- Email: [email protected]
The effect of bacterial solution on the wettability index and residual oil saturation in sandstone
Journal of Petroleum Science and Engineering 69 (2009) 255–260
Contents lists available at ScienceDirect
Journal of Petroleum Science and Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p e t r o l
Research paper
The effect of bacterial solution on the wettability index and residual oil saturation in sandstone M. Shabani Afrapoli ⁎, C. Crescente 1, S. Alipour, O. Torsaeter Norwegian University of Science and Technology, NTNU, Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 19 December 2008 Accepted 9 September 2009 Keywords: microbial flooding special core analysis wettability alteration Amott test sandstone and adsorption
a b s t r a c t Wettability is a major factor controlling the location, flow and distribution of fluids in a reservoir rock. The wettability of a core will have effects on almost all types of core analyses, hereby wettability changes and remaining oil saturation during microbial enhanced oil recovery processes. The objective of this study was to determine the influence of a bacterial solution on the Amott wettability index and the residual oil saturation resulting from the forced imbibition process in the wettability test. The experiments were performed both on water wet and neutral wet core plugs at laboratory temperature using NaCl brine and dodecane in one set of experiments and dodecane and a solution of brine and Rhodococcus sp 094 bacteria in another set of experiments. The Rhodococcus sp 094 bacteria were isolated from sea water and are alkane oxidizing bacteria that have the ability to form extremely stable crude oil-in-water emulsions. The core material was Berea sandstone from the same block where samples were cut to equal dimensions. To evaluate the effect of the bacterial solution on wettability in both water wet and neutral wet systems, some initially water wet Berea sandstone cores were chemically treated to obtain neutral wet properties. The wettability indices were determined by the Amott test involving spontaneous uptake of fluids and forced displacement steps. A comparison between wettability alteration and the remaining oil saturations by bacterial application on both water wet and neutral wet sandstone cores has been made. The results show that bacteria resulted in wettability changes on both systems, water wet and neutral wet, and influenced the remaining oil saturations. The change in wettability is probably due to adsorption of ingredients from the bacterial solution on the surface of the rock. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The concept of the use of microorganisms to recover more oil from depleted petroleum reservoirs is not new. In 1926 Beckman was the first to suggest the use of microorganisms to increase oil recovery and in the 1940s ZoBell started a systematic laboratory investigation on the subject (Donaldson et al., 1989). Some of the potential of microbes has been realized (Gregory, 1984), but the Microbial Improved Oil Recovery (MIOR) technology is still in an early stage of development and more experimental research and field tests are needed in order to develop tools for optimizing the MIOR process. Many laboratory and field studies on MIOR have demonstrated that there are several mechanisms by which bacteria can increase oil recovery from a reservoir. Possible mechanisms proposed are (Bryant, 1989): (1) Gas production: gases produced during bacterial activity in the reservoir can assist the oil recovery by increasing pressure, reducing viscosity and by swelling oil droplets. ⁎ Corresponding author. Tel.: +47 735 94 986; fax: +47 73944472. E-mail address: [email protected] (M.S. Afrapoli). 1 Now with StatoilHydro, Trondheim, Norway. 0920-4105/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2009.09.002
(2) Interfacial tension (IFT) reduction: bacteria can produce surface active components (surfactants) that are capable to reduce the interfacial tension between the oil and the brine in the reservoir. (3) Improving sweep efficiency: bacteria and their products can selectively plug the high pore channels in the reservoir rock and thus improve sweep efficiency. Bioproducts also can increase the viscosity of the water in the waterfloods. (4) Changes in permeability: the bacterial production of organic acids can dissolve the carbonate materials in the rock and increase the rock permeability. An opposite effect is obtained if bacterial growth on the pore surface plugs the high permeable zones and thereby reduces the total permeability. (5) Wettability changes: biosurfactants may alter the wettability of the reservoir, thus modifying relative permeability. The wettability of a rock–fluid system is one of the most important properties influencing the amount of residual oil in a reservoir and its ease of recovery. The wettability of a reservoir rock system will depend on factors such as reservoir rock material and pore geometry, geological mechanisms, composition and amount of oil and brine, physical conditions; pressure and temperature and mechanisms occurring during production; i.e. changes in saturation, pressure and composition (Anderson, 1986a,b).
256
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
Experimental studies on wettability have therefore been numerous and many reviews of wettability and its effect on oil recovery have been published (Anderson, 1986b, Cuiec, 1990, Morrow, 1990). The wetting properties of a rock sample can be altered by the addition of surface active material to the fluid system (Cuiec, 1990, Morrow et al., 1986). The concentration of surface active material has been shown to be of great importance, as well as the type of additive used (Alveskog et al., 1998, Tweheyo et al., 1999, Kowalewski et al., 2002). MIOR laboratory studies have resulted in wettability changes both towards more water wet (Mu et al., 2002) and towards more oil wet (Polson et al., 2002) as a result of bacterial applications, depending on properties of the rock, the fluids and the metabolites. Changes in wettability will change the drainage/imbibition processes. A wettability change to less extreme wettability status (less water wet or less oil wet) will in most cases be positive and increase oil recovery. Many different methods have been proposed for measuring the wettability of a system, including quantitative and qualitative methods. In this investigation the traditional Amott index method was used (Amott, 1959), for quantifying wettability and remaining oil saturations after bacterial solution flooding of water wet and neutral wet cores. Such quantification of the effect of bacterial solution on the wettability index and residual oil saturation will improve our understanding of the MIOR processes and thus help us to predict its behavior.
