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.


The Pre-Khuff formations typically encountered between 13,000 and 17,000 ft TVD in Saudi Arabia are the 'Unayzah, Jauf, Tawil, Sharawra, Qusaiba, Sarah Qasim and Saq. These formations are mainly sandstones interbedded with shale, limestone, dolomite, anhydrite and siltstone. The two major gas bearing reservoirs, the 'Unayzah and Jauf are generally characterized by hard and abrasive sandstones interbedded with shale and siltstone. The 'Unayzah is the most difficult to drill in general, harboring rock UCS up to 35,000 psi with internal friction angles from 25 to 60°.

A previous study of Pre-Khuff drilling operations in Saudi Arabia by Simpson et al.1 showed that although 5–7/8" and 8–3/8" TCI bits have been plagued numerous problems including undergauge wear, seal and bearing failures, cone loss and subsequent fishing operations, the average CPF over the last 22 years remained lower for the TCI's than that of fixed cutter bits run on PDM's or Turbines. This was attributed to TCI bits being much cheaper, no additional cost for running a PDM or Turbine and in many cases the TCI bits actually drilled faster with similar total footage but less rotating hours.

Practically every problem experienced drilling the 'Unayzah vertically was exasperated drilling it horizontally, except wellbore stability, which was greatly improved by drilling parallel to the maximum horizontal stresses. Bit sticking / severe stuck pipe occurred on three vertical wells while Turbo-drilling with Impreg bits in the 'Unayzah and Jauf formations. Bit sticking / severe stuck pipe also occurred twice while PDM-drilling with impregnated bits, once in the Jauf formation of a vertical well, and the other in the 'Unayzah formation of a horizontal well.

A recent study of Pre-Khuff drilling challenges and improvements by Simpson et al.2 reviewed recent motor drilling experiences in the Pre-Khuff. This paper reviews and analyzes 5–7/8" and 8–3/8" bit performance, overall costs and optimization in the five most recent 'Unayzah horizontal wells.

Horizontal Drilling in 'Unayzah

Saudi Aramco initiated a 'Unayzah Horizontal drilling campaign in 2001. Since then, five wells were drilled and completed. Previously, eight Khuff horizontal wells and one 'Unayzah horizontal well were drilled in the Ghawar field. Most of these horizontal wells prior to 2001 were drilled due south or due north, which is perpendicular to the maximum horizontal in-situ stress direction (refer to Temeng et al.3). These wells experienced numerous instances of tight hole and stuck pipe while drilling the reservoir section.

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