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Effects of connate water on chemical flooding processes in porous media
Journal of Petroleum Science and Engineering 19 Ž1998. 159–169
Effects of connate water on chemical flooding processes in porous media L. Thibodeau, G.H. Neale
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Department of Chemical Engineering, UniÕersity of Ottawa, Ottawa, Ont., Canada K1N 6N5 Received 11 March 1997; accepted 5 September 1997
Abstract This work examines the effects of connate water on the unstable displacement patterns and fractional oil recoveries observed when simulated secondary and tertiary oil displacement processes are conducted in the laboratory. Three displacement systems were investigated; a non-reacting system without surfactant Žwater displacing non-acidic paraffin oil., a non-reacting system with externally added surfactant Žsodium dodecyl sulfate solution displacing non-acidic paraffin oil., and a reacting system with internally generated surfactant Žsodium hydroxide solution displacing acidified paraffin oil.. A consolidated porous medium constructed of fused glass beads was used to simulate the underground oil reservoir. Each of the three basic displacement processes was conducted both in the absence and presence of connate water, and at three different flow rates. The results obtained indicate that the presence of connate water can exert a significant influence on the displacement pattern with the non-reacting systems, particularly at low flow rates. Some of these effects include an increase in the number of fingers andror much thinner fingers with many tiny branches. At low flow rates, the presence of connate water tends to decrease the breakthrough oil recovery for the non-surfactant system, but increases the breakthrough recovery for the surfactant-containing systems. At high flow rates, connate water has a much less pronounced effect due to the dominance of viscous forces. q 1998 Elsevier Science B.V. Keywords: oil recovery; porous media
1. Introduction When a fluid of lower viscosity Že.g., water. displaces one of higher viscosity Že.g., oil. in a porous medium, the displacement front between the fluids invariably becomes unstable. These interfacial instabilities, which are initiated by local heterogeneities in the porosity and permeability of the porous medium, result in ‘fingering’ of the displac)
Corresponding author. Tel.: q1-613-5625800. Fax: q1-6135625172. E-mail: [email protected]
ing fluid into the displaced fluid. This so-called viscous fingering phenomenon is responsible for premature breakthrough of the displacing fluid, thereby leading to a reduced displacement efficiency Žrecovery. of the displaced fluid. This has serious economic implications during secondary and tertiary recovery of oil from underground reservoirs by aqueous fluid injection. Connate water is the term used to describe the naturally occurring aqueous fluid that is contained within the oil phase in underground petroleum reservoirs, and which was deposited along with the oil in
0920-4105r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 0 - 4 1 0 5 Ž 9 7 . 0 0 0 4 3 - 0
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L. Thibodeau, G.H. Neale r Journal of Petroleum Science and Engineering 19 (1998) 159–169
prehistoric times. Depending on the preferential wettability of the reservoir rock, this aqueous phase Žwhich contains various dissolved salts. may occur as ganglia trapped between the rock pores, andror as a thin film covering some or all of the rock surface ŽPaterson et al., 1984a.. The objective of the present preliminary study was to determine the qualitative effects that connate water Žby virtue of the inevitable chemical dilution effects that it causes. can have on simulated chemical flooding recovery processes conducted in the laboratory, with a view to determining the importance of including connate water during such laboratory studies. Chuoke et al. Ž1959. and Peters and Flock Ž1981. studied viscous fingering instabilities during immiscible waterroil displacement processes conducted in unconsolidated opaque three-dimensional sand packs contained in cylindrical tubes, both in the presence and absence of connate water. The porous medium chosen for the present study was consolidated Žso that the permeability and porosity of the medium would remain constant., and two-dimensional Žthereby allowing the progress of the microscopic fingering pattern to be observed visually and monitored photographically.. Paterson et al. Ž1984b. studied viscous fingering in an oil-wet porous medium in the presence of connate water and concluded that connate water adds additional perturbations to the displacement interface leading to more irregular fingering patterns. A variety of water injection experiments was subsequently performed in this laboratory involving the visualization of displacement processes. Page et al. Ž1993. and Guo and Neale Ž1996. studied miscible displacement processes, whereas Thirunavu and Neale Ž1995. studied immiscible processes. These studies were performed in the absence of connate water. More recently, Thibodeau et al. Ž1997. performed a preliminary screening study of the effects of connate water on the fingering patterns and oil recoveries observed in immiscible waterroil displacement processes in the complete absence of chemical additives such as surfactants and alkalis. To increase oil recovery in practice, two basic approaches are possible: Ž1. the viscosity of the displacing aqueous phase can be increased Žby adding polymers., andror Ž2. the interfacial tension ŽIFT. between the two phases can be lowered Žby introduc-
ing surfactants, either externally or by in situ chemical reaction.. The present work focuses on the lowering of IFT by using chemical additives. Paterson et al. Ž1984a. compared the viscous fingering patterns observed for the cases of pure water and an aqueous surfactant solution displacing oil from an oil-wet porous medium and concluded that the surfactant induces more dispersive viscous fingers, with new fingers being created after breakthrough. A variety of work has been performed since the pioneering study of Jennings et al. Ž1974. on the displacement of acidic oils by alkaline solutions Žusually NaOH. for both naturally acidic crude oils and artificially acidified synthetic oils. When acidic oil is displaced by a caustic solution, interfacial reactions resulting in the in-situ generation of surfactants will occur. These surfactants tend to adsorb at the oilrwater interface and reduce the IFT between the two fluids. The interfacial reaction mechanisms involved have been discussed in detail by Rudin and Wasan Ž1992. and by Touhami et al. Ž1997., while their effects on displacement efficiency have been examined by Hornof and Baig Ž1995. using a Hele–Shaw cell. In reacting situations, the IFT initially decreases sharply towards a minimum value, and then increases steadily towards an equilibrium value as the reaction approaches completion. This so-called dynamic IFT behavior can have significant effects, which are usually difficult to predict and interpret, on the fingering pattern and subsequent oil recovery efficiency.
