Water-Sensitive Shales of Gulf of Mexico: Correlation with CEC Values for PDC Bit Applications

David E. Cook

The use of the Formation Methylene Blue Test (FMBT), an indicator of shale reactivity, has proven to be a quick and easy qualitative method of defining application windows for polycrystalline diamond compact (PDC) bits used to drill Gulf of Mexico clay shales using water-based drilling fluid. This paper presents the results of a laboratory test program wherein a correlation was established between FMBT corrected for pH effects and quantitative Cation Exchange Capacity (CEC) values on selected samples of various shales. Relationships were determined for drill cuttings and sidewall core samples from the Gulf of Mexico. Mineralogical identification of selected shale samples and Gulf of Mexico well samples was performed using x-ray diffraction techniques. CEC was done by wet- hemistry using both barium and ammonium acetate methods. Other factors influencing shale reactivity in the field data are local temperature and pressure gradients as related to growth faults or other geologic processes. A relationship was noted between FMBT and gamma-ray log response.

The program responsible for this work has saved significant dollars in drilling costs since its inception. Other developments have been innovative improvements through use of latest technology in bit hydraulics, design, and materials.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.