Effect of SC-CO2 - acid injection mode on fracture morphology and conductivity of CO2-energized acid-fracturing in tight carbonate rocks: An experimental study

CO 2 -energized acid-fracturing (CEAF) improves oil and gas recovery, geothermal extraction, and CO 2 storage in carbonate reservoirs by combining supercritical CO 2 (SC-CO 2 ) and acid to enhance fracture conductivity. However, how SC-CO 2 and acid injection modes affect acid-etched fracture morphology and conductivity remains unclear, limiting CEAFʼs widespread application. To study this, a self-developed SC-CO 2 acid-etching device simulated two injection modes-SC-CO 2 /acid mixed injection (SCM) and alternative (SCA)-compared with fracturing fluid/acid alternative (FA). 3D laser scanner captured fracture morphology, and conductivity was measured experimentally. Results show that acid-etching fracture behavior, morphology, and acid-rock reaction rate depend on non-reactive mineral distribution and injection mode. In impure limestone, acid-etched fractures are morphologically complex but consistently nail-type across all injection modes. In calcite and dolomite, injection mode significantly affects etching: SCA and SCM produce vertical-channel patterns; FA produces horizontal-channel patterns. Reaction rates are highest in calcite, then impure limestone, and lowest in dolomite. SCM retards the reaction due to SC-CO 2 -acid mixing; SCA slightly increases it. For conductivity, SCA works best in limestone; FA is most effective in dolomite.