2. Experimental The two-dimensional consolidated porous medium employed in this study was constructed from almost monosized glass particles of median diameter 0.448 mm. The porous medium was completely water-wet and had a thickness of 3.0 mm, a pore volume of 19.24 ml, and a porosity of 0.30. This medium was sintered between two identical glass plates with dimensions of 150 mm = 150 mm = 5 mm. The sintering process was performed by slowly heating the medium between the plates to 6608C. This temperature was then maintained for two hours. After cooling, the cell edges were sealed with an epoxy resin, and two sets of three equally-spaced ports of diameter 2.0 mm were drilled into opposite sides of the
L. Thibodeau, G.H. Neale r Journal of Petroleum Science and Engineering 19 (1998) 159–169
cell. The ports were 15 mm from the outer edge of the cell, and they permitted convenient access to the porous medium during both the waterroil displacement process and the subsequent cleaning process. 2.1. Experiments performed All of the experiments were performed in the horizontal flow mode to minimize the complicating effects of buoyancy forces. Three flow rates, namely 1.10 mlrh, 5.50 mlrh, and 96.0 mlrh were used for the experiments. For each flow rate, three basic displacement systems were employed: SYSTEM A – distilled water displacing nonacidic paraffin oil; SYSTEM B – a 50 mM aqueous solution of sodium dodecyl sulfate ŽSDS. surfactant displacing non-acidic paraffin oil; and SYSTEM C – a 250.0 mM Ž1.0 wt%. aqueous solution of sodium hydroxide ŽNaOH. displacing a 10.0 mM solution of linoleic acid in non-acidic paraffin oil. Although systems B and C were selected arbitrarily, they are typical of those employed in previous simulated waterroil displacement processes conducted in the laboratory ŽRudin and Wasan, 1992; Touhami et al., 1997.. They are not necessarily representative of real reservoir situations, although they could be in certain instances. Moreover, it should be stressed that some of the chemical additives andror reaction products will be adsorbed to varying and unknown degrees onto the porous medium surface, thereby reducing the concentrations of the active species. In view of this fact, the observed trends should be considered as strictly qualitative rather than quantitative in nature. Each of the above three displacement processes was conducted in both the absence and presence of
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connate water, making a total of 6 distinct experiments for each individual flow rate. Linoleic acid was used as the acidifying organic acid because it is soluble in heavy paraffin oil and is colorless. Also, previous IFT measurements ŽRudin and Wasan, 1992; Touhami et al., 1997. have shown that linoleic acidrparaffin oil mixtures display similar time-dependent IFT behavior as that which occurs between natural heavy crude oils and alkaline solutions. The displacing aqueous phases were dyed with 0.025 wt% bromocresol green for visualization purposes. This dye was selected because it does not react or deteriorate in the presence of NaOH, and it retains its distinctive color. Tables 1 and 2 display the properties of the various fluid systems employed at 25.0 " 0.58C. It should be noted that the dye acted as a mild surfactant, reducing the IFT slightly ŽTable 2.. All interfacial tensions were measured using a commercial drop volume tensiometer. 2.2. Procedures 2.2.1. Experiments without connate water The porous medium cell was first evacuated, and deaerated oil was drawn slowly into the cell. The dyed displacing fluid was then injected into the cell through the central inlet port using a constant flow rate syringe pump. The displaced oil was recovered from the opposite central port. The elapsed time was recorded from the time when the displacing fluid first entered the porous medium to the time that it first reached the outlet port. This so-called breakthrough time allows the fractional oil recovery at breakthrough to be calculated exactly from the equation R s Qt br rVp , where Vp denotes the pore volume of the cell, t br the breakthrough time, and Q the volumetric injection flow rate. Photographs of the developing fingering pattern were taken until the
Table 1 Properties of fluids at 25.0 " 0.58C Fluid
Density Žgrcm3 .
Viscosity ŽmPa P s.
Non-acidic paraffin oil 10.0 mM linoleic acid in non-acidic paraffin oil Žacid numbers 0.6 mg KOHrg of oil mixture. Dyed distilled water Dyed 5.0 mM sodium dodecyl sulfate solution Dyed 250.0 mM Ž1.0 wt%. NaOH solution
0.8699 0.8727 0.9971 0.9974 1.0074
132.42 137.23 0.8779 0.8823 0.9016
Note: all aqueous solutions were dyed with 0.025 wt% bromocresol green for visualization purposes.
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Table 2 Fluid systems employed and their interfacial tensions at 25.0 " 0.58C Fluid system
System code
IFT ŽmNrm.
Dyed distilled waterrnon-acidic paraffin oil Dyed 5.0 mM sodium dodecyl sulfate solutionrnon-acidic paraffin oil Dyed 250.0 mM NaOH solutionr10.0 mM linoleic acid in non-acidic paraffin oil solution
A B C
38.1 w52.2x 14.3 w16.0x 0.34 w0.46x Žafter 25 s.
Note: the numbers in brackets wx represent IFT measurements in the absence of dye.
breakthrough condition was reached. Following each experiment, the cell was cleaned using a standardized procedure, during which several pore volumes of propanol-2, water, acetone, and nitrogen were consecutively flushed through the cell. 2.2.2. Experiments with connate water When performing experiments in the presence of connate water, the same basic procedure as that outlined above was employed, but with some slight modifications. In this case, the distilled water used as the connate water phase was first evacuated to remove any dissolved air. After evacuating the cell, this water was drawn slowly into the cell. The oil phase was then injected into the cell to displace the bulk of the water. Since the oil had a much higher viscosity than the water, the displacement was very stable Ži.e., plug flow., resulting in the removal of the bulk of the water. The small Žirreducible. amount of water that remained constituted the connate water phase. Once the oil completely saturated the cell Žapart from the connate water., the cell was then ready for the waterroil displacement process as described in the previous section. In principle, the fractional volume Žsaturation. of the connate water phase can be measured, but in practice saturation is difficult to measure accurately
because of its low value. Approximate measurements involving both the mass and volume of the cell and its contents indicated that the connate water occupied about 5% " 2% of the total pore volume of the cell. It should be noted that the previously discussed equation for calculating the fractional recovery of the oil phase Ž R s Qt br rVp . remains valid provided that the connate water fraction in the displaced oil phase remains unchanged from that in the oil that was originally present in the porous medium, which is a reasonable assumption. 2.2.3. Capillary number and nature of the flow field Since the fluid injection and fluid recovery take place at single points as in real petroleum reservoirs, rather than along straight lines, the displacing fluid streamlines will always be diverging Ži.e., of decreasing velocity. near the inlet and converging Ži.e., of increasing velocity. near the outlet of the porous medium. It is not, therefore, meaningful to calculate the familiar capillary number, Ca s m w Urg Žwhich compares the relative magnitudes of the viscous and interfacial forces., since this definition is traditionally based on a constant linear frontal velocity, U, which does not exist in the present experiments. This is the reason why capillary numbers were not employed in the present work.
Table 3 Experimental data for low injection flow rate Ž Q s 1.10 mlrh. Expt. No.
Fluid system
Connate water?
Breakthrough time, t br Žs.
Breakthrough recovery, R Ž%.
