Reservoir fluid applications of a pseudo-component delumping new analytical procedure

Abstract

Compositional reservoir simulations require a huge number of flash calculations. The problem dimensionality (and thus the computational effort) is usually reduced by lumping several (often many) individual components into pseudo-components. Typically, less than 10 pseudo-components are used for full-scale field simulations. However, the detailed fluid phase split is important for surface process simulations. The detailed phase compositions resulting from a flash calculation performed on a lumped mixture can be estimated using a delumping (inverse lumping) procedure. In fact, the delumping acts like an interface between reservoir and surface simulations. We have recently proposed a delumping procedure based on the reduction method for phase equilibrium calculations, which is (i) analytical, (ii) consistent, (iii) applicable for equations of state (EoS) with non-zero binary interaction parameters (BIPs), and (iv) its applicability holds for a large variety of EoS, provided suitable mixing rules are used. In this paper, the new delumping method is tested for several reservoir fluids and reservoir processes (such as gas injection), focusing on how severe changes in composition are affecting its accuracy. We examine a series of flashes, considered to be representative of reservoir processes (differential expansion and constant volume depletion, multistage separations), and swelling tests. Non-zero BIPs between hydrocarbon components and classical contaminants, as well as between methane and heavier hydrocarbon components are considered in all cases. For all test examples, phase mole fractions and the vapor mole fraction of the delumped mixture are in excellent agreement with the values obtained by flashing the original mixture. Advancing to the next pressure step, the detailed composition obtained by delumping preserves a high accuracy in predicting detailed fluid properties even for drastic compositional changes.