New Insight into the Mechanisms of Shale Inhibition Using Water Based Silicate Drilling Fluids

Abstract

Abstract There has recently been a resurgence in the use of water based muds containing sodium silicate for drilling reactive shale sections, particularly in the North Sea. The performance of these silicate water-based muds (SWBM) has been mixed: they are extremely effective in inhibiting swelling and dispersion of claystone and chalk, however some concerns exist over the stability of the system in terms of fluid properties, long term wellbore stabilisation and lubricity. This paper details new results, previously unreported, which give a better insight into the mechanism of clay stabilisation using SWBM. Recent literature has suggested that silicate fluids are as effective as oil-based muds (OBM) in wellbore stabilisation. This paper describes research and development indicating that silicate precipitation occurs within shales to form a barrier or membrane that hinders ion movement. However, there are distinct differences between the efficiency of membranes produced by oil based and silicate water based muds. Results are included which demonstrate that the ion- exclusion membrane is not as effective with Silicate systems as it is in OBM. New core flow experiments demonstrate that a permeability reduction with silicates and a slow rate of ionic equilibration are the principal causes for the osmotic barrier seen in swelling tests. Further results indicate a change in the physicochemical nature of clays after exposure to high pH silicate fluids. It appears that redistribution of alumino-silicate occurs which may play an important role in the inhibitive nature of these fluids. These evaluations are placed in context with well data in which primary considerations are the nature of any lost time incidents and overall SWBM performance. P. 897