Clay Stabilization During Fracturing Treatments With Hydrolyzable Zirconium Salts
Authors: Peters F.W. , Stout C.M. 
Published: December 2007

This paper discusses the mechanism of clay damage and application of a zirconium salt in stabilizing clays to prevent this damage. Three years of production results from applications in the Clinton formation are used to substantiate this method of clay stabilization.

Introduction

Oil and gas production in the Appalachian area has frequently exhibited poor decline performance. High early rates are followed by rapid decline, and stabilized production rates are generally very low, often near the border line economically. In the Appalachian area, it is believed that formation damage caused by the migration of clay particles is the primary cause of rapid decline. Furthermore, it is believed that the dispersion and migration are the results of clay sensitization by conventional stimulation fluids. Core studies have shown that clays are certainly present in Appalachian formations. Table 1 shows the general composition of many of these formations. All contain clays ranging from 2 to 24 percent by weight of the rock matrix. percent by weight of the rock matrix. It is important to note that all formations contain illite and chlorite. Kaolinite also has been noted in some Appalachian formations. These minerals are the so-called "nonswelling" clays. They actually do swell to some extent when contacted by fresh water, but the swelling is not of a magnitude to cause extensive permeability damage. They can, however, cause severe permeability permeability damage. They can, however, cause severe permeability reduction by dispersion and migration. This simply means that when the clays are contacted by some fluids, the tiny platelets of which they are made will disperse. These small particles are then free to migrate with flowing fluids. Subsequently, the particles can agglomerate in narrow pore passages and block flow channels. Several studies confirm the dispersion mechanism of clay damage. Bertness called this the "dispersion-movement and blocking" mechanism. These studies also attempted to define the possible causes of this phenomenon. Salinity of native and induced fluids has long been considered the major factor affecting the clay sensitization. It is generally believed that, as they exist in a reservoir, clay particles are in flocculated equilibrium with the native fluids. particles are in flocculated equilibrium with the native fluids. According to Hewitt, and Moore, reduction of cation concentrations surrounding clay particles by dilution with induced fluids will promote dispersion of clays. Jones concluded that the type of promote dispersion of clays. Jones concluded that the type of cation as well as the concentration could affect clay damage. He found that divalent calcium is more effective than monovalent sodium in preventing dispersion, and that calcium seemed to dominate in a mixed brine if it composed at least 10 percent of the cations present. Jones further stated that any rapid decrease in salinity may trigger clay dispersion, regardless of the salts involved. He postulated that a chemical potential between a stronger brine in postulated that a chemical potential between a stronger brine in the clay matrix and a weaker brine surrounding it results in a phenomenon similar to osmosis that creates a differential pressure. phenomenon similar to osmosis that creates a differential pressure. The weaker solution forces its way into the clay lattice in an attempt to dilute the dense atmosphere of cations surrounding each platelet in the lattice. platelet in the lattice. Salinity may not be the only fluid property that affects the clay-damage mechanism. Bernard and Holbrook found that, in some cases, changes in pH of test fluids altered the degree of formation damage by clays.

JPT

P. 187