Energy &Fuels 1994,8, 1217-1222
1217
Profile Modification Due to Polymer Adsorption in Reservoir Rocks Liaqat Mi* and Maria A. Barrufet Department of Petroleum Engineering, Kuwait University, 13060 Safat, Kuwait Received February 2, 1994. Revised Manuscript Received August 15, 1994@
Flow behavior of a newly developed starch-based biopolymer is studied in the laboratory to gain insight into its ability and effectiveness to recover additional oil. This study analyzes permeability modification due to polymer adsorption in the porous media. Experiments are performed to determine the permeability reduction and the effects of polymer adsorption on capillary pressure curves. A model is developed to estimate the amount of polymer adsorbed in the porous media. The difference in pore size distribution before and after polymer flow indicates the effectivenessof polymer to modify the permeability profile. The differencein capillary pressure curves before and after polymer flow indicates the extent of polymer adsorption in the porous media. It is observed in this study that permeability reduction is a function of both initial permeability and polymer concentration. This study shows that starch-based biopolymers are not only cost effective and environmentally safe but also are very effective for profile modification. Therefore, these polymers can be used to recover additional oil where channeling and bypassing of oil by water are the main problems.
Introduction The most extensively studied and field-tested polymers are poly(acry1amide)s and poly(sa~charide)s.l-~ However, economical and environmental concerns make their use unattractive. The biodegradable starch becomes very attractive when compared to the nondegradable synthetic polymers consideringthe recent environmental regulatory laws, such as the Resource Conservation and Recovery Act (RCRA). The main degradation products of starch include C02 and HzO which are harmless to the environment. Moreover, the processed food grade starch costs 60-65QAb4which is still lower than most of the commercial polymers ($1-3Abh5 In a previous study we proved that starch-based biopolymers are excellent enhanced oil recovery (EOR)agents.6 Polymer flooding has proved to be a promising method of oil recovery. Conventional secondary recovery methods sweep oil from high permeability zones, bypassing substantial amounts of oil trapped in the lower permeability zones. Effective techniques and inexpensive materials to modify the permeability scenarios are essential for improving the volumetric sweep efficiency and lowering the operating cost. Polymer flooding offers a significant economic potential over conventional water flooding techniques because it improves the efficiency of oil recovery by two principal means: (1)by decreasing the mobility of water, and (2) by selective modification of the permeability t o water. Abstract published in Advance ACS Abstracts, September 15,1994. (1) Zaitoun, A.; Potie, B. Proc.: Int. Symp. Oilfield Geothermal Chem., Denver, CO, June 1-3,1983,SPE 11785. (2)Davidson, P.; Mentzer, E. Proc.: 55th Fall ConL Dallas, l X 1980, SPE 9300. (3)Ryles, R. G. Proc.: Int. Symp. Oilfield Geothermal Chem. Phoenix, Az,April 9-11, 1986,SPE 13585. (4)A. E. Staley Manufacturing Co., Decatur, IL. Personal communication, 1993. (5)Taber J. J. I n Situ 1990,14 (4),345-405. (6) Barrufet, M. A.; Perez, J. M.; Mandava, S. S.; Ali, L.; Poston, S. W. Proc.: 67th Annu. Fall Tech. Conf. Exhibition, Washington, DC, Oct. 4-7, 1992,SPE 24809.
Polymer is adsorbed as it flows through the porous rock. Polymer adsorption results in the reduction of pore sizes which leads to an increased resistance t o the flow of water. This property of polymer adsorption is generally desired where channeling and bypassing of oil are the main problems. However, polymer adsorption may represent a substantial loss in concentration and viscosity of the polymer which is detrimental to the success of a polymer project. Higher cost of polymer treatment may offset the beneficial properties of a polymer. The environmental compatibility and the excellent properties of starch-based biopolymers have led us to make a thorough evaluation of their ability to enhance oil recovery. This study is mainly focused on polymer adsorption. Polymer adsorption is generally evaluated by displacement experiments. We attempted in this study to evaluate polymer adsorption qualitatively from centrifuge data. A resistance to water flow, which is defined by a residual resistance factor, F, (also called permeability reduction), persists after the polymer solution has been displaced by water from the porous media. The permeability reduction is a measure of the degree of polymer adsorption in the porous media. Permeability reduction can be determined from the following expression if the injection rate is kept constant.