composition is the same that already published by Crescente et al. (2005). The synthetic hydrocarbon employed to saturate the cores was n-dodecane (CH3(CH2)10CH3 or C12H26), also referred to as C12. The densities and viscosities for the different fluids used were measured at room temperature of 20 °C and given in Table 2. To obtain samples with initially different wetting properties, a chemical product called SurfaSilTM was used. It is a short chain, clear polymeric silicone fluid consisting primarily of dichlorooctamethyl-tetrasiloxane (C8H24Si4O3Cl2). 3.3. Bacteria An alkane oxidizing bacterium, Rhodococcus sp. 094, was used in the experiments. These bacteria were isolated from seawater from the fjord of Trondheim and have been studied for the purpose of cleaning oil contaminated environments due to its ability to form extremely stable crude oil-in-water emulsions (Bredholt et al., 1998). Two growth-variants of the bacteria have been studied, Table 2. (1) Cells of Surfactant Producing Bacteria (SPB) which are known to produce surfactant. Dodecane is used as the only carbon source for this variant. (2) Cells of Non-surfactant Producing Bacteria (NSPB) which are known not to produce surfactants. Acetate served as the carbon source for the NSPB.
2. Experimental work 4. Methods A number of laboratory experiments have been carried out by the Rhodococcus sp. 094 bacteria mainly on Berea sandstone core plugs and presented earlier (Crescente et al., 2005, 2006). Those experiments entirely consisted of flooding processes while the present paper presents experimental work on the effect of bacterial activity on wettability. Amott tests performed on core plugs have been the main source of information. 3. Materials 3.1. Core samples The core materials were outcrop Berea sandstones from three blocks with different permeabilities where the plugs were cut to equal dimensions. The porosity of the samples varied between 17% and 23%, and the air permeability from 80 md to 410 md classified into three categories; low, medium and high permeability cores. Table 1 lists the general core properties. The core plug identification letters denote: U—untreated, T—treated (with chemicals), and H, M and L refer to high, medium and low permeability, respectively. The letter ‘A’ refers to Amott test. 3.2. Fluids The fluids used in these experiments were brine, n-dodecane and two growth-variants of bacteria. The brine is a NaCl brine and the
Table 1 General core plug properties. Core no.
UHA UMA ULA THA TMA TLA
Length
Diameter
Bulk volume
Pore volume
Helium porosity
Absolute permeability
(cm)
(cm)
(cm3)
(cm3)
(%)
(mD)
3.98 3.96 3.97 3.99 3.98 3.99
3.77 3.79 3.72 3.77 3.79 3.72
44.4 44.7 43.1 44.5 44.9 43.3
10.31 8.85 7.73 10.32 8.88 7.64
23.2 19.8 17.9 23.2 19.8 17.7
410 235 85 400 230 80
4.1. Bacteria cultivation Both variants of the bacteria were cultivated in our biotechnology laboratory. The growth medium for these variants was the same medium already published (Crescente et al., 2005). First different salts were weighted and added to 1 L of prepared distilled water in a flask. The medium was then heated to 30 °C and mixed until the salts were totally dissolved in the distilled water. Bicine was used as a buffer in the medium and the pH was adjusted to 8.3 at 30 °C. The medium was then sterilized in an autoclave to kill all possible bacteria. The autoclave runs at 120 °C at 1.5 bar for 20 min for the one-liter flasks to make sure all of the liquid is heated to 120 °C. Finally bacteria were cultured and grown either on acetate (for NSPB) or on dodecane (for SPB) as sole carbon and energy source. Colony Forming Unit (CFU) is a measure of viable bacterial numbers. A sample of bacteria is poured on a surface of an agar plate and left to incubate, thereby the number of colonies formed is counted. The bacterial concentration of 1 × 107 CFU/mL was used in these experiments. 4.2. Core preparation The prepared cores were cleaned in a Soxhlet apparatus by using methanol and toluene in cycles of at least 36 h each. The cores were then dried at 60 °C for at least 24 h. Core dimensions, dry weights, porosity and Klinkenberg corrected absolute permeabilities were determined and are shown in Table 1. To perform the experiments on samples with different original wettability, some initially water wet Berea sandstone cores were chemically treated to obtain neutral wet Table 2 Properties of studied fluids. Fluid
Dodecane Brine NSPB SPB
Density
Viscosity
(g/cm3)
(cp)
0.75 1.03 1.02 1.02
1.47 1.04 0.98 0.98
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
