A multiphase constant volume depletion simulation considering critical flow saturation

The constant volume depletion (CVD) test is widely regarded as a key pressure–volume–temperature experiment for capturing changes in fluid composition during depletion. However, this view is challenged by multiphase systems, as conventional oil–gas systems neglect the aqueous phase and its interactions with hydrocarbons and CO2. This work proposes a novel CVD simulation method that employs the Cubic-Plus-Association equation of state to perform multiphase flash calculations. During the depressurization process, the expelled phase and the corresponding volume are determined and allocated based on the phase's saturation and the proportion of its real-time mobility, which allows the model to reflect multiphase flow characteristics more realistically. The process is divided into three sequential stages based on the bubble point pressure (Ppb) and the gas phase critical flow pressure (Pcf) of the system: the elastic energy stage, the solution gas drive stage, and the gas convergence stage. For each stage, the differences in mobile phases, drive mechanisms, and recovery are analyzed in detail. Comparative analysis shows that the new model exhibits higher application potential and prediction accuracy in terms of production performance while also revealing the significant influence of the aqueous phase. Additionally, a sensitivity analysis is performed to evaluate the effects of water saturation (movable/irreducible water), injected CO2 mole fraction, and rock type (distinguished by pore size) on component recovery.