How Hydrolyzable Metal Ions React with Clays to Control Formation Water Sensitivity

Abstract

Hydrolyzed metal ions prevent clay dispersion by displacing the exchangeable cations and becoming firmly attached to clay surfaces. Formations treated with soluble compounds of hydrolyzable metals are highly resistant to permeability damage from clay movement. Introduction Water-sensitive formations present severe problems in drilling, testing, completion, production and waterflooding operations. Sensitive formations are characterized by a sharp loss of permeability on contact with water, but sometimes this permeability loss is accompanied by formation disintegration and sand production. A formation's sensitivity to fresh water is usually increased by exposure to monovalent cations (e.g., Na+, NH+, Li+, K+) and decreased by exposure to divalent cations (e.g., Ca++, Mg++, Ba++). Therefore, fresh water is more likely to cause permeability damage when it follows sodium chloride brine than when it follows calcium chloride brine. This phenomenon has been reported several times, but no information was found concerning the effects of trivalent or tetravalent cations on sensitive formations. Since formations treated with divalent cations are more stable (less easily damaged) than those treated with monovalent cations, it seems reasonable that treatment with more highly charged cations should produce even greater stability. The purpose of this study was to investigate the effects of highly charged cations on formation water sensitivity. The primary interest was the effect of such cations on water permeability. Consequently, most of the laboratory work consisted of direct permeability studies. permeability studies. Damage Theory The permeability of any argillaceous (clay bearing) sediment depends on how tightly the individual clay particles are bound together. Permeability will be particles are bound together. Permeability will be maximum if all the particles are bound together in compact, oriented aggregations too large to be transposed by moving fluids. Permeability will be minimum if these particles completely disperse and become entrained in the moving fluids. Such entrained particles form microscopic filter cakes on narrow pore particles form microscopic filter cakes on narrow pore openings and very effectively restrict fluid flow. Intermediate permeability losses will be noted when the compact aggregations only partially disperse, i.e., separate into smaller aggregations. In this case, the entrained solids are larger and form filter cakes that are considerably less effective in blocking fluid flow. It follows that sensitivity is related to how extensively the clay particles can separate or disperse. In all clay-water systems, there are certain factors tending to disperse the clay particles and certain other factors tending to hold or drive them together. The relative combined strengths of these opposing factors determine whether the particles will disperse or stick together. Any fluid flowing through a formation will cause permeability damage if the clay-dispersing factors exceed the binding factors. JPT P. 1111