Studies Dispel Myths, Give Guidance on Formulation of Drilling Fluids for Shale Stability

Abstract Shale instability problems when using water-base drilling fluids have remained unresolved for decades because of a lack of knowledge and understanding of the shale hydration mechanisms. The industry has relied upon hydrocarbon-base drilling fluids for combating shale problems, but misconceptions have kept even those fluids from being utilized to their fullest advantage. With the use of hydrocarbon-base fluids now being curtailed because of environmental concerns, costs due to shale problems could escalate. The understanding of shale instability problems has been hindered by inadequate laboratory means of simulating contact of drilling fluid and shale under downhole conditions of stress and temperature. To address this situation Gas Research Institute has conducted a project in which laboratory equipment and procedures were developed to permit preserved specimens of downhole shale (cored in hydrocarbon-base mud) to be restored to in situ axial stress, horizontal stress, pore pressure and temperature prior to being drilled at a selected borehole pressure. Provisions were made for measurement of fluid transport in either direction between the circulating drilling fluid and the shale during an extended period of exposure. The borehole pressure was then reduced incrementally to observe for borehole failure and obtain a measure of effect of the drilling fluid on the relative stability of the shale. The above procedures have been used to study a well- known troublesome Cretaceous shale cored using oil-base mud at a depth of about 5,500 ft in Block 4 of the U.K. sector of the North Sea. This paper presents data showing that the aqueous activity of either a water-base or hydrocarbon-base emulsion drilling fluid can be adjusted to develop osmotic pressure that will cause water to enter or be extracted from a low-permeability shale. The hydraulic differential between the borehole pressure and far-field shale pore pressure is also shown to be a driving force affecting the transfer of water. Discussion of these test results dispels several existing myths and provides guidelines for more effective use of current drilling fluids. The results also provide guidance for development of new environmentally acceptable water-base systems for combating shale problems where the use of hydrocarbon-base fluids is not permitted. P. 731