Copyright 2005, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the 2005 SPE International Symposium on Oil Field Chemistry held in Houston, Texas U.S.A., February 2-5, 2005. This paper was selected for presentation by an SPE Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract Waterflooding recovers little oil from fractured carbonate reservoirs, if they are oil-wet or mixed-wet. Surfactant-aided gravity drainage has the potential to achieve significant oil recovery by wettability alteration and interfacial tension (IFT) reduction. The goal of this work is to investigate the mechanisms of wettability alteration by crude oil components and surfactants. Contact angles are measured on mineral plates treated with crude oils, crude oil components, and surfactants. Mineral surfaces are also studied by atomic force microscopy (AFM). Surfactant solution imbibition into parallel plates filled with a crude oil is investigated. Wettability of the plates is studied before and after imbibition. Results show that wettability is controlled by the adsorption of asphaltenes. Anionic surfactants can remove these adsorbed components from the mineral surface and induce preferential water wettability. Anionic surfactants studied can imbibe water into initially oil-wet parallel plate assemblies faster than the cationic surfactant studied.

Introduction

Waterflooding is an effective method to improve oil recovery from reservoirs. For fractured reservoirs, water flooding is effective only when water imbibes into the matrix spontaneously. If the matrix is oil-wet, the injected water displaces the oil only from the fractures. Water does not imbibe into the oil-wet matrix because of negative capillary pressure, resulting in very low oil recovery. Thus there is a need of tertiary or EOR techniques like surfactant flooding1-5 to maximize production from such reservoirs. These techniques were developed in 1960s through 1980s for sandstone reservoirs, but were not widely applied because of low oil prices. Austad et al.6-9 have recently demonstrated that surfactant flooding in chalk cores can change the wettability from oil-wet to water-wet conditions, thus leading to higher

oil recovery (~70 % as compared to 5% when using pure brine). In 200310-12, they identified cheap commercial cationic surfactants, C10NH2 and bioderivatives from the coconut palm termed Arquad and Dodigen (priced at US $3 per kg). These surfactants could recover 50-90% of oil. However, the cost involved is still high due to the required high concentration (~1 wt%) and thus there is a need to evaluate other surfactants. The advantage of using cationic surfactants for carbonates is that they have the same charge as the carbonate surfaces and thus have low adsorption. Nonionic surfactants and anionic surfactants have been tested by Chen et al.13 CT scans revealed that surfactant imbibition was due to counter current flow in the beginning and due to gravity-driven flow during the later stages. The basic idea behind these techniques is to alter wettability (from oil-wet to water-wet) and lower interfacial tension. Hirasaki and Zhang14 have studied different ethoxy and propoxy sulfates to achieve very low interfacial tension and alter wettability from oil-wet to intermediate-wet. The presence of Na2CO3 reduces the adsorption of anionic surfactant by lowering the zeta potential of calcite surfaces, thus dilute anionic surfactant/alkali solution flooding seems to be very promising in recovering oil from oil-wet fractured carbonate reservoirs.

It is very important to understand the mechanism of wettability alteration to design effective surfactant treatments and identify the components of oil responsible to make a surface oil-wet. It is postulated that oil is often produced in source rocks and then migrates into originally water-wet reservoirs. Some of the ionic/polar components of crude oil, mostly asphaltenes and resins, collect at the water/oil interface15 and adsorb onto the mineral surface thus rendering the surface oil-wet.

In this work, we try to understand the nature of the adsorbed components by atomic force microscopy (AFM). Recently, AFM has been used extensively to get the force-distance measurements between a tip and a surface. These force measurements can be used to calculate the surface energies using Johnson-Kendall-Roberts (JKR) theory, Derjaguin-Muller-Toporov (DMT) theory etc.16,17 AFM is also used extensively for imaging surfaces. It can be used in the contact mode for hard surfaces and in the tapping mode for soft surfaces. It can be used to image dry surfaces or wet surfaces; tapping mode in water is a relatively new technique. AFM images have been used to confirm the deposition of oil components on mineral surfaces.18,19 In this work, crude oil treated mica surface is probed using atomic force microscopy before and after surfactant treatment to study the effects of

SPE 93009

Atomic Force Microscopy Study of Wettability Alteration K. Kumar, SPE, E.K. Dao, and K.K. Mohanty, SPE, U. of Houston

2 SPE 93009

surfactant. AFM measurements are correlated with contact angle measurements. We also study surfactant solution imbibition into an initially oil-wet parallel plate assembly to relate wettability to oil recovery. Our experimental methodology is described in the next section, the results are discussed in the following section, and the conclusions are summarized in the last section. Methodology Substrates. Silicon and mica were used as substrates in our experiments with SARA (saturates, aromatics, resins and asphaltenes) fractions. Both mica and silicon surfaces are atomically smooth and well suited for AFM imaging. Silicon was used because silica is a major component of sandstones. Being alumino silicate, mica has properties similar to clay minerals in sandstones.20 Mica, obtained from Ted Pella Inc. was in the form of slip disks of diameter 9 mm. Silicon, obtained from Silicon Inc. as wafers of 20 cm in diameter with a single polish side RI� ���� P� WKLFN� 6L22 layer, was cut to small (10 mm) square plates.

