Steam Condensation Front Research Articles

Negative-saturation approach for (non)-isothermal compositional three-phase flow simulations in porous media

This article describes the extension of the two-phase (non)-isothermal negative-saturation solution approach to the three-phase (non)-isothermal negative-saturation (NegSat3) solution approach. The NegSat3 solution approach solves efficiently any (non)-isothermal compositional flow problem in porous media that involves phase transitions between different phase states when the maximum number of phases is less than four. The solution approach circumvents using different equations and primary variables for single-phase, two-phase, and three-phase regions in porous media. Consequently, the NegSat3 solution approach avoids switches or variable substitutions at phase transitions where the convergence of the Newton-Raphson procedure is hampered by oscillations between m-phase and n-phase states. The NegSat3 solution approach can be implemented efficiently in numerical simulators to deal with modeling issues for thermal recovery processes, CO2 sequestration, and for multicontact miscible gas injection in oil reservoirs if the number of phases is less than four. We illustrate the NegSat3 solution approach by way of example to steam injection in a 1D heavy-oil reservoir. However, the NegSat3 solution approach can be used for any n ≤ 3-dimensional, multicomponent three-phase problems. The example solution is compared with a standard numerical solution that is analytically verified by the method of characteristics and shows excellent agreement. The results show that the oil recovery depends critically on whether the boiling temperature of the volatile oil is around the water boiling temperature, or much below or above it. These boiling-temperature ranges give rise to three different types of wave structures. When the boiling temperature of the volatile oil is near the boiling temperature of water, the striking result is that the speed of the evaporation front is equal or somewhat larger than the speed of the steam-condensation front. Thus, the volatile oil condenses at the location where the steam condenses too, yielding virtually complete oil recovery. Conversely, if the boiling temperature is too high or too low, there is incomplete recovery.

Effect of Methane Co-Injection in SAGD--Analytical and Simulation Study

Summary Steam-assisted gravity drainage (SAGD) is a commercially viable recovery method for oil sands of Athabasca used where other methods have been unsuccessful. In one variation of SAGD, a small amount of a noncondensable gas is added to the injected steam to maintain pressure in the chamber while using the energy in place, reducing steam consumption and providing thermal insulation from overburden heat losses. The role of gas during steam-gas co-injection processes, in terms of its effects on chamber development, bitumen flow rates, and heat losses, is not fully understood, and therefore is the main focus of this work. A new analytical model for gas injection in SAGD is derived, taking into account the three-phase flow of gas, oil, and water in the reservoir. The analytical theory is used to predict the fluid flow rates as well as phase mobility, relative permeability, and saturation profiles in the mobile oil region. The theoretical results are replicated by fine-grid numerical simulations. Methane was used as the noncondensable gas for the purpose of this study because it is the main solution gas in most reservoirs. It is, however, believed that the findings of this study are equally applicable to other noncondensable gases such as nitrogen, air, helium, and others. Fine-grid numerical simulations were performed to gain a visual understanding of gas distribution in a SAGD chamber and its effect on in-situ steam quality, overburden heat losses, phase saturations, and fluid-flow rates. The simulation results support the predictions of the mathematical theory. The results of the analytical and numerical study reveal that methane co-injection with steam is in general unfavorable in a SAGD operation. The injected methane tends to accumulate at the steam condensation front, which lowers the heat transfer rate of steam to the adjacent oil, resulting in lower oil production rates and slower growth of the chamber.

Maximal oil recovery by simultaneous condensation of alkane and steam

This paper deals with the application of steam to enhance the recovery from petroleum reservoirs. We formulate a mathematical and numerical model that simulates coinjection of volatile oil with steam into a porous rock in a one-dimensional setting. We utilize the mathematical theory of conservation laws to validate the numerical simulations. This combined numerical and analytical approach reveals the detailed mechanism for thermal displacement of oil mixtures discovered in laboratory experiments. We study the structure of the solution, determined by the speeds and amplitudes of the several nonlinear waves involved. Thus we show that the oil recovery depends critically on whether the boiling-point of the volatile oil is around the water boiling temperature, or much below or above it. These boiling-point ranges correspond to three types of wave structures. When the boiling point of the volatile oil is near the boiling point of water, the striking result is that the speed of the evaporation front is equal or somewhat larger than the speed of the steam condensation front. Thus the volatile oil condenses at the location where the steam condenses too, yielding virtually complete oil recovery. Conversely, if the boiling point is too high or too low, there is incomplete recovery. The condensed volatile oil stays at the steam condensation location because the steam condensation front is a physical shock.

