering everything from initial concept up to drilling the first well.


Located in the Emirate of Dubai, United Arab Emirates, the Margham onshore gas field is a limestone reservoir originally drilled in the 1980's for its condensate yield. As the original reservoir pressure declined so did the associated condensate and gas, such that the gas produced became the important product from the field, which in turn lead, in the 1990's, to the building of a gas plant to supply the Emirate with gas. Over time the reservoir pressure declined to an extent where it became necessary to drill laterals to access untapped parts of the reservoir. As a result it was decided to drill underbalanced with coil tubing using through tubing whipstocks to mill a window and allow the laterals to be drilled to access these untapped parts of the reservoir.

The generation of a project template, having documented all the necessary steps into a logical framework, was a vital asset in project start up and would be beneficial to areas where they are considering starting a UBCTD project for the first time, delivering value for both the provider and the client.

This value has already been proven through the assemblage of a UBCTD package developed over time from identifying the critical paths based on lessons learned during a previous UBCTD multi-year campaign. By applying procedures learned through experience, it was possible to deliver a similar successful UBCTD package to client within 3 months without delay.

Process Overview

The planning involved in an UBCTD operation is more extensive and complex than the planning of a normal conventional well. However, if correctly planned with a chronological template which identifies the critical path components, the probabilities for success at the implementation stage are much greater than in absence of such a tool. Both Service Companies and Operators stand to benefit from applying the proposed delivery template.

As there is numerous criteria involved, the template can be broken down into 6 main components; Basis of Design, Letter of Intention & Contracts, General Planning & Administration, Well Classification Criteria, Operations Planning and QHSE. Key to its success is planning and preparation utilizing a multidisciplinary team throughout the entire process. Each of these main components consists of several subgroups, all of which address the more detailed aspects of that particular phase.

Regardless, the earlier the project manager can be assigned to initialize the template process, the more effective the execution of a long-term underbalanced coiled tubing multilateral drilling project will be.

Basis of Design (BOD)

This approach uses reservoir data as an input to several modeling techniques to determine a complete operational scenario such that the viability and manageability of the CTUBD process is established.

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