Both mica and silicon have an isoelectric point around pH=3 and so are negatively charged at normal pH. Calcite has an isoelectric point around pH=10.21 This makes mica and silicon unsuitable to be used as representative of carbonate rocks. Calcite cannot be used for AFM study EHFDXVH� LW� LV�QRW�DWRPLFDOO\�VPRRWK��)LJ����VKRZV�D��� P�[���P�VFDQ�RI�D�FDOFLWH�VXUIDFH��7KH�]-scale is of the order of 20

nm. (Z-scale is defined as the difference in height between the highest and bottom most point on the area scanned). It should be mentioned that on cleaving the calcite surface, we obtain some very smooth areas on the calcite plate but it is very difficult to locate these areas after aging the calcite surface with oil. So we use mica coated with APTES (3-aminopropyltriethoxy silane) or AP mica as a substitute for calcite surface. The choice of this substrate was justified by contact angle experiments. The water-crude oil contact angle on a calcite surface was found to be 35o (Fig. 2) whereas for AP mica the contact angle was 33o. Calcite was obtained from Ward’s Scientfic Inc., AP mica from Novascan Technologies Inc. and microscope glass slides from Corning Glass Works. Chemicals. Distilled, de-ionized water was passed through Milli Q cartridge filters to obtain water of resistivity 18.2 0 �FP�DQG�S+���� �7KLV�XOWUD�SXUH�ZDWHU�ZDV�XVHG�WR�SUHSDUH�synthetic brine (brine with sodium chloride) and field brine. The composition of field brine is listed in Table 1. Toluene, methanol, APTES, trifluoroacetic acid and acetone were supplied by Sigma-Aldrich, Isopropyl alcohol and nitric acid from EM Science, hydrochloric acid from EMD, nitric acid, acetone and hydrogen peroxide were obtained from Mallinckrodt. All these chemicals were used as received.

The properties of surfactants evaluated are listed in Table 2. Alf-38 and Alf-68 were supplied by Sasol. Alf-38 had a carbon chain length of 14 whereas Alf-68 had a carbon chain length of 12-13. Both the above surfactants had 8 propoxy groups present and were anionic. Dodecyl trimethyl ammonium bromide (DTAB) was also used as a reference. The solutions of Alf-38 (0.05, 0.26 and 1 wt%) were prepared in 0.2 M Na2CO3 and the solution of Alf-68 (0.05 wt%) was prepared in 0.3 M Na2CO3. The surfactants were made in

Na2CO3 solution and the salinities were chosen to give the lowest interfacial tension for water-oil ratio equal to 1.22 The DTAB solution (1 wt%) was prepared in field brine. Two different crude oils were used: Crude A for SARA fractions and Crude B to evaluate surfactants. Crude A has a density of 25.7 o API and 38 cp viscosity while Crude B has a density of 28.2 o API, 19.1 cp viscosity, 0.2 acid number and 1.17 base number.

Background. In AFM force measurements, a tip is brought close to a surface and the deflection of the cantilever is measured as a function of tip-sample distance as shown in Fig. 3. At large tip-sample separations (position 1), there are no detectable interaction forces. As the separation distance decreases (position 2), forces such as electrostatic, van der Waals, specific interactions etc. come into play. As the tip is brought closer to the sample, at some point (position 3) the gradient of the attractive force exceeds the spring constant of the cantilever and the cantilever tip jumps into contact with the sample (position 4). This vertical distance between (position 3) and (position 4) gives us a measure of the jump to contact force. From position (position 4) to (position 5), the tip and the sample remain in contact with each other until the tip starts to retract. As the tip retracts, at a particular point (position 6) the spring constant exceeds the gradient of the force of adhesion between the tip and the sample and the tip suddenly breaks away from the sample to its equilibrium position (position 7). The value of the spring force at position 6 is called the force of adhesion or pull-off force or jump-off contact force. This force of adhesion can be related to the surface energy using the JKR theory of adhesion mechanics as16,17

Fadh ����� 5:abc , (1) where Wabc is the work of adhesion to pull the tip off the sample, R is the radius of curvature of the tip, a represents the cantilever tip, b represents the medium and c is the surface. Wabc can be expressed as a function of surface energies as Wabc= Wab +Wbc –Wac , where Wab is the tip surface free energy in equilibrium with the medium, Wbc is the sample surface free energy in equilibrium with the medium and Wac is the interfacial free energy of the tip-sample contact interface.

