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Title: Time-scales for the development of thermodynamic equilibrium in hydrocarbon reservoirs
Author: Besong, Donald Ojong
ISNI:       0000 0004 2688 633X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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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.
Supervisor: Vesovic, Velisa ; Muggeridge, Ann Sponsor: BP
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral