The Hassi Messaoud field (Figure 1), discovered in 1956, is the largest oil field in Algeria, covering about 2000 Km2 and currently including over 1000 Wells. The field is divided aerially into 25 zones that are separated from each other by low permeability barriers, usually faults. The Cambrian Reservoir is subdivided into four lithological zones designated from base to top as R3, R2, Ra and Ri. The Ra sub reservoir shows the best petrophysical properties with a maximum thickness of 150 m in the western part of the field. The Ra is farther subdivided into five subzones: D1, ID, D2, D3, and D4 based on reservoir properties and depositional conditions.

Well MD-330 was the 4th re-entry well in the Sonatrach campaign. A total of 330 m from landing point were successfully drilled in a continuous underbalanced condition inside D4. As a result, the well productivity was significantly improved, compared with offset wells, due to one of the positive impacts of UBD operations, namely that of the elimination of formation damage.

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 a single phase 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 production zone.

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 successful application in Algeria.


Drilling in the Cambrian Reservoir, in the peripheral zone of Hassi Messaoud field, specifically the 1A, 1B and 1C zones (Figure 2) is a real challenge, basically due the petrophysical reservoir complexity, the low reservoir pressure, the extremely hard and abrasive formation associated with numerous vertical and subvertical 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 of production history, a large number of them are closed due suffering from severe corrosion.

This is the first of its kind Coiled Tubing - Underbalaned Drilling project undertaken in Hassi Messaoud field. The candidate wells are from a pool of vertical wells drilled in the western area of the field and have ceased to produce with varying completion strings and production casing. The increase in number of candidates for slim hole re-entries added to the depletion of the reservoir have raised the importance of using the combined Coiled Tubing - Underalanced drilling technologies in Hassi Messaoud field. The original reservoir pressure was 6860 psi and the average permeability ranges from 0.5 to 1.0 md but can reach up to 1000 md in cases where open fractures are encountered. Production is 43.7 - 45 º API oil with original average GOR of about 200 m3/m3 and a formation temperature of approximately 120 ºC. Until 1964, the production mechanism was only depletion above bubble point. Gas injection and water injection started after that year to improve the production.

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