When control drilling interbedded formations due to lost circulation challenges, Bi-Center bits often fail to open the hole to the expected drill size. Possible causes include inconsistent downhole weight on bit due to formation changes, lateral and torsional vibration. Each leads to the pilot of the Bi-Center bit drilling an oversized hole which results in the bit drilling off-center, giving an undersized hole. To combat this, a patented double profile Bi-Center bit was developed. Its elongated pilot section includes a unique midreamer section which improves lateral stability due to more balanced cutter forces. Also, the mid-reamer provides an additional gauge section immediately below the reamer which in conjunction with the pilot gauge has proven to reduce tilt and keep the pilot bit centralized, providing more consistent full gauge hole. Additionally, vibrations generated by the drillstring are common during the hole-opening process. These affect not only the drilling performance but also the quality of the wellbore. A unique eccentric stabilizer, ideally located in tension in the BHA, provides vibration dampening in the drill-string. The V-Stab interrupts harmonic modes vibration as well as reducing the magnitude of shocks due to its unique geometry. Caliper logs from field trials of a 10–5/8 × 12-/14" CSDX6413S-B1 Bi-Center bit with a 10–5/8 × 12" vibration dampening tool demonstrate excellent hole opening performance and improved borehole quality. In contrast, logs for conventional Bi-Center bits run with the vibration dampening tool showed inconsistent hole opening and poor borehole quality in the same application. This paper describes the benefits of the unique combination of these new technologies, presenting detailed drilling performance data showing how they resulted in significant performance improvement and cost savings in southern Saudi Arabia. Introduction Lukoil Saudi Arabia drills some challenging exploration wells in southern Saudi Arabia. These wells have encountered several difficulties involving severe borehole stability problems. On the well "D", the 13–3/8 casing was planned to be set on top of Sulaiy Formation at around 9,258 ft. However, the 16" phase was unable to reach this depth, having to be stopped at a much shallower depth of 7,625 ft because of lost circulation problems Additionally, the 13–3/8" casing could not be run to 16" hole total depth due to restriction in the hole. The 13–3/8" casing was set at 6,865ft and the rest of the 16" open hole was finally covered by 11–3/4" expandable casing that was set at 7,617ft to ensure that the 9–5/8" casing setting depth could be reached as per the drilling program Fig.1. Since the 11–3/4" expandable casing has 11.286" drift ID after expansion, a hole opening tool was required to open the hole from the casing pass through size to 12–1/4" hole size, to allow the 9–5/8" casing to be set properly.

To drill deepening hole below 7" liner through deep section of compact interbedded carbonate formation results slow rate of penetration (ROP). Beside the limberness of BHA configuration (which is stiffer for the larger sizes of BHA) thus generates various string vibrations, the used of heavier mud weight can also effect overbalance pressure thus reduces drilling ROP. This paper outlines the two stages diameter PDC bit, that proficiently increases drilling efficiency in order to produce faster ROP through small hole, deep section drilling. When the PDC bits cutting structure is separated into two stages of hole diameters, the mechanical energy that is required to destroy a given volume of the rock to drill can be significantly reduced. This energy reduction can be equivalent to the percentage of the hole size reduction relative to the final hole diameter. The reduced hole diameter that is described as pilot hole will be drilled by the first stage of the bit. The second stage of the bit that is called as reamer section simply enlarges a stress relieved pilot hole, to a final hole diameter. The Mechanical Specific Energy (MSE) concept that is defined as work that is required to fail a given volume of rock has been formulated proportionally with formation rock strength. MSE can also be defined as input energy to result ROP. The ROP to drill a smaller hole is faster than ROP to drill larger hole although with similar BHA, whilst ROP to enlarge a pilot hole always faster than ROP to drill the pilot hole. Because the bi-center bit has a pilot section that can be used for drilling a smaller pilot hole and a reamer section for enlarging pilot hole to a final hole size, therefore it shoud be able to produce faster ROP compared to conventional one stage PDC bit. The field results revealed that all of 5–3/4" x 6–1/2" and 6" x 7" bi-center bits that were run below 7" casing by Kuwait Oil Company in Kuwait - through deep wells, have drilled the intervals more than 30% faster than conventional PDC bit. Introduction Kuwait Oil Company Deep Well Division have drilled several deepening hole below the 7" liner in order to produce hydrocarbon from the lower intervals of the existing production wells through several fields that were explored in 1950's decade. These deepening well drilling campaigns have been actively performed through the oil field located on the northern area of Kuwait that is shown on map, Figure-1.