Chapter 31
Formation Wettability Studies that Incorporate the Dynamic Wilhelmy Plate Technique 1
2
2
Dale Teeters, Mark A. Andersen, and David C. Thomas
1Chemistry Department, University of Tulsa, Tulsa, OK 74104 Amoco Production Company, Tulsa, OK 74102 2
The dynamic Wilhelmy plate technique, a new method for characterizing oil reservoir w e t t a b i l i t y , gave quanti tative values of wetting preference and comparisons of surface energy values related to wetting properties. Water-wetting and oil-wetting systems were d i s t i n guished r e a d i l y , as were hybrid-wetting systems which have both types of wetting behaviors, and i n t e r f a c i a l properties were quantified. Oxygen contamination caused inconsistent wetting behavior for some crude o i l / b r i n e / s o l i d systems, but operating i n a newly developed anaerobic vessel gave reproducible wetting behavior. The contact angle hysteresis of dolomite, marble, glass and polytetrafluoroethylene i n a series of solvents gave a q u a l i t a t i v e evaluation of surface energies. Dolomite and marble had similar surface energies which correlated to the wetting behavior for these solids obtained with the dynamic Wilhelmy plate technique. Other advantages of the Wilhelmy technique in studying reservoir w e t t a b i l i t y are discussed.
The preferential wetting c h a r a c t e r i s t i c s of crude oil/water/rock systems play an important r o l e i n characterizing o i l reservoirs. Formation wetting preference affects the success of most conven tional and enhanced recovery methods. Waterflood performance depends on the amount of imbibition which can be expected of a res ervoir and the selection of enhanced o i l recovery methods are affected by the formation w e t t a b i l i t y . Matching and predicting per formance successfully depends on the a b i l i t y to determine the degree of wetting preference of the formation. The r e l a t i v e permeability, c a p i l l a r y pressure, e l e c t r i c a l response, and occasionally the rock mechanical response a l l depend on the position of the f l u i d s i n the pores. The importance of these aspects of formation w e t t a b i l i t y has been been covered i n a series of thorough review papers by Anderson (1- 6). 0097-6156/89/0396-0560$06.00/0 © 1989 American Chemical Society
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561
Contact angle measurement i s one method of obtaining quantita tive wettability values and i s usually done i n the petroleum indus try by the sessile drop method (7- 10) or a modification of this technique (11_, 12). The contact angle i s measured at the edge of a drop of crude o i l placed between p a r a l l e l crystals i n a brine bath. One c r y s t a l i s displaced, creating a new contact angle when the water advances over a portion of the c r y s t a l formerly covered by o i l , and another new angle on the other side of the drop when the o i l advances over a portion of the c r y s t a l formerly covered by water. The displacement i s repeated u n t i l the equilibrium contact angle has been reached. The sessile drop method has several drawbacks. Several days elapse between each displacement, and t o t a l test times exceeding one month are not uncommon. It can be d i f f i c u l t to determine that the interface has actually advanced across the face of the c r y s t a l . Displacement frequency and distance are variable and dependent upon the operator. Tests are conducted on pure mineral surfaces, usually quartz, which does not adequately model the heterogeneous rock sur faces i n reservoirs. There i s a need for a simple technique that gives reproducible data and can be used to characterize various min eral surfaces. The dynamic Wilhelmy plate technique has such a potential. This paper discusses the dynamic Wilhelmy plate appara tus used to study wetting properties of l i q u i d / l i q u i d / s o l i d systems important to the o i l industry. The Dynamic Wilhelmy Plate Method The Wilhelmy hanging plate method (13) has been used for many years to measure i n t e r f a c i a l and surface tensions, but with the advent of computer data c o l l e c t i o n and computer control of dynamic test condi tions, i t s u t i l i t y has been greatly increased. The dynamic version of the Wilhelmy plate device, i n which the l i q u i d phases are i n motion r e l a t i v e to a s o l i d phase, has been used i n several surface chemistry studies not d i r e c t l y related to the o i l industry (14- 16). Fleureau and Dupeyrat (17) have used this technique