Restoring Permeability to Clay-Containing Water Damaged Formations
Authors: Slobod Robert L. 
Published: December 2007

This paper was prepared for the 44th Annual Fall Meeting of the of the Society of Petroleum Engineers of AIME, to be held in Denver, Colo., Sept. 28-Oct. 1, 1969. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal, provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
The idea of using multivalent ions such as Ca++ and Al+++ to change the properties of water sensitive clays is not new. From the literature one gains the impression that a core with permeability reduced by expanded [hydrated] clays might be expected to show increased permeability immediately after treatment with Ca permeability immediately after treatment with Ca ion. The study reported herein shows that such improvement does not occur in wet cores. The exchange of ions does occur and the clay is modified chemically, but the volume occupied by the clay does not change while the clay is wet. The clay must first be dried during which step the clay shrinks. This clay, upon re-wetting, now acts as the Ca-clay should and shows only a modest amount of expansion from hydration. With respect to field application, this action means that a reservoir which has been damaged by fresh water will not exhibit much improvement from a treatment with a solution containing a multivalent ion as long as the clay stays in the wet, expanded state. If some means can be used, however, to dry the clay in place prior to its re-exposure to fresh water, the treatment will have value. The dried clay will not expand to its former permeability damaging size when re-exposed to water. Data are presented from the experimental work substantiating this idea, and possible means for applying the idea to the field are presented.

Introduction
The fact that clay constituents in consolidated rocks expand when contacted with fresh water and reduce the permeability is a well known phenomenon. For production operations, the most serious aspect of this problem is the marked reduction in flow rate which occurs in the field when certain clay-bearing formations are exposed to fresh water. In some cases complete blockage of flow occurs.
By way of review we should note that the constituent of clay which causes the most trouble is the montmorillonite which can be visualized as occurring in layers which are close together when the clay is dry. Positive ions are present in the system as part of these clay layers, and the property of the clay is very dependent upon the identity of the ion which is present. When H and Na are present, the clay generally shows the ability to absorb a great deal of water causing the layers of the clay to separate. This action produces the so-called clay swelling which acts to shut off the flow of fluids in the pores. The clays also exhibit ion exchange ability which means that the H of the Na can be replaced in the clay structure by other positive ions. Such an exchange can take place when the clay is exposed to a solution containing a sufficient concentration of the ion doing the replacement.