Exploration for, and production of, hydrocarbons in Kazakhstan frequently involves drilling extremely hard-carbonate sequences, which contain high concentrations of chert. Historically, in almost all cases, bit records show that these formations have been drilled with tungsten carbide insert (TCI) roller-cone style bits and occasionally polycrystalline diamond compact (PDC) bits. Both rotary and positive displacement motor (PDM) assemblies have been used.

Drilling extremely hard-carbonate lithologies has been accomplished with PDC and TCI bits, but with the presence of chert, this becomes a considerable challenge. Depending on the nature of the chert (e.g., disseminated or nodular) it may well be impossible for a PDC bit to drill with the PDC cutting structure failure mechanism being catastrophic breakage sustained by the high modulus of elasticity (i.e., brittle) PDC diamond tables.

The cutting structure failure mechanism for a tungsten carbide bit tends to be the degradation, through breakage and chipping, of the gauge row cutting structure. This in turn will induce severe wear of the heel rows and shirt tail areas, as they attempt to maintain gauge. Severe shirt tail wear and subsequent loss of shirt tail material will invariably result in premature seal exposure and seal/bearing failures.

Despite advances in PDC and roller-cone tungsten carbide insert designs, the drilling of extremely hard carbonates characterized by chert concentrations as high as 65% remains a considerable challenge with slow rates of penetration (ROP) and costly trips to change out bits.

This paper outlines the introduction of a new drilling system for this type of application in Kazakhstan, in the form of natural-diamond impregnated bits run in conjunction with high-speed turbines. A discussion of the drive systems and drill-bit evolution is presented. The definitive parameter for quantifying drilling efficiency is cost per meter (CPM), and several such examples of resounding CPM reduction are illustrated.

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