Energy & Fuels 1989,3, 299-303
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Chemical and Wetting Interactions between Berea Sandstone and Acidic, Basic, and Neutral Crude Oil Components Joseph P. Smith,*p+Manuel A. Francisco,$ and Pamela J. Houserf Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77001, and Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received April 12, 1988. Revised Manuscript Received January 9, 1989 Rock wettability has an important influence on multiphase fluid flow behavior in petroleum reservoirs. Although the adsorption of crude oil components by rock surfaces is believed to be a controlling factor in wettability, no satisfactory correlation currently exists between crude oil composition and wetting tendency. We have investigated the chemical and wetting interactions between Berea sandstone and acidic, basic, and neutral crude oil components isolated by ion-exchange chromatography. Acidic and basic crude oil components greatly reduced the rate of spontaneous brine imbibition by Berea sandstone. This indicated that these components had a strong effect on rock wettability. Neutral crude oil components had a minimal effect on rock wettability. Spectrophotometric measurements showed that crude oil and its acidic and basic components contained aromatic species that were adsorbed by Berea sandstone in the presence of a brine phase. Aromatic species in the neutral crude oil component were not adsorbed under these conditions. Both acidic and basic crude oil components altered wettability, indicating that Berea sandstone does not exhibit an exclusively acidic or basic character with respect to its interactions with crude oil. Both acidic and basic crude oil components must be considered when predicting wetting tendencies in sandstones.
Introduction Rock wettability has an important influence on observables such as capillary pressure and relative permeability that are used to describe the properties of multiphase fluid systems in porous media. Suspected alteration of wettability during recovery of reservoir rock samples introduces uncertainty into the results of laboratory core analyses. A substantial effort has consequently been expended in the development of techniques to preserve native wettability during coring operations or to restore native wettability after a1teration.l The factors that control wettability in the petroleum reservoir environment are not fully understood. Since laboratory experiments show that crude oils differ significantly in their tendency to wet Berea sandstone2 or smooth mineral surfaces? it is usually assumed that crude oil composition is an important determinant of wettability. There is, however, no satisfactory correlation between crude oil composition and wetting behavior. Our approach to the study of the chemical basis of wettability is predicated on the hypothesis that acid-base interactions play an important role in rock-wetting phenomena. Simple organic acids and bases can alter the wettability of mineral surfaces.H Since organic acids and bases are found in crude oils, research has naturally focused on the involvement of these species in wetting7s8and other interfacial p h e n ~ m e n a . ~ We have investigated the role played by petroleum acids and bases in wettability by integrating ion-exchange chromatographic techniques for resolving crude oil into acidic, basic, and neutral components; spectrophotometric measurements for detecting retention of oil components by rocks; and spontaneous brine imbibition measurements for assessing rock wettability. Spontaneous brine imbibition is the process whereby a rock containing oil and
* To whom correspondence should be addressed. 'Exxon Production Research Co. Exxon Research and Engineering Co.
*
0887-0624189 12503-0299$01.50 I O
brine spontaneously takes up brine and expels oil. Strongly water-wet rocks imbibe brine rapidly. Reduced imbibition rates indicate an alteration of wettability. By this approach, we separated crude oil into fractions containing acidic, basic, and neutral species and then examined both the interactions of these species with the rock surface and their effect on the fluid flow properties of the rock.