properties. In the alteration process, depending on the degree of wettability alteration, a certain amount of the chemical was added to Pentane. Pentane has been used because of its high volatility and no effect on the wettability of the quartz surface. For a certain volume of solution, a mixture with 1 volume part of Surfasil and 9 volume parts of Pentane solution was used. Samples were saturated with the mixture solution. Since SurfaSilTM fluid is flammable, corrosive and moisture-sensitive, the procedure was carried out under a fume hood and then the cores were placed in an oven at 60 °C for at least 24 h. The water wet and neutral wet cores were saturated with brine and the brine saturated porosity was determined. Saturations were therefore corroborated by weighting the dodecane and brine saturated cores. 4.3. Amott wettability tests The wettability indices were determined using the Amott method (Amott, 1959). This method includes quantitative measures of wettability derived from imbibition and drainage flow processes. In our experiments the core samples were saturated with n-dodecane, and placed in an imbibition cell surrounded by brine or bacterial solution (Fig. 1-a). The brine or bacterial solution is allowed to imbibe into the core sample within a week and hereby displacing oil until equilibrium is reached. The volume of water imbibed or oil produced is measured, VO1. The core sample is then removed and the remaining oil in the sample is forced down to residual saturation by displacement with brine or bacterial solution in a centrifuge. The following centrifuge speeds were used; 500, 1000, 1500, 2000–4000 rpm. When no more oil production is obtained at the set speed equilibrium, a new speed is selected. The whole process with centrifuging takes 3 days. The volume of oil displaced may be measured directly or determined by weight measurements, VO2. The core now saturated with brine or bacterial solution at residual oil saturation is placed in an imbibition cell surrounded by oil (Fig. 1-b). The oil is allowed to imbibe into the core displacing the brine or bacterial solution out of the sample for a maximum week period, and the volume is measured, VW1. The core is removed from the cell after equilibrium is reached, and some of the remaining brine or bacterial solution in the core is forced out by displacement in a centrifuge, VW2. By recording all volumes produced, the wettability index, WI, is calculated by Eq. (1). WI = Iwater Ioil =
VO1 VW1 − : VO1 + VO2 VW1 + VW2
ð1Þ
Iwater is the displacement with brine (or bacterial solution) ratio and Ioil is the displacement with oil ratio. The wettability index will be a number between −1.0 and 1.0 where a water wet system has WI in
257
the range of +1 to +0.3, slightly water wet (+0.3 to +0.1), neutral wet (+0.1 to − 0.1), slightly oil wet (−0.1 to −0.3) and oil wet (−0.3 to − 1.0) (Amott, 1959, Cuiec, 1984). 5. Interfacial tension and contact angle measurements The authors have conducted experiments with an automated goniometer to measure interfacial tension (IFT) and contact angle in order to help in understanding the active mechanisms. Interfacial tension was measured between dodecane drop as oil phase and different fluids; e.g. Brine, Surfactant Producing Bacteria–SPB and Non-surfactant Producing Bacteria–NSPB as a surrounding phase. Contact angle of the dodecane drop placed on the bottom of a quartz plate submerged in the same fluids (Brine, SPB and NSPB) was measured to determine wettability. Both IFT and contact angle experiments were conducted in a static condition with a constant volume of the surrounding phase. The temperature was kept at 30 °C in all cases. Each experiment has been repeated at least once. Table 3 gives the stabilized IFT and contact angle values. 6. Results and discussion Crescente et al. (2005) have reported positive effects on the final recovery from conducting numerous core flood experiments using bacterial solutions as displacing fluid. The subsequent laboratory studies are performed to investigate the active mechanism(s) that increase additional oil recovery. One of the main suspended mechanisms by which the bacteria enhance the oil recovery may be wettability changes. Therefore, in the present work, Rhodococcus sp. 094 has been assessed and examined both as surfactant producing bacteria (SPB) and non-surfactant producing bacteria (NSPB) for altering wettability. The effects of the bacteria on wettability alteration are assessed on initial different wettability properties. For that purpose, both neutral wet and water wet Berea sandstone cores were employed because the flow behavior and the distribution of fluids in the two systems are different (Fig. 2). In water wet sandstone, water is in contact with the sand grains and occupies the narrow cracks and small spaces between sand grains while oil is out in the center of the pore space, and flows more freely. On the other hand, in oil/neutral wet sandstone, oil is in the small spaces in contact with sand grains and can remain continuous to much lower saturations. The results of the wettability tests in two systems, water wet and neutral wet are presented in Tables 4 and 5. All fluid volumes were measured and hereby the displacement by water ratio (Iwater) and the displacement by oil ratio (Ioil) or in general the wettability indices (WI) were calculated. Figs. 3 and 4 show the wettability index versus residual oil saturation in water wet and neutral wet cores, respectively. The residual oil saturation was calculated from the initial oil volume minus the sum of the oil volume produced during spontaneous uptake of imbibition and the oil volume from forced displacement. The results obtained in our investigations coincide and are complementary with the results from Crescente et al. (2005, 2006) and this makes it possible to explain the different ability of NSPB and SPB to modify the
Table 3 Interfacial tension and contact angle results. System
Stabilized interfacial tension Drop size
Fig. 1. Imbibition cell with oil saturated core plug surrounded by water/bacteria suspension in brine (a) and water saturated core plug surrounded by oil (b).
