Drilling in the Cambrian Reservoir, in the peripheral zone of Hassi Messaoud field, specifically the 1A, 1B and 1C zones (Figure 1) is a real challenge, basically due the petrophysical reservoir complexity, the low reservoir pressure, the extremely hard and abrasive sandstone and quartzite formation associated with numerous vertical and sub vertical fractures that connect the reservoir with deeper formations leading to major problems related with the salt saturated water production. Many of the vertical wells drilled in that zones have more than 30 years from completed and a large proportion of them have been shut in since being drilled.

Well E was the 5th re-entry well in the Sonatrach Coiled Tubing – Underbalanced Drilling campaign. A total of 372 m from KOP and 316 m from landing point were successfully drilled in a continuous underbalanced condition inside D1 formation. As a result, the well productivity while drilling, the Reservoir characterization and rate of penetration was significantly improved, compared with offset wells, due to the positive impacts of UBD operations.

The combination of Underbalanced and Coiled Tubing Drilling technologies has been 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.

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, the prevention of conventional drilling problems such as fluid circulation loses and related differential sticking events, the 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 and implementation of the Coiled Tubing Underbalanced Drilling project in Hassi Messaoud and highlights the operational key factors and challenges faced during this particular and successful application in Algeria.

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