Mixing During Two-Phase, Steady-State Laboratory Displacements in Sandstone Cores
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This paper presents simulation results for water shielding in strongly water-wet Berea rock. The analysis of experimental data indicates three oil phase fractions; flowing, trapped and dendritic (refers to isolated oil fraction which is in contact with flowing oil phase), which are different for different Berea core samples. Model calculations demonstrate that the noted differences are significant and that data measured from one sample cannot be used to predict displacement performance in other samples. Berea rock exhibits relatively small amounts of dentritic oil; therefore, formulating a dendritic oil fraction in model calculations is not as critical as correctly estimating the trapped fraction. The trapped oil saturation can be estimated from the oil primary drainage and imbibition relative permeability data. Model validation using only an oil trapping function is supported by good agreement between model and experimental oil recoveries over a wide range of water/solvent injection ratios for tertiary CO2 displacements of a West Texas oil from water-wet Berea rock. Also presented are the results of a parametric study to identify key variables and their parametric study to identify key variables and their effect on water shielding phenomena. Introduction Miscible gas flooding applications usually employ an injected gas which has a lower viscosity than reservoir oil. This leads to unfavorable mobility ratios and lower sweep efficiency which can be treated by the water-alternate-gas (WAG) process. WAG may lead to high mobile water saturation which can result in oil entrapment in water-wet porous media. Several authors have discussed this oil trapping phenomenon as it occurs in first-contact miscible (CM) as well as multiple-contact miscible (MCM) displacements in water-wet rocks. Investigators have proposed a trapping mechanism whereby water shields oil from being displaced by solvent, and verified experimentally that the trapping phenomenon occurs for the nonwetting phase. Thus, oil-wet porous media exhibit almost no oil trapping. Early attempts to model water shielding include simple equations or correlations, e.g., Raimondi and Torcaso and Shelton and Schneider estimated the trapped oil saturation from the relative permeability data under steady-state conditions. Chase and Todd incorporated a modified version of the Raimondi and Torcaso correlation into their finite difference miscible flood simulator to predict flood performance under unsteady-state conditions. Salter performance under unsteady-state conditions. Salter and Mohanty extended the efforts by Stalkup and introduced a sophisticated description of water shielding whereby each phase was characterized in terms of three fractions: flowing, dendritic and trapped. The flowing fraction was allowed to flow due to a convective flux, while both dendritic and trapped fractions were treated as immobile. The dendritic fraction was, however, allowed to exchange mass with the flowing fraction according to a first-order rate law. Salter and Mohanty used a modified Coats and Smith dispersion-capacitance model to history-match effluent tracer histories which were run under steady-state conditions. Later, Dai and Orr developed a one-dimensional, four-component (water and three hydrocarbon species), finite-difference model based on Salter and Mohanty's modeling work. Dai and Orr used their model to predict the performance of unsteady-state displacements. Since their model was closely patterned after the work of Sa lter and Mohanty, it reproduced Salter and Mohanty's e xperimental data well. P. 243
This paper presents simulation results for water shielding in strongly water-wet Berea rock. The analysis of experimental data indicates three oil phase fractions; flowing, trapped and dendritic (refers to isolated oil fraction which is in contact with flowing oil phase), which are different for different Berea core samples. Model calculations demonstrate that the noted differences are significant and that data measured from one sample cannot be used to predict displacement performance in other samples. Berea rock exhibits relatively small amounts of dentritic oil; therefore, formulating a dendritic oil fraction in model calculations is not as critical as correctly estimating the trapped fraction. The trapped oil saturation can be estimated from the oil primary drainage and imbibition relative permeability data. Model validation using only an oil trapping function is supported by good agreement between model and experimental oil recoveries over a wide range of water/solvent injection ratios for tertiary CO2 displacements of a West Texas oil from water-wet Berea rock. Also presented are the results of a parametric study to identify key variables and their parametric study to identify key variables and their effect on water shielding phenomena. Introduction Miscible gas flooding applications usually employ an injected gas which has a lower viscosity than reservoir oil. This leads to unfavorable mobility ratios and lower sweep efficiency which can be treated by the water-alternate-gas (WAG) process. WAG may lead to high mobile water saturation which can result in oil entrapment in water-wet porous media. Several authors have discussed this oil trapping phenomenon as it occurs in first-contact miscible (CM) as well as multiple-contact miscible (MCM) displacements in water-wet rocks. Investigators have proposed a trapping mechanism whereby water shields oil from being displaced by solvent, and verified experimentally that the trapping phenomenon occurs for the nonwetting phase. Thus, oil-wet porous media exhibit almost no oil trapping. Early attempts to model water shielding include simple equations or correlations, e.g., Raimondi and Torcaso and Shelton and Schneider estimated the trapped oil saturation from the relative permeability data under steady-state conditions. Chase and Todd incorporated a modified version of the Raimondi and Torcaso correlation into their finite difference miscible flood simulator to predict flood performance under unsteady-state conditions. Salter performance under unsteady-state conditions. Salter and Mohanty extended the efforts by Stalkup and introduced a sophisticated description of water shielding whereby each phase was characterized in terms of three fractions: flowing, dendritic and trapped. The flowing fraction was allowed to flow due to a convective flux, while both dendritic and trapped fractions were treated as immobile. The dendritic fraction was, however, allowed to exchange mass with the flowing fraction according to a first-order rate law. Salter and Mohanty used a modified Coats and Smith dispersion-capacitance model to history-match effluent tracer histories which were run under steady-state conditions. Later, Dai and Orr developed a one-dimensional, four-component (water and three hydrocarbon species), finite-difference model based on Salter and Mohanty's modeling work. Dai and Orr used their model to predict the performance of unsteady-state displacements. Since their model was closely patterned after the work of Sa lter and Mohanty, it reproduced Salter and Mohanty's e xperimental data well. P. 243
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