Brine–quartz–C12 NSPB–quartz–C12 SPB–quartz–C12
IFT
(μl)
(mN/m)
9 9 9
18.3 13.6 10.4
Stabilized contact angle–water wet quartz Time
3h 5.5 h 6h
Drop size
Contact angle
Time
o
(μl)
( )
12 12 12
35.5 39.5 47.8
2.5 h 6h 6h
258
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
Fig. 2. Schematic of a pore cross-section in a porous medium. (a) Water wet rock grains are surrounded by a thin film of brine and are not contacted by oil. (b) Neutral wet pore has adjacent oil wet and water wet regions.
wettability. The results show that NSPB and SPB have different behaviors regarding the change of wettability. The wettability in the case of brine flooding is the base line. It is clear that NSPB tends to give the rock even more water wet characteristics. This trend is similar for all permeabilities and for both initially water wet and initially neutral wet samples. The trend for the SPB goes to less water wet characteristics both for the initially water wet cores and the initially neutral wet core samples. However the change in wettability is less significant in the case of initially neutral wet core samples. The general observable trend is that there are stronger changes in wettability in the water wet cases than in the neutral wet cases, but the changes are not very significant for any of the experiments. The different behavior of NSPB and SPB to change the wettability may have several reasons. The first reason may be due to bacteria characteristics. From the work by Bredholt et al. (1998) it can be seen
that SPB is hydrophobic and will tend to form clumps so that each cell has the smallest possible part of its surface in contact with water and the bacteria travel through the porous space in larger pieces than the NSPB. On the other hand, NSPB cells are hydrophilic and travel as loose particles. These results are consistent with the results obtained by contact angle measurements. The stabilized contact angles given in Table 3 indicate wettability properties resulting from interaction of the forces existing between three oil/brine or culture/quartz interfaces. The contact angles measured with the bacteria are compared with the results obtained with the brine. The contact angle with brine indicates the initial wettability status of the plates. It is clear that the NSPB in comparison to the brine indicates more water wet characteristics while the SPB shows less water wet characteristics. Consequently the results show that NSPB and SPB compared to brine have hydrophilic and hydrophobic properties. The hydrophobicity of the SPB means that
Table 4 Wettability test results in water wet (untreated) Berea sandstone cores. Core number
UHA
UMA
ULA
Displaced fluid
Dodecane
Displacing fluid
NSPB
Brine
SPB
NSPB
Brine
SPB
NSPB
Brine
SPB
Vo1 (cm3) Vo2 (cm3) Residual oil saturation after centrifugation in water Vw1 (cm3) Vw2 (cm3) Water index, Iw Oil index, Io WI = Iw − Io Wettability type
6.60 0.70 0.29 0.00 5.6 0.90 0.00 0.904 SWW⁎
5.90 0.90 0.34 0.00 4.9 0.87 0.00 0.868 SWW
5.80 1.20 0.32 0.00 4.7 0.83 0.00 0.829 SWW
5.30 0.50 0.34 0.00 3.3 0.91 0.00 0.914 SWW
4.50 0.60 0.42 0.00 3.2 0.88 0.00 0.882 SWW
4.60 0.90 0.38 0.00 3.1 0.84 0.00 0.836 SWW
4.30 0.30 0.40 0.00 2.2 0.93 0.00 0.935 SWW
3.70 0.40 0.47 0.00 2.3 0.90 0.00 0.902 SWW
3.70 0.60 0.44 0.00 2.0 0.86 0.00 0.860 SWW
Dodecane
Dodecane
SWW⁎: strongly water wet.
Table 5 Wettability test results in neutral wet (treated) Berea sandstone cores. Core number
THA
TMA
TLA
Displaced fluid
Dodecane
Dodecane
Dodecane
Displacing fluid
NSPB
Brine
SPB
NSPB
Brine
SPB
NSPB
Brine
SPB
Vo1 (cm3) Vo2 (cm3) Residual oil saturation after centrifugation in water Vw1 (cm3) Vw2 (cm3) Water index, Iw Oil index, Io WI = Iw − Io Wettability type
0.00 8.60 0.17 0.00 6.2 0.00 0.00 0.000 NW⁎
0.10 8.30 0.19 0.10 5.6 0.01 0.02 − 0.006 NW
0.20 8.30 0.18 0.30 6.3 0.02 0.05 − 0.022 NW
0.10 7.00 0.20 0.00 6.0 0.01 0.00 0.014 NW
0.10 6.70 0.23 0.00 5.7 0.01 0.00 0.015 NW
0.30 6.80 0.20 0.30 6.0 0.04 0.05 − 0.005 NW
0.20 5.80 0.21 0.00 3.5 0.03 0.00 0.033 NW
0.20 5.50 0.25 0.00 3.8 0.04 0.00 0.035 NW
0.30 5.60 0.23 0.10 3.4 0.05 0.03 0.022 NW
NW⁎: neutral wet.