The interaction forces in water between chemically modified spherical tips and the oil treated surfaces in water ZHUH� PHDVXUHG� XVLQJ� WKH� $)0�� &DQWLOHYHUV� ZLWK� � P�diameter, COOH-terminated borosilicate spherical tips (from Novascan Technologies Inc.) were used. These cantilevers had a nominal spring constant of 0.32 N/m. When the spherical tip is brought close to the sample, there are issues of water drainage from the thin water film and when the tip retracts, we have water suction into the film. The dynamics of the film is quite complicated to model. As an alternative, we use the adhesion force or pull-off force to characterize the surface. Force Measurements were done in the contact mode. While measuring the interaction forces, the whole tip was kept inside water in the fluid cell. Moreover, the tip was wetted with water before it was placed on AFM in order to avoid trapping of any air bubble at the tip or capillary effects. The force of adhesion also depends on Z scan rate23, which was taken to be

SPE 93009 3

0.199 Hz in our experiments. The scan rate is kept low in order to minimize the effects of viscosity.

Procedure SARA Fractions. The SARA fractions (Saturates / Aromatics / Resins / Asphaltenes) were obtained for the crude oil A using an ASTM technique.24 In order to deposit SARA fractions onto the surface of the substrates, we first dissolved these fractions in toluene at 70oC. The solutions were prepared separately for resins, aromatics and asphaltenes. Mica and silicon substrates, immersed in brine for a day, were aged in these individual SARA solutions for 7 days at approximately 80oC. The aging was carried out in a closed container at an elevated temperature to enhance the kinetics of adsorption of crude oil components. After aging in oil, the substrate was rinsed with toluene for a few minutes. In order to remove the excess solvent, the substrate was further washed with IPA followed by methanol and finally by water. Care was taken not to let the sample dry at any point of time, which is a more realistic condition in a reservoir. Force measurements were done on these prepared samples using COOH terminated tips. Force Measurements on AP Mica. AP mica is aged in synthetic brine (0.5 % sodium chloride) for a day. An oil droplet is placed on it using an inverted needle as shown in Fig. 4 and aged for 21 days. The excess oil is then washed with water and the surface was scanned and force measurements were done using AFM. The AP mica was then treated with different surfactants for a day and the surfaces were scanned and force measurements were done again. Parallel Plate Imbibition. The petri dish was first washed in “piranha solution” (3.5% H2O2 in 18 M H2SO4), followed by rinsing in water and acetone.25 Glass slides (1”x 3”) were first washed with HCL/HNO3 (3:1) once and then cleaned with distilled water in an ultrasonic bath for 10 minutes. The glass slides were then aged with trifluoroacetic acid for 90 minutes and then stored in vacuum for 10 hours over solid KOH for 15 hours. These slides were then silanized using APTES (2% in 99% acetone) for 5 minutes and then washed with acetone. The silane linkages on the glass slides (AP glass) were then cured in an oven at 110 oC and stored under vacuum if not used instantaneously.

Two AP glass were brought together with a gap of 30 P� EHWZHHQ� WKHP�� WKH� JDS� ZDV� ILOOHG� ZLWK� RLO� %�� DQG� WKH�

parallel plate assembly was then immersed in a surfactant solution such that the top edge of the AP glass was approximately 2 mm above the surfactant solution. Fig. 5 shows a schematic of the side view of the set up. The height of the surfactant solution is then monitored for 48 hours using a digital camera. The glass slides are then taken apart and the excess oil is washed with water. A contact angle goniometer was used to measure the advancing and receding water/oil contact angles by the sessile drop method as described in section below. All the contact angles were measured through the water phase. Contact Angles. An optical glass cell, filled with the aqueous phase, was placed between a light source and a microscope. A

horizontal mica/silicon plate was placed inside the aqueous phase. A U-shaped stainless steel needle attached to a motor driven syringe was used to introduce an oil droplet at the bottom surface of the plate. Decane was used as the probe oil. The volume of oil droplet was increased until the contact line moved outwards. A video frame grabber captured the image of the drop and contact angles were measured by fitting the drop shape to a solution of the Young-Laplace equation. The contact angle measured was the receding angle. The volume of the oil droplet was later decreased using the motor driven syringe until a neck forms. The angle formed by the droplet just before the detachment from the tip of the syringe is known as the advancing angle. On further reducing the volume of the oil droplet, the neck got thinner and finally broke. The oil volume was increased or decreased slowly in order to reduce the effect of viscous forces. Results and Discussion Force Measurements. A large number of force measurements ZHUH� FRQGXFWHG� XVLQJ� D� �� P�&22+� WHUPLQDWHG�ERURVLOLFDWH�particle at several spots in order to get an estimate of the adhesive forces. The force of adhesion or the jump-off contact force has been modeled by Maugis23 as being proportional to the surface energy of the sample. Van der Vegte and Hadziioannou16 have measured the force of attraction between the COOH surface and COOH tip in ethanol and the surface energy was found to be 4.5mJ/m2. We carried out a similar experiment and the surface energy was calculated as 3.7mJ/m2 using Eq. 1. We used a tip of radii 500nm as compared to 35nm used by Van der Vegte and Hadziioannou.16

SARA Fractions. Fig. 6 shows the force of adhesion for the different SARA fractions (solubilized in toluene) along with that of the Crude A (washed subsequently in toluene). The error bars in the histogram shows the minimum and the maximum values of the force obtained for each sample. The force of adhesion for the case of the whole oil (~15 nN) is very close to the one obtained from the apshaltene solution. It can be inferred that the adsorbed material left behind when the cores/samples are washed with toluene is primarily asphaltenic in nature.

