Energy Fuels 2010, 24, 1813–1821 Published on Web 02/01/2010
: DOI:10.1021/ef901277v
First-Contact-Miscible and Multicontact-Miscible Gas Injection within a Channeling Heterogeneity System Yahya M. Al-Wahaibi* Petroleum & Chemical Engineering Department, Sultan Qaboos University, Muscat 123, Oman Received November 1, 2009. Revised Manuscript Received January 14, 2010
Experimentally measured recoveries, gas cuts, and residual saturations, as well as visual observations, were used to quantify the effects of a channeling heterogeneity system on the efficiency and gas/oil nonequilibrium of first-contact-miscible (FCM) and multicontact-miscible (MCM) displacements. These experiments are the first of their type, because they have enabled a direct insight into the mechanisms of gas/oil flow occurring within such type of heterogeneities, and particularly have provided a firmer understanding of the MCM processes. The key finding in this work is the fact that the produced fluids in all MCM experiments were not in compositional equilibrium. The effect of channeling heterogeneity was to reduce mass transfer between the oil and MCM gas phases throughout the porous medium as a whole, thereby driving the system to be more submiscible and, as such, reducing the sweep and increasing the bypassing. These results were also reflected in the increase in nonequilibrium between gas and oil phases. This work has proved that the channeling heterogeneities, even with small permeability contrast, can distort FCM and MCM displacement patterns considerably. In addition, the results suggested that the performance of MCM processes decrease significantly as the injection rate increases. This was probably due to an interplay between capillary and viscous forces in the heterogeneous model, causing the gas at the highest rate to flow faster into the high permeability stripe, therefore resulting in a larger transition zone, shorter miscibility region, greater nonequilibrium, and, hence, a lesser-efficient flood. This study has important implications for the correct interpretation of core data, and for scale-up processes to reservoir scale, particularly for handling gas/oil nonequilibrium when modeling MCM displacements.
(a vaporizing gas drive) or from the gas to the oil (a condensing gas drive). Unfortunately, many phenomena conspire to limit the efficiency of the miscible flooding process. A strong limitation on recovery arises from permeability heterogeneity, which can cause flow channeling and poor sweep efficiency. Heterogeneity is present at many scales in clastic reservoirs. Small-scale heterogeneities are particularly problematic for all secondary and tertiary recovery processes, because they can have a significant effect on recovery yet cannot be modeled explicitly in field-scale simulations.6-8 One of the most common small-scale heterogeneities is channeling (channeling could also refer to similar permeability variation at larger scales). No one has yet investigated the influence of channeling heterogeneity on multicontact-miscible (MCM) displacement processes and how they might alter the development of miscibility. This study seeks to remedy this deficiency and, in addition, investigate the mechanisms of FCM gas/oil flow occurring within such types of heterogeneities.
1. Introduction The ultimate recovery governed by immiscible gas injection is limited primarily by three factors: (1) areal sweep efficiency, (2) volumetric sweep efficiency, and (3) microscopic sweep efficiency. Because of viscous fingering, gravity segregation, permeability stratification, interfacial tension, wettability, and pore structure, ultimate oil recovery is always much less than 100%. Certainly, the lure of a more-efficient recovery explains the interest in the miscible injection method.1-5 However, it is often not economical, and sometimes not technically feasible, to inject a gas that is first-contact-miscible (FCM) with the oil. Instead, the injected gas is designed to develop miscibility by the net transfer of components from the oil into the gas *Tel.: (þ968) 24-14-2546. Fax: (þ968) 24-14-1354. E-mail: ymn@ squ.edu.om. (1) Leach, M. P.; Yellig, W. F. Compositional model studies;CO2 oil-displacement mechanisms, SPE Paper 8368; Presented at the SPE 54th Annual Technology Conference and Exhibition, Las Vegas, NV, Sept. 23-26, 1981. (2) Stalkup, F. I., Jr. Miscible Displacement; SPE Monograph Series 8; Society of Petroleum Engineers: New York, 1983. (3) Giordano, R. M.; Salter, S. J.; Mohanty, K. K. The effects of permeability variations on flow in porous media, SPE Paper 14365; Presented at the 60th SPE Annual Technology Conference and Exhibition, Las Vegas, NV, Sept. 22-25, 1985. (4) Pande, K. K.; Sheffield, J. M.; Emanuel, A. S.; Ulrich, R. L.; Dezabala, E. F. Scale-up of near-miscible gas injection processes: Integration of laboratory measurements and compositional simulation. Pet. Geosci. 1996, 2, 343-349. (5) Wylie, P.; Mohanty, K. K.: Effect of wettability on oil recovery by near-miscible gas injection. SPE Reserv. Eval. Eng. 1999, 2 (December), 558-564. r 2010 American Chemical Society
(6) Kjonsvik, D.; Doyle, J.; Jacobsen, T. The Effects of Sedimentary Heterogeneities on Production from a Shallow Marine Reservoir-What Really Matters?, SPE Paper 28445; Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, LA, 1994. (7) Jones, A. D. W.; Verly, G. W.; Williams, J. K. What reservoir characterization is required for predicting waterflood performance in a high net-to-gross fluvial environment? In North Sea Oil and Gas Reservoirs III; Aasen, J. O., Berg, E., Buller, A. T., Hjelmeland, O., Holt, R. M., Kleppe, J., Torsæter, O., Eds.; Springer: New York, 1994; pp 223-232. (ISBN 978-0792323044.) (8) Jones, A. D. W.; Doyle, J. D.; Jacobsen, T.; Kjonsvik, D. Which Subsurface Heterogeneities Influence Waterflood Performance? A Case Study of a Low Net to Gross Fluvial Reservoir. In New Developments in Improved Oil Recovery; De Haan, H. J., Ed.; Geological Society Special Publication No. 84; Geological Society: London, 1995; pp 5-18.
