Interfacial Effects in Gas-Condensate Recovery and Gas-Injection Processes
- Y. Melean (IFP) | N. Bureau (IFP) | D. Broseta (IFP)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2003
- Document Type
- Journal Paper
- 244 - 254
- 2003. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.2.2 Fluid Modeling, Equations of State, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements, 4.3.3 Aspaltenes, 4.6 Natural Gas, 5.8.8 Gas-condensate reservoirs, 5.1 Reservoir Characterisation, 4.1.5 Processing Equipment, 4.3.4 Scale, 5.4.2 Gas Injection Methods
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This paper evaluates the behavior of the dimensionless numbers that gauge the gas/oil interfacial tension (IFT) with respect to the other forces (viscous and buoyancy forces) involved in two-phase flow through porous media. These numbers, referred to respectively as the capillary and Bond numbers, diverge on approach to gas/oil complete miscibility, meaning that viscous and buoyancy forces become dominant over capillary forces. The divergence behaves as a power of the distance to complete miscibility as quantified, for instance, by the difference in densities between the oil and gas phases. The exponents of these power laws are "universal," whereas the prefactors vary between gas/oil systems. This allows the classification of the most common injection gases with respect to their efficiency in reducing the gas/oil IFT. This efficiency increases with the miscibility of the injected gas in the oil: supercritical CO2 is more effective in reducing IFT than off-critical CO2 or CH4, which themselves are more effective than N2.
A simple procedure is then introduced to determine the wetting (or spreading) behavior of oil on a porous substrate covered with water, as often occurs in practice. The only inputs required are the composition and densities of the three coexisting phases: water, oil, and gas. When they are not measured, these quantities can be calculated by means of an appropriate equation of state. CO2 turns out to be the most effective for promoting the spreading of the oil on water, followed by CH4 and then by N2.
Interfacial forces play an important role in many oil-recovery schemes. The gas/oil IFT and the wetting behavior of oil on the porous substrate control the distribution of fluids within the pore space. Therefore, these quantities affect the two-phase flow parameters, such as the capillary pressure, the phase permeabilities, and the quantity of oil remaining after drainage with gas.
These interfacial properties are strongly dependent on thermodynamic conditions such as pressure, temperature, and phase composition. The IFT between the gas and oil (or condensate) phases may vary by several orders of magnitude in the primary production of near-critical gas condensates or volatile oils and in nearmiscible gas-injection processes. Upon such variations, the flow regime changes from an emulsion-like flow at very low IFT to a capillary-dominated flow at high IFT. These changes are reflected in the parameters characterizing two-phase flow through porous media. Residual saturations are higher, and permeabilities are lower for capillary-dominated flows (high IFTs), while for very low IFTs, residual saturations tend to zero and the oil and gas relative permeabilities tend to the corresponding phase saturations. 1,2 Furthermore, the wetting behavior of oil changes with thermodynamic conditions. A transition from partial wetting (i.e., the oil forms lenses on the porous substrate) to complete wetting (i.e., a thick oil layer covers the susbstrate) is expected when the conditions for complete miscibility of the gas and oil phases are approached.3 This transition is referred to as the wetting transition. Significant changes in flow processes are expected at this transition.4,5
The purpose of this paper is to analyze how the gas/oil IFT and the oil-wetting behavior depend on thermodynamic conditions. Rather than analyzing the gas/oil IFT alone, it is preferable to examine the dimensionless groups that gauge the gas/oil IFT with respect to the other forces involved in the flow process (the buoyancy forces and the viscous forces). Flow parameters (phase permeabilities and residual saturations) are in fact correlated with these groups rather than with the IFT alone. These groups are referred to as the Bond number (buoyancy vs. IFT) and the capillary number (viscous forces vs. IFT). A generic or "universal" scaling behavior should characterize the dependence of these groups with the degree of miscibility between the gas and oil phases.2,4,6 We will examine this behavior in detail, our main purpose being to evaluate and compare the efficiencies of various injection gases (such as N2 or the greenhouse gases CH4 and CO2) in reducing capillary forces relative to buoyancy or viscous forces.
We will then address the wetting behavior of the oil phase and present a simple procedure that allows an accurate estimate of the wetting transition when the substrate is covered with an aqueous (brine) phase, as is often the case. This procedure, which requires the precise knowledge of the compositions and densities of the water, oil, and gas phases, has recently been validated against a series of careful experiments with systems of pure alkanes and their mixtures on water.7 Here, we use this procedure to assess the effect of the common injection gases (N2, CH4, or CO2) on the wetting (or spreading) of oil on a water-covered substrate.
The outline is as follows: the next section is a literature review, in which we introduce the dimensionless groups involving the gas/oil IFT. We will then discuss their effects, as well as those of the oil-wetting behavior, on oil- and gas-recovery processes. Nearmiscible gas injection in fractured reservoirs is then analyzed in a separate section; this analysis, which extends some previous work by Schechter et al.,6 shows how both the oil/gas IFT and the oil-wetting behavior affect the ultimate oil recovery. Finally, the last sections of this paper analyze and discuss successively the dimensionless groups involving the gas/oil IFT and the wetting (or spreading) behavior of oil on water.
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