Influence of Chemical Composition of Water on Clay Blocking of Permeability
Authors: Jones Frank O. 
Published: December 2007

JONES JR., FRANK O., PAN AMERICANPETROLEUM CORP., TULSA, OKLA. MEMBER AIME

Abstract
The capabilities of small proportions of divalent cations, such as calcium or magnesium, for controlling clay blocking are reported. Potentially sensitive formations can be exposed to fresh water if at least one-tenth of the salts dissolved in both the native water and invading fresh waterare calcium and magnesium salts. Often, even less suffices. The behavior evidently depends upon the cation exchange properties of the clays, which favor adsorption of calcium and magnesium over sodium. Clays having sufficient adsorbed calcium and magnesium resist dispersion by water and consequent blocking of permeability. An allied phenomenon, the dependency of clay blocking on salinity contrast, is also reported. Abrupt change from highly saline to fresh water can cause blocking which may not occur if salinity is lowered slowly. A process related to osmosis is thought responsible for this behavior. Applications of finding to drilling, fracturing and water flooding are discussed.

Introduction
Many formations, when exposed to fresh water, canlose permeability through clay effects. For example, drilling-mud filtrate can cause oil permeability decreases, which persist long enough to interfere with drill-stemtesting and well completion. Ultimate productivity may suffer when clay damage is severe, especially if drawdown pressures are low.' The injection pressures and the time required for water flooding can increase if clay blocking occurs. Earlier work indicates that clay blocking is caused by obstruction of flow channels by clays or other mineral fines dispersed by fresh water. It is well known that increasing salt concentrations in water tends to prevent clay blocking. Also it is well established, although perhaps not as widely known, that the nature of the dissolved solids is also important. If, for instance, formation clays are exposed to calcium chloride solution and then exposed to weaker solutions of calciumchloride or distilled water, considerably less permeability damage results than if the clays had been exposed instead to sodium chloride solutions. The reasons for this behavior are:clays are cation-exchange, or base-exchange, materials similar in this respect to zeolites or exchange resins, andclays in the calcium form do not disperse easily in fresh water, whereas clays in the sodium form do.
The base-exchange form of a clay is easily altered by flowing a solution through it. For example, asodium clay can be changed to a calcium clay simply bypassing a calcium chloride solution through the clay bed. There is general agreement that the reason calcium and magnesium clays do not disperse easily in fresh water is because the calcium ions do not ionize easily from the clay surfaces. Sodium clays, however, are believed to allow the sodium ions to ionize from the clay particle surfaces. In fresh water, this allows the particle to accumulate a net negative electrical charge great enough for the particles to repel one another and, therefore, dispersemore easily. The foregoing suggests that, even in the presence of a large excess of sodium, a relatively small ratio of divalentcation, such as calcium or magnesium, might serve to prevent clay dispersion. The divalent ion probably adsorbs to a disproportionately large extent, because of being adsorbed more tightly, with the result that the proportion of divalent cation on the exchange positions of the clay might then be great enough for the clays to resist dispersion. Consequently, an investigation was conducted to find how mixtures of salts in solution influence clay blocking.

APPARATUS AND PROCEDURE
Water permeability tests were conducted using conventional techniques. The equipment is illustratedschematically in Fig. 1. Flow rates of solutions driven through small core samples at known temperatures and pressures were measured. Regulated compressed air, usually at 10 to15 psig, was used to drive the solutions. Core samples0.75 in. in diameter and I in. long were used for the most part. Cores were saturated by evacuating and then admittinga de-aerated solution. Tight core samples were first saturated with carbon dioxide gas and then evacuated and saturated with the test solution. After saturation, 140 psig pressure was imposed for several hours.
JPT
P. 441^