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Compositional numerical simulation is usually used to predict the performance of these recovery schemes on the basis of equation-of-state properties determined from the regression on data obtained from laboratory experiments. The accuracy of these predictions is critically dependent on the validity of the assumptions used in these simulations. The most critical of these assumptions is that of complete vapor/liquid equilibrium in each grid block.2 This allows the calculation of different phase saturations from the composition of the different hydrocarbon components in that grid block. However, in many field scale simulations, the size of the grid blocks (>100 m) and the time step between solutions (days) is such that there may not be time for diffusion to equilibrate phase compositions across the entire grid block.9-11 In addition, fluid saturation distributions may be nonuniform within the volume equivalent to a simulation grid block. In this case, equilibrating compositions across the entire grid block volume may not be physically correct. Viscous fingering, gravity override, and channeling through subgrid heterogeneities are all possible reasons why there may not be nonuniform saturations.12-16
Figure 1. Schematic diagram of the beadpack used for the experiments, showing (a) inlet/outlet ports and (b) inlet/outlet reservoirs; the permeability ratio is given as K2/K1 = 2.5.
negligible. This was essential to minimize the influence of the outlet tubing on the mixing between the produced fluids. The porous medium was filled with different-sized Ballotini glass beads in selected areas, to represent a typical channeling heterogeneity structure (see Figure 1). The layer in the middle is composed of Ballotini grade 9 beads (310-425 μm) and is sandwiched between two layers composed of finer grains (grade 11 (160-200 μm)). This flow geometry is one of the most common small-scale sedimentary structures found in sandstone reservoirs, especially in fluvial deposits. The artifical laminae were constructed using very thin Perspex baffles placed inside the Perspex case at the desired spacing to separate zones of different permeability. These baffles had the same width as the internal cross section of the Perspex box, to prevent mixing of bead sizes between layers. After packing, the baffles were carefully withdrawn and further beads were packed as required. Carbon dioxide was injected at low pressure through the pack, to displace the air. Several pore volumes of the desired displaced fluid were then injected into the model to displace and absorb the carbon dioxide. During displacement of CO2, the exit end of the model was raised above the inlet level to ensure a stable uniform displacement, thereby reducing trapping. Thereafter, the pack was mounted horizontally, to eliminate gravity effects. All displacements were performed at a flow rate of 3 mL/min, unless otherwise stated. The porosity and absolute permeabilities of each of the different grades of glass beads comprising the channeling heterogeneity were measured using a homogeneous porous medium. The values are given in Table 1. The uniformity of packing in the homogeneous beadpacks was checked by performing a unit mobility ratio miscible displacement (undyed water displacing dyed water) through them. In each case, the linearity of the displacement front was clearly observed. Fluid System. For all MCM displacements, mixtures of cyclohexene (C), isopropyl alcohol (IPA), and water (W) were used. This three-component, two-phase fluid system exhibits an upper critical point under ambient conditions and forms up to two liquid phases in equilibrium (see Figure 2).17-20
