The first ‘Unayzah horizontal well (W-4) with a 5-7/8″ lateral was drilled with considerable difficulty. The 5,732 ft lateral at 14,000 ft TVD traversed the most productive, albeit hardest and most abrasive section of the ‘Unayzah B with average unconfined compressive strength of 22,000 psi, with hard streaks up to 35,000 psi; and internal angles of friction ranging from 39 to 60°.

Drilling horizontally, the TCI bits were subjected to increased bearing stress and gauge wear. Moreover, they had to be run on PDM's for directional control at minimum bit speeds of 130 – 170 rpm, in the sliding and rotational modes respectively, which substantially cut on bottom rotating time. The PDC and Impregnated bit performance diminished drilling horizontally, as did PDM stator and bearing life with static BHT of 278° F. However, since W-4 required a pilot hole, and two sidetracks it offered a unique opportunity to compare drive mechanism, TCI, PDC, TSP and Impregnated bit performance at equivalent TVD's and formations in the same well.

This paper presents a summary of primarily 5-7/8″ lateral drilling experience in the ‘Unayzah B reservoir along with cost and performance comparisons with other ‘Unayzah horizontal wells. Valuable lessons learned are also discussed.

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