Micromodels with simplified porous microfluidic systems are widely used to mimic the underground oil-reservoir environment for multiphase flow studies, enhanced oil recovery, and reservoir network mapping. However, previous micromodels cannot replicate the length scales and geochemistry of carbonate because of their material limitations. Here a simple method is introduced to create calcium carbonate (CaCO₃) micromodels composed of in situ grown CaCO₃. CaCO₃ nanoparticles/polymer composite microstructures are built in microfluidic channels by photopatterning, and CaCO₃ nanoparticles are selectively grown in situ from these microstructures by supplying Ca²⁺, CO₃^2- ions rich, supersaturated solutions. This approach enables us to fabricate synthetic CaCO₃ reservoir micromodels having dynamically tunable geometries with submicrometer pore-length scales and controlled wettability. Using this new method, acid fracturing and an immiscible fluid displacement process are demonstrated used in real oil field applications to visualize pore-scale fluid-carbonate interactions in real time.