2. Design of Experiments Pack Design, Construction, and Properties. All the displacement experiments were performed in linear, visual beadpacks made from a 25 cm 10 cm 0.6 cm, rectangularshaped sealed Perspex box filled with Ballotini glass beads (Figure 1). The pack’s thickness was determined by the requirement that the flow be essentially two-dimensional (2D), so that direct comparison with 2D numerical simulations could be made. The transparent Perspex material used has excellent optical properties, permitting visualization of fluid movement within the models. Fluids entered the beadpack through an inlet channel that was connected to the pack via small holes drilled along its length and separated from the beads by a fine mesh screen. This inlet channel was designed to ensure a uniform front. At the outlet, the pack was designed so that the dead volume between the porous medium and the collection point was (9) Haajizadeh, M.; Fayers, F. J.; Cockin, A. P.; Roffey, M.; Bond, D. J. On the importance of dispersion and heterogeneity in the compositional simulation of miscible gas processes, SPE Paper 57264; Presented at the SPE Asia Pacific Improved Oil Recovery Conference, Kuala Lumpur, Malaysia, Oct. 25-26, 1999. (10) Ballin, P. R.; Clifford, P. J.; Christie, M. A. Cupiagua: Modeling of a complex fractured reservoir using compositional upscaling. SPE Reserv. Eval. Eng. 2002, (December), 488-498. (11) Ajose, D.; Mohanty, K. K. Compositional upscaling in heterogeneous reservoirs, effect of gravity, capillary pressure and dispersion, SPE Paper 84363; Presented at the SPE Annual Technical Conference and Exhibition, Denver, CO, Oct. 5-8, 2003. (12) Christie, M. A.; Bond, D. J. Detailed simulation of unstable flows in porous media. SPE Reserv. Eng. 1987, 2, 514-522. (13) Christie, M. A.; Jones, A. D. W. Comparison between Laboratory Experiments and Detailed Simulation of Miscible Viscous Fingering; Presented at the 4th European Symposium on Enhanced Oil Recovery; Hamburg, Germany, 1987. (14) Christie, M. A.; Jones, A. D. W.; Muggeridge, A. H. Comparison between laboratory experiments and detailed simulations of unstable miscible displacement influenced by gravity. In North Sea Oil and Gas Reservoirs II; Buller, A. E., Berg, E., Hjelmeland, O., Kleppe, J., Eds.; Graham and Trotman: London, 1990; pp 245-300. (15) Davies, G. W.; Muggeridge, A. H.; Jones, A. D. W. Miscible Displacements in a Heterogeneous Rock: Detailed Measurements and Accurate Predictive Simulation, SPE Paper 22615; Presented at the SPE Annual Technical Conference and Exhibition, Dallas, TX, Oct. 6-9, 1991. (16) Muggeridge, A. H.; Jackson, M. D.; Al-Mahrooqi, S.; Al-Marjabi, M.; Grattoni, C. A. Quantifying Bypassed Oil in the Vicinity of Discontinuous Shales, SPE Paper 77487; Presented at the SPE Annual Technical Conference and Exhibition, San Antonio, TX, Sept. 29-Oct. 2, 2002.
(17) Al-Wahalbi, Y. M.; Grattoni, C. A.; Muggeridge, A. H. Drainage and imbibition relative permeabilities at near miscible conditions. J. Pet. Sci. Eng. 2006, 53, 239-253. (ISSN No. 0920-4105.) (18) Al-Wahaibi, Y. M.; Muggeridge, A. H.; Grattoni, C. A. Experimental and numerical studies of gas/oil multicontact miscible displacements in homogeneous and crossbedded porous media. SPE J. 2007, 12, 62-76. (ISSN No. 1086-055.) (19) Al-Wahaibi, Y. M.; Grattoni, C. A.; Muggeridge, A. H. Physical properties (density, viscosity, surface tension, interfacial tension, and contact angle) of the system isopropyl alcohol plus cyclohexene plus water. J. Chem. Eng. Data 2007, 52, 548-552. (ISSN No. 0021-9568.) (20) Al-Wahaibi, Y.; Muggeridge, A.; Grattoni, C. Gas-oil nonequilibrium in multicontact miscible displacements within homogeneous porous media. J. Pet. Sci. Eng. 2009, 68, 71-80. (ISSN No. 0920-4105.)
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Table 1. Porosities and Permeabilities of the Different Bead Sizes Used in the Construction of the Channeling Heterogeneity Beadpack porosity (%)
permeability, D
42 38
25 10
grade 9 beads grade 11 beads
Figure 3. Experimental displacement fronts for the FCM displacement at different pore volumes injected within the heterogeneous pattern. Here, blue-colored gas is displacing clear oil at a rate of 3 cm3/min. The model is placed horizontally.