1 2 3 4 5 6
A A B B C C
no yes no yes no yes
4388.1 3542.1 4415.2 5655.3 4132.2 6228.3
6.97 5.63 7.01 8.98 6.56 9.89
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3. Results and discussion
3.1. Low flow rate displacements (Q s 1.10 ml r h)
The effects of connate water were investigated, on System A Žthe non-reacting system without surfactant., on System B Žthe non-reacting system with externally added surfactant., and on System C Žthe reacting system with internally generated surfactant..
Table 3 displays the breakthrough recoveries for this particular flow rate which were quite low Žin the 5%–10% range. because of the high oilrwater viscosity ratio. These results indicate that the presence of connate water leads to a decrease in oil recovery
Fig. 1. Low flow rate Ž Q s 1.10 mlrh. displacement patterns for systems A, B and C in absence and presence of connate water. Ža. System A without connate water Ž R s 6.97%.; Žb. System A with connate water Ž R s 5.63%.; Žc. System B without connate water Ž R s 7.01%.; Žd. System B with connate water Ž R s 8.98%.; Že. System C without connate water Ž R s 6.56%.; Žf. System C with connate water Ž R s 9.89%.. ŽThe fluid injection point is at the left-hand side in all pictures.
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when surfactant is absent ŽSystem A., but to an increase in recovery when surfactant is present ŽSystems B and C.. 3.1.1. System A In the absence of connate water, the non-reacting displacement produced a single, broad finger as shown in Fig. 1Ža.. However, as seen in Fig. 1Žb., the presence of connate water changes the fingering pattern significantly. The finger is now lighter with many twists and tiny extensions. There also exists a large tributary to the main finger. At a low flow rate and a high IFT, such as in this case, capillary forces dominate the displacement. Therefore, when a highviscosity oil is being displaced by a low-viscosity aqueous phase, the displacing fluid tends to enter the smaller pores of the water-wet medium where the capillary pressure is high Žspontaneous imbibition.. Due to imbibition, viscous fingers develop in many directions, including backwards towards the inlet. The connate water may have promoted the backward moving fingers because the displacing phase naturally tends to coalesce with the connate water phase while moving through the cell. 3.1.2. System B The IFT between the oil and the surfactant solution is mid-range and remains constant. In the absence of connate water, the fingering pattern is similar to that for the non-reacting system but with more branches, as shown in Fig. 1Žc.. The slight lowering of the IFT causes an increase in the number of branches because the fluid can create paths through the porous medium with less expenditure of energy ŽPaterson et al., 1984b.. However, in the presence of connate water, the displacing fluid followed a broad
network of paths through the porous medium ŽFig. 1Žd.. because the connate water adds additional perturbations to the interface.
3.1.3. System C The reacting system displayed a much different displacement pattern from those for the two non-reacting systems. Due to the very low IFT and its dynamic Ži.e., time-dependent. nature, the viscous fingering pattern consists of many long, dendritic fingers which diverge radially from the inlet, as shown in Fig. 1Že.. Generally speaking, the reacting displacements tend to exhibit a greater areal sweep in comparison with the two non-reacting systems. However, patches of oil still remain trapped between and around the fingers. It is also noteworthy that these patterns are quite symmetrical about the centerline of the cell, which is indicative of uniform chemical reaction and associated molecular diffusion processes. With connate water present in the cell ŽFig. 1Žf.., the overall viscous fingering pattern is superficially similar to that produced in the absence of connate water ŽFig. 1Že.., although in Fig. 1Žf. the edges of the fingers are slightly more hazy and thicker than those in Fig. 1Že... A possible explanation for the hazy finger tips is that an emulsion is created in the vicinity of the interface as a result of the chemically generated surfactant. The reacting displacements are quite complicated because several inter-related processes are occurring. First, both reactants Žthe acid and the alkali. must diffuse to the interface, and once there, the chemical reactions discussed earlier will take place. In addition, there are adsorption and desorption processes occurring, to and from, the bulk phases and the
Table 4 Experimental data for intermediate injection flow rate Ž Q s 5.50 mlrh. Expt. No.
Fluid system
Connate water?
Breakthrough time, t br Žs.
Breakthrough recovery, R Ž%.
7 8 9 10 11 12
A A B B C C
no yes no yes no yes
1537.9 878.5 1027.4 1389.4 714.4 882.3
12.21 6.98 8.16 11.03 5.67 7.01
L. Thibodeau, G.H. Neale r Journal of Petroleum Science and Engineering 19 (1998) 159–169
interface. Furthermore, convective currents exist during the displacement as a result of the advancing interface. Finally, if the concentration of one of the reactants is not great enough, or if the diffusion of one of the reactants to the interface is very slow, then the reaction will slow down or even cease, and the IFT will start to increase.
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3.2. Intermediate flow rate displacements (Q s 5.50 ml r h) The average frontal displacement velocity corresponding to this flow rate is close to that which is typically used in practice during secondary and tertiary oil recovery operations in real crude oil reser-
Fig. 2. Intermediate flow rate Ž Q s 5.50 mlrh. displacement patterns for Systems A, B and C in absence and presence of connate water. Ža. System A without connate water Ž R s 12.21%.; Žb. System A with connate water Ž R s 6.98%.; Žc. System B without connate water Ž R s 8.16%.; Žd. System B with connate water Ž R s 11.03%.; Že. System C without connate water Ž R s 5.67%.; Žf. System C with connate water Ž R s 7.01%.. ŽThe fluid injection point is at the left-hand side in all pictures.
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voirs Ži.e., about 30 cm a day.. The displacement is governed by both capillary and viscous forces, which represents the most stable displacement regime for waterflooding. Table 4 displays the breakthrough recovery results for this particular flow rate. According to these results, the presence of connate water leads to a decrease in oil recovery Ži.e., a decrease in breakthrough time. when surfactant is absent ŽSystem A., but to an increase in oil recovery when surfactant is present ŽSystems B and C.. These observations are the same as for the low flow rate.
