The Hassi Messaoud field, discovered in 1956, is the largest oilfield in Algeria, covering about 2000 km2 and currently including over 1000 wells. Production is from a thick, Cambrian age sandstone, at a vertical depth of approximately 3400 m. This sandstone is extremely hard, abrasive, and slow to drill. Sonatrach introduced Underbalanced Drilling (UBD), primarily in an effort to improve penetration rates, though the possibility of other benefits from UBD experienced elsewhere, such as improved production characteristics, was also acknowledged. Several factors requiring special consideration were addressed from candidate selection to equipment and procedures. The combination of depth and required bottomhole circulating pressure approach the limits of what is achievable with concentric gas injection, the only locally available UBD technology. The methods used in conducting this project are good examples of how to approach and successfully execute such a project.

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