Innovative Encapsulating Acid with Release Dually Controlled by the Concentration of Hydrogen Ions and Temperature

Acid fracturing is a stimulation method in carbonates, in which the acid
is injected into a hydraulically created fracture. The encapsulating/coating
acid proposed in this paper can realize deep penetrating acid fracturing,
increase the effective action distance of acid solution, and improve the flow
conductivity at the far end of fractures. For this purpose, a solid
acid encapsulating/coating material is proposed, which maintains its
structural integrity and isolates the solid acid at higher H+ ion concentrations and lower temperatures. The solid acid is released
at lower H+ concentrations and higher temperatures due
to the fracture of the encapsulating material. Thus, the acid release
process is dually controlled by the concentration of hydrogen ions
and temperature. The molecular structure, characteristic viscosity,
and solubility of the encapsulating material were experimentally determined,
as well as the effective content, release performance, corrosion performance,
and dissolution performance. The encapsulating material was found
to be composed of acrylamide and other polymers, belonging to the
high polymer group. Its characteristic viscosity was approximately 2.445 dL/g,
making it mainly insoluble in a high-concentration acid (>16%).
The
effective content of the encapsulating acid reached 85%, the critical
release concentration of hydrochloric acid was about 14%, and the
critical release temperature was about 95 °C (for hydrochloric
acid concentration not exceeding 14%). Its corrosion performance was somewhat better
than that of hydrochloric acid of the same concentration. The encapsulating/coating
acid proposed in this paper has promising application prospects in
the acid fracturing reconstruction of high-temperature deep carbonate
reservoirs.