Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Petroleum Engineering

First Advisor

Adam T. Bourgoyne, Jr


This study was performed to diagnose the cause of poor PDC bit performance in deep, overpressured shales. Deep, overpressured wells account for a disproportionate share of all drilling costs, about 27 percent. The effect of bit performance on this cost is significant, about $2.9 billion annually in overpressured formations alone. Bit performance in deep, overpressured formations when using water-based muds is especially poor, adding at least \$500 million annually to worldwide drilling costs. This problem is described as the slow drilling shale problem and is shown to exist worldwide. Potential causes of this problem identified in the technical literature include various kinds of balling, strong rock within shaly intervals, confining pressure effects, chip hold-down effects, and shale plasticity. The strategy of this study is to identify the characteristic symptoms of the actual problem in the field and to match them to the symptoms resulting from different possible causes in controlled laboratory tests. Field data in an area identified as exemplitting the problem was reviewed to develop reliable symptoms. Seven bit runs had adequate drilling and log data for detailed analysis that yielded examples of effective and ineffective performance and their respective symptoms. Ineffective performance was confirmed to occur principally in shales. Full scale laboratory drilling tests confirmed that similar symptoms could be reproduced in a laboratory environment using outcrop shales. The range of physical and mineralogical characteristics of deep, overpressured shales was determined, and outcrop rocks were acquired that represent a similar range. Single cutter tests were the cornerstone of this study. The tests allowed differentiation of several of the possible causes, and observation of whether there was any evidence that a particular cause actually occurred during a test. Global bit balling, cutter balling, confining pressure effects, and a strong rock were all explicitly tested with single cutter experiments. Global bit balling was shown to be the principal cause of the slow drilling shale problem. Although all of the other effects also impede bit performance, only global balling caused all of the symptoms observed in the field. Conventional laboratory tests were also performed to better understand the problem.