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
259
Fig. 3. Residual oil saturation vs. wettability index for water wet cores.
when a group of SPB comes in contact with a drop of dodecane, each cell will try to have as much of its hydrophobic surface as possible. Another reason may be related to interfacial tension between dodecane and bacterial solutions. Looking at the IFT measurements from Table 3 it is clear that there is a significant change in the interfacial tension between two variants of the bacteria for the observation period. The reduction of interfacial tension is more significant with SPB than with NSPB. The result for NSPB cells is close to 13.6 mN/m and for SPB it seems to be around 10.4 mN/m. The reason is related to the fact that the bacteria are aerobic, so they use oil as nutrients and oxygen during their metabolic activities that lead to reduction of interfacial tension. Another observation is that the residual oil saturations are reduced when the NSPB and SPB were used in the flooding experiments compared to pure brine flooding. Both variants of bacteria have demonstrated that after the spontaneous and forced imbibition processes, the residual oil saturations are less than when brine solution is used. The
reduction of residual oil saturation is more significant in water wet cores than in neutral wet cores. NSPB has reduced more of the residual oil saturation than SPB in both water wet and neutral wet cores. The reason could be that SPB surrounds the drops of oil it comes in contact with much faster than NSPB, thereby increasing the chance of the oil drop being trapped and if that happens the bacteria and oil drop will not move throughout the rest of the experiment. On the other hand NSPB will spread slower on the surface of the dodecane, giving more chance of interfacial tension reduction, allowing more of the oil to be released. It is obvious that biosurfactants are produced as a byproduct of metabolism. These chemicals lower the IFT at the oil–water interface. Capillary trapped residual oil is reduced by lowering IFT and this leads to displacement of oil that cannot be displaced by water alone. NSPB cells need more time than SPB cells to produce the biomass which leads to the creation of an emulsification phase between oil and water. Therefore the SPB cells are quickly adapted to metabolize on
Fig. 4. Residual oil saturation vs. wettability index for neutral wet cores.
260
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
dodecane, while NSPB needs some time to attach to the dodecane and to be able to produce surfactant. It is interesting that there is almost no spontaneous imbibition of brine or bacterial solution in the neutral wet cores, while there is significant production from the spontaneous imbibition in the water wet cores. However, the forced imbibition process in the neutral wet cores results in lower final residual oil saturation than the water wet cores. The final residual oil saturation in neutral wet cores is less than that for water wet cores for both brine and bacterial flooding. It is also obvious that with increased permeability the residual oil is decreasing. The drainage/imbibition process depends on the capillary pressure curve which is a function of the wettability properties. Hence the changes in wettability will change the drainage/imbibition processes and the recovery of oil. Wettability change occurs mostly by bacterial mass attaching to the grain surfaces and adsorption of ingredients from the bacterial solution on the surface of the rock. Further research will hopefully reveal the mechanisms that result in the wettability modifications observed in the present work. 7. Conclusion The most important conclusions from this work are the following: (1) Mixture of SurfaSilTM and Pentane solution changed the original wettability of Berea sandstone to neutral wet properties. (2) Changes in wettability were obtained with both SPB and NSPB. This indicates that wettability alteration is a possible recovery mechanism. (3) The magnitude of the wettability changes of originally water wet cores was larger than for the neutral wet cores. For the neutral wet cores, the change in wettability was insignificant. (4) NSPB changed the wettability of the originally water wet cores to be even more water wet. SPB changed the originally water wet cores to a state of wettability that was slightly less water wet than the initial wettability. (5) Reduction in residual oil saturation was observed for both water wet and neutral wet cores when exposed to SPB and NSPB. This means that wettability changes have effects on the overall recovery.
References Alveskog, P.L., Holt, T., Torsaeter, O., 1998. The effect of surfactant concentration on the Amott wettability index and residual oil saturation. J. Pet. Sci. Eng. Amott, E., 1959. Observation relating to the wettability of porous rock. Trans. AIME 216, 156–162. Anderson, W.G., 1986a. Wettability literature survey: part 2 — wettability measurement. J. Pet. Technol. Nov. Anderson, W.G., 1986b. Wettability literature survey: part 6 — the effect of wettability on waterflooding. J. Pet. Technol. Dec. Bredholt, H., Josefsen, K., Vatland, A., Bruheim, P., Eimhjellen, K., 1998. Emulsification of crude oil by alkane oxidizing Rhodococcus species isolated from seawater. Can. J. Microbial. 