Interpretation of the Unusual Fluid Distribution in the Yufutsu Gas-Condensate Field
- Kassem Ghorayeb (Reservoir Engineering Research Inst.) | Abbas Firoozabadi (Reservoir Engineering Research Inst.) | Toshiyuki Anraku (Japan Petroleum Exploration Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- June 2003
- Document Type
- Journal Paper
- 114 - 123
- 2003. Society of Petroleum Engineers
- 5.8.8 Gas-condensate reservoirs, 5.2 Reservoir Fluid Dynamics, 5.2.2 Fluid Modeling, Equations of State, 5.9.2 Geothermal Resources, 5.2.1 Phase Behavior and PVT Measurements, 2.2.2 Perforating, 5.6.5 Tracers, 5.3.2 Multiphase Flow
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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.
Hydrocarbon formations are large porous media saturated with hydrocarbon fluid mixtures in the Earth's subsurface with a thickness of as much as 1 km or more, and horizontal dimensions on the order of kilometers or more. In these formations, denser fluid mixture is at the bottom, and lighter fluid mixture is at the top. The species distribution follows density distribution; methane, which is usually the main component and often the smallest molecule of the fluid mixture, is more concentrated at the top, while large molecules are more concentrated at the bottom. These variations are commonplace despite the geothermal temperature gradient in the Earth.
The variation of species and density in an isothermal medium can be described by invoking the Gibbs criterion of thermodynamic equilibrium1 in the gravity field for an n-component fluid mixture, which leads to the following differential equation2:
In the above equation, the chemical potential of component i, µi, is a function of composition and pressure; it varies with vertical position z (z is chosen to be positive downward in Eq. 1). Other symbols in Eq. 1 include Mi, the molecular weight of component i, and g, the gravitational acceleration. Given composition and pressure at a reference point in the z-direction, Eq. 1 can be used to calculate the variation of composition and pressure as a function of depth (z) or height (-z).
Various authors have used Eq. 1 to investigate the distribution of species in hydrocarbon formations.3,4 In such calculations, the fluid density increases with depth. The concentration of the smallest molecular weight component, that is, methane, decreases with depth,2 and that of heavy species increases with depth. Abundant data from hydrocarbon formations from different parts of the world (both onshore and offshore) are in qualitative agreement with predictions from Eq. 1.3,5-7 The general consensus among geoscientists, and especially petroleum engineers, is that there is always vapor at the top and liquid at the bottom whenever there is more than one phase. Results from Eq. 1 are also in support of phase segregation.
We have recently conducted an extensive set of pressure, composition, and temperature measurements in a large gas field in Japan in order to investigate its unusual fluid distribution. Data in several 5-km-deep vertical wells that extend from the Earth's surface to a depth of 5 km, and in the 1-km-thick formation, are measured. The data from the wells show that there exists a motionless liquid in the middle with lighter vapor at the top and the bottom of the well. This is the first report of such an observation in geoscience and petroleum engineering literature to the best of our knowledge. Careful measurements in the hydrocarbon formation show that
Methane concentration increases with depth.
The concentration of heavy species, which are grouped as heptane-plus (heptane and heavier species), decreases with depth.
The fluid density decreases with depth.
These are also the first reports of their kind in the literature to the best of our knowledge. Previously, Temeng et al.8 reported a heptane-plus decrease with depth in a hydrocarbon formation in Saudi Arabia without conclusive evidence. Our measurements both in the formation and in the wells (which are connected) confirm the existence of a dense fluid mixture floating on the top of a light fluid mixture.
In this article, we first present the measured data in detail, and then use a model based on the irreversible thermodynamics to interpret the fascinating trends in the data.
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