light blue in the oil. The exception was close to the critical point in the MCM, where, because of partition, both phases were light blue (as the color gets diluted through dissolving in a larger volume). The effluent was collected and analyzed to follow the evolution of the recovery process. In the MCM displacements, the refractive index21 was used to determine the IPA concentration in the oil and gas phases. 3. Results and Discussion The series of well-characterized laboratory experiments that were performed in this study demonstrated particularly in situ gas-oil mixing behavior and the development of miscibility through miscible displacements in a channelling heterogeneity porous model. These laboratory measurements form a benchmark dataset suitable for testing the compositional simulation of FCM and MCM gas injection without history matching. 3.1. FCM Displacements. In this experiment, a clear water mixture was displaced by colored water through the heterogeneous model, as shown in Figure 3. This was an FCM displacement with a mobility ratio of 2. Glycerol was added to the displaced fluid to achieve this mobility ratio. This was required so that almost the same mobility ratio as that in the MCM displacements could be obtained. The ultimate oil recovery following the FCM displacement seems to be affected only slightly by heterogeneity, because of the fact that, in FCM displacements, there is no fluid-fluid interface and no capillary pressure. However, breakthrough recovery and the number of pore volumes injected to recover the oil were noticeably affected. The gas flowed most quickly through the high-permeability layer and more slowly through the lower-permeability layers. This can be explained by the refraction equation derived by King Hubbert22 and Bear,23 which describes the flow of a single fluid through media of different permeabilities.24 However, because the mobility ratio was low (2), there was negligible viscous fingering and the overall sweep was good. Hence, by the end of the displacement, almost all oil that was originally in place had been produced. Figure 4 shows that, during FCM flooding, the presence of heterogeneities resulted in early breakthrough and delay in the recovery. Approximately 4% of the oil remained unrecovered after 2PVI, because of bypassing of the
Figure 2. Isopropanol-water-cyclohexene equilibrium ternary diagram. Table 2. Initial Viscosity Ratio and Compositions of the Fluids Used in the Condensing and Vaporizing Drive Experiments IPA Water ConcenConcentration (%) tration (%) type of drive process condensing drive vaporizing drive
initial viscosity ratio gas 1.9 2.1
70 18
Cyclohexene Concentration (%)
oil
gas
oil
gas
oil
24 65
0 0
76 35
30 82
0 0
The fluid compositions used for the condensing and vaporizing displacements are given in Table 2, where, for the sake of brevity, the displacing phase is labeled “gas” and the displaced phase is labeled “oil”. In the condensing drive, the fluid compositions were chosen so that miscibility developed as IPA condensed from the displacing gas into the oil. In the vaporizing drive, the fluid composition was such that miscibility developed by IPA vaporizing from the oil into the displacing gas. These choices of fluid compositions, combined with the use of untreated glass beads, mean that the oil is always the more wetting phase in all displacements. In the FCM experiment, dyed water displaced a mixture of undyed water and glycerol. The glycerol was used to obtain a viscosity ratio of 2, similar to that observed in the MCM displacements. Measurement Techniques. In all experiments, the gas phase was colored with a Waxolene Blue AP-FW oil-soluble dye, to allow the movement of the displacement front to be recorded photographically and by sequence video recording. The dye was used at a concentration of 0.01% (by weight). It dissolves preferentially in cyclohexene and slightly in IPA. Thus, although there was initially no dye in the oil phase, as the cyclohexene concentration increased in the oil phase, so did the blue coloration. Generally, the mixtures used in the experiments had a very strong blue color in the gas and a very
(21) Grattoni, C. A.; Dawe, R. A.; Yen, C. S.; Gray, J. D. Lower critical solution coexistence curve and physical properties (density, viscosity, surface tension, and interfacial tension) of 2,6-lutidine þ water. J. Chem. Eng. Data 1993, 38, 516–519. (22) Hubbert, M. K. Darcy’s Law and the Field Equations of the Flow of Underground Fluids. In The Theory of Ground-Water Motion and Related Papers; Hafner: New York, 1969; pp 261-300. (23) Bear, J. Dynamic of Fluids in Porous Media; Dover Publications: New York, 1998. (Originally published in 1972 by Elsevier (New York)). (24) Roti, Oe.; Dawe, R. A. Modelling Fluid Flow in Cross-bedded Sections. Transport Porous Media 1993, 12, 143–159.
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Figure 5. Experimental displacement fronts for the condensing gasdrive displacement at different pore volumes injected. Here, the blue-colored gas is displacing clear oil at a rate of 3 cm3/min.
