One aspect of an emulsion includes an internal phase including acid, an external phase including supercritical carbon dioxide, and multiple nanoparticles to stabilize the internal phase and the external phase. The acid can include hydrochloric acid. The hydrochloric acid can include 15% hydrochloric acid. The nanoparticles can include hydrophobic nanoparticles. A concentration of nanoparticles in the emulsion can be at least 0.1% by weight. The emulsion can include a corrosion inhibitor. A concentration of the corrosion inhibitor can be in a range of between 0.25% and 0.6% by volume. A ratio of a concentration of the acid to a concentration of the supercritical carbon dioxide can be in a range between 30% and 70%.