Experimental Section Reagents. Cyclohexane, toluene, and methanol were HPLCgrade solvents. Reagent grade isopropylamine and glacial acetic acid were used as received. Tetrahydrofuran was freshly distilled from lithium aluminum hydride before use. Acidic (Amberlyst 15 (Rohm and Haas)) and basic (Amberlite IRA-904 ( R o b and Haas)) ion-exchange resins were extracted with methanol in a Soxhlet apparatus for 8 h and then dried before use. Characterization of Crude Oil. The crude oil sample used for this work was characterized by measurement of its density, viscosity, nitrogen content, total acid number, and total base number. The asphaltene content was determined by n-pentane precipitation. The saturate, aromatic, and nitrogen-sulfuroxygen-containing component contents were estimated by adsorption chromatography. Preparation of Crude Oil Components. Whole crude oil (100.4 g) was heated at 90 "C under vacuum (5 mmHg) to produce a stabilized crude oil (86.94 g) that lost no further weight under (1) Wendel, D. J.; Anderson, W. G.; Myers, J. D. Presented at the 60th Annual Conference and TechnicalExhibition of the Societv of Petroleum Engineers, Las Vegas, NV, September 1985; Paper 14298. (2) McGhee. J. W.: Crocker. M. E.: Donaldson. E. C. Relative Wettine Properties of Crude Oils in Berea Sandstone. U S . Dep. Energy, R e i Invest. 1979, No. BETCIRI-78/79. (3) Trieber, L. E.; Archer, D. L.; Owens, W. W. SOC.Pet. Eng. J . 1972, 12, 531-540. (4) McCaffery, F:G.; Mungan, N. J. Can. Pet. Technol. 1970, 9(JulySept), 185-196. (5) Mungan, N. SOC. Pet. Eng. J. 1964, 4 , 115-123.
(6)Morrow, N. R.; Cram, P. J.; McCaffery, F. G. SOC. Petr. Eng. J.
1973,13,221-232.
(7)Benner, F. C.; Bartell, F. E. In Drilling and Production Practice, American Petroleum Institute: New York, 1941; pp 341-348. (8) Denekas, M. 0.; Mattax, C. C.; Davis, G. T. Trans. Am. Inst. Min., Metall. Pet. Eng. 1959,216, 330-333. (9) Seifert, W. K.; Howells, W. G. Anal. Chem. 1969, 41, 554-562.
0 1989 American Chemical Society
Smith et al.
300 Energy & Fuels, Vol. 3, No. 3, 1989 these conditions. The basic crude oil component was isolated from stabilized crude oil by ion-exchange chromatography. Acidic ion-exchangeresin (358 g) was slurried in water and packed into a glass column (4 cm i.d. X 60 cm). The resin was activated by sequential elution with 1 N aqueous NaOH (3 L), water (1 L), 1 N HCl(3 L), and water (1 L). A flow rate of ca. 180 mL/h was used for all elutions. This sequence was repeated once and then followed by elution with 25 vol % methanol/water (0.3 L), methanol (0.7 L), toluene (1 L), and cyclohexane (1L) to change from aqueous to nonaqueous conditions. Stabilized crude oil (30.00 g) was dissolved in cyclohexane (175 mL) and passed through the activated resin. Nonbases were eluted with cyclohexane (ca. 3 L) until the eluates were clear and relatively colorless (containing less than 0.1 g/500 mL). Cyclohexane eluates were combined. Bases were eluted with isopropylamine/methanol/ tetrahydrofuran (10/10/80 vol %) until eluates were clear. Evaporation of eluates on a rotary evaporator followed by drying in a vacuum oven at 90 "C for 16-24 h yielded 2.52 g of basic component and 27.28 g of nonbasic component. The acidic crude oil component was isolated from the nonbasic component. Basic ion-exchange resin (587 g) was slurried in water and packed into a glass column (4.8 cm i.d. X 130 cm). The resin was activated by sequential elution with 1N aqueous HCl(6 L), water (3 L), 1 N aqueous NaOH (6 L), and water (3 L). This sequence was repeated once and then followed by elution with 25 vol % methanol/water (0.6 L), 50 vol % methanol/water (0.6 L), 75 vol % methanol/water (0.6 L), methanol (1.2 L), toluene (5 L), and cyclohexane (5 L) to change from aqueous to nonaqueous conditions. Nonbasic component (27.28 g) was dissolved in cyclohexane (150 mL) and passed through the activated resin. Neutral components were eluted with cyclohexane (ca. 4 L) until eluates were colorless. Acids were eluted with glacial acetic acid/methanol/tetrahydrofuran(10/10/80 vol 90)until eluates were colorless (ca.4 L). Evaporation and drying of eluates yielded 2.26 g of acidic crude oil component and 24.81 g of neutral crude oil component. Rock Samples. Berea sandstone, in the form of 3.8-cm-diameter by 61-cm-longcylinders, was purchased from the Cleveland Quarry Co., Amherst, OH. The cylinders were cut into 7.6-cm-long samples by using fresh water as a cutting fluid. X-ray diffraction and elemental analyses showed that the rock contained,by weight, 89% quartz, 3% potassium feldspar, 3% kaolinite, 2% ankerite, and 2% illite. Traces (less than 1%) of plagioclase, smectite, and chlorite were detected. The BET surface area of the