Traveling waves in a finite condensation rate model for steam injection

Steam drive recovery of oil is an economical way of producing oil even in times of low oil prices and is used worldwide. This paper focuses on the one-dimensional setting, where steam is injected into a core initially containing oil and connate water while oil and water are produced at the other end. A three-phase (oil, water, steam) hot zone develops, which is abruptly separated from the two-phase (oil + water) cold zone by the steam condensation front. The oil, water and energy balance equations (Rankine–Hugoniot conditions) cannot uniquely solve the system of equations at the steam condensation front. In a previous study, we showed that two additional constraints follow from an analysis of the traveling wave equation representing the shock; however, within the shock, we assumed local thermodynamic equilibrium. Here we extend the previous study and include finite condensation rates; using that appropriate scaling requires that the Peclet number and the Damkohler number are of the same order of magnitude. We give a numerical proof, using a color-coding technique, that, given the capillary diffusion behavior and the rate equation, a unique solution can be obtained. It is proven analytically that the solution for large condensation rates tends to the solution obtained assuming local thermodynamic equilibrium. Computations with realistic values to describe the viscous and capillary effects show that the condensation rate can have a significant effect on the global saturation profile, e.g. the oil saturation just upstream of the steam condensation front.

Co-injection of air and steam for the prevention of the downward migration of DNAPLs during steam enhanced extraction: An experimental evaluation of optimum injection ratio predictions

When steam is injected into soil containing a dense volatile non-aqueous phase liquid contaminant, the DNAPL vaporized within the heated soil region condenses and accumulates ahead of the steam condensation front. If enough DNAPL accumulates, gravitational forces can overcome trapping forces allowing the liquid contaminant to flow downward. By injecting air with steam, a portion of the DNAPL vapor remains suspended in equilibrium with the air, decreasing liquid contaminant accumulation ahead of the steam condensation front, and thus reducing the possibility of downward migration. In a previous work, a theoretical model was developed to predict the optimum injection ratio of air to steam that would eliminate accumulation of DNAPL ahead of the temperature front and thus minimize the potential for downward migration. In this work, the theoretical model is summarized, and an experiment is presented in order to evaluate the optimum injection ratio prediction. In the experiment, a two-dimensional water saturated sand pack is contaminated with a known mass of TCE (DNAPL). The system is then remediated by co-injecting air and steam at the predicted optimum injection ratio, calculated based on the average contaminant soil concentration in the sand pack. Results for the co-injection of air and steam are compared to results for the injection of pure steam or pure air. Injection at the predicted optimum injection ratio for a volumetric average NAPL saturation, reduced accumulation of the contaminant ahead of the condensation front by over 90%, as compared to steam injection alone. This indicates that the optimum injection ratio prediction is a valuable tool for limiting the spreading of DNAPL during steam-enhanced extraction. Injection at the optimum injection ratio resulted in earlier recovery of contaminant than for steam injection alone. Co-injection of steam and air is also shown to result in much higher recovery rates than air injection alone.

A theoretical model of air and steam co-injection to prevent the downward migration of DNAPLs during steam-enhanced extraction

When steam is injected into soil containing a dense volatile non-aqueous phase liquid contaminant the DNAPL vaporized within the heated soil region condenses and accumulates ahead of the steam condensation front. If enough DNAPL accumulates, gravitational forces can overcome trapping forces allowing the liquid contaminant to flow downward. By injecting air with steam, a portion of the DNAPL vapor remains suspended in equilibrium with the air, decreasing liquid contaminant accumulation ahead of the steam condensation front, and thus reducing the possibility of downward migration. In this work, a one-dimensional theoretical model is developed to predict the injection ratio of air to steam that will prevent the accumulation of volatile DNAPLs. The contaminated region is modeled as a one-dimensional homogeneous porous medium with an initially uniform distribution of a single component contaminant. Mass and energy balances are combined to determine the injection ratio of air to steam that eliminates accumulation of the contaminant ahead of the steam condensation front, and hence reduces the possibility of downward migration. The minimum injection ratio that eliminates accumulation is defined as the optimum injection ratio. Example calculations are presented for three DNAPLs, carbon tetrachloride (CCl 4), trichloroethylene (TCE), and perchloroethylene (PCE). The optimum injection ratio of air to steam is shown to depend on the initial saturation and the volatility of the liquid contaminant. Numerical simulation results are presented to validate the model, and to illustrate downward migration for ratios less than optimum. Optimum injection ratios determined from numerical simulations are shown to be in good agreement with the theoretical model.

