Most bits are categorized according to the type of formation hardness that they can drill. The usual classifications are soft, medium-soft, medium, medium-
Drill Rate, ft/hr
Fig. 7-35 Decision-making plot for conversion to a long-life bit hard, and hard. Admittedly, these classifications are vague. Unfortunately, no other widely used system is available and, as such, this classification will be followed.
Many formations are erroneously defined as "medium to hard" due to low drill rates. This occurs in areas such as South Louisiana and the Gulf of Mexico and is obviously the result of poor bit selection or operating conditions. Many drilling supervisors have tolerated low drill rates in past years due to uncertainty as to proper selection of bit operating conditions.
The three common bit types can effectively drill soft formations with low compressive strengths. The actual bit to be used depends on other conditions, such as mud types and bit sizes. Bit selection in soft formations is reduced to a matter of delming the conditions that will provide minimum-cost drilling.
Hard and abrasive formations usually fail in a compressive mode. As a result, bits that fail the rock in a shear mode are not very successful. Roller cone bits in the 1ADC codc range of 6,1,7 or greater are usually successful in hard rocks, tn addition, these bits have been developed to minimize abrasive wear, which can be very damaging to certain shear-failure bits.
Formations with sticky characteristics require special consideration. The stickiness often results from clay rocks that are hydratable. The cuttings stick to the teeth or hit structure and impede drilling. Bits designed for sticky formations have a high degree of teeth interlit and good hydraulics, such as centerjet capabilities. PDC, diamond, and short-tooth roller cone bits have not been particularly successful in sticky formations (unless oil muds are used with PDC bits).
Generally, PDC bits drill faster than mill tooth or diamond bits in soft to medium-soft rocks unless they are sticky. Numerous test results are available to substantiate this observation. A PDC bit manufacturer reports on performance tests on 54 North Sea wells as follows:
Bit Type Footage Drill Rate
Roller cone (steel) 3.75X IX
Roller cone (carbide insert) 5.75X 2X
Diamond 0.5X 3X
Oil-based muds often rcducc drill rates for cone bits. PDC hits, and diamond bits to some degree, arc not affected by oil muds. In fact, many groups believe that oil muds enhance PDC performance since they inhibit clay hydration and stickiness.
Air drilling generally requires the use of cone bits by necessity of design. Air does not have the cooling effect on the bit and tooth structures that liquid (oil or water) muds possess. Therefore, bit failure resulting from frictional overheating can occur
Cone bits are available with internal circulating ports that direct the air stream to the bearings. The bearings remain cool and continue to operate. Although diamond and PDC bits do not have any moving parts, their matrix structure may become weak and shuck the diamonds or diamond blanks if overheated. Diamonds themselves can fail above 750°C (poly crystal line) to 1,200°C (natural).
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