rock, determined by nitrogen adsorption, was 0.6 m2/g. Rock samples were cleaned of any oily contaminants by overnight extraction with methanol/toluene (50/50 ~ 0 1 %in ) a Dean-Stark apparatus. Solvents were removed by evaporation first in air and then in a vacuum oven at 160 "C for 6 h. The rock samples were then made strongly water wet by heating in air at 400 "C for 6 h. The dry weights of the rock samples at this point were used as a basis for the calculation of brine saturations. Wettability Tests. A rock sample was mounted in a rubber-sleeve core holder, evacuated, and saturated with a brine containing, by weight, 2.3% NaCl and 0.3% CaC1,. The pH of brine effluent from Berea samples used in this work ranged from 6.4 to 7.0. The permeability to brine was measured, and a low brine saturation was established by centrifugation of the rock sample in air at 2000 rpm for 24 h. The rock sample was weighed to determine the brine saturation and then sealed in a core holder equipped with a special slotted end piece that allowed brine to be pumped across one face of the rock sample during imbibition measurements. The air in the rock sample was displaced with 2 pore volumes (PV) of water-saturated ethane flowing at 4 cm3/min. Solutions of stabilized crude oil or of isolated crude oil components were prepared at 1% (by weight) of their respective concentrationsin the whole crude oil. In this way, dilute solutions of components were used, but the relative concentrations of components were the same as in the whole crude oil. This technique permitted the use of an absorbance detector to monitor effluent concentration and allowed comparison of the relative effect of different components on rock wettability. Pure toluene or a toluene solution of crude oil components was injected into the rock sample at 1 cm3/min against a back pressure of 15-20 psig. The effluent from the rock sample was passed through an absorbance monitor set at a wavelength of 350 nm. After injection
Table I. Crude Oil Chemical and Physical ProDerties 0.894 density, g/cm3 (22.7 " C ) viscosity, cSt (22.7 " C ) 35.8 0.07 total acid no., mg of KOH/g 0.56 total base no. mg of KOH/g 2040 nitrogen content, ppm CI6+ .. fraction compn, wt % asphaltenes 7.6 paraffins-naphthenes 39.9 aromatics 37.5 nitrogen-sulfur-oxygen (NSO) compounds 15.0
I
300
-',
I
I
400
500
-.-i 600
../.L.\
700
Wavelength(nm) Figure 1. Absorption spectra of toluene solutions of the basic component (solid line), the acidic component (dashed line), and the neutral component (dash-dot line). of the toluene phase, rock samples were incubated at room temperature under a positive pore pressure for 19 days. Imbibition tests were carried out by pumping brine through the slot in one end piece of the core holder and across one face of the cylindrical rock sample. The brine displaced the toluene phase that previously fiied the slot. The rock face, now in contact with the brine, could spontaneously imbibe brine and expel toluene phase. Any toluene phase produced by imbibition was swept out of the end piece by the brine stream and into a collection vessel. The volume of toluene phase produced as a function of time after the start of brine flow across the rock face was recorded. Interfacial tensions between brine and toluene phases were measured by the pendant-drop technique.
Results The oil sample used for this work was a medium-weight crude oil with a low acid number, a high nitrogen content, and substantial quantities of polar (NSO)components (Table I). The results of a typical ion-exchange separation of crude oil components are presented in the Experimental Section. Four separations were performed for this work. The overall recovery of materials applied to chromatographic columns averaged 99.2 f 0.9%. The average contents of acidic, basic, and neutral components, expressed as a weight percent of the whole crude oil were 5.0 f 1.6%, 9.2 f 2.7570, and 70.1 f 0.7%, respectively. Volatile components accounted for the balance of the whole crude oil. Absorption spectra of toluene solutions of the crude oil components show no distinct peaks or shoulders in the 300-700-nm wavelength range (Figure 1). The neutral component has the smallest absorption coefficient, on a weight basis, of the three components due to its high content of aliphatic hydrocarbons, which do not absorb light in the 300-700-nm region. A wavelength of 350 nm was chosen for monitoring the absorbance of effluents from Berea samples because the 1% solutions of crude oil or of the isolated components in toluene had experimentally accessible absorbance values at that wavelength. The ratio of effluent absorbance to influent absorbance was calculated from the output of the absorbance monitor.