low-permeability layers. As the flood continued, gas swept more and more through the low-permeability regions with the help of mass transfer between gas and oil phases, which caused a reduction in interfacial tension (IFT); hence, miscibility (or near miscibility) was achieved, although still less than what was observed in the homogeneous model17-20 experiments. Breakthrough was earlier than in the homogeneous porous media, and the flow was dispersed upon passing through the boundary; hence, component concentration distributions were smeared. The same was also found by Caruana and Dawe25 for immiscible displacements. They found that, during a waterflood, high-permeability and high-oil-weight regions were bypassed because of capillary pressure differences, giving rise to high residual oil saturations in these regions. Through material balance calculations, they showed that the residual oil saturation in heterogeneous systems was, in most cases, at least 20% higher than that in a homogeneous model. The main difference between our findings and their observations is that the effects of heterogeneities on MCM displacements were less than for the immiscible displacements. The reason may be explained as follows. The impact of capillarity on MCM displacement characteristics is minimal, compared to its effect on immiscible displacement.9 Capillary pressure effects are expected to be significant mainly in the transition zone, where saturation gradients are sharper. However, in this zone, IFT and consequently gas-oil capillary pressure, which is proportional to IFT, are reduced substantially, because of the approach to miscibility. As depicted in Figure 5, the channelling had driven the system toward a more submiscible process, because the volume of oil that was not in contact with gas was increased, hence reducing the transfer of components between the bypassed region (low-permeability layers) and the partially mixed region (high-permeability layer). 3.2.2. Effects of Channeling Heterogeneity on Recovery and Gas Cut. The effluent data have been plotted as recovery and gas cut versus pore volume injected and are presented in Figures 6 and 7. As implied in Figure 6, breakthrough and total oil recovery values of 57% and 87%, respectively, were achieved. The breakthrough and total recovery values for the homogeneous model17-20 experiments were higher by 15% and 13%, respectively, than those achieved in the channelling heterogeneity media. Sure enough, such differences were due to the presence of heterogeneities, which resulted in bypassing, because of the capillary pressure differences between different permeability regions. During condensing gas-drive flood, we found that the produced effluent was not at equilibrium and there was still mass transfer between the two phases just after
Figure 4. Comparison of measured FCM flood: (a) cumulative oil recovery and (b) gas cut in homogeneous17-20 and heterogeneous models.
low-permeability layers. The recovery in the porous media with channelling heterogeneity is 30% lower than that in the homogeneous model.17-20 3.2. Condensing-Gas Drive Displacement. In this set of experiments, oil was composed of 24% IPA and 76% water and was displaced by gas composed of 70% IPA and 30% cyclohexene (Table 2). When the enriched gas comes into contact with the oil, IPA condenses from the gas into the oil, making the oil lighter. The equilibrium gas is more mobile than the oil, so it moves on ahead and is replaced by fresh injection gas, from which more IPA condenses, making the oil even lighter. This continues until the oil is light enough to be completely miscible with the injection gas at IPA concentration of 56% (by volume). The gas is stripped off its intermediates as it moves ahead of the enriched oil and contacts the original reservoir oil. 3.2.1. Permeability Channeling Effects on Displacement Characteristics. Figure 5 shows the displacement patterns at different pore volumes injected for the gas-oil system. The figure demonstrates that, during condensing gas-drive displacement in the presence of permeability variation heterogeneity, the gas filled the high-permeability layer very efficiently. At first, the oil, which was displaced from the low-permeability regions, was less than that from the high-permeability layer. Therefore, at breakthrough, a considerable amount of movable oil was bypassed in the
(25) Caruana, A.; Dawe, R. A. Experimental Studies of the Effects of Heterogeneities on Miscible and Immiscible Flow Processes in Porous Media. Trends Chem. Eng. 1996, 3, 185–203.
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Figure 6. Comparison of measured condensing drive oil recovery in homogeneous17-20 and channelling heterogeneity models.
Figure 8. Water, IPA, and cyclohexene components’ pre-equilibrium to equilibrium mole fractions in condensing drive plotted against total produced IPA mole fraction in the (a) oil phase and (b) gas phase.
Comparing pre-equilibrium and equilibrium oil phase volumes (Figure 8) showed an increase in the oil-phase volume after equilibration, which was an indication of a condensing gas-drive displacement where components from the gas phase were condensing into the oil phase. As portrayed in the figure, some of the ratios are greater than unity, whereas others are less than unity. During the flooding, the oil phase loses some of its water component to the gas phase, so allowing effluent to equilibrate implies that the concentration of the water component in the oil phase will continue to decrease until reaching equilibrium, whereas the opposite will occur in the gas phase. Hence, the ratios of pre-equilibrium to equilibrium mole fractions for the water component are greater than unity in the oil phase and less than unity in the gas phase. On the other hand, during the displacement, the oil phase gains IPA and cyclohexene components from the gas phase, thus allowing effluent to equilibrate means that IPA and cyclohexene concentrations in the oil phase will continue to increase until reaching equilibrium, whereas the opposite will occur in the gas phase. Consequently, the ratios of pre-equilibrium to equilibrium mole fractions for the IPA and cyclohexene components are less than unity in the oil phase and greater than unity in the gas phase. Figure 8 also reveals that, for the condensing drive, the pre-equilibrium to equilibrium ratios for both phases for all components are close to one (i.e., the experiment is closest to equilibrium) for mole fractions of IPA of ∼30% and become significantly different from one as IPA mole fraction increases toward the critical point. This is attributed to the fact that, just after breakthrough, the fraction of gas volume
Figure 7. Comparison of experimental condensing drive pre-equilibrium and equilibrium gas cut curves.
