Bit Balling in Water-Reactive Shale During Full-Scale Drilling Rate Tests

Abstract

ABSTRACT This paper demonstrates a useful means of studying bit balling in laboratory drilling tests. Full-scale tests were conducted using a new 6-1/8 in. (156 mm) tricone insert-type bit, realistic muds, and Pierre shale, which is similar to water-reactive shales which are apt to cause balling in the field. Balling ranged from slight to severe with an uninhibited water-base mud, and did not occur with an oil-base mud. The ratio of bit torque to weight-on-bit was a reliable indicator for detecting the degree of balling. The results show that proper modeling of bit balling in the laboratory must address the degree of overbalance which results from insufficient mud weight in the field. They also indicate that increasing mud weight, in the field is an effective means to reduce or prevent bit balling if, and only if, balling is caused by mechanical (not chemical) wellbore instability. Although bit balling would become (irreversibly) severe within seconds, it could be prevented by significantly reducing weight-on-bit quickly enough after incipient balling.