Microencapsulation of Acids by Nanoparticles for Acid Treatment of Shales

Acid treatment is one of the common well-stimulation techniques to improve the production in tight carbonates or calcite-rich shale reservoirs. Because the reaction rates are often too large at the reservoir temperatures, retarded acid systems such as emulsified acids, foamed acids, and polymer-gelled acids are used. The retarded acids also minimize damage to the wellbore and propagate acids long-distance from the wellbore in fractured shales. In this work, a novel encapsulation method of highly concentrated acid (∼10 wt % HCl) is reported as an alternative retarded acid system. Microencapsulation of these acids is performed using highly hydrophobic silica nanoparticles. The mixing of these particles with acid under high shear rates results in the formation of acid-in-air powders. The release of acid from these powders could be triggered by external stimuli such as mechanical pressure or surfactant addition. The thermal stability, corrosion inhibition efficiency, and shale surface reactivity are compared to conventional acid-in-oil emulsions, which are typically used for shale acidization processes. The encapsulation is found to be quite robust at 80 °C, and half-life (time taken to release 50% of encapsulated acid) is increased by more than 2 orders of magnitude as compared to acid-in-oil emulsions. A gravimetric measurement technique is used to quantify the iron corrosion rate for these two systems. The corrosion rate for the case of powders is found to be lower than that of emulsions at all of the studied temperatures (25, 60, and 80 °C). The shale surface topography is analyzed using an optical profilometer before and after the surface treatment with these two encapsulated-acid systems. The release of acid from powders, triggered via mechanical pressure, results in surface etching, which could potentially increase the fracture conductivity in shales. The release of acid can also be enhanced by the addition of anionic surfactants in the carrier fluid. The simplicity of the acid-in-powder generation process along with excellent thermal stability and corrosion inhibition efficiency makes the proposed acid-encapsulation technique lucrative for acid treatment applications.