breakthrough. As such, instantaneous oil and gas volumes, and volumes after a few hours from production, were measured and compared. From now onward, we will call the first “pre-equilibrium” volumes, whereas the second will be called “equilibrium” volumes. Since the flooding was a condensing gas-drive, allowing effluent to equilibrate resulted in more condensation from the gas phase to the oil phase, thus the oil phase volumes at equilibrium are higher than pre-equilibrium volumes (see Figure 7). Comparing Figure 7 and the work reported by Al-Wahaibi et al.17-20 suggests that the average nonequilibrium, in the case of the channelling heterogeneity model, was 7% higher than that in the homogeneous model. This is attributed to the decrease in the chances for better transfer of components between gas and oil phases, because of the presence of layers with different permeabilities. 3.2.3. Impact of Channelling Heterogeneity on Miscibility Development. One method that facilitates the understanding of the mechanism by which channelling heterogeneity impacted development of miscibility is to examine the ratio of pre-equilibrium to equilibrium component’s mole fraction in the oil and gas phases, calculated as a function of the total IPA mole fractions (see Figure 8). 1817
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in the tube (used to collect effluent at each time step) was small, compared to that of oil. Hence, the room for the oil phase to increase (by condensation of gas phase components) during equilibration was minimal. As displacement continued, however, the gas volume fraction increased in each collected tube until close to the end of the displacement, at which point most of the tube volume was occupied by gas. This was accompanied by an increase in the room for the oilphase volume to increase during equilibration (through the condensation of gas-phase components). Comparing Figure 8 and the work reported by Al-Wahaibi et al.17-20 reveals that, although the trends are similar, the values calculated for the condensing drive within the channelling heterogeneity model were higher than those for the homogeneous porous media. This is true for the three components in both gas and oil phases, which reveals, as discussed earlier, that the heterogeneity reduced chances for complete mixing between the phases, and, hence, the nonequilibrium increased. As a result, for the heterogeneous model, we believe that the transition zone was larger than that in the homogeneous model; the miscibility region was shorter, which, in turn, resulted in a less-efficient displacement than in the homogeneous porous media. 3.3. Vaporizing-Gas Drive Displacement. We performed a vaporizing gas-drive displacement where oil that had a composition of 65% IPA and 35% water was displaced by gas that had a composition of 18% IPA and 82% cyclohexene (see Table 2). In this displacement, when the gas comes into contact with the oil, IPA vaporizes from the oil into the gas, making the gas richer. The gas is more mobile than the oil, so it moves ahead and contacts new oil, from which more IPA vaporizes, making the gas even richer. This continues until the gas is rich enough to be miscible with the oil at an IPA concentration of 56% (by volume). 3.3.1. Permeability Channeling Effects on Displacement’s Characteristics. The experimental displacement patterns at different pore volumes injected are shown in Figure 9. The figure reveals that, in the presence of permeability heterogeneity, the same qualitative behavior that occurred in the condensing drive was seen during vaporizing gas-drive displacement. The low-permeability layers acted as temporary flow barriers. This means that the high-permeability region could be well-swept before gas continued toward the outlet. Gas broke through, having swept only a small fraction of the low-permeability layers. However, as the IFT decreased, because of the development of miscibility, the capillary forces were reduced and more gas entered the low-permeability stripes, although still less than in the homogeneous model experiments.17-20 In addition, Figure 9 indicates that the flow, at the trailing edge of the transition zone, was dispersed on passing through the boundary; hence, component concentration distributions were smeared. For the interface between the highpermeability stripe and the low-permeability regions, as the flow came closer to the interface, it tried to remain in the more-permeable medium and became more tangential to the boundary. Upon crossing into the lower-permeability zone, flow gradually became less tangential and reverted into their original direction.24 3.3.2. Effects of Channeling Heterogeneity on Recovery. The behavior of oil recovery and gas cut versus pore volume injected were similar to that observed in the condensing drive (see Figures 10 and 11). The produced fluids were not in equilibrium in the experiment, although, in this case, the gas
Figure 9. Experimental displacement fronts for the vaporizing gasdrive displacement at different pore volumes injected. Here, the blue-colored gas is displacing clear oil at a rate of 3 cm3/min.
Figure 10. Comparison of measured vaporizing drive oil recovery in homogeneous17-20 and heterogeneous models.
