Experimental Study On Water- Supercritical CO₂ Two-phase Flow And Displacement Using Sandstone Cores And Micromodel

Geological carbon sequestration（GCS） in deep aquifer has been considered as an effective technology to reduce the greenhouse effect by controlling the emission of anthropologic CO₂. In modeling of geological carbon storage, dissolution of supercritical CO₂（scCO₂） is often assumed to be instantaneous with equilibrium phase partitioning. However, the low viscosity of scCO₂ comparing to formation brine,complicated and heterogeneous pore structure and pore-network topology may result in scCO₂-water non-equilibrium dissolution by limited interface area. Moreover, the interaction and coupling of scCO₂-water two phase flow and dissolution needs a comprehensive and systematical study.In this phD dissertation, experimental study of scCO₂ dissolution and mass transfer was conducted at core- and pore-scale. At core-scale, displacement experiments were conducted at 8-10 MPa,40°C by using representative sandstone samples from GCS site, Ordas Basin. The rock core was first saturated with deionized（DI） water and drained by injecting scCO₂ to establish a stable scCO₂ saturation. DI water was then injected at constant flow rates after scCO₂ drainage was completed.We observed the time-dependent dissolved CO₂（dsCO₂） concentration in the outflowing water, 1-2 orders of magnitude smaller than the CO₂ solubility under experimental conditions, indicating the non-equilibrium CO₂ dissolution. This observation was further studied by sensitive analysis on water injection rates and dsCO₂ concentration in the displacing water. A model is presented to describe the observed S-shaped function of non-equilibrium mass transfer rate with core mass ratio and the dsCO₂ differential concentration in injected water.To further investigate the core-scale CO₂ dissolution and mass transfer, pore-scale scCO₂ dissolution experiments in a 2D heterogeneous, sandstone-analogue micromodel were conducted at supercritical conditions（9 MPa and 40 °C）. High resolution time-lapse images of scCO₂ and water distributions were obtained during imbibition and dissolution by a fluorescent imaging system. Through the images,direct observation and evidence on the CO₂ non-equilibrium dissolution was obtained:the channeling flow of water along the high permeability regions and bypassed of the majority scCO₂ limited the two phase interfacial area, leading to the dsCO₂ 1-3orders of magnitude smaller than the CO₂ solubility under the experimental conditions.In addition, the measured relative permeability of water, affected by scCO₂ dissolution and bi-modal permeability, shows a non-monotonic dependence on saturation, which differs from the classic models showing of a monotonic relationship between relative permeability and saturation. Results from the pore-scale experiments provided direct evidence and explanations on previous core-scale observations, which may help in understanding the mechanisms transition from residual trapping to dissolution trapping. The cross scale studies on coupling between dissolution and multi-phase flow may also have a good insight on other resource exploitations, such as hydraulic fracturing of shale, CO₂-EOR, CO₂-ECBM and CO₂-EGS.