observed at the lower flow rate Žcompare Fig. 2Že. with Fig. 1Že... Long, fine fingers extend radially from the inlet, the probable result of IFT gradients caused by the interfacial chemical reaction. In the presence of connate water ŽFig. 2Žf.., the general shape of the displacement pattern is similar to that without connate water ŽFig. 2Že... However, the fingers are more closely spaced, and the generation of emulsion again makes the edges of the fingers look hazy and indistinct when connate water is present. 3.3. High flow rate displacements (Q s 96.0 ml r h)
3.2.1. System A In the absence of connate water, the non-reacting system is characterized by broad fingers, as shown in Fig. 2Ža.. The presence of connate water increases the irregularity of the fingers, as displayed in Fig. 2Žb.. Having connate water in the cell reduces the oil recovery by about 5%. 3.2.2. System B In the absence of connate water, the surfactant solution displacement displays a different fingering pattern from the non-reacting system as may be seen by comparing Fig. 2Žc. with Ža.. The fingers are relatively smooth and straight with a number of small branches extending from the main fingers. In the presence of connate water, the fingers become more dispersive and irregular, and the areal sweep increases Žcompare Fig. 2Žd. with Žb... The number of fingers has increased significantly, presumably due to the lower IFT and to the reduced flow resistance associated with the presence of connate water. 3.2.3. System C In the absence of connate water, the reacting system displays a similar displacement pattern to that
Table 5 displays the breakthrough recoveries for this particular flow rate. According to these results, both the non-reacting system ŽSystem A. and the external surfactant system ŽSystem B. exhibit low recoveries, whereas the in situ generated surfactant system ŽSystem C. exhibits a very high recovery. The presence of connate water leads to an increase in oil recovery in the absence of chemical reaction ŽSystems A and B., but to a decrease in oil recovery in the presence of chemical reaction ŽSystem C., which is significantly different behavior from those observed at the low and intermediate flow rates. The highest flow rate exhibits the most significant differences in breakthrough recovery amongst the three different displacement systems since viscous forces are so dominant that other effects are small or insignificant. 3.3.1. System A In the absence of connate water, the non-reacting system exhibits the expected viscous fingering pattern, as shown in Fig. 3Ža.. The increased viscous forces lead to an increase in the number of finger branches and result in the displacing fluid being
Table 5 Experimental data for high injection flow rate Ž Q s 96.0 mlrh. Expt. No.
Fluid system
Connate water?
Breakthrough time, t br Žs.
Breakthrough recovery, R Ž%.
13 14 15 16 17 18
A A B B C C
no yes no yes no yes
35.0 42.0 39.7 46.4 150.4 112.3
4.85 5.82 5.50 6.43 20.85 15.56
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forced directly towards the exit port, thereby displacing little oil. In this high flow rate region, the displacement pattern is quite uniform and predictable, in contrast to that in the low flow rate region. From Table 5, it is observed that the recovery is low both in the absence and presence of connate water. Fig. 3Žb. shows that rather than proceeding
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directly towards the outlet, some of the displacing fluid branches off in various directions. This is partly due to the effects of mixing between the displacing aqueous phase and the connate water. It is apparent that in the absence of connate water ŽFig. 3Ža.. the fingers are relatively smooth at the tips, but with connate water present ŽFig. 3Žb.., the fingers are
Fig. 3. High flow rate Ž Q s 96.0 mlrh. displacement patterns for Systems A, B and C in absence and presence of connate water. Ža. System A without connate water Ž R s 4.85%.; Žb. System A with connate water Ž R s 5.82%.; Žc. System B without connate water Ž R s 5.50%.; Žd. System B with connate water Ž R s 6.43%.; Že. System C without connate water Ž R s 20.85%.; Žf. System C with connate water Ž R s 15.56%.. ŽThe fluid injection point is at the left-hand side in all pictures.
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slender and follow more circuitous routes through the porous medium. 3.3.2. System B In the absence of connate water ŽFig. 3Žc.., the surfactant solution displacement exhibits a symmetric diverging–converging pattern with a low recovery. The presence of connate water has relatively little effect on the displacement other than slightly decreasing the finger width ŽFig. 3Žd... At high flow rates, the viscous forces are so dominant that the chemical dilution effects caused by the connate water are largely overshadowed. 3.3.3. System C For the reacting system, the displacement patterns appear to be somewhat similar to those encountered in miscible displacement processes ŽGuo and Neale, 1996., thus indicating that the IFT has been reduced to very low values by virtue of chemical reaction that occurs throughout the entire displacement. Although there are patches of oil left unrecovered ŽFig. 3Že. and Žf.., thick macroscopic fingers comprised of many tiny fingers result in an efficient displacement of oil. The percentage of oil recovered is much higher than for any of the other systems and flow rates. With connate water present ŽFig. 3Žf.., the recovery was lower because the displacement pattern exhibited slightly lower areal sweep efficiency. 3.4. Comments and recommendations The work reported here, albeit preliminary in nature, reveals that the presence of connate water, by virtue of the inevitable chemical dilution effects that it causes, can significantly modify fingering patterns, areal sweep efficiencies and fractional oil recovery efficiencies, during the displacement of oil by aqueous solutions in a porous medium. This work highlights the major importance of including connate water whenever simulating practical secondary and tertiary crude oil displacement processes in the laboratory. In addition, it indicates that the results of laboratory studies that have been conducted in the absence of connate water must be interpreted cautiously when extrapolating their conclusions to real petroleum reservoir field conditions.
It should be stressed that the present preliminary study was conducted using a water-wet porous medium and distilled water as the connate water phase. To make this study more relevant in the context of laboratory simulations of real oil reservoir displacement processes, the following additional factors should be investigated: Ø the effects of porous medium wettability Ži.e., oil-wet vs. water-wet.. Ø the effects of connate water composition, because real connate waters contain various ionic species, some of which are known to adversely affect the IFT-lowering capabilities of surfactants. Ø the effects of acid concentration and alkali concentration in the case of reacting systems because chemical additives will always be diluted when coming into contact with connate water. Ø the effects of connate water on oil recovery after the breakthrough condition has been reached.
4. List of symbols Ca IFT Q R t br U Vp g mw
capillary number Žs m w Urg . interfacial tension volumetric injection flow rate fractional recovery of oil at breakthrough breakthrough time characteristic velocity of displacement front pore volume of porous medium oilrwater interfacial tension viscosity of aqueous phase
Acknowledgements The financial support for this work provided by the Natural Sciences and Engineering Research Council of Canada ŽNSERC. is gratefully acknowledged.
References Chuoke, R.L., van Meurs, P., van der Poel, C., 1959. The instability of slow, immiscible, viscous liquid–liquid displacements in permeable media. Trans. AIME 216, 188–194.
L. Thibodeau, G.H. Neale r Journal of Petroleum Science and Engineering 19 (1998) 159–169 Guo, T., Neale, G.H., 1996. Effects of buoyancy forces on miscible liquid–liquid displacement processes in a porous medium. Powder Tech. 86, 265–273. Hornof, V., Baig, F.U., 1995. Influence of interfacial reaction and mobility ratio on the displacement of oil in a Hele–Shaw cell. Exp. Fluids 18, 448–453. Jennings, H.Y., Johnson, C.E., McAuliffe, C.D., 1974. A caustic waterflooding process for heavy oils. J. Pet. Tech. 26, 1365– 1374. Page, C.A., Brooks, H.J., Neale, G.H., 1993. Visualization of the effects of buoyancy of liquid–liquid displacements in vertically aligned porous medium cells. Exp. Fluids 13, 472–474. Paterson, L., Hornof, V., Neale, G., 1984a. Water fingering into an oil wet porous medium saturated with oil at connate water saturation. Rev. Inst. Fr. Pet. 39, 517–521. Paterson, L., Hornof, V., Neale, G., 1984b. Visualization of a surfactant flood of an oil-saturated porous medium. SPEJ 24, 325–327.
