Stabilization of Formation Clays with Hydroxy-Aluminum Solutions
Authors: Reed M.G. 
Published: December 2007

To protect water-sensitive formations against permeability damage by fresh water, a material is needed that will adhere very tenaciously to both external and interlayer surfaces of clay minerals. This paper describes the use of hydroxy-aluminum solutions to stabilize formation clays against dispersion and structural expansion.

Introduction

Nearly all formations of interest to the petroleum industry contain clay minerals. Serious permeability decreases can occur when clay minerals obstruct flow by either expanding to fill pore spaces or dispersing and lodging in restrictions. Structural expansion occurs when additional water is adsorbed between clay layers. Montmorillonite under reservoir salinity conditions has two or three layers of water between interlayer surfaces. This gives a basal spacing of 15o to 18A. If the formation brine is diluted with fresher water, additional water is absorbed between theo layers until the basal spacing is perhaps 30A or more. If large amounts of expanding clays (often called swelling clays) are present, a very significant fraction of the flow channels may be consumed by the increased clay volume. Generally, however, expanding clay minerals are present in relatively small amounts, and problems from structural expansion are not so widespread as was once thought.

Particle dispersion and migration is a much more important damage mechanism, because it can cause serious damage with only very small amounts of clay present. Clay particles dispersed in flow channels present. Clay particles dispersed in flow channels are carried downstream until they lodge in constrictions. Miniature filter cakes are formed by these obstructions, plugging the pores, and reducing permeability according to the fraction of pores plugged. permeability according to the fraction of pores plugged. A specific force acting on expanding clays to expand their structure is caused by the affinity that exchangeable interlayer cations and interlayer surfaces have for water. A logical means of decreasing this tendency to expand in fresh water is to replace the exchangeable cations with cations less inclined to attract water to interlayer sites. Amines have been used but they have the disadvantages of (1) being expensive, (2) tending to oil wet the formation surfaces, and (3) eventually being displaced from surfaces by reservoir brines.

Another force that causes both structural expansion and particle dispersion results from the inherent negative charge on almost all clay minerals. This charge is neutralized by adsorption of cations on clay surfaces. Since the cations tend to dissociate, a positive ion swarm is established in the solution near positive ion swarm is established in the solution near the surface of the particle and, of course, a negative charge exists within the particle. This is normally referred to as an electric double layer. Particles with such double layers repel each other; hence, they tend to disperse. Since the strength of repulsion is directly related to the dissociative tendency of the adsorbed cations, more firmly attached cations tend to decrease the double-layer thickness thus the tendency of the particles to disperse. Multivalent cations such as particles to disperse. Multivalent cations such as calcium, magnesium, and aluminum are much more tightly adsorbed than monovalent cations such as sodium, potassium, and lithium. Multivalent cations prevent clay damage while they remain in place and prevent clay damage while they remain in place and Ca has been used successfully to treat water-sensitive formations.

JPT

P. 860