A Fast 3D Interface Simulator for Steamdrives

Summary Here we describe a fast 3D steam drive simulator. We use an interface model, where the single-phase steam zone is separated from the two-phase liquid zone by the steam condensation front (SCF) which constitutes the interface. Steady-state heat balances applied at the interface reduce the steam problem to a problem of gas/oil/water flow. Heat losses are treated by a prescribed conversion of steam to water. The model incorporates gravity, viscous and capillary forces and handles arbitrary permeability distributions and well configurations. We use a multigrid method to solve the pressure equation. The steam zone development is determined by a probabilistic method, which ensures that instability phenomena are properly treated. The oil/water flow problem in the liquid domain is solved as in conventional reservoir simulators. We validate the model with analytical models. Example calculations for a thin medium-viscosity oil field show that a transition zone with a reduced oil viscosity just downstream of the SCF has a pronounced stabilizing effect. This, and the global heat loss effects are the reason for the high displacement efficiency of steam drives.

Uniqueness conditions in a hyperbolic model for oil recovery by steamdrive

In this paper we study a one-dimensional model for oil recovery by steamdrive. This model consists of two parts: a (global) interface model and a (local) steam condensation/capillary diffusion model. In the interface model a steam condensation front (SCF) is present as an internal boundary between the hot steam zone (containing water, oil and steam) and the cold liquid zone (containing only water and oil). Disregarding capillary pressure away from the SCF, a 2× 2 hyperbolic system arises for the water and steam saturation. This system cannot be solved uniquely without additional conditions at the SCF. To find such conditions we blow up the SCF and consider a parabolic transition model, including capillary diffusion. We study in detail the existence conditions for traveling wave solutions. These conditions provide the missing matching conditions at the SCF in the hyperbolic limit. We show that different transition models yield different matching conditions, and thus different solutions of the interface model. We also give a relatively straightforward approximation and investigate its validity for certain ranges of model parameters.

An investigation of the vertical sweep efficiency of steam drive in a layered reservoir

Abstract This paper presents results obtained in a theoretical and experimental study on the efficient use of steam injection for the recovery of oil from a layered, dipping reservoir. For the experimental part, we use a scaled transparent vacuum model. An infrared camera follows the evolution of the steam condensation front. For the theoretical part, we use an interface model to describe the behaviour of the steam condensation front. Two approximations for the flow field are examined, leading to a Dietz-Dupuit model and a cross-flow model respectively. The theoretical results are compared to the experimental values to investigate the validity of the models. We conclude that vertical sweep efficiency improves with dip. Furthermore the Dietz-Dupuit model leads to a reasonable agreement in the case with a high permeable layer at the top. With the high permeable layer at the bottom, however, discrepancies occur. The simple cross-flow model shows agreement with all of the gravity stable experiments, but lacks a firm physical basis.

The effects of steam injection on the electrical conductivity of an unconsolidated sand saturated with a salt solution

The spatial and temporal variation of electrical conductivity in saturated sands during steam injection has been measured and modelled. Experiments consisted of introducing steam into one end of a tube filled with sand saturated with a slightly saline solution. A steam condensation front formed, separating the mixed‐phase steam zone from the liquid zone. Measurements of electrical conductivity were made at 10 locations along the tube using a four‐electrode technique. Results show that conductivity starts at a constant value, decreases before the steam front arrives and then, immediately prior to the steam front arrival, goes through a maximum before dropping by a factor of about 25. These variations can be explained by first, a dilution of the interstitial solution causing the initial drop in conductivity; second, an increase in temperature of the solution immediately prior to the arrival of the steam front causing the conductivity maximum; and finally, the large drop in conductivity due to the combined effects of a decrease in saturation and dilution of the residual liquid in the two‐phase zone. Mathematical solutions of a set of differential equations that take into consideration all of these effects are presented. These solutions reproduce the significant features of the conductivity data. This study suggests that the measurement of changes in the subsurface conductivity field during steam injection operations may indicate the location of ionic concentration, temperature, and steam saturation fields.

Stability of Piston-Like Displacements of Water by Steam and Nitrogen in Porous Media

Abstract Downhole steam generation leads to consideration of reservoir fluid displacement by a mixture of steam and nitrogen. The linear stability analysis of the steam condensation front has been generalized to include a non-condensing gas. Roughly speaking, the addition of nitrogen increases the likelihood of having fingers, but, compared with the no-nitrogen case, the fingers will grow more slowly.