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Peters, E.J., Flock, D.L., 1981. The onset of instability during two-phase immiscible displacement in porous media. SPEJ 21, 249–258. Rudin, J., Wasan, D.T., 1992. Mechanisms for lowering of interfacial tension in alkaliracidic oil systems: Part 2. Theoretical studies. Colloids Surfaces 68, 81–94. Thibodeau, L., Guo, T., Neale, G.H., 1997. Effects of connate water on immiscible displacement processes in porous media. Powder Tech. 93, 209–217. Thirunavu, S.R., Neale, G.H., 1995. Effects of buoyancy forces on immiscible waterroil displacements in a vertically-oriented porous medium. Rev. Inst. Fr. Pet. 50, 517–536. Touhami, Y., Hornof, V., Neale, G.H., 1997. Dynamic interfacial tension behavior of acidified oilrsurfactant-enhanced alkaline systems: Part 2. Theoretical studies. Colloids and Surfaces A – Physicochemical and engineering aspects, in press.
Effects of connate water on chemical flooding processes in porous media L. Thibodeau, G.H. Neale
)
Department of Chemical Engineering, UniÕersity of Ottawa, Ottawa, Ont., Canada K1N 6N5 Received 11 March 1997; accepted 5 September 1997
Abstract This work examines the effects of connate water on the unstable displacement patterns and fractional oil recoveries observed when simulated secondary and tertiary oil displacement processes are conducted in the laboratory. Three displacement systems were investigated; a non-reacting system without surfactant Žwater displacing non-acidic paraffin oil., a non-reacting system with externally added surfactant Žsodium dodecyl sulfate solution displacing non-acidic paraffin oil., and a reacting system with internally generated surfactant Žsodium hydroxide solution displacing acidified paraffin oil.. A consolidated porous medium constructed of fused glass beads was used to simulate the underground oil reservoir. Each of the three basic displacement processes was conducted both in the absence and presence of connate water, and at three different flow rates. The results obtained indicate that the presence of connate water can exert a significant influence on the displacement pattern with the non-reacting systems, particularly at low flow rates. Some of these effects include an increase in the number of fingers andror much thinner fingers with many tiny branches. At low flow rates, the presence of connate water tends to decrease the breakthrough oil recovery for the non-surfactant system, but increases the breakthrough recovery for the surfactant-containing systems. At high flow rates, connate water has a much less pronounced effect due to the dominance of viscous forces. q 1998 Elsevier Science B.V. Keywords: oil recovery; porous media
1. Introduction When a fluid of lower viscosity Že.g., water. displaces one of higher viscosity Že.g., oil. in a porous medium, the displacement front between the fluids invariably becomes unstable. These interfacial instabilities, which are initiated by local heterogeneities in the porosity and permeability of the porous medium, result in ‘fingering’ of the displac)
Corresponding author. Tel.: q1-613-5625800. Fax: q1-6135625172. E-mail: [email protected]
ing fluid into the displaced fluid. This so-called viscous fingering phenomenon is responsible for premature breakthrough of the displacing fluid, thereby leading to a reduced displacement efficiency Žrecovery. of the displaced fluid. This has serious economic implications during secondary and tertiary recovery of oil from underground reservoirs by aqueous fluid injection. Connate water is the term used to describe the naturally occurring aqueous fluid that is contained within the oil phase in underground petroleum reservoirs, and which was deposited along with the oil in
0920-4105r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 0 - 4 1 0 5 Ž 9 7 . 0 0 0 4 3 - 0
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prehistoric times. Depending on the preferential wettability of the reservoir rock, this aqueous phase Žwhich contains various dissolved salts. may occur as ganglia trapped between the rock pores, andror as a thin film covering some or all of the rock surface ŽPaterson et al., 1984a.. The objective of the present preliminary study was to determine the qualitative effects that connate water Žby virtue of the inevitable chemical dilution effects that it causes. can have on simulated chemical flooding recovery processes conducted in the laboratory, with a view to determining the importance of including connate water during such laboratory studies. Chuoke et al. Ž1959. and Peters and Flock Ž1981. studied viscous fingering instabilities during immiscible waterroil displacement processes conducted in unconsolidated opaque three-dimensional sand packs contained in cylindrical tubes, both in the presence and absence of connate water. The porous medium chosen for the present study was consolidated Žso that the permeability and porosity of the medium would remain constant., and two-dimensional Žthereby allowing the progress of the microscopic fingering pattern to be observed visually and monitored photographically.. Paterson et al. Ž1984b. studied viscous fingering in an oil-wet porous medium in the presence of connate water and concluded that connate water adds additional perturbations to the displacement interface leading to more irregular fingering patterns. A variety of water injection experiments was subsequently performed in this laboratory involving the visualization of displacement processes. Page et al. Ž1993. and Guo and Neale Ž1996. studied miscible displacement processes, whereas Thirunavu and Neale Ž1995. studied immiscible processes. These studies were performed in the absence of connate water. More recently, Thibodeau et al. Ž1997. performed a preliminary screening study of the effects of connate water on the fingering patterns and oil recoveries observed in immiscible waterroil displacement processes in the complete absence of chemical additives such as surfactants and alkalis. To increase oil recovery in practice, two basic approaches are possible: Ž1. the viscosity of the displacing aqueous phase can be increased Žby adding polymers., andror Ž2. the interfacial tension ŽIFT. between the two phases can be lowered Žby introduc-
ing surfactants, either externally or by in situ chemical reaction.. The present work focuses on the lowering of IFT by using chemical additives. Paterson et al. Ž1984a. compared the viscous fingering patterns observed for the cases of pure water and an aqueous surfactant solution displacing oil from an oil-wet porous medium and concluded that the surfactant induces more dispersive viscous fingers, with new fingers being created after breakthrough. A variety of work has been performed since the pioneering study of Jennings et al. Ž1974. on the displacement of acidic oils by alkaline solutions Žusually NaOH. for both naturally acidic crude oils and artificially acidified synthetic oils. When acidic oil is displaced by a caustic solution, interfacial reactions resulting in the in-situ generation of surfactants will occur. These surfactants tend to adsorb at the oilrwater interface and reduce the IFT between the two fluids. The interfacial reaction mechanisms involved have been discussed in detail by Rudin and Wasan Ž1992. and by Touhami et al. Ž1997., while their effects on displacement efficiency have been examined by Hornof and Baig Ž1995. using a Hele–Shaw cell. In reacting situations, the IFT initially decreases sharply towards a minimum value, and then increases steadily towards an equilibrium value as the reaction approaches completion. This so-called dynamic IFT behavior can have significant effects, which are usually difficult to predict and interpret, on the fingering pattern and subsequent oil recovery efficiency.