Ww(after polymer flow) F, = A ?', (before polymer flow)
(1)
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0887-0624/94/2508-1217$04.50/0
If there is reduction in permeability by the flow of polymer solution through the porous media, the value of the residual resistance factor will always be greater than unity. The residual resistance factor (permeability reduction) is related to the adsorbed layer thickness as
F, = (1 -
)e;
-4
The amount of polymer adsorbed per unit mass of the
0 1994 American Chemical Society
Ali and Barrufet
1218 Energy & Fuels, Vol. 8, No. 6, 1994 rock (pglg) can be computed as
400
-5 I
300
u
100
n
where is the porosity, epis the density of the polymer, emis the grain density of the rock, and L is the length of the core. This equation was derived from Poiseuille's equation assuming that t h e porous media is composed of a series of capillaries whose cross sections have been reduced by a uniform layer of adsorbed polymer. The complete derivation may be found e l ~ e w h e r e . ~
-t 3
200
0
00
Experimental Section Starch-based biopolymer solutions are prepared by dissolving the corresponding weight percent of powdered starch in distilled water and heated to 95 "C (203 OF) while stirring continuously. Starch polymerizes at 95 "C (203 OF). The polymer solutions are allowed t o cool t o ambient conditions and used subsequently in the displacement experiments. Two types of outcrop sandstones were used in this study: Elgin and Okesa. All core samples were cut parallel to the bedding plane from the outcrop rock samples with a core bit using water as the cutting fluid. The freshly-cut core plugs were washed and dried in an oven at 150 "C (302 "F)for 24 h and the dried samples were weighed. The core plugs used for capillary pressure measurements were 1 in. in length and 1 in. in diameter. The core plugs used in displacement studies were 3 in. long and 1in. in diameter. The core samples were vacuum saturated with degassed brine and the porosities were determined by gravimetric method. A Model L8-6OMP ultracentrifuge was used to obtain air-brine displacement data. The core flooding apparatus consisted of a HPLC pump, a core holder and a fraction collector. Since the relative permeability t o oil is nearly unaffected by the flow of polymer,8all experiments were conducted at zero residual oil saturation.
Results and Discussion Effect of Polymer Treatment on Capillary Pressure Curves. As polymer flows through the porous rock, it adsorbs onto the surfaces of the pores thereby reducing their size. This adsorption reduces water flow by as much as 60-95%9 and increases the irreducible water saturation.1° For a polymer concentration of 1 wt% the irreducible water saturation after the polymer treatment increased from 6.5 to 14.9%in Elgin sandstone (Elgin SS), whereas the irreducible water saturation increased from 9.5 to 26% in Okesa sandstone (Okesa SS) as shown in Figure 1. This increase in irreducible water saturation has been attributed to the hydration water from the polymer and to some increase in inaccessible pore volume available to the nonwetting phase due to the presence of adsorbed polymer in smaller pores.1° Since Okesa SS contains twice as much clays as Elgin SS7 and clays have high water retaining capacity due to microporosity, these observations could also be attributed to the amount of clays present in these rocks. Two experiments with different polymer concentrations were conducted with Okesa SS to observe the corresponding effect on the capillary pressure curves. (7) Ali, L. Ph.D. dissertation, Department of Petroleum Engineering, Texas A&M University, College Station, TX,1993. ( 8 ) Zaitoun, A.; Kohler, N. Proc.: 4th Eur. Symp. EOR,Hamburg, October 27-29, 1987, 839. (9) White, J. L.; Goddard, J. E.; Philips, H. M. J.Pet. Technol. 1973, February, 143-150. (10)Zaitoun, A.; Kohler, N. Proc.: 63rd Annu. Tech. Conf. Erhibition, Houston, TX, Oct. 2-5, 1988,SPE 18085.
02
04 06 Water Saturation (Fraction)
08
I O
Figure 1. Effect of polymer treatment on capillary pressure curves in Elgin SS (E-56)and Okesa SS (0-23)with a polymer concentration of 1.0%.
0 00
02
04
06
0.8
10
Water Saturation (fracuon)
Figure 2. Effect of polymer concentration on capillary pressure curve in Okesa SS (0-24)with a polymer concentration of 0.5%.
Y
."P 50
0 0.0
0.2
0.4 0.6 Water Saturation (fraction)
0.8
I .0
Figure 3. Effect of polymer concentration on capillary pressure curve in Okesa SS (0-25) with a polymer concentration of 0.2%. Figure 2 shows that the irreducible water saturation increased from 7.3 to 14.2%for a polymer concentration of 0.5 wt %. Figure 3 shows that irreducible water saturation increased from 9.1 to 13.2%for a polymer concentration of 0.2 wt%. It can be concluded from these observations that higher polymer concentrations cause more polymer adsorption in the porous media and hence increase the irreducible water saturation. These results agree with the results presented by Zaitoun and Kohler.lo Though the increase in the irreducible water saturation is not significant, the permeability was reduced by about 5 times of the initial permeability for a polymer concentration of 0.5%and this reduction was 3 times of the initial permeability for a 0.2%polymer concentration as reported in Table 1 which will be discussed later.
