An Experimental Study on Carbonated Water Imbibition in Ultra-low Permeability Tight Cores Using Nuclear Magnetic Resonance

The development of tight reservoirs is frequently constrained by significant flow resistance resulting from nanoscale pore throats, which often limits the efficiency of conventional water flooding and leads to suboptimal production performance. CO₂ fracturing is a highly promising technology for reservoir stimulation and enhanced oil recovery. During CO₂ fracturing operations, dissolution of CO₂ into formation water enables in-situ generation of carbonated water (CW). This process facilitates the simultaneous utilization of capillary-driven imbibition and CO₂ mass transfer/diffusion mechanisms to enhance oil recovery (EOR). To investigate the EOR mechanisms of CW imbibition during CO₂ fracturing in tight reservoirs, a comparative imbibition study was conducted using carbonated water and conventional brine on core samples. An integrated experimental methodology demonstrates that CW imbibition exhibits superior performance compared to conventional brine in tight sandstone formations. The technology significantly improves recovery efficiency, increasing the ultimate imbibition recovery factor from 19.6% to 27.15%, representing a substantial enhancement in oil displacement performance. Macroscopic analysis reveals that CW imbibition achieves faster frontal advancement and superior sweep efficiency within the reservoir, resulting in more thorough displacement of crude oil from the pore network. This observation is visually confirmed through NMR imaging, which shows rapid and pronounced contraction of oil-saturated regions. At the microscopic scale, carbonated water mainly functions by dissolving the rock matrix to enhance pore connectivity. The effect significantly improves oil mobilization from micropores, increasing their production contribution from approximately 10% to 20%, thereby optimizing the overall pore-scale production profile. This study demonstrates that carbonated water imbibition represents a highly promising EOR technology for tight oil reservoirs, exhibiting particular effectiveness in accessing and producing from underutilized micropore networks while simultaneously enhancing both ultimate recovery and the operational effectiveness of the imbibition fluid.