2. Experimental The two-dimensional consolidated porous medium employed in this study was constructed from almost monosized glass particles of median diameter 0.448 mm. The porous medium was completely water-wet and had a thickness of 3.0 mm, a pore volume of 19.24 ml, and a porosity of 0.30. This medium was sintered between two identical glass plates with dimensions of 150 mm = 150 mm = 5 mm. The sintering process was performed by slowly heating the medium between the plates to 6608C. This temperature was then maintained for two hours. After cooling, the cell edges were sealed with an epoxy resin, and two sets of three equally-spaced ports of diameter 2.0 mm were drilled into opposite sides of the
L. Thibodeau, G.H. Neale r Journal of Petroleum Science and Engineering 19 (1998) 159–169
cell. The ports were 15 mm from the outer edge of the cell, and they permitted convenient access to the porous medium during both the waterroil displacement process and the subsequent cleaning process. 2.1. Experiments performed All of the experiments were performed in the horizontal flow mode to minimize the complicating effects of buoyancy forces. Three flow rates, namely 1.10 mlrh, 5.50 mlrh, and 96.0 mlrh were used for the experiments. For each flow rate, three basic displacement systems were employed: SYSTEM A – distilled water displacing nonacidic paraffin oil; SYSTEM B – a 50 mM aqueous solution of sodium dodecyl sulfate ŽSDS. surfactant displacing non-acidic paraffin oil; and SYSTEM C – a 250.0 mM Ž1.0 wt%. aqueous solution of sodium hydroxide ŽNaOH. displacing a 10.0 mM solution of linoleic acid in non-acidic paraffin oil. Although systems B and C were selected arbitrarily, they are typical of those employed in previous simulated waterroil displacement processes conducted in the laboratory ŽRudin and Wasan, 1992; Touhami et al., 1997.. They are not necessarily representative of real reservoir situations, although they could be in certain instances. Moreover, it should be stressed that some of the chemical additives andror reaction products will be adsorbed to varying and unknown degrees onto the porous medium surface, thereby reducing the concentrations of the active species. In view of this fact, the observed trends should be considered as strictly qualitative rather than quantitative in nature. Each of the above three displacement processes was conducted in both the absence and presence of
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connate water, making a total of 6 distinct experiments for each individual flow rate. Linoleic acid was used as the acidifying organic acid because it is soluble in heavy paraffin oil and is colorless. Also, previous IFT measurements ŽRudin and Wasan, 1992; Touhami et al., 1997. have shown that linoleic acidrparaffin oil mixtures display similar time-dependent IFT behavior as that which occurs between natural heavy crude oils and alkaline solutions. The displacing aqueous phases were dyed with 0.025 wt% bromocresol green for visualization purposes. This dye was selected because it does not react or deteriorate in the presence of NaOH, and it retains its distinctive color. Tables 1 and 2 display the properties of the various fluid systems employed at 25.0 " 0.58C. It should be noted that the dye acted as a mild surfactant, reducing the IFT slightly ŽTable 2.. All interfacial tensions were measured using a commercial drop volume tensiometer. 2.2. Procedures 2.2.1. Experiments without connate water The porous medium cell was first evacuated, and deaerated oil was drawn slowly into the cell. The dyed displacing fluid was then injected into the cell through the central inlet port using a constant flow rate syringe pump. The displaced oil was recovered from the opposite central port. The elapsed time was recorded from the time when the displacing fluid first entered the porous medium to the time that it first reached the outlet port. This so-called breakthrough time allows the fractional oil recovery at breakthrough to be calculated exactly from the equation R s Qt br rVp , where Vp denotes the pore volume of the cell, t br the breakthrough time, and Q the volumetric injection flow rate. Photographs of the developing fingering pattern were taken until the
Table 1 Properties of fluids at 25.0 " 0.58C Fluid
Density Žgrcm3 .
Viscosity ŽmPa P s.
Non-acidic paraffin oil 10.0 mM linoleic acid in non-acidic paraffin oil Žacid numbers 0.6 mg KOHrg of oil mixture. Dyed distilled water Dyed 5.0 mM sodium dodecyl sulfate solution Dyed 250.0 mM Ž1.0 wt%. NaOH solution
0.8699 0.8727 0.9971 0.9974 1.0074
132.42 137.23 0.8779 0.8823 0.9016
Note: all aqueous solutions were dyed with 0.025 wt% bromocresol green for visualization purposes.
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Table 2 Fluid systems employed and their interfacial tensions at 25.0 " 0.58C Fluid system
System code
IFT ŽmNrm.
Dyed distilled waterrnon-acidic paraffin oil Dyed 5.0 mM sodium dodecyl sulfate solutionrnon-acidic paraffin oil Dyed 250.0 mM NaOH solutionr10.0 mM linoleic acid in non-acidic paraffin oil solution
A B C
38.1 w52.2x 14.3 w16.0x 0.34 w0.46x Žafter 25 s.
Note: the numbers in brackets wx represent IFT measurements in the absence of dye.
breakthrough condition was reached. Following each experiment, the cell was cleaned using a standardized procedure, during which several pore volumes of propanol-2, water, acetone, and nitrogen were consecutively flushed through the cell. 2.2.2. Experiments with connate water When performing experiments in the presence of connate water, the same basic procedure as that outlined above was employed, but with some slight modifications. In this case, the distilled water used as the connate water phase was first evacuated to remove any dissolved air. After evacuating the cell, this water was drawn slowly into the cell. The oil phase was then injected into the cell to displace the bulk of the water. Since the oil had a much higher viscosity than the water, the displacement was very stable Ži.e., plug flow., resulting in the removal of the bulk of the water. The small Žirreducible. amount of water that remained constituted the connate water phase. Once the oil completely saturated the cell Žapart from the connate water., the cell was then ready for the waterroil displacement process as described in the previous section. In principle, the fractional volume Žsaturation. of the connate water phase can be measured, but in practice saturation is difficult to measure accurately
because of its low value. Approximate measurements involving both the mass and volume of the cell and its contents indicated that the connate water occupied about 5% " 2% of the total pore volume of the cell. It should be noted that the previously discussed equation for calculating the fractional recovery of the oil phase Ž R s Qt br rVp . remains valid provided that the connate water fraction in the displaced oil phase remains unchanged from that in the oil that was originally present in the porous medium, which is a reasonable assumption. 2.2.3. Capillary number and nature of the flow field Since the fluid injection and fluid recovery take place at single points as in real petroleum reservoirs, rather than along straight lines, the displacing fluid streamlines will always be diverging Ži.e., of decreasing velocity. near the inlet and converging Ži.e., of increasing velocity. near the outlet of the porous medium. It is not, therefore, meaningful to calculate the familiar capillary number, Ca s m w Urg Žwhich compares the relative magnitudes of the viscous and interfacial forces., since this definition is traditionally based on a constant linear frontal velocity, U, which does not exist in the present experiments. This is the reason why capillary numbers were not employed in the present work.