Wetting in Berea Sandstone
Energy & Fuels, Vol. 3, No. 3, 1989 301 Table 111. Interfacial Tensions of Fluid Pairs Used in Imbibition Measurementsa interfacial tension against brine, dyn/cm oil phase (22 "C)
6 0.9
B a 0.86'
"
2
"
"
"
4 6 8 1 Volume lnjected(PV)
'
0
Figure 2. Absorbance-ratiodata for the injection of toluene solutions of stabilized crude oil (solid line) and of neutral component (dashed line) into Berea sandstone.
.P
c
8
-
neat toluene 1% crude oil 1%neutral component 1 % acidic component 1 % basic component
32.3 (1.2) 17.5 (0.5) 22.9 (2.7) 20.5 (0.8) 22.3 (0.2)
Numbers in parentheses are standard deviations of average interfacial tensions.
1.0 -
,
//--
-/--
g 0.9 :: 9
0.e0
2
4 6 8 1 Volume Injected (PV)
20
0
Figure 3. Absorbance-ratiodata for the injection of toluene solutions of acidic component (dashed line) and of basic component (solid line) into Berea sandstone. Table 11. Properties of Berea Sandstone Samples initial brine satuoil Dhase Figure bilitv, md Dorositv ration 4 827 0.226 0.110 neat toluene 0.224 0.084 1% crude oil 2, 4 702 0.221 0.107 1% neutral component 2, 4 710 0.227 0.113 1% acidic component 3, 6 705 0.113 1% basic component 769 0.224 3, 6 0.124 379 0.212 5 neat toluene 0.122 443 0.211 1% crude oil 5 424 0.212 0.119 1 % neutral component 5 569 0.224 0.115 7 1 % acidic component 584 0.223 0.115 7 1% basic component
permea-
An absorbance ratio less than unity indicates that the rock has retained components that absorb light at 350 nm. The absorbance-ratio data for injection of toluene solutions of stabilized crude oil or of neutral component (Figure 2) show that Berea sandstone retained species from the stabilized crude oil but not from the neutral component. Berea sandstone retained species from both the isolated acidic and the isolated basic crude oil components (Figure 3). The offset in the data for injection of the solution of basic components resulted from a drift in the detector base line during an unexpected shutdown of the injection pump. The effect of crude oil components on rock wettability was assessed by comparing the rates of brine imbibition by rocks containing toluene solutions of crude oil components with the rate of brine imbibition by a rock containing neat toluene as the oil phase. A separate sample of Berea sandstone was used for each wettability test. Although Berea sandstone is a relatively homogeneous material, intersample variations in rock properties, as well as differences in wettability, may have contributed to the observed differences in imbibition rates. In addition to rock wettability, imbibition rates depend on rock permeability, porosity, and initial brine saturation and on the interfacial tension between the brine and the toluene phases.1° The
30
40
Figure 4. Spontaneous brine imbibition by Berea sandstone samples (average permeability, 750 md) containing neat toluene (solid line),a toluene solution of neutral component (dashed line), and a toluene solution of stabilized crude oil (dash-dot line) as the oil phase.
h
/------
/-
20
IO t"2
__-30
40
k"'2)
Figure 5. Spontaneous brine imbibition by Berea sandstone samples (average permeability, 415 md) containing neat toluene (solid line),a toluene solution of neutral component (dashed line) and a toluene solution of stabilized crude oil (dash-dot line) as the oil phase. Berea samples used for experiments reported here had similar porosities and initial brine saturations (Table 11). Direct comparisons of imbibition rates were made only for samples with similar (&lo0 md (md = millidarcies)) permeabilities. Solutions of crude oil components in toluene displayed significantly lower interfacial tensions against brine than did neat toluene (Table 111). Berea sandstone containing a toluene solution of stabilized crude oil had a markedly lower rate of brine imbibition than did Berea samples containing either neat toluene or a toluene solution of neutral crude oil component (Figure 4). A set of Berea samples with 40% lower average permeability displayed the same trend (Figure 5). Both acidic and basic crude oil components had a strong effect on brine-imbibition rates (Figures 6 and 7). There was greater intersample variability in imbibition rates for Berea exposed to acidic or basic components. There was (IO) Mattax, C. C.; Kyte, J. R. soc. Pet. Eng. J. 1962,2, 177-184.
