Effect of Phase Behavior on Bypassing in Enriched Gasfloods

Abstract

Summary Enriched gasfloods incorporate a complex interaction of heterogeneity, fingering, multiphase flow, and phase behavior. Experiments and simulations indicate that the optimum solvent enrichment in high-viscosity-ratio secondary gasfloods can be below minimum miscibility enrichment (MME). The compositional path and resulting mobility profile in multidimensional multiple-contact miscible (MCM) or immiscible floods are different from their 1D counterparts for high-viscosity-ratio floods in heterogeneous media. Introduction The objective of this work is to study the effect of phase behavior on bypassing in laboratory gasfloods by combined use of compositional modeling and laboratory computed tomography (CT) scanning. Oil was displaced from a heterogeneous core by several solvents at constant, high viscosity ratio (1,600). Displacement was vertical to avoid gravity override. The bypassing of the oil during the flood was monitored by a vertical CT scanner. A 2D compositional model was used to simulate these displacements and a model three-component system at viscosity ratios of 22 to 200. The experimental data indicate that bypassing decreases as immiscibility increases. The solvent finger moved fastest in the single-phase displacement and slowest in the three-phase displacement. Compositional simulation of these floods was unstable at a 1,600 viscosity ratio. Model system simulation indicates that as viscosity ratio increases, sweep efficiency in first-contact-miscible (FCM) solvents deteriorates sharply. Sweep in near- and below-MME solvents does not decrease as sharply because of multi phase flow. Optimum solvent enrichment in high-viscosity-ratio, secondary gasfloods can be below MME. The compositional path and resulting mobility profile in multidimensional MCM or immiscible floods are different from their 1D counterparts for high-viscosity-ratio floods in heterogeneous media. Blunt et al.'s1 theory of compositional fingering does not work for the heterogeneous medium studied. Background The economics of hydrocarbon solvent flood projects depends on factors that include the enrichment level of the solvent as well as the slug size and WAG ratio. Similarly, the economics of CO2 flood projects depends heavily on the injection pressure. Common industry practice is to use a hydrocarbon solvent at or above its MME2 or to use CO2 at or above its minimum miscibility pressure (MMP).3 In 1D displacements, MME and MMP are the optimum levels of enrichment or pressure, respectively, for the injection solvent. However, reservoir flow is 3D. Rock heterogeneity, viscous fingering, gravity override, diffusion, dispersion, and presence of mobile water may cause optimum enrichment (or pressure) in a reservoir flood to be different from that in a 1D flood, especially in a slim-tube test. Injected solvent composition (or pressure) affects not only the local displacement efficiency (i.e., that evaluated by 1D experiments or calculation) but also the sweep efficiency.4 The mobility ratio and density contrast are large in most solvent floods. Sweep efficiency can be low as a result of fluid channeling, viscous fingering, and gravity override and plays a crucial role in determining the overall recovery efficiency. Simulations of secondary and tertiary solvent floods in several heterogeneous permeability fields have shown that floods with solvent enrichment at or below that required for development of multicontact miscibility in 1D flow can perform as well or better than floods with richer solvents.4 Pande and Orr5 showed, by method-of-characteristics calculations, that the optimum pressure can be lower than MMP in a two-layer reservoir. These results, if valid, are very important to solvent flood economics because they can reduce solvent cost. Such simulations, however, are always open to questions regarding inclusion of all types of crossflow (e.g., capillary and dispersive), realistic spatial permeability variations, history-dependent relative permeability and capillary pressure hysteresis, and numerical dispersion.6 One objective of this work is to conduct solvent floods in a heterogeneous rock to determine whether optimum enrichment levels can be lower than MME at laboratory scale. This would validate the simulation results reported by Pande4 and Pande and Orr.5 Generally speaking, as enrichment (or pressure) increases, microscopic displacement efficiency increases before leveling off at MME (or MMP). However, sweep efficiency can decrease as enrichment (or pressure) increases.4 This decrease is not because of viscosity ratio or density contrast, which decrease as enrichment (or pressure) increases. However, it may be caused by the interactions of phase behavior and heterogeneous flow field.7,8 The second objective of this work is to study this interaction at a laboratory scale. Blunt et al.1 and Blunt and Christie9 have advanced the empirical theory for viscous fingering significantly. Application of this theory to compositional floods assumes that the 1D average compositional path in fingered floods is the same as in 1D floods. This was the case for their 2D fine-grid simulations in a low-heterogeneity permeability field in the absence of gravity segregation. Fingers were small compared with the widths of the systems in all their examples. The third objective of this study is to determine the effect of bypassing on composition path of our laboratory floods and to verify whether the assumption of Blunt et al.'s1 theory is applicable in these corefloods. In the next two sections, we describe our experimental program and discuss the results. Then, we describe the modeling of experimental floods, the simulation of a three-component model system, and the interactions between phase behavior and flow bypassing. The last section summarizes our findings. Experimental Procedure The experimental program consisted of several corefloods on a vertically mounted 8-in.-long by 1.5-in.-diameter core (Fig. 1). Flow direction was from top to bottom. The experimental setup included a composite core holder with a constant-temperature jacket, an injection module consisting of two pressure vessels for fluid transfer and gas injection and for overburden control, and a production module with a backpressure regulator (BPR) and a graduated centrifuge tube for recording recovery volumes. A Techni care Deltascan 2020 CT scanner oriented for vertical corefloods was used for the experiments. To achieve a complete core scan within 20 minutes, we used a 4.9-in. scan diameter, a 0.3-in. scan thickness, and 16 slices. Five corefloods were conducted: a matched-density/-viscosity miscible flood, a matched-density/adverse-viscosity miscible flood, an ethane flood of oil at 10 mL/hr, an ethane flood at 1 mL/hr, and a hydrocarbon gasflood of oil. All floods were conducted at irreducible water saturation, a 1,650-psi outlet pressure, and a 65°F system temperature. After each experiment, the core was cleaned with decalin, then resaturated with the particular "oil." The oil viscosity was ˜80 cp, and the ethane and hydrocarbon-solvent viscosities were ˜0.05 cp.