44, 330–340. Bryant, R.S., 1989. Review of microbial technology for improving oil recovery, SPE paper 16646. SPE Reserv. Eng. J. 4 (2) May. Crescente, C., Rasmussen, K., Torsaeter, O., Stroem, A., Kowalewski, E., 2005. An experimental study of microbial improved oil recovery by using Rhodococcus sp. 094, SCA2005-45. SCA2005-45, presented in the SCA Annual Conference in Toronto, Canada, pp. 21–25. Sep. Crescente, C., Torsaeter, O., Hultmann, L., Stroem, A., Rasmussen, K., Kowalewski, E., 2006. An experimental study of driving mechanisms in MIOR processes by using Rhodococcus sp. 094. SPE paper 100033, presented at the International Symposium on Improved Oil Recovery, Tulsa, Oklahoma, 22–26 April. Cuiec, L.E., 1984. Rock/crude oil interaction and wettability: an attempt to understand their interrelation. Paper SPE 13211 presented at the SPE Annual Technical Conference and Exhibition, Houston, Sep. 16–19. Cuiec, L.E., 1990. Evaluation of reservoir wettability and its effect on recovery. In: Morrow, N.R. (Ed.), Interfacial Phenomena in Oil Recovery. Marcel Decker, New York, pp. 375–391. Donaldson, E.C., Chilingarian, G.V., Yen, T.F., 1989. Microbial Enhanced Oil Recovery, Developments in Petroleum Science 22. Elsevier Science Publications, Amsterdam. p. 220. Gregory, A.T., 1984. Fundamental of the microbial enhanced hydrocarbon recovery. SPE 12947. Kowalewski, E., Holt, T., Torsaeter, O., 2002. Wettability alterations due to an oil soluble additive. J. Pet. Sci. Eng. 33. Morrow, N.R., 1990. Wettability and its effect on oil recovery. JPT. Dec. Morrow, N.R., Lim, H.T., Ward, J.S., 1986. Effect of crude oil induced wettability changes on oil recovery. SPE Form. Eval. Feb. Mu, B., Wu, Z., Chen, Z., Wang, X., Ni, F., Zhou, J., 2002. Wetting behavior on quartz surfaces by the microbial metabolism and metabolic products. Paper presented at the 7th International Symposium on Wettability and its Effect on Oil recovery, Tasmania, Australia, March 12–14. Polson, E.J., Buckman, J.O., Bowen, D., Todd, A.C., Gow, M.M., Cuthbert, S.J., 2002. An ESEM investigation in to the effect of microbial biofilms on the wettability of the quartz. Paper presented at the 7th International Symposium on Wettability and its Effect on Oil recovery, Tasmania, Australia, March 12–14. Tweheyo, M.T., Holt, T., Torsaeter, O., 1999. An experimental study of the relationship between wettability and oil characteristics. J. Pet. Sci. Eng. 24.
Glossary Acknowledgments We would like to thank Mona Myren at the Laboratory of Biotechnology at NTNU for providing us with the bacterial strain studied here. Thanks also to Post-Doc Hassan Karimaie and Laboratory Engineer Roger Overaa for laboratory support.
VO1: oil volume produced during spontaneous water imbibition VO2: oil volume produced during water flooding (forced imbibition) VW1: water volume produced during spontaneous uptake of oil VW2: water volume produced during oil flooding (forced drainage) Iwater: displacement-with-water ratio Ioil: displacement-with-oil ratio WI: wettability index
Contents lists available at ScienceDirect
Journal of Petroleum Science and Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p e t r o l
Research paper
The effect of bacterial solution on the wettability index and residual oil saturation in sandstone M. Shabani Afrapoli ⁎, C. Crescente 1, S. Alipour, O. Torsaeter Norwegian University of Science and Technology, NTNU, Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 19 December 2008 Accepted 9 September 2009 Keywords: microbial flooding special core analysis wettability alteration Amott test sandstone and adsorption
a b s t r a c t Wettability is a major factor controlling the location, flow and distribution of fluids in a reservoir rock. The wettability of a core will have effects on almost all types of core analyses, hereby wettability changes and remaining oil saturation during microbial enhanced oil recovery processes. The objective of this study was to determine the influence of a bacterial solution on the Amott wettability index and the residual oil saturation resulting from the forced imbibition process in the wettability test. The experiments were performed both on water wet and neutral wet core plugs at laboratory temperature using NaCl brine and dodecane in one set of experiments and dodecane and a solution of brine and Rhodococcus sp 094 bacteria in another set of experiments. The Rhodococcus sp 094 bacteria were isolated from sea water and are alkane oxidizing bacteria that have the ability to form extremely stable crude oil-in-water emulsions. The core material was Berea sandstone from the same block where samples were cut to equal dimensions. To evaluate the effect of the bacterial solution on wettability in both water wet and neutral wet systems, some initially water wet Berea sandstone cores were chemically treated to obtain neutral wet properties. The wettability indices were determined by the Amott test involving spontaneous uptake of fluids and forced displacement steps. A comparison between wettability alteration and the remaining oil saturations by bacterial application on both water wet and neutral wet sandstone cores has been made. The results show that bacteria resulted in wettability changes on both systems, water wet and neutral wet, and influenced the remaining oil saturations. The change in wettability is probably due to adsorption of ingredients from the bacterial solution on the surface of the rock. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The concept of the use of microorganisms to recover more oil from depleted petroleum reservoirs is not new. In 1926 Beckman was the first to suggest the use of microorganisms to increase oil recovery and in the 1940s ZoBell started a systematic laboratory investigation on the subject (Donaldson et al., 1989). Some of the potential of microbes has been realized (Gregory, 1984), but the Microbial Improved Oil Recovery (MIOR) technology is still in an early stage of development and more experimental research and field tests are needed in order to develop tools for optimizing the MIOR process. Many laboratory and field studies on MIOR have demonstrated that there are several mechanisms by which bacteria can increase oil recovery from a reservoir. Possible mechanisms proposed are (Bryant, 1989): (1) Gas production: gases produced during bacterial activity in the reservoir can assist the oil recovery by increasing pressure, reducing viscosity and by swelling oil droplets. ⁎ Corresponding author. Tel.: +47 735 94 986; fax: +47 73944472. E-mail address: [email protected] (M.S. Afrapoli). 1 Now with StatoilHydro, Trondheim, Norway. 0920-4105/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2009.09.002
