Experimental and Simulation Study of High-Temperature Foam Displacement in Porous Media

Abstract

Summary High-temperature surfactant foams are simulated by modifying gas-phasemobility in a conventional thermal simulator. Both surfactant-alternating-gas(SAG) and gas/liquid-coinjection processes are modeled. Foam generation byleave-behind and snap-off as well as foam coalescence and trapping mechanismsare incorporated in the model by an equation for the number density of foambubbles; gas-phase relative permeability and apparent viscosity are modifiedaccording to the bubble density. Pressure and saturation data of laboratorycorefloods are successfully history matched with simulation results. Field-scale sensitivity studies of the steam-foam-drive process demonstrate howthe coalescence rate affects the extent of steam diversion. Introduction Gases (such as steam, CO2, and nitrogen) are injected into oil reservoirs asdrive fluids in some EOR processes. Early gas breakthrough can occur atproducing wells owing to override and channeling, resulting in low oil-recovery efficiency. Injecting surfactant to create foam can reduce gasmobility and improve volumetric sweep efficiency in oil reservoirs. Foams usedin both mature and infant steamdrives have resulted in incremental oilproduction in California heavy-oil reservoirs. The behavior of foam in porousmedia is complex, and the mechanisms governing its flow are not yet fullyunderstood. Laboratory and theoretical studies have investigated foamgeneration, bubble coalescence, and the effect of oil on foam stability. A fewinvestigators have begun to develop a comprehensive model of foam flow inporous media, but only few experiments have been modeled. This paper offers amathematical model that includes the principal mechanisms that govern foamdisplacement in porous media. The effect of foam on gas-phase relativepermeability and apparent viscosity is included in the model. Both static orcontinuous-gas foams and "strong" or discontinuous-gas foams aremodeled. In the first case, static foam lamellae block pore throats for gasflow, decreasing the gas-phase relative permeability. In the second case, foambubbles are displaced through the pore network and the flow behavior iscontrolled by the rheology and the generation, trapping, and coalescence of theflowing foam bubbles. As proposed by Falls et al. and Patzek, an equation isincorporated to calculate the flowing-foam-bubble density, which, in turn, dictates how the flowing-foam mobility is modified. Chaser SD1000 (TM), asurfactant developed for steam-foam applications, was the sole chemical used inthis study. Model parameters are obtained from corefloods and by historymatching nitrogen foam floods in Berea sandstone cores. Two laboratorycorefloods are compared to simulations with the chosen parameters. Field-scalesimulations of the steam-foam-drive process are then presented for a range of bubble coalescence rates. Mechanism of Phase-Mobility Reduction With Foam In porous media, foams have a continuous liquid phase, including thelamellae, and a gas phase, which is either continuous or dispersed. Leave-behind lamellae occur when a continuous gas phase invades asurfactant-laden region. These lamellae are stationary and block part of thepore network to gas flow, reducing the gas-phase relative permeability. Leave-behind lamellae dominate gas-mobility reduction at low velocities. Snap-off and lamella division create discrete gas bubbles that either lodge atsome point in the porous medium, blocking gas pathways as with the leave-behindmechanism, or flow. Gas flowing as discrete bubbles causes a much greaterresistance to flow than gas flowing as continuous gas. Discrete bubblesdominate the reduction of gas mobility at high injection rates. The principal mechanisms of gas-mobility reduction caused by foam inreservoir applications depend partly on the injection mode. Surfactant solutionand gas may be alternately injected (SAG) or coinjected at a set fractionalflow. To model all possible flow conditions, criteria (discussed later) areneeded that indicate the primary mechanism (or combination of mechanisms)responsible for controlling the gas-phase mobility. Mathematical Formulation A conventional thermal simulator essentially identical to the formulationproposed by Coats was expanded to include the steam-foam process. Foam onlymodifies the gas-phase mobility (relative permeability and viscosity). Aconservation equation for surfactant and a population-balance equation were thenew additions. The following assumptions were made.Darcy's law applies;foam affects only the gas-phase relative permeability and viscosity.Foamcannot exist if the oil saturation is larger than a limiting oil saturation, or if the surfactant concentration is less than a limiting concentration, . These criteria are determined from experimental data.Surfactant adsorptionon the rock can be modeled with a Langmuir-type model and is unaffected by foamlamellae.The process of foam-bubble trapping is instantaneous, and theaverage number density of stationary bubbles is in equilibrium with the averagenumber density of flowing bubbles. Here we present only the mass-balanceequation for surfactant and the population-balance equation.