Smith et al.
302 Energy & Fuels, Vol. 3, No. 3, 1989
0.4
1 I
0.0
IO
20 11'2
30
40
(min"2)
Figure 6. Spontaneous brine imbibition by Berea sandstone samples (average permeability, 740 md) containing a toluene solution of basic component (solid line) and a toluene solution of acidic component (dashed line) as the oil phase.
-> Q
0.4
l
'
I
O.OL-'
0
/
/
'
1
20
IO
30
40
t"2
Figure 7. Spontaneous brine imbibition by Berea sandstone samples (average permeability, 576 md) containing a toluene solution of basic component (solid line) and a toluene solution of acidic component (dashed line) as the oil phase. no clear indication that either component had the stronger effect on wettability.
Discussion The ion-exchange separation technique is well-suited for probing the involvement of petroleum acids and bases in wetting phenomena. Compared with methods based on adsorption chromatography used in previous wettability studies?" ion exchange provides a separation based more upon the presence or absence of acidic or basic functional groups than upon combinations of properties such as molecular weight, polarity, or solubility. The basic component contains species with aromatic, nitrogen-containing basic functionalities. These include alkylpyridines, alkylquinolines, alkylisoquinolines and their higher homol ~ g u e s . ' ~ JThe ~ acidic component contains nonbases with carboxylic acid or phenolic functional groups. Spectroscopic data characterizing acidic functional groups in another oil have recently been p~b1ished.l~The ion-exchange technique permits high recovery of sample material applied to chromatographic columns. The importance of complete sample recovery is emphasized by our finding that fractions constituting only 5 w t % of the oil had a major effect on rock wettability. Adsorption chromatography techniques lose up to 10% of the crude oil sample due to irreversible adsorption on the chromatographic materia1.l' The previous use of adsorption chromatogra(11)Crocker, M.E.; Marchm, L. M. Presented at the SPE/DOE Fifth Symposium on Enhanced Oil Recovery of the Society of Petroleum Engineers, Tulsa, OK, April 20-23, 1986; Paper 14885. (12)Lochte, H.L.; Littmann, E. R. The Petroleum Acids and Bases; Chemical Publishing Co., Inc.: New York, 1955;pp 324-339. (13)Burchill, P.; Herod, A. A.; Mahon, J. P.; Prichard, E. J. Chro-
matogr. 1983,265,223-238. (14)Rose, K.D.; Francisco, M. A. J. Energy Fuels 1987,1, 233-239.
phy for separations in wettability studies8J1 may have resulted in the overestimation of the relative effect of the recovered components on wettability due to the loss of the most highly surface-active species through irreversible adsorption. Separation procedures used in this and other studies may have caused an alteration of the interfacial activity of components isolated from crude oil. In particular, the exposure of the oil to heat during the removal of volatile components and to oxygen during subsequent chromatographic separation could have altered interfacial behavior. The experiments reported here did not address this possibility, but future applications of the techniques described here could integrate anaerobic technique into the ion-exchange separation and apply it to crude oils sampled and handled in an oxygen-free environment. Toluene was used as a solvent for crude oil components in order to minimize possible plugging of Berea samples by undissolved asphaltenes. This is only an approximation to the solvent environment of polar crude oil components in the reservoir where a wide distribution of aliphatic and aromatic species are present. The effect of solvent composition on relative wetting tendencies was not studied in this work. Absorbance-ratio measurements showed that species in the crude oil and in its acidic and basic components that absorb 350-nm light were retained by partially brine-saturated Berea sandstone. These species were absent from the neutral component of the oil. The neutral component contained species that absorb 350-nm light, but these species were not retained by the rock. The retention of neutral species that do not absorb 350-nm light could not be detected by the absorbance-ratio method. Crude oil species that absorb at 350 nm include polynuclear aromatic hydrocarbons as well as NSO compounds with smaller aromatic systems. The retained species thus include crude oil acids and bases with aromatic functionalities. Nonaromatic acids are present in crude OilsJz and may also have an effect on wettability. Since these species do not absorb at 350 nm, the absorbance-ratio measurement cannot detect their retention by Berea. The wavelength dependence of the