Interpretation and practical applipations of flow phenomena in clayey media

Abstract

The presence of clay in sandstone reservoirs complicates the interpretation of electric logs and can influence the flow properties of fluids. The water-clay interaction and, in particular, the double ionic layer concept has been applied to the study of macroscopic fluid flow phenomena from two points of view: [1] the mechanical driving of clay particles and [2] the ionic equilibrium which allows a generally reversible swelling of the clay to take place. In the latter case, an effective porosity has been defined in keeping with the fact that the swelling water in the double stratum does not participate in the flow, and the resulting permeability reduction has been calculated in function of the ionic concentration of the aqueous phase. Experimental proof of this theory is very effective in artificial formations. To study the practical problems raised by clayey sandstones, sandstones containing illite and kaollnite were examined with standard electrolytes [NaCl, CaCl2, KCl]. The permeability decreases conformed to the theory and depend on the nature of the clay, its distribution in the sandstone and the nature of the compensating ion in the clay. Clay particle driving occurred in both sandstones and artificial media, thus enabling the chances of plugging linked to salt-content variations to be evaluated. A study of this sort brings out the precautions that should be taken in drilling into clayey reservoirs and in making petrophysical studies.