The capability to provide workover services on wells without having to kill or balance the well with workover fluid has been one aspect of coiled tubing service that has made it invaluable in the oilfield. However, coiled tubing pipe and units are limited in size, and therefore, are limited in their scope of workover potential. This paper will discuss a scenario in which there was need to provide underbalanced well workovers that could not be performed by coiled tubing.

The Dayung Gas Field in south Sumatra has a reservoir with natural fractures and decreasing mud weight requirements relative to depth changes. Excessive fluid loss is common during the drilling and workover phases of wells in this field. Therefore, drilling and workovers in this field are candidates for underbalanced operations.

Performance of underbalanced operations can assist in the management of problems created 1) by excessive fluid loss in reservoirs prone to lose circulation, 2) in depleted or low pressure reservoirs, which are fluid sensitive, 3) in reservoirs susceptible to skin damage, or 4) by difficulties experienced in killing the well. After consideration of the limitations of coiled tubing and the advantages of performing workovers in an underbalanced condition, the decision was made to review other options.

This paper will discuss the decision-making process employed to select the type of unit to best perform the required services, The paper will also present a case history of the workover project in Dayung with the "wins" of the project, the lessons learned during the project, and the corrective actions taken to enhance job procedures.

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