Figure 11. Comparison of experimental vaporizing drive pre-equilibrium and equilibrium gas-cut curves.
cut was higher after the produced fluids had been allowed to equilibrate (as would be expected from a vaporizing drive where IPA vaporizes from the oil phase into the gas phase, making the gas richer). As demonstrated in Figure 10, the breakthrough and total oil recovery values were 50% and 80%, respectively. In the homogeneous model experiments, the breakthrough and total recovery values were higher (by 29% and 20%, respectively) than those achieved in the channeling heterogeneity porous media. These significant differences were due to the capillary pressure differences between different permeability regions, which, in turn, yielded the smearing of components’ concentrations. Note that, in the heterogeneous 1818
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Figure 13. Measured oil recovery for FCM, condensing-drive, and vaporizing-drive floods within the heterogeneous porous model.
the oil phase will continue to increase until reaching equilibrium, whereas the opposite will occur in the gas phase. As such, the ratios of pre-equilibrium to equilibrium mole fractions for the cyclohexene component are less than unity in the oil phase and greater than unity in the gas phase. In contrast to what is observed in the condensing drive, the ratios of pre-equilibrium to equilibrium mole fractions are closest to 1 for IPA mole fractions of ∼50% and become significantly different from unity at low IPA mole fractions in the vaporizing gas-drive flood. The cause of this is the fact that, just after breakthrough, the fraction of oil volume in the tube was high, compared to that of gas. Hence, the room for the gas phase to increase (by vaporization of oil-phase components) during equilibration was highest. As displacement continued, however, the oil volume fraction decreased in each collected tube until close to the end of the displacement, when most of the tube volume was occupied by gas. This was accompanied by decreased room for the gas-phase volume to increase during equilibration through the vaporization of oil-phase components into gas phase. The ratios calculated for the channeling heterogeneity porous media were higher than those for the homogeneous model. This is obvious if we compare Figure 12 and earlier work reported by Al-Wahaibi et al.17-20 This is attributed to the presence of heterogeneity, which caused a larger transition zone, shorter miscibility region, and hence a lesserefficient flood when compared to that for the homogeneous porous media. The comparison also reveals that the trend of ratios for IPA, water, and cyclohexene components obtained for both porous media was similar. 3.4. Comparison between FCM and MCM Displacements. The comparisons of oil recoveries of FCM, condensingdrive, and vaporizing-drive displacements are shown in Figure 13. The FCM injection outperformed the MCM floods. As demonstrated in the figure, recovery increased monotonically with enrichment. The cause of these differences has been explained previously. Our results agree with those reported by Burger and Mohanty,26 who observed the same trend for gas floods in a water-wet core in the absence of water, and contradict those of Wylie and Mohanty,5 who found that, for horizontal
Figure 12. Water, IPA, and cyclohexene components’ pre-equilibrium to equilibrium mole fractions in the vaporizing drive plotted against total produced IPA mole fraction in the (a) oil phase and (b) gas phase.
model, the condensing drive outperformed the vaporizing drive, with a recovery of 7% more at breakthrough and the end of displacement. Since flooding is via vaporizing gas-drive, allowing effluent to equilibrate resulted in more vaporization from oil phase to gas phase; thus, the oil-phase volumes at equilibrium are lower than the before-equilibrium volumes. Comparing Figure 11 and the work reported by Al-Wahaibi et al.17-20 shows that the nonequilibrium in the case of heterogeneous model was higher than that in the homogeneous model, by 9%. 3.3.3. Impact of Channeling Heterogeneity on Miscibility Development. To illustrate the effects of channeling heterogeneity on the nonequilibrium and development of miscibility for vaporizing gas-drive, the ratios of pre-equilibrium to equilibrium component’s mole fraction in the oil and gas phases were calculated and plotted as a function of total IPA concentrations (see Figure 12). During the vaporizing drive, the oil phase loses water and IPA components to the gas phase, so allowing effluent to equilibrate implies that concentrations of both components in the oil phase will continue to decrease until reaching equilibrium, whereas the opposite will occur in the gas phase. Hence, the ratios of pre-equilibrium to equilibrium mole fractions for the water and IPA components are greater than unity in the oil phase and less than unity in the gas phase. In addition, during the flood, the oil phase gains cyclohexene component from the gas phase; therefore, allowing effluent to equilibrate means that the cyclohexene concentration in
(26) Burger, J. E.; Mohanty, K. K. Mass transfer from bypassed zones during gas injection. SPE Reserv. Eng. 1997, 12 (May), 124-130.
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Figure 14. Experimental displacement fronts for the condensing gas-drive displacement at different pore volumes injected. Here, the blue-colored gas is displacing clear oil at an injection rate of 0.3 cm3/ min.
Figure 15. Experimental displacement fronts for the condensing gas-drive displacement at different pore volumes injected. Here, the blue-colored gas is displacing clear oil at an injection rate of 6 cm3/ min.