Table 3 Experimental data for low injection flow rate Ž Q s 1.10 mlrh. Expt. No.
Fluid system
Connate water?
Breakthrough time, t br Žs.
Breakthrough recovery, R Ž%.
1 2 3 4 5 6
A A B B C C
no yes no yes no yes
4388.1 3542.1 4415.2 5655.3 4132.2 6228.3
6.97 5.63 7.01 8.98 6.56 9.89
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3. Results and discussion
3.1. Low flow rate displacements (Q s 1.10 ml r h)
The effects of connate water were investigated, on System A Žthe non-reacting system without surfactant., on System B Žthe non-reacting system with externally added surfactant., and on System C Žthe reacting system with internally generated surfactant..
Table 3 displays the breakthrough recoveries for this particular flow rate which were quite low Žin the 5%–10% range. because of the high oilrwater viscosity ratio. These results indicate that the presence of connate water leads to a decrease in oil recovery
Fig. 1. Low flow rate Ž Q s 1.10 mlrh. displacement patterns for systems A, B and C in absence and presence of connate water. Ža. System A without connate water Ž R s 6.97%.; Žb. System A with connate water Ž R s 5.63%.; Žc. System B without connate water Ž R s 7.01%.; Žd. System B with connate water Ž R s 8.98%.; Že. System C without connate water Ž R s 6.56%.; Žf. System C with connate water Ž R s 9.89%.. ŽThe fluid injection point is at the left-hand side in all pictures.
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when surfactant is absent ŽSystem A., but to an increase in recovery when surfactant is present ŽSystems B and C.. 3.1.1. System A In the absence of connate water, the non-reacting displacement produced a single, broad finger as shown in Fig. 1Ža.. However, as seen in Fig. 1Žb., the presence of connate water changes the fingering pattern significantly. The finger is now lighter with many twists and tiny extensions. There also exists a large tributary to the main finger. At a low flow rate and a high IFT, such as in this case, capillary forces dominate the displacement. Therefore, when a highviscosity oil is being displaced by a low-viscosity aqueous phase, the displacing fluid tends to enter the smaller pores of the water-wet medium where the capillary pressure is high Žspontaneous imbibition.. Due to imbibition, viscous fingers develop in many directions, including backwards towards the inlet. The connate water may have promoted the backward moving fingers because the displacing phase naturally tends to coalesce with the connate water phase while moving through the cell. 3.1.2. System B The IFT between the oil and the surfactant solution is mid-range and remains constant. In the absence of connate water, the fingering pattern is similar to that for the non-reacting system but with more branches, as shown in Fig. 1Žc.. The slight lowering of the IFT causes an increase in the number of branches because the fluid can create paths through the porous medium with less expenditure of energy ŽPaterson et al., 1984b.. However, in the presence of connate water, the displacing fluid followed a broad
network of paths through the porous medium ŽFig. 1Žd.. because the connate water adds additional perturbations to the interface.
3.1.3. System C The reacting system displayed a much different displacement pattern from those for the two non-reacting systems. Due to the very low IFT and its dynamic Ži.e., time-dependent. nature, the viscous fingering pattern consists of many long, dendritic fingers which diverge radially from the inlet, as shown in Fig. 1Že.. Generally speaking, the reacting displacements tend to exhibit a greater areal sweep in comparison with the two non-reacting systems. However, patches of oil still remain trapped between and around the fingers. It is also noteworthy that these patterns are quite symmetrical about the centerline of the cell, which is indicative of uniform chemical reaction and associated molecular diffusion processes. With connate water present in the cell ŽFig. 1Žf.., the overall viscous fingering pattern is superficially similar to that produced in the absence of connate water ŽFig. 1Že.., although in Fig. 1Žf. the edges of the fingers are slightly more hazy and thicker than those in Fig. 1Že... A possible explanation for the hazy finger tips is that an emulsion is created in the vicinity of the interface as a result of the chemically generated surfactant. The reacting displacements are quite complicated because several inter-related processes are occurring. First, both reactants Žthe acid and the alkali. must diffuse to the interface, and once there, the chemical reactions discussed earlier will take place. In addition, there are adsorption and desorption processes occurring, to and from, the bulk phases and the
Table 4 Experimental data for intermediate injection flow rate Ž Q s 5.50 mlrh. Expt. No.
Fluid system
Connate water?
Breakthrough time, t br Žs.
Breakthrough recovery, R Ž%.
7 8 9 10 11 12
A A B B C C
no yes no yes no yes
1537.9 878.5 1027.4 1389.4 714.4 882.3
12.21 6.98 8.16 11.03 5.67 7.01
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interface. Furthermore, convective currents exist during the displacement as a result of the advancing interface. Finally, if the concentration of one of the reactants is not great enough, or if the diffusion of one of the reactants to the interface is very slow, then the reaction will slow down or even cease, and the IFT will start to increase.
165
3.2. Intermediate flow rate displacements (Q s 5.50 ml r h) The average frontal displacement velocity corresponding to this flow rate is close to that which is typically used in practice during secondary and tertiary oil recovery operations in real crude oil reser-
Fig. 2. Intermediate flow rate Ž Q s 5.50 mlrh. displacement patterns for Systems A, B and C in absence and presence of connate water. Ža. System A without connate water Ž R s 12.21%.; Žb. System A with connate water Ž R s 6.98%.; Žc. System B without connate water Ž R s 8.16%.; Žd. System B with connate water Ž R s 11.03%.; Že. System C without connate water Ž R s 5.67%.; Žf. System C with connate water Ž R s 7.01%.. ŽThe fluid injection point is at the left-hand side in all pictures.
