Hydraulic Fracture Performance of Carbonate Rocks with Novel Molecular Film Composite Acid: Experiments and Field Application

Carbonate rocks, characterized by complex porous media with vugs and fractures, are critical reservoirs for oil, gas, and geothermal energy. Acid fracturing is the primary stimulation method for these formations; however, conventional single-component acid systems are insufficient for complex carbonate reservoirs. This study investigates a molecular film composite acid (MFC acid) formulated with a thickener and a molecular film agent, and systematically assesses its effects on acid-etched fracture morphology and conductivity via static dissolution and acid-fracture conductivity experiments. Static dissolution experiments reveal a significant synergistic retardation effect driven by a dual defense mechanism, where interfacial shielding and fluid viscosity-induced mass-transfer resistance mutually reinforce each other. Although acid viscosity increases exponentially with thickener concentration, the acid-rock reaction rate is jointly governed by both additives and exhibits nonmonotonic behavior. A thickener proportion of 1/2 serves as a critical threshold for etching morphology: proportions below 1/2 promote highly nonuniform etching and maintain fracture conductivity above 3 D·cm at 60 MPa, whereas proportions at or above 1/2 yield uniform face dissolution and rapid conductivity decay below 2 D·cm at 60 MPa. Consequently, an MFC acid with a 1/3 thickener proportion is identified as the optimal formulation, effectively interconnecting natural fractures and achieving a superior conductivity of 5.1 D·cm at 60 MPa. Field applications in the Sichuan Basin confirm its ability to reduce pumping pressure and improve acid injection efficiency, providing a reliable reference for the design and application of novel MFC acid systems.