Another important thing observed was that the force was the largest (28-42 nN) for the case of resins. It is believed that resins are more liquid like than asphaltenes. So, when the tip approaches the surface, resins engulf a larger area of the spherical tip than the other fractions and thus have a larger force of adhesion. This may be also due to the high compressibility of resin films adsorbed.26 Asphaltenes are envisioned as crystalline and thus have a relatively smaller area of contact. The force of adhesion for aromatics is the least. The trend is similar for mica and silicon surfaces. The force of adhesion for asphaltenes was similar for both mica and silicon substrates. This might imply that the adsorbed asphaltenes on both mica and silicon are very similar. It was also observed that resins have the highest jump to contact force (not shown). For the case of asphaltenes and aromatics, the jump to contact force was negligible. For asphaltenes, small long-range repulsive forces were also observed. AFM force measurements give us insights into the nature of adsorbed materials.

4 SPE 93009

Surfactants. Force measurements were also carried out after the AP mica was treated with surfactants. The idea behind force measurements is that if the surfactant actually removes the oil molecules from the surface, the force of adhesion between the functionalized tip and the initially oil-wet area would become similar to the water-wet area. Fig. 7 shows the force of adhesion for AP mica surfaces aged with crude oil for 15 days and 50 days using COOH terminated tip. Fig. 7 also shows the force of adhesion for the oil-aged AP mica surfaces treated with different surfactants. It was observed that the force of adhesion for the oil wet and water wet area became similar for the case of Alf-38 and Alf-68. But there was no change in the force values when the surface was treated with DTAB. This implies that DTAB was not effective in removing oil molecules from the AP mica surface. This was further confirmed by AFM scans (see the next section). It was also observed that the force of adhesion for the oil-wet area before surfactant treatment was approximately the same when the surface was aged for 15 days or 50 days. This meant that the adsorption of polar components of oil onto AP mica reaches equilibrium in less than 15 days.

A similar trend was also observed when the force measurements were done with an NH2 terminated tip. Fig. 8 shows the force of adhesion for oil-aged AP mica surface and oil-aged AP mica surfaces which have been treated with different surfactants. However in this case, the force of adhesion between the initially oil-wet area and NH2 terminated tip was higher than 50 nN and could not be measured using the tip with a spring constant of 0.32 N/m. This was also observed when the oil-wet area was treated with DTAB. This implies that there is still a large amount of oil molecules adsorbed on the AP mica surface after treating with DTAB. This is in conjunction with the force measurements using COOH terminated tip.

Surface Topography. After washing the excess oil on the AP mica surface by water, the oil-wet and water-wet regions were VFDQQHG��)LJ����UHSUHVHQWV�D���� P�[���� P�VFDQ��7KH�OHIW�KDOI�shows an area which was exposed to oil and thus was oil-wet. The white dots represent the oil-wet patches. The z scale of the white dots is of the order of 700 nm. The right side of Fig. 9, which has very few white dots, represents the water-wet area. The same area as viewed in Fig. 9 is scanned (Fig. 10) after treatment with Alf-38 for 15 minutes, followed by washing with water. Fig. 10 shows that the number of white dots (oil-wet patches) has greatly reduced. The z scale of Fig. 10 is only 100 nm. This signifies that the surfactant actually removes the adsorbed oil molecules.

The AP mica was again dipped in surfactant for a day and scanned after washing off the excess surfactant with ZDWHU�� )LJ�� ��� UHSUHVHQWV� D� �� P�[� �� P� VFDQ� RI� WKH� RLO-wet area treated with Alf-38 for a day. The z scale is 50 nm and it is clearly seen that Alf-38 has removed a lot of adsorbed oil molecules. The oil-wet patches (shown by white dots) have diameters around 50 nm. Fig 12 and Fig 13 show the oil-wet areas which have been treated with Alf-38 and DTAB respectively. The z-scales are 100 nm and 200nm respectively. The Alf-68 treated surface shows oil adsorbed patches with radius ranging from 50 nm to 500 nm whereas the DTAB

