Non-equilibrium Phase Behavior of Hydrocarbons in Compositional Simulations and Upscaling

Numerical models widely used for multiphase flow simulations are based on the assumption of equilibrium phase behavior of hydrocarbons. However, it is not uncommon for oil and gas-condensate reservoirs to exhibit essentially non-equilibrium phase behavior, e.g., in the processes of secondary recovery after primary pressure depletion below saturation pressure. Also, the ability to match field data with an equilibrium model depends on simulation scale. The only method to account for non-equilibrium phase behavior adopted by the majority of flow simulators is the option of limited gas dissolution (condensate evaporation) rate in black oil models. For compositional simulations, no practical method has been presented so far, except for some upscaling techniques in problems of oil recovery by gas injection. Rarely presented previous models for non-equilibrium compositional simulations have a common problem of doubling the number of flow equations and unknowns at each simulation step compared to equilibrium formulation. This significant drawback obstructs their incorporation to existing simulators and widely adopted simulation algorithms. We suggest a physically-consistent formulation for the problem of compositional flow simulation with non-equilibrium phase behavior which has yet shown the following benefits. 1. The new formulation is based on the same form of flow equations in as in the equilibrium case, with modified flash equations. This makes it possible to incorporate the non-equilibrium model in existing simulators without significant extra computational costs and principal modifications to flow simulation modules. 2. A consistent technique has been developed for upscaling an equilibrium or non-equilibrium model into a coarse-scale non-equilibrium model with account for influence of scale effect on non-equilibrium phase behavior. 3. A model for component interphase mass transfer rate in the non-equilibrium flash equations has been presented which considers phase composition relaxation dynamics under changing pressure and overall composition and provides an efficient and robust tool for history matching in non-equilibrium phase behavior conditions. A number of simulation cases for real oil and gas-condensate mixtures are to be presented. Main contributions: 1. A new formulation has been presented for multiphase compositional flow model with account for non-equilibrium phase behavior suitable for practical reservoir simulations and implementation in existing compositional simulators. 2. Relation between problem scale and relevance of non-equilibrium effects has been physically justified. An upscaling technique has been developed for equilibrium and non-equilibrium compositional models based on the proposed non-equilibrium model formulation. 3. A model has been derived for component interphase mass transfer rates in non-equilibrium flash equations to provide robust, efficient and physically-consistent history matching of compositional models.