Purpose-Built PDC Bit Successfully Drills 7-in. Liner Equipment and Formation: An Integrated Solution
Authors: Punnel J.G.A. , Huppertz A. , van Gellekom J.B. , Huizing J. 
Published: December 2007

Abstract
Historically, drilling out the 7-in. liner equipment has been a time consuming operation with a limited success ratio. The success of the operation is highly dependent on the type of drill bit employed. Tungsten carbide mills and mill tooth rock bits required from 7.5 to 11.5 hours respectively to drill the pack-off bushings, landing collar, shoe track and shoe. Rates of penetration dropped dramatically when drilling the float equipment. While conventional PDC bits have drilled the liner equipment successfully (averaging 9.7 hours), severe bit damage invariably prevented them from continuing to drill the formation at cost-effective penetration rates.
This paper describes the integrated development and application of an IADC M433 Class PDC bit, which was designed specifically to drill out the 7-in. liner equipment and continue drilling the formation at satisfactory penetration rates. The development was the result of a joint investigation where the operator and bit/liner manufacturers shared their expertise in solving a drilling problem.
The heavy-set bit was developed following drill-off tests conducted to investigate the drillability of the 7-in. liner equipment. Key features of the new bit and its application onshore The Netherlands will be presented and analyzed.

Introduction
A typical well drilled onshore The Netherlands employs a four-plug, rotating 7-in. liner system. The liner equipment, which is shown in Fig. 1, is cemented in the Zechstein Group of evaporates between 3,000-3,500 meters TVD or at long- hole depth up to 5,400 meters.
At these depths, a liner system is more economic than casing. More specifically, the squeezing salts characteristic of liner setting depth in this environment makes the four-plug system preferable over a two-plug liner configuration. The former better isolates the mud from the cement, and optimum cementation is critical in the Zechstein, which has been the cause of numerous drilling/production problems. Many of these problems have centered on collapsed casing, resulting from point loading on uncemented areas.
Oil-based muds and the heated mud system (KMG) have proven effective in controlling the large washouts in the squeezing salt layers that precipitate poor cementation and eventual casing or liner failures.
Fig. 2 compares the two-plug and four-plug liner systems. Essentially, a two-plug liner system has only one plug behind the cement, which basically displaces the cement around the pipe and landing collar. Thus, separation is only accomplished behind the cement.
With a four-plug system, an additional plug is ahead of the cement, providing for much better cleaning and separation between the cement and drilling mud.
Further, rotating the four-plug liner system puts the cement in turbulent flow, thereby preventing the channeling associated when cement is in laminar flow. Two-plug systems can also be rotated; however, as mentioned, they do not provide the additional isolation benefits of their four-plug counterparts.
In the 1980's, only a few attempts were initiated to drill out the 7-in. liner equipment. In earlier onshore wells, the liner was installed across the reservoir.
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