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" × 6-1/2" and 6" × 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.

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