Effects of drilling fluid/shale interactions on borehole stability: Studies using speeton shale. Topical report, June 1994-November 1996

Laboratory equipment and procedures have been developed to permit specimens of downhole shale cored in oil-base mud to be restored to in situ temperature and stresses prior to being drilled with a fluid to be evaluated, preventing the introduction of a contaminating gaseous phase that has flawed most other laboratory investigations. These studies show that the aqueous activity of either a water-base or oil base emulsion drilling fluid can be adjusted to cause water to enter or be extracted from a low-permeability shale. The relative activities developed at downhole conditions determine the chemical potential driving force that will either support or oppose any hydraulic potential driving force caused by difference between the borehole pressure and the formation pore pressure. The results provide guidance for the development of a new class of water-base fluids that can be an environmentally acceptable alternative to hydrocarbon-base fluids for drilling troublesome shales.