Summary

Bit balling is a major problem that occurs during drilling of a formation containing water-sensitive clays, such as shales. This paper discusses a new technique for eliminating this problem that establishes an electric potential between the formation and the bit with the bit as the cathode. Laboratory drilling experiments in Pierre shale showed that, under favorable conditions, bit balling was reduced. The rate of penetration (ROP) doubled when the bit was negatively charged with respect to the rock, compared with the case where no potential was applied. This method suggests a new approach to reducing bit balling without toxic chemicals or oil-based muds, which are subject to serious environmental restrictions.

Introduction

About 60% of oilwell footage is drilled in clays and shales. These rocks are neither hard nor abrasive and thus should be easy to drill. Problems arise, however, because of the soft and sticky nature of the cuttings produced. These cuttings absorb water from the drilling fluid, swell, and stick to the bit. This gives rise to bit balling, with the following undesirable effects.

  1. Because cuttings stick to the spaces between bit teeth, the penetration depth is reduced.

  2. The nozzles are partially blocked; thus, flushing-fluid flow is reduced around the bit.

  3. During drilling with roller-cone bits, individual cones may stop rotating, leading to excessive shear and bit-tooth wear.

Bit balling is common in the field and has been addressed by various authors. The simplest method to reduce bit balling is a sufficient flow of flushing fluid at the work front to remove cuttings as they are produced, as Cunningham and Eeninkand Garnier and van Lingen discussed. Feenstra and van Leeuwen and Feenstra and Zijsling indicated that tooth scavenging could be used to reduce bit balling efficiently. Alternatively, Zijsling and Illerhaus proposed bit-structure modification by establishing a maximum clearance between the teeth and the bit body and removing as much of the supporting structure as possible, thereby reducing the chances of debris accumulation around the cutter structure (called the "eggbeater" design). Although attractive, these methods are still experimental, and it is uncertain whether the open structure of eggbeater designs will be strong enough to withstand field conditions or whether very high mud flow rates will cause borehole erosion or be too demanding on the pumping system.

Another method to reduce bit balling consists of modifying the mud to prevent water transfer to the formation. The muds are in two main classes: inhibitive water-based muds and oil-based muds.

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