Well treatments with only HCl are potentially damaging in formations that contain aluminosilicate minerals (clays, feldspars, and micas) because HCl reacts significantly with aluminosilicates, leaching aluminum and forming hydrated silica. Fluid flow past leached mineral particles liberates mineral fragments and amorphous silica within the pore network. These fines can block pore throats, reducing the rock permeability. Both field data and reservoir-condition coreflood test results are consistent with this chemical reaction and permeability impairment mechanism. The average permeability reduction in our corefloods was 35%.