A Phase-State-Radius-Based Multiphase-State Identification Acceleration Approach of the Hydrocarbons-CO2-H2O System for Compositional Reservoir Simulation

Abstract

Summary The CO2-enhanced oil recovery (CO2-EOR) technique could increase the oil recovery factor and achieve subsurface CO2 storage. To evaluate the feasibility of CO2-EOR operation, an equation-of-state-based (EOS-based) compositional simulation is inevitable. However, time-consuming multiphase flash calculations limit the application of the conventional EOS-based compositional simulation in large-scale reservoir simulations. Therefore, there is an urgent need to accelerate the EOS-based compositional simulation by reducing the time spent on the multiphase flash calculations. In this study, we develop a fast, multiphase-state identification approach to achieve high computational efficiency of the compositional simulation. First, taking one given point in the compositional space composed by pressure and the overall composition as the original point, the phase-state radius (PSR) is defined as a radius inside which all points have the same phase state as the given point. Subsequently, the results of the multiphase flash calculations of the points in the compositional space are stored. Finally, when a new point is inside the PSR of one calculated point, the stored results are used to accelerate the new multiphase flash calculations of the new point by omitting the phase stability analysis (PSA) and possible additional multiphase split (MS) calculations. Case studies show a 6–10% reduction in multiphase flash computation time compared with two existing methods when the fluid is far from the phase boundary and a reduction of at least 17.5% when the fluid is near the phase boundary.