Figure 17. Condensing gas-drive equilibrium and pre-equilibrium gas-cut curves for (a) 6 cm3/min and (b) 0.3 cm3/min.
recovered only 72%. They attributed this to the gravity override they noticed in FCM displacements. 3.5. Effect of Injection Rate. To investigate the effects of injection rate on MCM process recovery and gas/oil nonequilibrium, further condensing and vaporizing gas-drive displacements were performed at 6 cm3/min and 0.3 cm3/ min. However, the results from the vaporizing drives were similar to those from the condensing drive where only the condensing drive results are reported. Figures 14 and 15 show the flood patterns at different pore volumes injected for the 0.3 cm3/min and 6 cm3/min condensing gas-drive displacements, respectively. As observed in these two figures, the flood at a rate of 0.3 cm3/min outperformed the other two displacements. At 2 PVI, recoveries achieved in the 6 cm3/min, 3 cm3/min and 0.3 cm3/min displacements were 78%, 86%, and 96%, respectively (see Figure 16a). This implies that the performance decreased with the increase in injection rate. The flood at 6 cm3/min broke through earlier than the other two floods (see Figure 16b). These observations suggest that the effect of rate on condensing gas-drive displacement in channeling heterogeneity porous media was significant. This was probably due to an interplay between capillary and viscous forces in the heterogeneous model causing the gas (at highest rate) to flow at a faster rate into the high-permeability stripe, with capillary forces causing the gas to flow into the
Figure 16. Comparison between condensing gas drive (a) recovery and (b) gas cut at injection rates of 6, 3, and 0.3 cm3/min.
gas floods on an oil-wet homogeneous core at 0% water saturation, the increase in recovery with enrichment is nonmonotonic. Wylie and Mohanty5 observed that MCM gas solvent recovered the most oil (85%), whereas the FCM gas 1820
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• The mass-transfer rate and overall sweep decreased because of the channeling heterogeneity. The effect of this heterogeneity was to lower mass transfer between oil and MCM gas phases throughout the porous model as a whole, thereby driving the system to be more submiscible and as such reducing the sweep and increasing the bypassing. Consequently, the displacing gas moved faster and broke through earlier. These results were also reflected in the increase in nonequilibrium between gas and oil phases. • During both condensing and vaporizing gas-drive floods, the gas swept the high-permeability layer very efficiently, whereas the flood front was far behind in the low-permeability regions. Thus, at breakthrough, a considerable amount of movable oil was bypassed in the low-permeability layers. However, as the flood continued, gas swept more and more through the low permeability layers with the help of mass transfer between gas and oil phases, which caused a reduction in interfacial tension (IFT) and, hence, miscibility (or near miscibility) achievement. • The performance of condensing and vaporizing gas-drive processes decreased significantly as the injection rate increased. This is due to the fact that, at higher rates, there was less time for mass transfer between a fast-moving gas and a slower-moving oil at any given location in the pore space, thus less time to develop miscibility. Moreover, this is can be explained by the measure of pre-equilibrium gas-cut curve departure from the equilibrium curve. As proved experimentally in the channeling heterogeneity porous model, the gas/oil nonequilibrium was higher at higher rates and was higher than that observed for homogeneous porous media.
high-permeability layer and viscous forces helping the gas to penetrate the high-permeability layer faster, therefore resulting in an earlier breakthrough, reduced breakthrough recovery, and more bypassing of oil. Our findings may be explained by the fact that, at higher rates, there is less time for mass transfer between a fastmoving gas and a slower-moving oil at any given location in the pore space, and, thus, there is less time to develop miscibility. Hence, the larger physical dispersion at higher rates may inhibit the development of miscibility. This was also supported by the measure of pre-equilibrium gas-cut curve departure from the equilibrium curve. As depicted in Figures 17a and 17b, the gas/oil nonequilibrium was higher at a higher rate. Doubling the rate (from 3 cm3/ min to 6 cm3/min) caused the maximum nonequilibrium (which is the maximum difference between equilibrium and pre-equilibrium gas cuts at the same PVI) to increase by as much as 5%. For the 0.3 cm3/min injection rate case, although the rate was low, the system was still away from equilibrium by as much as 9%. 4. Conclusions Multicontact-miscible (MCM) and first-contact-miscible (FCM) displacement experiments were performed to evaluate the effect of channeling heterogeneity on their efficiencies and on gas/oil nonequilibrium. The principal observations from the experiments were as follows: • The FCM injection outperformed the MCM floods, i.e., recovery increased monotonically with enrichment. • In the channeling heterogeneity porous media, the experimentally observed produced oil and gas were not in equilibrium in the condensing and vaporizing gas-drive displacements.
Acknowledgment. I thank Petroleum Development Oman for its financial support of this work.
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