Interpretation of the Unusual Fluid Distribution in the Yufutsu Gas-Condensate Field

Abstract

It is generally believed that at steady state, a heavy fluid mixture cannot float, without motion, at the top of a light fluid mixture in a cavity. The expectation is that because of pressure diffusion, segregation occurs with the light fluid at the top and the heavy fluid at the bottom. We present, for the first time, an extensive set of measurements in 5-km vertical wells in a large hydrocarbon formation of 1-km thickness with horizontal dimensions on the order of several kilometers that show a high-density fluid mixture at the top of a light-density fluid mixture at steady state. The data in the 5-km wells show liquid in the middle, and vapor at the top and bottom. In the hydrocarbon formation, there is a gradual decrease of density with depth.A theoretical model based on the thermodynamics of irreversible processes is used to provide an interpretation of the unusual density variation vs. depth both in the hydrocarbon formation and in the long wells, as well as the unusual species distribution in the hydrocarbon formation. The results reveal that thermal diffusion (caused by geothermal temperature gradient) causes the segregation of heavy components in the subsurface fluid mixture to the cold side in the Earth (that is, the top), overriding pressure and molecular diffusion (Fickian diffusion). As a consequence of the competition of these three diffusion effects, a heavy fluid mixture can float at the top with a light fluid mixture underneath. In the past, thermal diffusion has been thought of as a second-order effect. For the fluid mixture in our work, thermal diffusion is the main phenomenon affecting the spatial density and species distribution.

Impact of Thermodiffusion on the Initial Vertical Distribution of Species in Hydrocarbon Reservoirs

“…However, other phenomena may affect the compositional grading in real reservoirs (Holt et al 1983;Whitson and Belery 1994;Montel et al 2007). Among them is thermodiffusion, or Soret effect, induced by the geothermal gradient (Ghorayeb et al 2003;Montel et al 2007), which is a phenomenon that couples heat and mass fluxes (de Groot and Mazur 1984). The contribution of thermodiffusion is difficult to quantify, mainly due to a lack of experimental data as well as accurate modelling for multi-component mixtures.…”

Thermodiffusion of the tetrahydronaphthalene and dodecane mixture under high pressure and in porous medium

“…Since conditions under which the crude oil is found underground imply high pressure (HP) and diffusion in porous medium, it is important to analyze the influence of the pressure and the interaction with a porous medium on the transport properties of dense mixtures. This is done in order to improve the description of the initial distribution of components within a reservoir [6,7]. Nevertheless, thermal diffusion data under reservoir conditions, i.e.…”

Study of the thermal diffusion behavior of simple binary mixtures

“…where D th is thermal diffusivity and the source term S(x,t) is given by 58) where α is an optical absorption coefficient, c p the specific heat at constant pressure, ρ is the density and I(x,t) is the intensity of the writing beam. Eq.…”

“…is of practical relevance, because the determination of reliable model parameters is essential for the characterization of geological fields [30,57,58]. Additionally the effect plays also an important role in separation techniques for liquid mixtures (see e.g.…”

“…The effect has important technical applications for example in the modeling of the separation of crude oil components under the influence of thermal diffusion in geological conditions [30,57,58] ; it also plays an important role in separation techniques for liquid mixtures (see e.g. Refs.…”