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voirs Ži.e., about 30 cm a day.. The displacement is governed by both capillary and viscous forces, which represents the most stable displacement regime for waterflooding. Table 4 displays the breakthrough recovery results for this particular flow rate. According to these results, the presence of connate water leads to a decrease in oil recovery Ži.e., a decrease in breakthrough time. when surfactant is absent ŽSystem A., but to an increase in oil recovery when surfactant is present ŽSystems B and C.. These observations are the same as for the low flow rate.
observed at the lower flow rate Žcompare Fig. 2Že. with Fig. 1Že... Long, fine fingers extend radially from the inlet, the probable result of IFT gradients caused by the interfacial chemical reaction. In the presence of connate water ŽFig. 2Žf.., the general shape of the displacement pattern is similar to that without connate water ŽFig. 2Že... However, the fingers are more closely spaced, and the generation of emulsion again makes the edges of the fingers look hazy and indistinct when connate water is present. 3.3. High flow rate displacements (Q s 96.0 ml r h)
3.2.1. System A In the absence of connate water, the non-reacting system is characterized by broad fingers, as shown in Fig. 2Ža.. The presence of connate water increases the irregularity of the fingers, as displayed in Fig. 2Žb.. Having connate water in the cell reduces the oil recovery by about 5%. 3.2.2. System B In the absence of connate water, the surfactant solution displacement displays a different fingering pattern from the non-reacting system as may be seen by comparing Fig. 2Žc. with Ža.. The fingers are relatively smooth and straight with a number of small branches extending from the main fingers. In the presence of connate water, the fingers become more dispersive and irregular, and the areal sweep increases Žcompare Fig. 2Žd. with Žb... The number of fingers has increased significantly, presumably due to the lower IFT and to the reduced flow resistance associated with the presence of connate water. 3.2.3. System C In the absence of connate water, the reacting system displays a similar displacement pattern to that
Table 5 displays the breakthrough recoveries for this particular flow rate. According to these results, both the non-reacting system ŽSystem A. and the external surfactant system ŽSystem B. exhibit low recoveries, whereas the in situ generated surfactant system ŽSystem C. exhibits a very high recovery. The presence of connate water leads to an increase in oil recovery in the absence of chemical reaction ŽSystems A and B., but to a decrease in oil recovery in the presence of chemical reaction ŽSystem C., which is significantly different behavior from those observed at the low and intermediate flow rates. The highest flow rate exhibits the most significant differences in breakthrough recovery amongst the three different displacement systems since viscous forces are so dominant that other effects are small or insignificant. 3.3.1. System A In the absence of connate water, the non-reacting system exhibits the expected viscous fingering pattern, as shown in Fig. 3Ža.. The increased viscous forces lead to an increase in the number of finger branches and result in the displacing fluid being
Table 5 Experimental data for high injection flow rate Ž Q s 96.0 mlrh. Expt. No.
Fluid system
Connate water?
Breakthrough time, t br Žs.
Breakthrough recovery, R Ž%.
13 14 15 16 17 18
A A B B C C
no yes no yes no yes
35.0 42.0 39.7 46.4 150.4 112.3
4.85 5.82 5.50 6.43 20.85 15.56
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forced directly towards the exit port, thereby displacing little oil. In this high flow rate region, the displacement pattern is quite uniform and predictable, in contrast to that in the low flow rate region. From Table 5, it is observed that the recovery is low both in the absence and presence of connate water. Fig. 3Žb. shows that rather than proceeding
167
directly towards the outlet, some of the displacing fluid branches off in various directions. This is partly due to the effects of mixing between the displacing aqueous phase and the connate water. It is apparent that in the absence of connate water ŽFig. 3Ža.. the fingers are relatively smooth at the tips, but with connate water present ŽFig. 3Žb.., the fingers are
Fig. 3. High flow rate Ž Q s 96.0 mlrh. displacement patterns for Systems A, B and C in absence and presence of connate water. Ža. System A without connate water Ž R s 4.85%.; Žb. System A with connate water Ž R s 5.82%.; Žc. System B without connate water Ž R s 5.50%.; Žd. System B with connate water Ž R s 6.43%.; Že. System C without connate water Ž R s 20.85%.; Žf. System C with connate water Ž R s 15.56%.. ŽThe fluid injection point is at the left-hand side in all pictures.
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slender and follow more circuitous routes through the porous medium. 3.3.2. System B In the absence of connate water ŽFig. 3Žc.., the surfactant solution displacement exhibits a symmetric diverging–converging pattern with a low recovery. The presence of connate water has relatively little effect on the displacement other than slightly decreasing the finger width ŽFig. 3Žd... At high flow rates, the viscous forces are so dominant that the chemical dilution effects caused by the connate water are largely overshadowed. 3.3.3. System C For the reacting system, the displacement patterns appear to be somewhat similar to those encountered in miscible displacement processes ŽGuo and Neale, 1996., thus indicating that the IFT has been reduced to very low values by virtue of chemical reaction that occurs throughout the entire displacement. Although there are patches of oil left unrecovered ŽFig. 3Že. and Žf.., thick macroscopic fingers comprised of many tiny fingers result in an efficient displacement of oil. The percentage of oil recovered is much higher than for any of the other systems and flow rates. With connate water present ŽFig. 3Žf.., the recovery was lower because the displacement pattern exhibited slightly lower areal sweep efficiency. 3.4. Comments and recommendations The work reported here, albeit preliminary in nature, reveals that the presence of connate water, by virtue of the inevitable chemical dilution effects that it causes, can significantly modify fingering patterns, areal sweep efficiencies and fractional oil recovery efficiencies, during the displacement of oil by aqueous solutions in a porous medium. This work highlights the major importance of including connate water whenever simulating practical secondary and tertiary crude oil displacement processes in the laboratory. In addition, it indicates that the results of laboratory studies that have been conducted in the absence of connate water must be interpreted cautiously when extrapolating their conclusions to real petroleum reservoir field conditions.
It should be stressed that the present preliminary study was conducted using a water-wet porous medium and distilled water as the connate water phase. To make this study more relevant in the context of laboratory simulations of real oil reservoir displacement processes, the following additional factors should be investigated: Ø the effects of porous medium wettability Ži.e., oil-wet vs. water-wet.. Ø the effects of connate water composition, because real connate waters contain various ionic species, some of which are known to adversely affect the IFT-lowering capabilities of surfactants. Ø the effects of acid concentration and alkali concentration in the case of reacting systems because chemical additives will always be diluted when coming into contact with connate water. Ø the effects of connate water on oil recovery after the breakthrough condition has been reached.
4. List of symbols Ca IFT Q R t br U Vp g mw
capillary number Žs m w Urg . interfacial tension volumetric injection flow rate fractional recovery of oil at breakthrough breakthrough time characteristic velocity of displacement front pore volume of porous medium oilrwater interfacial tension viscosity of aqueous phase
Acknowledgements The financial support for this work provided by the Natural Sciences and Engineering Research Council of Canada ŽNSERC. is gratefully acknowledged.
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