treated surface show oil-adsorbed regions of radius from 50nm to 1 P��,W�LV�REYLRXV�IURP�WKH�)LJV���������DQG����WKDW�'7$%�is least effective in removing oil molecules from the oil-wet surface as compared to Alf-38 and Alf-68. Parallel Plate Imbibition. The parallel plate assembly (as described in methodology section) was immersed in the surfactant solution and the height of the surfactant solution imbibed between the parallel plates was recorded as a function of time. Fig. 14 shows a plot of height of the surfactant solution imbibed vs. time. It was observed that Alf-38 reaches the equilibrium height first followed by Alf-68, DTAB and Na2CO3 solution. The equilibrium height was approximately the same for all the anionic surfactants and Na2CO3 solution implying that the final capillary pressure is approximately equal. For DTAB, the aqueous phase-oil interface was wavy. Fig. 15 shows the images of the parallel plates dipped in Alf-68, DTAB and Na2CO3 solution. The error bars in Fig. 14 show the minimum and maximum heights of the DTAB aqueous phase observed between the parallel plates.

Fig. 16 shows a snapshot of the experiment with Alf-38 (0.05 %) at some intermediate time. Three distinct regions were observed: region 1- oil, region 2- transition region, and region 3- aqueous phase. The transition region represents the region with an oil film sticking to the AP glass surface. This transition region is very distinct in the case of Alf-38. Alf-68 shows a thin transition region, whereas DTAB does not show any. Alf-38 readily adsorbs at the oil-water interface and lowers the interfacial tension thus displacing the bulk oil out of the parallel plates. But the process to remove the adsorbed oil molecules on the AP glass surface may be slower than the lowering of the interfacial tension. Also, because the IFT is low, adhering oil film does not increase the free energy of the system significantly. So an oil film adheres to AP glass when the bulk oil is displaced from that region. This is also seen for Alf-68 and Na2CO3 solutions. The region 2 is absent for the DTAB solution clearly indicating a difference in mechanism of oil displacement by cationic and anionic surfactant solutions. The AFM scans and force measurements after this imbibition experiment show that DTAB leaves behind a lot of oil on AP mica surface.

Apart from changing wettability, surfactants also play an important role in changing the interfacial tension. It was seen that when the parallel plates were immersed in distilled water, there was no water imbibition. This meant that the capillary pressure was higher than the gravitational force, which is the driving force for surfactant imbibition. So both processes (reduction in interfacial tension and wettability alteration) can be achieved by surfactant-aided gravity drainage. Effect of Surfactant Concentration. The parallel plate imbibition experiment was conducted with three different surfactant concentrations of Alf-38: 0.05, 0.26 and 1 wt %. The parallel plate experiments show that the time taken to displace oil did not vary linearly with concentration. Fig. 17 shows the height of oil-water interface vs. time. It was seen that 0.26 wt% Alf-38 displaced oil between the parallel plates the quickest, followed by 0.05 wt% and finally 1 wt%. Also the equilibrium height for 0.05 wt% and 0.26 wt% solutions

SPE 93009 5

were approximately same, implying that the equilibrium interfacial tensions for both concentrations are equal. But for the case of 1 wt %, the equilibrium position was the lowest among the Alf-38 solutions. This means that interfacial tension depends on surfactant concentration nonmonotonically. The optimal salinity depends on the surfactant concentration; these experiments were conducted at a fixed salinity. Another interesting thing observed was that the transition region (region 2) for 1 wt % Alf-38 solution was very thin. Post-imbibition Wettability Tests. After treating the parallel plates with surfactant for two days, decane-water contact angles were measured. Fig. 18 summarizes the advancing and receding contact angles. It was again observed that oil-aged AP glass treated with Alf-38 and Alf-68 are more water wet as compared to those treated with DTAB and Na2CO3. This observation confirms the result from the AFM scans and force measurements. Proposed Mechanisms Wettability Alteration AP glass is positively charged at normal pH. During aging with oil, anionic molecules from the oil adsorb onto the AP glass. During surfactant solution imbibition, anionic surfactants (Alf-38 and Alf-68) lower the interfacial tension, the gravitational force exceeds the capillary pressure, and surfactant solutions invade the gap between the parallel plates, but leave behind a thin oil film (region 2). The oil in the film gets solubilized slowly by the surfactant micelles in the invading phase. Eventually the micelles solubilize the adsorbed anionic oil molecules leading to a change in wettability of the solid surface. The time-scale for the wettability alteration appears to be a lot higher than the time-scale of the oil/water meniscus movement due to IFT lowering. The cationic surfactant (DTAB) solution imbibes in a different way. Wettability alteration and meniscus movement happen simultenously. Because the IFT is high, thin films are not laid down (no region 2). Minor variations in wettability along the wall lead to the wavy nature of the water-oil front. The surfactant dissolves into the oil phase and desorbs the anionic oil molecules from the solid surface by forming ion pairs. Because the wettability alteration step is slow, the meniscus movement is slower in the case of the cationic surfactant.