absorbance-ratio effect was not investigated in this work but could provide additional information about the type of species retained by the rock. The effect of crude oil components on rock wettability was assessed by comparing the brine-imbibition behavior of Berea sandstone exposed to crude oil components with the corresponding behavior of similar rock samples exposed to neat toluene. We assume that rocks exposed only to brine and neat toluene are in the most strongly water-wet state attainable. Strongly water-wet rocks imbibe brine rapidly whereas imbibition rates are reduced for rocks that have been made less water wet through exposure to adsorbable organic material. Berea sandstone containing neat toluene as the oil phase had the most rapid brine imbibition of all samples. Berea containing a solution of neutral component had a slightly lower imbibition rate. Solutions of stabilized crude oil or of isolated acidic or basic crude oil components had a strong effect on the brine-imbibition behavior of Berea sandstone. The imbibition-rate results provided no clear indication that either the acidic or the basic component had the stronger effect on wettability. Berea sandstone thus did not exhibit an exclusively acidic or basic character with respect to its chemical interactions with crude oil components. Imbibition rates depend on oil-water interfacial tension as well as on rock wettability. A dimensionless time scalelo
Wetting in Berea Sandstone
L
0.0
v
0
Energy & Fuels, Vol. 3, No. 3, 1989 303
/------
1
50
I
100
I
150
I
200
112 'd
Figure 8. Imbibition curves from Figure 4 presented on a dimensionless time scale.
0.0
0
50
100
150
200
1/2 'd
Figure 9. Imbibition curves from Figure 5 presented on a dimensionless time scale.
can be used to compare imbibition curves for fluid pairs wettability is constant. The dimensionless time is defined as (k/r#J)1/2yt/pJ.,2, where k, 4, and L are the permeability, porosity, and length of the rock, p,,. is the aqueous-phase viscosity and y is the oil-water interfacial tension. Replotting the data in Figures 4 and 5 on a dimensionless time scale (Figures 8 and 9) demonstrated that the effect of the toluene solution of crude oil on imbibition behavior cannot be attributed to its reduced interfacial tension against brine. The interfacial tensions of 1?& solutions of
acidic or basic components were likewise not low enough to account for a significant fraction of the observed reduction in imbibition rates. On a dimensionless time scale, imbibition curves for Berea sandstone containing neutral component are closer to the corresponding curves for Berm sandstone containing neat toluene. The lack of complete agreement indicates the effect of intersample variability. Reduced interfacial tension could thus be partly responsible for the small effect of neutral component on imbibition rates. The dimensionless time plots show that reduced interfacial tension was not a significant factor in the effect of solutions of crude oil or of acidic or basic component on imbibitim rates. It is thus concluded that the crude oil and its acidic and basic components had a strong effect on wettability, whereas the neutral component had a minimal effect on wettability. We found that crude oil acids and bases interact with and alter the wettability of Berea sandstone. Removal of acids and bases from crude oil by ion-exchange chromatography yields a neutral fraction that has minimal effect on the wettability of Berea sandstone. The effect on wettability is correlated with the presence of aromatic species that are retained by the rock. An objective of wettability research in petroleum engineering is to rationalize and predict wetting phenomena by using information about the composition of the reservoir crude oil, rock, and brine. The major implication of this work is that sandstones do not display an exclusively acidic or basic character with respect to their interactions with crude oil. Attempts to correlate crude oil composition with wetting behavior must, in the general case, consider the involvement of both acidic and basic crude oil components. Correlations based on single parameters such as acid number or nitrogen content will not be robust predictors of wetting tendency. Designs for chemical treatments for the alteration of formation wettability should consider the possible use of both acidic and basic agents that might be more effective in disrupting existing rock-oil interactions when used in combination.
Acknowledgment. We acknowledge contributions by A. L. Pozzi, who developed the imbibition test apparatus, and by R. A. Humphrey, who provided the crude oil sample.