Polymer Adsorption in Reservoir Rocks
Energy & Fuels, Vol. 8, No. 6, 1994 1219
I .o 0.8
0.2 0.0'
IO.'
.
.
- - - * . . I
.
-
'
IO"
-
-
10:
IO2
Pore Entrance Diameter (micron)
Figure 4. Effect of polymer treatment on pore entrance size distributions for Elgin SS (E-56) and Okesa SS (0-23)with a polymer concentration of 1.0%. Figure 6. ESEM image of the profile modification due to polymer adsorption of Elgin SS. The sample was fractured parallel to the bedding plane.
Figure 5. ESEM image of the profile modification due to polymer adsorption of Elgin SS. The sample was fractured perpendicular to the bedding plane.
Effect of Polymer Treatment on Pore Size Distributions. Figure 4 shows how the polymer treatment changes the pore entrance size distributions. These changes indicate the degree of effectiveness of the polymer in modifjing the permeability profile in Elgin SS and Okesa SS. The frequency of the occurrence of pores in the interval rz-rl may be expressed as flr2-5) =
Jr2a(r> dr rl Jrm"a(r) dr rmin
(4)
where p c i @INi
a(r)= --
ri f l c i and a(r) is the pore size distribution function. The subscript i indicates Pc, S,, and r for various levels of desaturation in the centrifuge experiment. If r l = rmin and r2 = rmax, then f l r m a - r m i n ) = 1. Figure 4 indicates that the adsorbed polymer reduces the maximum pore entry diameters from 90 to 55 pm which results in an increase in cumulative frequency of pore entry diameters in the range of 1-50 pm for Elgin SS. This pore size reduction can be clearly seen in the ESEM photomicrographs (Figures 5 and 6). The coating of the pore walls, in addition to reducing the pore size, makes the pores relatively smoother which
is clearly seen in Figure 5. The swelling of this coating film will further reduce the pore size as describe in the later section. Figure 6 shows the cross section of the pores as they intersect in the vertical as well horizontal direction. In the middle of the photomicrograph an intersection of three pores is shown. The cross section of two pores parallel to the flow direction shows that the pore walls are relatively smoothly coated with the adsorbed polymer in the form of a film which is intersected and ruptured by a third pore in the vertical direction due to the flow of brine after the polymer treatment. This rupturing of film is also evident in other places in the same photomicrograph. It can be inferred from these observations that the overall reduction of larger pore openings and correspondingincrease in the cumulative frequency of middle range pore openings will translate into a better volumetric sweep efficiency. It can also be observed that pore entry diameters 10.11pm are completely blocked. A maximum of 20% decrease in cumulative frequency of these pore entry diameters is observed. This blocking results in a decrease of cumulative frequency of pore entry diameters in the range -0.15-0.5 pm. We believe that the blocking of these small pores will not affect the sweep efficiency significantly because the small pores in a water-wet rock are occupied by water. However, this kind of blocking will result in an increase in irreducible water saturation as indicated by the capillary pressure shown in Figure 1. Similar results have been obtained with other samples of Elgin SS.7 On the other hand, for Okesa SS pore entry diameters 222 pm and pore entry diameters 50.4 pm have been completely blocked. This observation can be easily verified from the ESEM images (Figures 7 and 8). Figure 7 shows a cross section of a pore which was formerly plugged with the polymer. Af'ter the polymer treatment, the brine appears to have drilled a hole through the plug. Also the coating of the pore walls is not as smooth as in Elgin SS. In the middle of the photomicrograph (Figure 8) two pores are shown intersecting each other. The pore plug is shown to be ruptured by the flow of brine and some pores are still partially blocked. This blocking of pore throats resulted in an overall decrease of cumulative frequency of pore
1220 Energy & Fuels, Vol. 8,No. 6,1994
Ali and Barrufet Table 1. Petrophysical Properties of Reservoir RocksQ
E-41 E-56 E-65 E-66 E-67 0-23 E-42 E-47 0-24 E-38 0-25
2029 1650 2080 2090 2405 1048 1551 986 1168 858 1414
0.259 0.234 0.258 0.240 0.256 0.267 0.250 0.264 0.244 0.256 0.241
1.0 1.0 1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.2 0.2
18.33 19.88 19.26 17.74 17.68 30.80 3.13 8.48 4.81 1.53 3.00
1.0452 1.0452 1.0452 1.0452 1.0452 1.0452 1.0336 1.0336 1.0336 1.0256 1.0256
2.516 2.521 2.456 2.437 2.440 2.695 2.474 2.579 2.524 2.544 2.482
2.497 2.578 2.642 2.642 2.624 2.540 2.543 2.535 2.507 2.544 2.616
35.22 29.74 33.63 33.40 36.32 25.16 17.15 19.96 18.22 5.32 14.81
a Permeabilityreduction and polymer adscorption data computed from eqs 1 and 2, respectively.
