A description is given of a single-cutter tester for studying the cutting process of PDCs in rock under simulated downhole conditions. The use of the tester is illustrated by a study of the effect of downhole pressures on the cutting process in shales. Prior to the tests the Pierre shale and Mancos shale outcrop samples used for the study were conditioned to enable proper downhole pressures to be simulated during the tests.

The total bottomhole pressure only was found to govern the cutting process in Mancos shale, whereas both the pore pressure and the total bottomhole pressure govern the cutting process in Pierre shale. The dilatancy characteristics of the rocks are shown to be responsible for this behaviour. In addition the tests unveiled the balling mechanism of the outcrop shales in water-based and oil-based mud environments.

On the basis of the insight gained from single-cutter tests, the paper discusses drilling characteristics of PDC bits in shales and bit and cutter design aspects that facilitate mechanical bit cleaning as a means of improving PDC bit performance in various shales.

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