to study the effects of an e l e c t r i c f i e l d on the formation of surfactants at oil/water/rock interfaces. The work presented here i s concerned with reservoir w e t t a b i l i t y . Figure 1 i s a schematic of the apparatus used i n our studies. A Cahn Model 29 microbalance and a stepper motor were interfaced to an IBM PC/XT through an RS-232 interface and an IEEE488 general pur pose interface bus (GPIB), respectively. The microbalance rested on top of a housing containing a f l a t platform on a v e r t i c a l stage moved by the stepper motor. The vessel holding the l i q u i d s rested on the platform. The plate hanging from the balance was immersed i n and removed from the l i q u i d s i n a continuous motion so that immer sion-emersion cycles or "wetting cycles" could be obtained. Clean surfaces on the plates were of the utmost importance for reproduci ble wetting cycles. The cleaning procedures for the plates used i n this work and the preparation of the mineral plates from bulk sam ples have been described elsewhere (18). The i n i t i a l plate position was t y p i c a l l y 4 mm above the liquid/vapor interface. A wetting cycle run consisted of moving the l i q u i d interface a certain d i s tance up onto the plate and then the same distance down at a stand ard speed of 0.127 mm/sec. During this movement, 720 data values
OIL-FIELD CHEMISTRY
562
were recorded by the microcomputer for t o t a l cycle distances of 50.4 mm and 360 values for cycles of 25.4 mm. Figure 2 i s a representation of the force balance on a Wilhelmy plate that has gone through one phase and has been wetted by a second phase. The three i n t e r f a c i a l tensions are related to the contact angle (measured through phase 2) by the f a m i l i a r Young equation
Y
1 2
cos0 = Y
- V
S L
(1)
S 2
where the subscripts 12, SI and S2 represent the phase 1/phase 2, solid/phase 1 and solid/phase 2 interfaces, respectively. In the experiment described above the force, F, on a plate in an a i r / l i q u i d system which i s p a r t i a l l y submersed in the l i q u i d i s F = p
V
cos 6
-
B
(2)
where p i s the perimeter of the plate, V i s the surface tension, 9 is the contact angle and B the buoyant force on the portion of the plate below the general surface. When two l i q u i d s such as o i l and water are involved, the force on a plate which has passed through the o i l layer into the water layer i s given by F = p V
A 0
cos 0
A Q
- B
Q
+ p V
O W
cos 9
0 W
- B„
(3)
where AO and OW indicate the a i r / o i l and oil/water interface respec t i v e l y , BQ i s the buoyant force caused by the o i l and B that caused by the water layer. The hexadecane/water/glass system was used as an i n i t i a l model for crude o i l / b r i n e systems. Characterization of wetting behavior of this system from the dynamic Wilhelmy plate data shown in Figure 3 i s interpreted by using Equation 3. The mass read by the balance was converted to a tension by multiplying by the g r a v i t a t i o n a l acceleration, g, and d i v i d i n g by the plate perimeter, p. As the plate was immersed, the tension increased when the s o l i d surface was wetted by the hexadecane ( Y ^ Q cos 9 < 9 0 ° in Equation 3). Further immersion of the plate into the nexadecane resulted in a s l i g h t decrease i n the tension because of the buoyant force. Another increase i n tension was observed at the hexadecane/water interface since the water wetted the glass surface in preference to hexadecane and the advancing contact angle as measured through the aqueous layer was again less than 9 0 ° . The buoyant force of the water caused the measured tension to s l i g h t l y decrease as the s l i d e went further into the water layer. The d i r e c t i o n of motion was reversed for the emersion half of the cycle and the contact angles in Equation 3 changed from advancing to receding angles. The d i f ference between these two angles caused the hysteresis observed in the wetting cycle shown i n Figure 3. The hexadecane/water/glass system i s a t y p i c a l example of water-wetting behavior. Oil-wetting systems can be modeled by replacing the glass plate with a plate of polytetrafluoroethylene (PTFE), as shown i n the hexadecane/water/PTFE system i n Figure 4. At the hexadecane/water interface the water phase did not wet the PTFE surface, the contact angle was thus greater than 9 0 ° and the tension decreased. The peak W
31. TEETERS ETAL.
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IBM PC/XT 1
STEPPER
563
n i
PLOTTER
MOTOR IEEE488
CONTROL
Figure 1. ratus.