Figure 7. ESEM image of the profile modification due to polymer adsorption of Okesa SS. The sample was fractured perpendicular to the bedding plane.
I
lp
1u
10’
IO*
IO’
lo4 K v lo5
lo6
10’
loR
0
Figure 9. Effect of initial permeability on permeability reduction.
entry diameters in the range -1-10 pm. The displacement of oil by water and polymer in these rocks is currently being studied in the laboratory to explain how these results would effect the overall sweep efficiency. However, relative permeability studies show that in addition to reduction in water relative permeability in Okesa SS relative permeability to .thenonwetting phase has been found to increase at a certain polymer concentration.7 Effect of Initial Permeability on Permeability Reduction. The residual resistance factor, F,, gives a measure of permeability reduction in the reservoir rocks. Several researchers have noted the effect of initial permeability on the permeability reduction produced by flow of HPAMliJ2 and biopolymers.6 The porous media in these studies were represented by several different kinds of synthetic porous media as well as reservoir sandstones. Table 1shows the values of permeability reduction. The permeabilityreduction ranges from 3 to 30 in Okesa SS whereas it varies from 1.5 to 20 in Elgin SS. These
observations indicate that having roughly the same initial permeability and porosity, the permeability reduction in Okesa SS is almost twice as much as in Elgin SS. It was also observed that the increase in irreducible water saturation due to polymer treatment is approximately double in Okesa SS compared to Elgin SS (Figure 1). This fluid-rock interaction could be the result of the amount and distribution of clays because Okesa SS contains twice as much clay as Elgin SS.7 Since the clays have more surface area they provide potential surfaces for the adsorption of polymers. Moreover, as the clay content of a rock increases the permeability decreases and polymer adsorption increases.l3 Table 1also shows that the permeability reduction is a function of both initial permeability and polymer concentration. Figure 9 presents permeability reduction data for starch-based biopolymers along with data taken from the literature.1°J3 The porous media used for this study are Elgin and Okesa sandstones. On the x-axis K,& is used instead of initial permeabilitybecause it represents the approximate “effective pore It is clear from this figure that, like WAM, the residual resistance due to the adsorbed layer of polymer increases as the permeability decreases. Also, an increase in polymer concentration results in an increase of residual resistance to the flow of water. As the polymer concentration increases, the shape of the curve does not change and the curves for different concentrations are parallel to each other. The lines have been drawn only to show a comparative trend.
(11)Smith,F. W. J. Pet. Technol. 1970, February, 148-156. (12)Zaitoun, A.; Kohler, N. Proc.: Int. Symp. Oilfield Chem., San Antonio, m, Feb. 4-6,1987, SPEDOE 21000.
(13)Jennings, R. R.; Rogers, J. H.; West, T. J. J. Pet. Technol. 1974, March, 391-401. (14)Wyllie, M. R. J.; Gardner, G. H. F. Word Oil 1968,March, 121.
I
i
i
Figure 8. ESEM image of the profile modification due to polymer adsorption of Okesa SS. The sample was fractured parallel to the bedding plane.
Energy & Fuels, Vol. 8, No. 6,1994 1221
Polymer Adsorption in Reservoir Rocks 102L
Elgin-I7
.-.su.
.e
* Relative Viscosity
0 0.5% HPAM (Ref. 15) A 0.1% Polysaccharide(Ref. 10) 0 0.25% PAM (Ref. 10) 0.05% PAM (Ref. 10)
e
2
.-5
0
101:
E
I
3
g
0
e CY
10'
a" A 100
-
=
-
D
. .
.
.
*
*
IO.*
10"
10''
10'
IO2
IO3
Shear Rate (sec")
Figure 12. Permeability reduction due to the effect of shear rate on adsorbed polymer and effect of shear rate on relative viscosity for a polymer concentration of 1.0%. h
12-
g
Thickness from tfisplacetnent data
10-
0
200
400
m
800
Pore Volume Injected
Figure 11. Effect of the swelling of hydrophilic film of polymer on permeability reduction for a polymer concentration of 1.0%.
