The combination of Underbalanced and Coiled Tubing Drilling technologies has proven to be a viable solution to successfully drill horizontal re-entry wells in mature fields where the reservoir pressure is adequate to induce underbalanced conditions using single phase or multiphase drilling fluid injection.

This is the first of its kind Underbalanced-Coiled Tubing Drilling project undertaken in Hassi Messaoud field. The candidate wells are from a pool of vertical abandoned wells drilled in the western area of the field and have seized to produce with varying completion string and production casing. The increase in the number of candidates for thru tubing re-entries in addition to the depletion of the reservoir, have raised the importance of using the combined Underbalanced-Coiled Tubing Drilling technologies in Hassi Messaoud field.

The experience in this field has validated the benefits of the combination of these technologies and has been attributed to the excellent balance between drilling cost and improved productivity. For example, the drilling cost in the short radius re-entry wells drilled with coiled tubing are highly reduced due to the elimination of pipe connection procedures, the increase in rate of penetration, prevention of conventional drilling problems such as fluid circulation loses and related differential sticking events, increase in the bit life and reduction in time and drilled distance to reach the profitable production level.

This paper describes the technical basis for the design of the Underbalanced-Coiled Tubing Drilling project in Hassi Messaoud field-Algeria, and highlights the operational results, key factors and challenges faced during three consecutive successful re-entry wells drilled in this field.

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