Stability of Clay Minerals in Acid

Abstract

Abstract Hydrochloric (HCl) acid is commonly used to acidize sandstone formations. Although much emphasis has been placed upon the dissolution and reprecipitation theories for acidization with hydrofluoric (HF) acid, little information is available concerning the effect of HCl acid on clay minerals commonly found in sandstone formations. While clays are not truly soluble in HCl acid, exposure to HCl acid does affect the structure of clay minerals. This paper presents results of acid stability studies conducted with chlorite, illite, and kaolinite clays. Initial testing utilized solubility analyses to determine which minerals are affected by acid. A second series of tests utilized x-ray diffraction analysis to determine the effect of HCl acid on the crystalline structure of the clays. Chlorite was found to be the most susceptible to acid attack. The reaction of acid on chlorite was found to be dependent upon HCl strength and temperature. A third series of tests evaluated the effects of weak HCl, formic, and acetic acid. Diffraction studies, coupled with fluid analyses, revealed that the mechanism of acid attack is the leaching of ions from the matrix of chlorite clay. By-products of the acid dissolution of chlorite clay are of concern because they can cause formation damage. When the crystalline structure is destroyed, a significant amount of amorphous residue remains. The leached ions (primarily iron and aluminum) can precipitate as the acid is neutralized. In addition to the diffraction studies permeability change flow tests were conducted utilizing permeability change flow tests were conducted utilizing formation cores with high chlorite content. The results of these tests demonstrate that formations containing chlorite clay can be acidized successfully, provided that clay content, acid strength, and provided that clay content, acid strength, and bottomhole temperature are considered in the treatment design. The flow tests also demonstrate that where extremely high chlorite contents cause acid sensitivity, organic acids may serve as acceptable breakdown and stimulation fluid systems. Introduction Acidization of sandstone formations is generally performed for one of three purposes:to open or performed for one of three purposes:to open or "break down" perforations,to remove acid-soluble scales, andto increase permeability in the near wellbore area. Regardless of the reason for acidizing, it is important to consider the composition of the formation when planning and designing an acid stimulation treatment. In particular, one must consider the minerals which are susceptible to acid attack. Sandstone formations are composed of quartz with varying amounts of feldspars, clays, and carbonates. Much emphasis has been placed upon dissolution and reprecipitation theories for the reaction of HF acid with these minerals. HF acidizing theory is beyond the scope of this paper, rather this paper deals with the effects of HCl acid on clay minerals such as kaolinite, illite, and chlorite. Clays are layer silicates formed by the chemical weathering of other rock-forming silicate minerals. The layers are composed of various combinations of two fundamental units:tetrahedra, layers consisting of linked silicon-oxygen tetrahedra, andoctahedral layers in which hydroxyl ions occur in two planes, one above and one below a plane of magnesium planes, one above and one below a plane of magnesium or aluminum ions. Each clay mineral has a specific arrangement of the two fundamental units (Fig. 1). A three-layer clay would have one octahedral sheet with tetrahedral sheets on each side. A pure crystal of this type is known as the clay mineral pyrophyllite. pyrophyllite. P. 201