Stimulation of Sandstone Formations with The Acid-Mutual Solvent Method
Authors: Gidley John L. 
Published: December 2007

In stimulating sandstone formations, if the acid treatment is followed with diesel oil containing a mutual solvent, ethylene glycol monobutyl ether, the water wetness of both fines and formation will be assured. Field results show that EGMBE effects five to six times as much increase in oil productivity as conventional hydrofluoric-hydrochloric acid treatments. productivity as conventional hydrofluoric-hydrochloric acid treatments.
Introduction
Often the cause of low productivity in sandstone formations is reduced formation permeability near the wellbore. This condition, called "formation damage", has been related to a variety of completion and drilling practices. For example, the perforating operation may practices. For example, the perforating operation may reduce permeability around the perforation by matrix crushing and compaction caused by the shaped charge or by gun debris. The loss of completion fluids, filtrates from drilling mud, or drilling mud particles may cause clay swelling, particle plugging by dispersed formation fines, particle invasion, or adverse changes in fluid saturation and formation wettability. Highly alkaline filtrates occurring with the cementing operation may reduce effective permeability near the wellbore. Finally, reduced permeability may be caused by certain processes that occur during production; an example is the precipitation of scales, asphaltenes, or other insolubles as formation pressures and temperatures are lowered. Many of the adverse permeability effects relate directly to the clay fraction of the matrix. Extensive research has shown that certain effects such as clay swelling, clay dispersion, and particle plugging can be reduced by solutions of inorganic salts such as potassium chloride, calcium chloride, and certain complex potassium chloride, calcium chloride, and certain complex transition metal salts. A more direct elimination of the problem involves solubilizing the clay fraction with problem involves solubilizing the clay fraction with mineral acid solutions of hydrogen fluoride.

Mechanics of Acid Attack
The value of hydrofluoric acid for removing clays and increasing permeability in sandstone formations has been known for a long time. Unfortunately the potential of this treatment has not been realized in practice because of the nature of hydrogen fluoride attack upon sandstone The mechanics of this reaction was first described by Smith and Hendrickson, who showed that the initial contact of a sandstone core with hydrofluoric acid causes a reduction in permeability. The extent of this reduction was found to be related to the HF concentration of the acid, the flow gradient imposed during acid treatment, and the mineralogical composition of the sandstone core. These results are typified by Fig. 1. The reduction in permeability is caused by the partial disintegration of the sandstone matrix by hydrogen fluoride, which dissolves matrix cementing material and loosens fine particles. These in turn flow downstream and plug pore channels. This phenomenon is not limited to HF attack; it has also been found that permeability reduction can be induced by acids containing no hydrogen fluoride, provided the matrix cementing material is acid soluble provided the matrix cementing material is acid soluble and can be contacted by the acid. Sandstone formations containing calcite often exhibit reduced permeability in the first stages of acid contact even with dilute hydrochloric acid. In core experiments, the initial reduction in permeability resulting from acid attack is overcome when permeability resulting from acid attack is overcome when enough acid is injected to dissolve plugging materials or to enlarge alternate flow channels.
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