Schematic diagram of the dynamic Wilhelmy Plate Appa
Phase 1
?12 Figure 2. Forces on a thin plate with a meniscus. The surface tensions are 7 and y ^ for the s o l i d against f l u i d s 1 and 2, respectively, and * i n t e r f a c i a l tension between the liquids at contact angle 0. The force on the plate F i s measured by a microbalance. g l
o r
t n e
564
OIL-FIELD CHEMISTRY
60Air
Water
Hexadecane
50-
40^ ^ ^ ^
30
r ->
\
^ ^ ^ ^
2010 Advancing Receding
-10
..
i
-5
0
5
10 Location (mm)
13
20
25
Figure 3* Hexadecane/water/glass wetting cycle exhibiting water-wetting behavior. (Reproduced with permission from Teet ers, D.; Wilson, J . F.; Andersen, M. A.; Thomas, D. C. J . Col l o i d Interface S c i . , 1988, 126 i n press. Copyright 1988 Academic Press.)
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observed at the hexadecane/water interface on emersion was t y p i c a l of other oil-wetting systems and i s believed to be caused by edge effects at the bottom of the plates. Crude/brine systems have been known to have large amounts of hysteresis with advancing contact angles greater than 90 and reced ing angles less than 90° (8). This type of wetting behavior i s shown i n Figure 5 which i s a hexadecane/water/glass system i n which the hexadecane phase contains o l e i c acid (0.8 molar). When the plate was immersed, the water did not displace the hexadecane-oleic acid phase and the advancing contact angle was greater than 90 • When the direction of motion was reversed, the l i n e of hexadecane/water contact on the plate was pinned and the magnitude of the tension increased u n t i l a stable meniscus with receding con tact angle less than 90° was formed. This wetting cycle demon strated oil-wetting behavior on immersion and water-wetting on emersion and has been termed a hybrid-wetting cycle. The large hys teresis i s most l i k e l y due to adsorption of o l e i c acid onto the glass surface during the immersion part of the cycle since Langmuir (19) has observed a marked difference between contact angles of an advancing and a receding water surface on glass covered by an o l e i c acid monolayer. The computer interface system lends i t s e l f well to the determi nation of i n t e r f a c i a l tension and contact angles using Equation 3 and the technique described by Pike and Thakkar for Wilhelmy plate type experiments (20). Contact angles for crude o i l / b r i n e systems using the dynamic Wilhelmy plate technique have been determined by this technique and a l l three of the wetting cycles described above have been observed i n various crude o i l / b r i n e systems (21) (Teeters, D.; Wilson, J . F.; Andersen, M. A.; Thomas, D. C ; J . C o l l o i d Inter face S c i . , 1988, 126, i n press). The dynamic Wilhelmy plate device also addresses other aspects of wetting behavior pertinent to petro leum reservoirs. Advantages of Dynamic Crude O i l Wetting Measurements The dynamic Wilhelmy plate technique d i r e c t l y measures the adhesion tension, which i s the product of the i n t e r f a c i a l tension (IFT) and the cosine of the contact angle, as i l l u s t r a t e d i n Equation 3. The degree of wetting of a system depends on the adhesion tension rather than the contact angle alone. As an example, consider the systems l i s t e d i n Table I. Using Craig's c r i t e r i a (22) the contact angles indicate that system A has intermediate wettability while system B i s oil-wetting. The IFT of system A i s higher than the IFT of system B, leading to a greater adhesion tension for the "intermedi ate" system than for the "oil-wetting" system. That i s , the o i l i n the "intermediate" system A adheres more strongly to the s o l i d sur face than the "oil-wetting" system B. The Leverett J-function i s used i n reservoir engineering (22) to relate the permeability k, porosity
