INFLUENCE OF PORE PRESSURE ON DRILLING RESPONSE IN HARD SHALES

Results are presented from laboratory drilling studies of two different hard shales similar to those found in some areas of the North Sea. Two laboratory drilling machines, with milled-tooth bits were used. Pore pressure was measured in all the experiments, and in some it was reduced to several megapascals below bottomhole pressure (BHP). Pore-pressure reduction was achieved by allowing the pore fluid to drain out of the shale while external stresses were maintained constant. The influence of BHP was studied for values from 3 to 33 MPa [435 to 4,786 psi]. The differential pressure, which equals BHP minus pore pressure, ranged from near balance to > 10 MPa [> 1,450 psi]. BHP was found to have a strong influence on drilling response, but differential pressure did not, contrary to conventional wisdom. The results are compared with other drilling experiments and single-cutter experiments in the literature.