(2) Interfacial tension (IFT) reduction: bacteria can produce surface active components (surfactants) that are capable to reduce the interfacial tension between the oil and the brine in the reservoir. (3) Improving sweep efficiency: bacteria and their products can selectively plug the high pore channels in the reservoir rock and thus improve sweep efficiency. Bioproducts also can increase the viscosity of the water in the waterfloods. (4) Changes in permeability: the bacterial production of organic acids can dissolve the carbonate materials in the rock and increase the rock permeability. An opposite effect is obtained if bacterial growth on the pore surface plugs the high permeable zones and thereby reduces the total permeability. (5) Wettability changes: biosurfactants may alter the wettability of the reservoir, thus modifying relative permeability. The wettability of a rock–fluid system is one of the most important properties influencing the amount of residual oil in a reservoir and its ease of recovery. The wettability of a reservoir rock system will depend on factors such as reservoir rock material and pore geometry, geological mechanisms, composition and amount of oil and brine, physical conditions; pressure and temperature and mechanisms occurring during production; i.e. changes in saturation, pressure and composition (Anderson, 1986a,b).
256
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
Experimental studies on wettability have therefore been numerous and many reviews of wettability and its effect on oil recovery have been published (Anderson, 1986b, Cuiec, 1990, Morrow, 1990). The wetting properties of a rock sample can be altered by the addition of surface active material to the fluid system (Cuiec, 1990, Morrow et al., 1986). The concentration of surface active material has been shown to be of great importance, as well as the type of additive used (Alveskog et al., 1998, Tweheyo et al., 1999, Kowalewski et al., 2002). MIOR laboratory studies have resulted in wettability changes both towards more water wet (Mu et al., 2002) and towards more oil wet (Polson et al., 2002) as a result of bacterial applications, depending on properties of the rock, the fluids and the metabolites. Changes in wettability will change the drainage/imbibition processes. A wettability change to less extreme wettability status (less water wet or less oil wet) will in most cases be positive and increase oil recovery. Many different methods have been proposed for measuring the wettability of a system, including quantitative and qualitative methods. In this investigation the traditional Amott index method was used (Amott, 1959), for quantifying wettability and remaining oil saturations after bacterial solution flooding of water wet and neutral wet cores. Such quantification of the effect of bacterial solution on the wettability index and residual oil saturation will improve our understanding of the MIOR processes and thus help us to predict its behavior.
composition is the same that already published by Crescente et al. (2005). The synthetic hydrocarbon employed to saturate the cores was n-dodecane (CH3(CH2)10CH3 or C12H26), also referred to as C12. The densities and viscosities for the different fluids used were measured at room temperature of 20 °C and given in Table 2. To obtain samples with initially different wetting properties, a chemical product called SurfaSilTM was used. It is a short chain, clear polymeric silicone fluid consisting primarily of dichlorooctamethyl-tetrasiloxane (C8H24Si4O3Cl2). 3.3. Bacteria An alkane oxidizing bacterium, Rhodococcus sp. 094, was used in the experiments. These bacteria were isolated from seawater from the fjord of Trondheim and have been studied for the purpose of cleaning oil contaminated environments due to its ability to form extremely stable crude oil-in-water emulsions (Bredholt et al., 1998). Two growth-variants of the bacteria have been studied, Table 2. (1) Cells of Surfactant Producing Bacteria (SPB) which are known to produce surfactant. Dodecane is used as the only carbon source for this variant. (2) Cells of Non-surfactant Producing Bacteria (NSPB) which are known not to produce surfactants. Acetate served as the carbon source for the NSPB.
2. Experimental work 4. Methods A number of laboratory experiments have been carried out by the Rhodococcus sp. 094 bacteria mainly on Berea sandstone core plugs and presented earlier (Crescente et al., 2005, 2006). Those experiments entirely consisted of flooding processes while the present paper presents experimental work on the effect of bacterial activity on wettability. Amott tests performed on core plugs have been the main source of information. 3. Materials 3.1. Core samples The core materials were outcrop Berea sandstones from three blocks with different permeabilities where the plugs were cut to equal dimensions. The porosity of the samples varied between 17% and 23%, and the air permeability from 80 md to 410 md classified into three categories; low, medium and high permeability cores. Table 1 lists the general core properties. The core plug identification letters denote: U—untreated, T—treated (with chemicals), and H, M and L refer to high, medium and low permeability, respectively. The letter ‘A’ refers to Amott test. 3.2. Fluids The fluids used in these experiments were brine, n-dodecane and two growth-variants of bacteria. The brine is a NaCl brine and the
Table 1 General core plug properties. Core no.
