Controlling mechanisms of CO₂ sequestration efficiency in tight carbonate gas reservoirs: experimental insights into pore-throat constraints and mineralogical responses

The injection of CO₂ into low-pressure tight gas reservoirs can achieve the purposes of enhancing reservoir energy, increasing gas reservoir recovery and reducing carbon emissions. For the CO₂ energized fracturing process, it can also improve the fracturing fluid flowback efficiency and reduce water blocking effects. In the context of "dual carbon" strategy, studying the CO₂ storage behavior during CO₂ injection in tight carbonate gas reservoirs is of great significance. In this paper, the CO₂ storage effect and influencing factors of CO₂ injection in tight carbonate core samples are experimentally investigated. The main factors affecting the bound CO₂ storage are analyzed by means of nuclear magnetic resonance (NMR), threshold pressure gradient testing, and X-ray diffraction. Additionally, the influence of dissolved-solidified CO₂ storage on mineral composition and pore size distribution is also investigated. The results show that the CO₂ injection pressure has a significant impact on the bound CO₂ storage. When the pressure is higher than the supercritical pressure, the bound CO₂ storage rate can reach over 60%. And the dissolved-solidified CO₂ storage rate is at its peak of 10-15% when the pressure is between 5 MPa and 7 MPa. With the decreasing core permeability and the increasing threshold pressure gradient, the bound CO₂ storage rate increases. For tight carbonate gas reservoirs, the dissolution and solidification storage of CO₂ mainly occurs in small pores, medium pores and large pores. The dissolved-solidified CO₂ storage rate is affected by the mineral composition. Dolomite and calcite are the main dissolution minerals of CO₂ in water, thereby changing the pore throat distribution of the reservoir. This study can provide theoretical guidance for optimizing CO₂ injection technology, predicting storage effects, and optimizing gas well production in tight carbonate gas reservoirs.