In this paper, the dependence of the drilling specific energy e on the virgin pore pressure p° is examined by analyzing the mechanics of a PDC cutter. On the basis of a simple failure mechanism involving a single shear plane, it is first established that the specific energy depends linearly on the difference between the bottom-hole pressure p h and the average pore pressure p h on the shear plane. The relationship between p b and p° is then determined by considering fluid mass balance across the shear plane. Different pore pressure regimes are identified that are controlled by a dimensionless number λ: low speed, transient, and high speed. In the high speed regime, the rock in the shear zone is undrained and pressure drop induced by shear induced dilatancy leads to cavitation, with the consequence that the specific energy depends only on the mud pressure. It is shown that the drilling conditions for a shale is in the high-speed regime, while for a permeable sandstone it is typically in the low speed regime.
|Original language||English (US)|
|Number of pages||9|
|State||Published - Jan 1 1991|
|Event||32nd U.S. Symposium on Rock Mechanics, USRMS 1991 - Norman, United States|
Duration: Jul 10 1991 → Jul 12 1991
|Other||32nd U.S. Symposium on Rock Mechanics, USRMS 1991|
|Period||7/10/91 → 7/12/91|