EFFECT OF CLAY POLYMER INTERACTIONS ON SHALE STABILIZATION DURING DRILLING

Adsorption measurements were used to investigate the mechanisms of shale stabilization by water-soluble polymers, which are commonly added to drilling fluids to control reactive shale formations. High molecular weight (10(7)) polyacrylamides of varying ionicity were adsorbed on well-characterized K-smectite, from solutions of varying electrolyte concentration. Polymer adsorption was found to be strongly dependent on the ionicity of the polymer and on the ionic strength of the medium, due to charge screening effects and the salt-dependent energy of tactoid assembly. The interaction of these polymers with a standard shale rock was investigated by conventional oilfield cuttings rolling tests and in an apparatus designed to simulate wellbore conditions of pressure and flow rates. The results show that under conditions promoting adsorption of polymers, disintegration and dispersion are inhibited, although the near wellbore material still imbibes water and ions. The stability of the wellbore is also dependent on the nature of the polymer-clay interaction; the strongly bound cationic polymer is able to limit clay swelling.