Parallel Plate Imbibition

The equations of motion of a liquid flowing into a capillary under the influence of surface tension and hydrostatic pressure was examined by Washburn in 1921.27 The Lucas-Washburn equation28,29 assumes a quasi-steady process where gravity and viscous forces are balanced by the capillary force. But this treatment neglects the inertial effects due to changes in momentum of the liquid column. These inertial effects were first accounted for by Bosanquet30 and later analyzed in detail by Zhmud et al.28 and Strarov.31 The above treatments were for a liquid imbibing into a capillary at a constant interfacial tension.

In this work, we present a very simple model for a surfactant solution imbibing into parallel plates where the

capillary pressure changes with time. The parallel plates are initially oil-wet and so the capillary pressure (Pc ≡ Po-Pw) is negative, thus opposing the imbibition of surfactant solution into the parallel plates. The system is modeled as a pseudo-steady state process (Poisueille flow) driven by gravitational forces and opposed by capillary and viscous forces (in both oil and water phases). The equation of motion is represented by,

( )

( )

dt

hLhdtdh

d

chLhdtdh

PhLghLg

t

tct

−+

+

−+=+−−−

)(

/)()()(

12

12112

ρρ

µµρρ

(2) with boundary conditions:

0@0 == th , (3)

0@1

112 =+−

= tL

PgLgLc

dtdh

t

ct

µρρ , (4)

where µ , ρ , cP , and c are viscosity, density, capillary pressure and a constant based on the plate geometry, respectively. The subscript 2 and 1 represent water and oil, respectively. The other variables are as shown in Fig. 5. In deriving equation 2, the capillary pressure at the top oil-air interface is neglected. The oil-water capillary pressure is given by

rrPc

γθγ ′== cos

, (5)

where θ and γ are dynamic contact angle and dynamic

interfacial tension, respectively. Note, γ ′ ≡ θγ cos . For the parameters of our experiment, the inertial term is negligible. Then equation 2 simplifies to a first order differential equation given by

( ) ( )ctt PhLghLgchLhdtdh +−−−=−+ )()()( 11212 ρρµµ

. (6) The above equation has an analytical solution for constant capillary pressure, given by

( ))(

)(ln

)()()()(

1112

12112

12

112

12

1

12

12

c

ct

ctt

PgLgLghPgLgL

gPgLgLL

hgc

t

−−−−−−

−

+−+

−+

−−

=

ρρρρρρ

ρρρρ

µµµ

ρρµµ

(7) The interaction between crude oil and aqueous phase

is taken into account by the capillary pressure term which depends on γ ′ . In our experiments, γ ′ changes with time at the beginning as the surfactant and oil molecules diffuse and try to to get to thermodynamic equilibrium. We have not captured this mixing mechanistically in this simplistic model. Instead we have introduced a kinetic rate constant, β, for the lowering of γ’, given by

( ) )/exp(0 βγγγγ t−′−′+′=′ ∞∞ (8)

6 SPE 93009

where 0γ ′ and ∞′γ are the initial and the equilibrium values of

γ ′ . Since the dynamic contact angle could not be measured,

the interfacial tension, γ and θcos are combined together as

γ ′ . Higher the value of β , slower γ ′ approaches its

equilibrium value. ∞′γ can be calculated from the final height

reached by the surfactant solutions. 0γ ′ can be calculated from equation 4 by measuring the initial velocity of the aqueous phase imbibing into the gap between the parallel plates. β can then be obtained by curve fitting the solution to equation 6 to the experimental data (Fig. 19). The values of β and

0γ for Alf-38, Alf-68 and 0.2 M Na2CO3 solution were found to be 12, 20, 24 hours and 0.21, 0.3, 0.31 dynes/cm, respectively. Conclusions • Wettability is controlled by the adsorption of asphaltenic

components. The force of adhesion for minerals aged with just the asphaltene fraction is similar to that of the whole oil. The force of adhesion for the minerals aged with just the resin fraction is highest of all the SARA fractions.

• Both anionic and cationic surfactants can help imbibe water into initially oil-wet capillaries. The imbibition is the fastest in the case of Alf-38 and slowest in the case of DTAB (among the surfactants studied).

• Force of adhesion studies and contact angle measurements show that greater wettability alteration is possible with these anionic surfactants than the cationic surfactant studied.

• The water imbibition rate does not increase monotonically with an increase in the surfactant concentration.

The anionic surfactants look promising but core-scale and field-scale experiments need to be conducted to better evaluate the surfactant methods for fractured media. Acknowledgment The authors thank G. Russell (Sasol) for the surfactants and NPTO/US Department of Energy for funding of this work. References 1. Bragg, J.R., Gale, W.W., McElhannon Jr., W.A.,

Davenport, O.W., Petrichuk, M.D. and Ashcraft, T.L.: “Loudon Surfactant Flood Pilot Test,” SPE/DOE 10862, SPE/DOE Third Joint Symposium on EOR, Tulsa, April 4-7, 1982.

