Introduction

Some of the oil-bearing formations in the Lake Maracaibo area are strongly contaminated by clays. Gamma-ray logging has detected the presence of clay between 2500 and 3000 ft where water is being injected for secondary recovery purposes.

The reduction observed in the permeability to oil because of the clay alteration due to drilling filtrate is not easy to identify and quantify. There are several empiric techniques to attenuate the damage caused by clay dispersion to the porous media.

From the crystalographic point of view, it is not correct to generalize the different types of clays, since because of their specific properties, they react differently to water invasion; it is proposed to study the crystalographic structure of proposed to study the crystalographic structure of the clays present in some of the Lake Maracaibo formations, observing their behavior under water invasion and trying to find an adequate treatment for the damage caused by clay alteration. After experimenting with several ionic solutions, it was found that a 6.5% A1K(SO4)12H2O solution was the best inhibitor to clay dispersion in the cases studied.

THEORETICAL CONSIDERATION OF CLAY DISPERSION IN WATER SOLUTION

Clays are alkaline silicates. The alkali ions saturate a lattice structure and maintain the clay electrostatic balance. The alkali ions easily can exchange places with other types of ions. This facility to exchange their exterior ions is the reason for the wide variety of-existing natural clays.

Figs. 1 and 2 depict two different crystalographic structures, illite and kaolinite.

The alkali ions easily absorb water because of their high affinity with its molecule dipole. This causes a double effect: first, clay swelling and, second, clay dispersion. These two effects are some of the reasons for formation damage and are well known in the oil production recovery by water injection.

IONIC EXCHANGE - PREVENTIVE TREATMENT TO CLAY DISPERSION

The oil formations under study contain two types of clays, illite and kaolinite, as shown n Figs. 1 and 2.

Illite is a clay with a triple-layer crystalographic unit, while kaolinite is a double-layer crystalographic unit. The triple layer of illite is bonded through electrostatic forces, generated between the oxygen ions in the structure, and the external cations, generated from isomorphic changes that occurred in the crystalographic structure during the process of crystalization. The cations that characterize the illite are: K+, H3O+, Na+, Fe2+, Ca2+, Mg2+, Fe2+, a.d A13+. Among them, only the monovalent ones can hydrate and hydrolize easily breaking the electrostatic bonds when they migrate.

EXPERIMENTAL PROCEDURE

Five representative samples of the oil formation in which the clay concentration changed were chosen. The kind of clays present were determined by means of diffraction and fluorescence X-rays. Illite and kaolinite concentration at the corresponding depths are presented in Table 1. The samples were washed with toluene and distilled water, and dried up to 230 degrees F (110 degrees C) during 24 hours.

The static effect of the different liquids under well pressure and temperature [2000 psi (136 atm), 175 degrees F (79-4 degrees C)] was determined during 3 hours. Afterward, the samples were screened to determine the size of the dispersed particles. Similar mesh-sized particles were analyzed by diffraction and fluorescence X-ray to determine the clay concentration for each particle size.

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