Time-scales for the development of thermodynamic equilibrium in hydrocarbon reservoirs
Author(s)
Besong, Donald Ojong
Type
Thesis
Abstract
This project investigates the time-scales for hydrocarbon components in an isothermal oil reservoir
column to reach thermodynamic equilibrium under the competing influences of molecular
diffusion and gravitational segregation by gravity diffusion. The influence of non ideal behaviour
on the equilibrium compositional profile, as well as on the equilibrium time for some
examples of binary and ternary hydrocarbon mixtures is also investigated.
When the variation of hydrocarbon composition within a field cannot be described by standard
steady-state models of gravity-diffusion equilibrium, it is usually assumed to be caused by
some degree of hindrance to the connectivity of the oil volume, a situation known as reservoir
compartmentalization. However, order of magnitude estimates of the time taken for thermodynamic
equilibrium to be established by diffusion are similar to the ages of many hydrocarbon
reservoirs (between 1 million and 100 million years). Thus it is possible that compositional
variations within a reservoir may be simply due to there having been insufficient time from
reservoir filling for diffusion to equilibrate compositions.
It is important to determine the time-scales for vertical compositional gradients to be established
in order to assess whether compositional profiles that are not in thermodynamic equilibrium
are indicative of barriers to flow within the reservoir or simply that the reservoir fluids
have not yet had time to establish a steady-state distribution.
A macroscopic, numerical model of the thermodynamic behaviour of the reservoir fluids has
been used for this investigation. The model has been validated against simple transient analytic
solutions for molecular diffusion, as well as steady-state solutions for molecular/gravity diffusion
in binary mixtures. It was found that a uniform mixture of methane and undecane will
segregate over a vertical distance of 300m in a few hundred thousand years and that this timescale
can be affected by non-ideal mixing and the relative proportions of the two components.
Equilibrium time was found to be a function of the competing influences of molecular diffusion
and gravitational segregation. We also use our model to investigate laboratory observations
of compositional grading in ternary mixtures of methane, n-pentane and 1-methylnapthalene
reported by Ratulowski et al. (2003) and why their numerical simulation produced an unexpectedly
smaller separation when methylnaphthalene was replaced by n-undecane, although
methylnaphthalene and undecane have almost the same molar weight. Our study also provides insight into the relative importance of density, molecular diffusion, initial composition and realmixture
modelling (as opposed to ideal mixture assumptions) on compositional grading in a
fluid more representative of a real crude oil.
column to reach thermodynamic equilibrium under the competing influences of molecular
diffusion and gravitational segregation by gravity diffusion. The influence of non ideal behaviour
on the equilibrium compositional profile, as well as on the equilibrium time for some
examples of binary and ternary hydrocarbon mixtures is also investigated.
When the variation of hydrocarbon composition within a field cannot be described by standard
steady-state models of gravity-diffusion equilibrium, it is usually assumed to be caused by
some degree of hindrance to the connectivity of the oil volume, a situation known as reservoir
compartmentalization. However, order of magnitude estimates of the time taken for thermodynamic
equilibrium to be established by diffusion are similar to the ages of many hydrocarbon
reservoirs (between 1 million and 100 million years). Thus it is possible that compositional
variations within a reservoir may be simply due to there having been insufficient time from
reservoir filling for diffusion to equilibrate compositions.
It is important to determine the time-scales for vertical compositional gradients to be established
in order to assess whether compositional profiles that are not in thermodynamic equilibrium
are indicative of barriers to flow within the reservoir or simply that the reservoir fluids
have not yet had time to establish a steady-state distribution.
A macroscopic, numerical model of the thermodynamic behaviour of the reservoir fluids has
been used for this investigation. The model has been validated against simple transient analytic
solutions for molecular diffusion, as well as steady-state solutions for molecular/gravity diffusion
in binary mixtures. It was found that a uniform mixture of methane and undecane will
segregate over a vertical distance of 300m in a few hundred thousand years and that this timescale
can be affected by non-ideal mixing and the relative proportions of the two components.
Equilibrium time was found to be a function of the competing influences of molecular diffusion
and gravitational segregation. We also use our model to investigate laboratory observations
of compositional grading in ternary mixtures of methane, n-pentane and 1-methylnapthalene
reported by Ratulowski et al. (2003) and why their numerical simulation produced an unexpectedly
smaller separation when methylnaphthalene was replaced by n-undecane, although
methylnaphthalene and undecane have almost the same molar weight. Our study also provides insight into the relative importance of density, molecular diffusion, initial composition and realmixture
modelling (as opposed to ideal mixture assumptions) on compositional grading in a
fluid more representative of a real crude oil.
Date Issued
2009-12
Date Awarded
2010-08
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Vesovic, Velisa
Muggeridge, Ann
Sponsor
BP
Creator
Besong, Donald Ojong
Publisher Department
Earth Science and Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)