2. Kalpakci, B., Arf, T. G., Barker. J. W., Krupa, A. S., Morgan, J. C., and Neira, R. D.: “The Low Tension Polymer Flood Approach to Cost Effective Chemical EOR,” SPE/DOE 20220, SPE/DOE Symposium on EOR, Tulsa, April 22-25, 1990.

3. Krumrine, P. H., Falcone, J. S. and Campbell, T. C.: “Surfactant Flooding 1: The Effect of Alkaline Additives on IFT, Surfactant Adsorption and Recovery Efficiency,” SPEJ (August 1982), 22, 503-513.

4. Krumrine, P. H., Falcone, J. S. and Campbell, T. C.: “Surfactant Flooding 2: The Effect of Alkaline Additives

on Permeability and Sweep Efficiency,” SPEJ (August 1982), 22, 983-992.

5. Falls, A. H., Thigpen, D. R., Nelson, R. C., Ciaston, J. W., Lawson, J. B., Good, P. A., Uber, R. C. and Shahin, G. T.: “A Field Test of Cosurfactant–Enhanced Alkaline Flooding,” SPE/DOE 24117, SPE/DOE Symposium on EOR, Tulsa, April 22-24, 1992.

6. Austad, T. and Milter, J.: “Spontaneous Imbibition of Water into Low Permeable Chalk at Different Wettabilities using Surfactants,” SPE 37236 presented at the International Symposium on Oilfield Chemistry held in Houston, February 18-21, 1997.

7. Standnes, D. C. and Austad, T.: “Wettability Alteration in Chalk 1: Preparation of Core Materials and Oil Properties,” J. Petroleum Science and Engineering (2000), 28, 111-122.

8. Standnes, D. C. and Austad, T.: “Wettability Alteration in Chalk 1: Mechanism for Wettability Alteration from Oil-wet to Water-wet using Surfactants,” J. Petroleum Science and Enginering (2000), 28, 123-143.

9. Standnes, D. C. and Austad, T.: “Wettability Alteration in Carbonates: Interaction between Cationic Surfactant and Carboxylates as a key factor in Wettability Alteration from Oil-wet to Water-wet Conditions,” Colloids and Surfaces A (2003), 216, 243-259.

10. Standnes, D. C. and Austad, T.: “Wettability Alteration in Carbonates: Low Cost Ammonium Surfactants based on Bio-derivatives from Coconut Palm as Active Chemicals to change the Wettability from Oil-wet to Water-wet Conditions,” Colloids and Surfaces A (2003), 218, 161-173.

11. Standnes, D. C. and Austad, T.: “Nontoxic Low-cost Amines as Wettability Alteration Chemicals in carbonates,” J. Petroleum Science and Enginering (2003), 39, 431-446.

12. Strand, S., Standnes, D. C. and Austad, T.: “Spontaneous Imbibtition of Aqueous Surfactant Solution into Neutral to Oil-wet Carbonate Cores: Effects of Brine Salinity and Composition,” Energy and Fuels (2003), 17, 1133-1133.

13. Chen, H. L., Lucas, N. R., Nogaret, L. A. D., Yang, H. D. and Kenyon, D. E.: “Laboratory Monitoring of Surfactant Imbibition using Computerized Tomography,” SPERE (2001), 4, 16-25.

14. Hirasaki, G. and Zhang, D.L.: “Surface Chemistry of Oil Recovery from Fractured, Oil-wet, Carbonate Formation,” SPE 80989 presented at SPE International Symposium on Oilfield Chemistry held at Houston, TX, February 5-8, 2003.

15. Freer, E. M.; Svitova, T. and Radke. J.: “The Role of Interfacial Rheology in Reservoir Mixed Wettability,” J. Petroleum Science and Engineering (2003), 39, 137-158.

16. Van der Vegte, E. W. and Hadziioannou, G.: “Scanning Force Microscopy with Chemical Specificity: AN Extensive study of Chemically Specific Tip-Surface Interactions and the Chemical Imaging of Surface Functional Groups,” Langmuir (1997), 13, 4375-4368.

17. Schneider, J.; Barger, W. and Lee, G. U.: “Nanometer Scale Properties of Supported Lipid Bilayers Measured with Hydrophobic and Hydrophilic Atomic Force Probes,” Langmuir (2003), 19(5), 1899-1907.

SPE 93009 7

18. Buckley, J. S. and Lord, D. L.: “ Wettability and Marphology of Mica Surfaces after Exposure to Crude Oil,” J. Petroleum Science and Engineering (2003), 39, 261-273.

19. Toulhoat, H., Prayer, C. and Rouquet, G.: “ Characterization by Atomic Force Microscopy of Adsorbed Asphaltenes,” Colloids and Surfaces A (1994), 91, 267-283.