It was assumed that all the polymers should fall on the same curve if the polymer properties are to be compared with each other. To normalize the different polymers for presentation on the same curve, K&p was divided by the corresponding polymer concentration. Figure 10 shows that the permeability reduction vs K d &p for all the polymers fall approximatelyon the same curve. Some deviations have been observed and these can be attributed to the uncertainties involved in using different porous media and different polymers. From these observations, it can be concluded that starchbased biopolymers are as effective as other polymers. Effect of Hydrophilic Film Swelling on Permeability Reduction. Figure 11 shows the effect of polymer film swelling on permeability reduction. Permeability reduction increases with increasing pore volume of water injected. The increase in permeability reduction for synthetic core is more pronounced than for Elgin SS core at equivalent pore volume injected. This increase could be attributed to the greater surface area available for polymer adsorption. This marked reduction in permeability due to the swelling of the polymer film indicates that starch-based biopolymers can be used to control water production. Since nearwellbore treatments do not require large amount of polymer, the economics of using starch for this purpose would be very favorable. Figure 12 shows the effect of shear rate on permeability reduction due to adsorbed polymer. Also shown in this figure is the relative viscosity us shear rate from viscometric measurements. Permeability reduction has been measured in these tests in the absence of residual oil saturation. Both sandstones show that permeability
F.-56
E-65
F-66
i-67
t-38
)-/I
' -)4
'
15
Figure 13. Comparison of adsorbed layer thickness from centrifuge and displacement data.
reduction is less pronounced at higher shear rates. In other words, permeability reduction is higher at lower shear rates. Permeability reduction in Okesa SS is greater a t equivalent shear rates when compared with that of Elgin SS. The decrease in viscosity (and thus the resistance to flow) with shear rate, commonly referred to as shear-thinning,is exhibited by many nonNewtonian polymer solutions. The shear-thinningfluid if described by power law model has a flow index less than 1. The decrease in relative viscosity with increasing shear rate is far greater than the decrease in permeability reduction at equivalentshear rates in both sandstones. Relative viscosity decreases rapidly as shear rate increases. Comparison of Adsorbed Layer Thickness from Centrifuge and PermeabilityReduction Data. The adsorbed layer thickness from both capillary pressure measurements and displacement data are compared in Figure 13. The average adsorbed layer thickness is the difference between the average pore entry radii before and after polymer flood when capillary pressure data are used. The average pore entry radius from pore size distribution is computed as
F= .
Jrm"ra(r) dr rmin
(6)
Jrmma(r) rmm dr
On the other hand, the adsorbed layer thickness from displacement data is computed as
+]
e = [yv2[1 -
(7)
The adsorbed layer thickness values determined from capillary pressure measurements are comparable for
1222 Energy & Fuels, Vol. 8, No. 6, 1994
Okesa SS whereas the difference in the two thicknesses is greater for Elgin SS. This discrepancy may be attributed to multilayer polymer adsorption as pointed by Zaitoun et a1.15 Photomicrographs of Elgin SS show that polymer adsorption may be occurring in multilayers (Figures 5 and 6). The differences in adsorbed layer thickness could also be attributed to the shear-thinning effects associated with the hydrodynamic measurements. The amount of polymer adsorbed computed from the model developed in this study (eq 2) ranges from about 5 to 36 pglg for starch biopolymer as shown in Table 1 which is substantially lower than the values reported for poly(saccharide)s and poly(acry1amide)s in the literature.1° Low adsorption values would mean lower overall cost. The model assumes that the porous medium is composed of a bundle of capillaries whose cross sections have been reduced by the adsorbed polymer.
Conclusions The results of this experimental study lead us to make several conclusions. The difference between capillary (15)Zaitoun, A.; Kohler, N. In Situ 1990,14 (2), 133-146.
Ali and Barrufet pressure curves before and after polymer flooding indicates clearly the degree of polymer adsorption in the reservoir rocks. The polymer adsorption increases the irreducible water saturation. This effect is enhanced as the polymer concentration is increased. The changes in pore entrance size distributions reflect the degree of effectiveness of polymer in modifymg the permeability profile. It can be surmised that Okesa sandstone will have a better volumetric sweep efficiency than Elgin sandstone. The permeability reduction is not only a function of initial permeability but also a function of polymer concentration. The permeability reduction increases as the initial permeability is decreased. An increase in polymer concentration also increases the permeability reduction. Starch biopolymer adsorption is substantially lower than the values reported for poly(saccharide)s and poly(acry1amide)s in the literature. Low adsorption values would mean lower operating or processing costs.
Acknowledgment. We thank Dr. Jorge Perez and Shanthi Mandava for their help in the experimental part of this work. This work was supported by US. DOE under Contract No. DE-GFG07-89BC14446 and the Center of Energy and Mineral Resources at Texas A&M University.
