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Reaction of Hydrothermally Altered Volcanic Rocks in Acid Solutions

Georgina Izquierdo Montalvo, Alfonso Aragón Aguilar, F. Rafael Gómez Mendoza and Magaly Flores Armienta

Rock matrix stimulation has been used to clean, to recover and to enhance well productivity in oil systems. Recently, for the same purposes this methodology began to be applied in geothermal systems. In order to investigate the solubility of altered volcanic rocks in acid solution used in rock matrix stimulation; experiments were carried out on samples of igneous hydrothermal altered rocks from the Los Humeros geothermal reservoir. Industrially, the common acid solutions used during acid well stimulation are HCl 10% and a mixture of HCl 10% and HF 5%. In this work, experiments were conducted in the laboratory using the referred acid solutions at atmospheric pressure and temperature of 110 ± 5°C.

The chemistry, the mineralogy and the permeability of selected rocks from Los Humeros geothermal field were determined before and after the reaction with each acid solution. Mineral dissolution is selective and depends on the permeability of the rocks, the type and the intensity of hydrothermal alteration.

As it is expected, Calcite readily reacts with acids leaving empty cavities, veins and micro fractures (worm holes). Calc-silicates are resistant to acid solutions. If Calcite is absent dissolution of minerals is observed in the external surfaces of the specimen in contact with the acid solution giving rise to a rough texture and leaving the rock matrix unreacted