20. Liu, L. and Buckley, J. S.: “ Alteration of Wetting of Mica Surfaces,” J. Petroleum Science and Engineering (1999), 24, 75-83.

21. Buckley, J.S., Takamura, K., and Morrow, N.R.: “ Influence of Electrical Surface Charges on the Wetting Properties of Crude Oils,” SPE 16964 presented at 62nd Annual Technical Conference and Exhibition of the Society of Petroleum Engineers held in Dallas, TX, September 27-30, 1987.

22. Seethepalli, A, Adibhatla, B. and Mohanty, K. K.: “ Wettability Alteration during Surfactant Flooding of Carbonate Reservoirs,” SPE 89423 presented at the SPE/DOE Symposium on IOR, Tulsa, April 17-21, 2004.

23. Capella, B. and Dietler, G.:“ Force-Distance Curves by Atomic Force Microscopy,” Surface Science Reports (1999), 34, 1-104.

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Trade Name Source Structural Name Active % Molecular

Weight

CMC (g/l) Type

Alf-38 Sasol Propoxylated

sufates-8PO

26.0% 715 0.005 Anionic

Alf-68 Sasol Propoxylated

sufates-8PO

30.6% 667 0.014 Anionic

DTAB Sigma Dodecyl trimethyl

ammonium bromide

308.3 4.625 Cationic

Table 1 - Surfactant Properties

Salt g/l

CaCl2.2H2O 2.942

MgCl2.6H2O 2.032

NaCl 5.815

Fe(NH4)2(SO4)2.6H2O 0.007

Na2SO4 0.237

Table 2 - Composition of Field Brine

8 SPE 93009

Fig. 1 -�$��� P�[��� P�VFDQ�RI�D�FDOFLWH�VXUIDFH Fig. 2 - A drop of crude oil attached to a calcite surface.

Fig. 3 - A typical cantilever displacement vs. separation

distance plot as obtained by AFM.

Fig. 4 - A schematic diagram of AP mica being with crude B.

Fig. 5 - A schematic for parallel plate experiments (side view)

Fig. 6 - Force of Adhesion for the different SARA fractions

30 m

h

(Lt – h)

2.54 cm

L1

Brine

Oil

AP Mica

Separation Distance

Approach

Retract

123

5

6

7

4Can

tilev

er D

efle

ctio

n

35

0

10

20

30

40

50

60

1 2 3 4

Fo

rce

of A

dh

esi

on

(n

N)

Mica Silica

Asphaltenes

Aromatics

Resins

Whole Oil

0 nm 20 nm

5.00 µm

SPE 93009 9

Fig. 7 - Force of adhesion for oil-aged AP mica treated with and without surfactant treatment using COOH terminated tip Fig. 8 - Force of adhesion for oil-aged AP mica treated with and without surfactant treatment using NH2 terminated tip

Fig. 9 - Oil-wet and Water-ZHW�,QWHUIDFH���� P�[���� P�

Fig. 10 - Oil-wet and water-wet interface treated with Alf-38 IRU����PLQ����� P�[���� P��

Fig. 11 - Oil aged AP mica treated with Alf-38 for 1 day �� P�[��� P��

0

5

10

15

20

25

30

35

15 days 50 days Alf 38 Alf 68 DTAB

Forc

e of

Adh

esio

n (n

N)

Oil wetWater Wet

0

5

10

15

20

25

30

35

40

alf 38 alf 68 DTAB

Forc

e of

Adh

esio

n (n

N)

oil-wetwater-wet

700 nm 0 nm

0 nm 100 nm

10 SPE 93009

Fig. 12 - Oil aged AP mica treated with Alf-���IRU���GD\��� P�[��� P��

Fig. 13 - Oil aged AP mica treated with DTAB for 1 day �� P�[��� P��

0

0.5

1

1.5

2

2.5

0 500 1000 1500 2000 2500

Time (min)

h(cm

)

na2co3alf 68alf 38DTAB

Fig. 14 - Height of surfactant solution vs. time

SPE 93009 11

Fig. 15 - Parallel plates dipped in Alf-68, DTAB and Na2CO3 solution after 150 minutes

Fig. 16 - Parallel plates in Alf-38 (0.05 %)

0

0.5

1

1.5

2

2.5

0 500 1000 1500 2000 2500 3000time (min)

heig

ht (c

m)

0.05%0.26%1%

Fig. 17 - Height of surfactant vs time for different concentration of Alf-38 solution.

0

20

40

60

80

100

120

140

160

alf-38 alf-68 Na2CO3 DTAB

Co

nta

ct A

ng

le

Receding AngleAdvancing Angle

Fig. 18 – Post-wettability contact angles for different surfactant solutions.

Fig. 19 - Experimental and theoretical curves for height of surfactant vs. time.

Alf -38 DTAB Na2CO3

Region 2

Region 3

Region 1