Microscopic Mechanisms of Oil Recovery By Near-Miscible Gas Injection

“…The results presented here illustrate a pore-scale mechanism, which could partly explain the previous reports (Shyeh-Yung 1991;Burger et al 1994;Thomas et al 1994;Soroush and Saidi 1999) that near-miscible gas injection could be a very effective oil recovery mechanism. The pore-scale experimental results presented here suggest that (to benefit from this mechanism) gas floods should be either near-miscible or multiple contact miscible (MCM) where two-phase flow is still possible behind the gas front.…”

“…2D simulation studies (Burger et al 1994; Thomas et al 1994) have demonstrated that injectant with less enrichment than that required for first-contact miscible with the oil often yields optimum oil recoveries owing to better sweep efficiencies in heterogeneous reservoir models. Field-scale simulations show that near-miscible solvents are attractive because of improved sweep over miscible solvents (Pande 1992).…”

“…Field-scale simulations show that near-miscible solvents are attractive because of improved sweep over miscible solvents (Pande 1992). Burger et al (1994) showed that economically optimum enrichment in high-viscosity-ratio secondary gas floods could be operated at pressures below the MMP. Thomas et al (1994) reported that gas-oil IFT can be reduced adequately under near-miscible conditions and that zero IFT is unnecessary for effective displacement processes in realistic porous media.…”

Visualisation of Residual Oil Recovery by Near-miscible Gas and SWAG Injection Using High-pressure Micromodels

“…The 2D simulation studies (Burger et al 1994; Thomas et al 1994) have demonstrated that injectant with less enrichment than that required for first-contact miscible (FCM) with the oil often yields optimum oil recoveries owing to better sweep efficiencies in heterogeneous reservoir models. Field-scale simulations show that near-miscible solvents are attractive because of improved sweep over miscible solvents (Pande 1992).…”

“…Field-scale simulations show that near-miscible solvents are attractive because of improved sweep over miscible solvents (Pande 1992). Burger et al (1994) showed that economically optimum enrichment in high-viscosity-ratio secondary gasfloods could be operated at pressures below the MMP. Thomas et al (1994) reported that gas-oil IFT can be reduced adequately under near-miscible conditions and that zero IFT is unnecessary for effective displacement processes in realistic porous media.…”