Simulating Early Gas Breakthrough in Undersaturated Oil Using Alpha-Factors

Abstract

A method is proposed for reproducing early gas breakthrough when performing full-field compositional simulation of gas injection in under-saturated oil. It was developed by solving the successive problems encountered on a real heterogeneous field with a gas injection pilotThe first problem was to correctly model transport in this large carbonate field using a coarse grid. It was dealt with by using alpha-factors. The second difficulty was related to the strong under-saturation of the oil: all injected gas dissolves until saturation pressure reaches actual block pressure. In practice, this means that gas will appear too late in the simulation. This is easily avoidable in black-oil simulation, but compositional simulators will usually not allow to circumvent the equation of state. We found out that correcting the critical transition allowed to speed up the appearance of the gas phase. A third difficulty was to history match the gas injection pilot without distorting the geological model. We thus wanted to reproduce the inherent instability of gas injection, which had not swept all layers homogeneously, as shown by time-lapse RST data. This problem was solved using an extra-capability of alpha-factors: stating that the transported fluid viscosity is lower than average grid-block viscosity, and integrating this new correction into the oil and gas alpha-factor table, allowed to successfully reproduce quick gas encroachment in permeable layers. The fourth task of imposing a residual oil saturation Sorm was solved using the end-point of the alphafactor table.Thus we propose a method for correcting several effects simultaneously: acceleration of the light components, acceleration of the critical transition from volatile oil to gas condensate, viscosity correction to reproduce gas fingering, and slow-down of the heavy components in order to force residual oil saturation. To our knowledge, documentation of the second and third corrections is new. All four corrections being justified, we used them all in a composite alpha-factor table.

Modelling Non-Equilibrium Phase Behavior of Hydrocarbon Mixtures

“…The proposed approach provides an alternative to standard techniques of compositional models upscaling (alpha factors) [10,11] and is characterized by more accurate representation of the physics, simplicity of implementation without intervention to flow simulation algorithms and applicability not only to oil recovery processes by gas injection, but to a wide variety of oil and gas flow problems with intensive phase transitions. Moreover, dealing with non-equilibrium compositional simulation, upscaling and history matching of compositional models on the same basis of the modified flash system (6) allows accurate simulation of non-equilibrium phase behavior occurring on different scales during petroleum reservoir development.…”

“…It should be pointed out that for the upscaling problem a similar method was presented in [11]. However, in that study both the formulation of left hand side of equations (6) and the calculations of its right hand side in the process of upscaling are based on differences not between logarithms but the values of components' fugacities themselves.…”

Modelling Non-Equilibrium Phase Behavior of Hydrocarbon Mixtures (Russian)

“…Предлагаемый подход является альтернативой к известной процедуре ремасштабирования композиционных моделей с использованием α-факторов [10,11] и отличается большей физической обоснованностью, простотой реализации без вмешательства в гидродинамическую подзадачу и пригодностью не только для процессов вытеснения нефти газовыми агентами, но и для широкого круга задач фильтрации нефтяных и газоконденсатных систем с интенсивными фазовыми превращениями. Кроме того, рассмотрение задач неравновесного композиционного моделирования, ремасштабирования и адаптации композиционных моделей на единой основе с использованием модифицированной системы термодинамических уравнений (6) позволяет корректно моделировать неравновесные эффекты, возникающие на различных масштабах в процессе разработки месторождений.…”

“…Следует отметить, что применительно к задаче ремасштабирования близкий метод предложен в работе [11]. Однако там как запись левой части системы (6), так и расчет ее правой части при ремасштабировании осуществляется на основе разности не логарифмов летучестей, а самих величин летучестей.…”

Modeling of capacitance flow behavior in EOS compositional simulation

“…Pseudo relative permeabilities are required at sub-miscible conditions. The presence of more phases yields more non-linearity and severer non-uniqueness of the history matching process with the alpha factors (Barker and Fayers, 1994;Christie and Clifford, 1998;Barker et al, 2005;Bourgeois et al, 2011Bourgeois et al, , 2012. Christie and Clifford (1998) combined the alpha-factor method with streamline techniques, and re-calculated the boundary conditions for certain grid blocks to deal with the potential error in multidimensional cases.…”