UHA UMA ULA THA TMA TLA
Length
Diameter
Bulk volume
Pore volume
Helium porosity
Absolute permeability
(cm)
(cm)
(cm3)
(cm3)
(%)
(mD)
3.98 3.96 3.97 3.99 3.98 3.99
3.77 3.79 3.72 3.77 3.79 3.72
44.4 44.7 43.1 44.5 44.9 43.3
10.31 8.85 7.73 10.32 8.88 7.64
23.2 19.8 17.9 23.2 19.8 17.7
410 235 85 400 230 80
4.1. Bacteria cultivation Both variants of the bacteria were cultivated in our biotechnology laboratory. The growth medium for these variants was the same medium already published (Crescente et al., 2005). First different salts were weighted and added to 1 L of prepared distilled water in a flask. The medium was then heated to 30 °C and mixed until the salts were totally dissolved in the distilled water. Bicine was used as a buffer in the medium and the pH was adjusted to 8.3 at 30 °C. The medium was then sterilized in an autoclave to kill all possible bacteria. The autoclave runs at 120 °C at 1.5 bar for 20 min for the one-liter flasks to make sure all of the liquid is heated to 120 °C. Finally bacteria were cultured and grown either on acetate (for NSPB) or on dodecane (for SPB) as sole carbon and energy source. Colony Forming Unit (CFU) is a measure of viable bacterial numbers. A sample of bacteria is poured on a surface of an agar plate and left to incubate, thereby the number of colonies formed is counted. The bacterial concentration of 1 × 107 CFU/mL was used in these experiments. 4.2. Core preparation The prepared cores were cleaned in a Soxhlet apparatus by using methanol and toluene in cycles of at least 36 h each. The cores were then dried at 60 °C for at least 24 h. Core dimensions, dry weights, porosity and Klinkenberg corrected absolute permeabilities were determined and are shown in Table 1. To perform the experiments on samples with different original wettability, some initially water wet Berea sandstone cores were chemically treated to obtain neutral wet Table 2 Properties of studied fluids. Fluid
Dodecane Brine NSPB SPB
Density
Viscosity
(g/cm3)
(cp)
0.75 1.03 1.02 1.02
1.47 1.04 0.98 0.98
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
properties. In the alteration process, depending on the degree of wettability alteration, a certain amount of the chemical was added to Pentane. Pentane has been used because of its high volatility and no effect on the wettability of the quartz surface. For a certain volume of solution, a mixture with 1 volume part of Surfasil and 9 volume parts of Pentane solution was used. Samples were saturated with the mixture solution. Since SurfaSilTM fluid is flammable, corrosive and moisture-sensitive, the procedure was carried out under a fume hood and then the cores were placed in an oven at 60 °C for at least 24 h. The water wet and neutral wet cores were saturated with brine and the brine saturated porosity was determined. Saturations were therefore corroborated by weighting the dodecane and brine saturated cores. 4.3. Amott wettability tests The wettability indices were determined using the Amott method (Amott, 1959). This method includes quantitative measures of wettability derived from imbibition and drainage flow processes. In our experiments the core samples were saturated with n-dodecane, and placed in an imbibition cell surrounded by brine or bacterial solution (Fig. 1-a). The brine or bacterial solution is allowed to imbibe into the core sample within a week and hereby displacing oil until equilibrium is reached. The volume of water imbibed or oil produced is measured, VO1. The core sample is then removed and the remaining oil in the sample is forced down to residual saturation by displacement with brine or bacterial solution in a centrifuge. The following centrifuge speeds were used; 500, 1000, 1500, 2000–4000 rpm. When no more oil production is obtained at the set speed equilibrium, a new speed is selected. The whole process with centrifuging takes 3 days. The volume of oil displaced may be measured directly or determined by weight measurements, VO2. The core now saturated with brine or bacterial solution at residual oil saturation is placed in an imbibition cell surrounded by oil (Fig. 1-b). The oil is allowed to imbibe into the core displacing the brine or bacterial solution out of the sample for a maximum week period, and the volume is measured, VW1. The core is removed from the cell after equilibrium is reached, and some of the remaining brine or bacterial solution in the core is forced out by displacement in a centrifuge, VW2. By recording all volumes produced, the wettability index, WI, is calculated by Eq. (1). WI = Iwater Ioil =
VO1 VW1 − : VO1 + VO2 VW1 + VW2
ð1Þ
Iwater is the displacement with brine (or bacterial solution) ratio and Ioil is the displacement with oil ratio. The wettability index will be a number between −1.0 and 1.0 where a water wet system has WI in
257
the range of +1 to +0.3, slightly water wet (+0.3 to +0.1), neutral wet (+0.1 to − 0.1), slightly oil wet (−0.1 to −0.3) and oil wet (−0.3 to − 1.0) (Amott, 1959, Cuiec, 1984). 5. Interfacial tension and contact angle measurements The authors have conducted experiments with an automated goniometer to measure interfacial tension (IFT) and contact angle in order to help in understanding the active mechanisms. Interfacial tension was measured between dodecane drop as oil phase and different fluids; e.g. Brine, Surfactant Producing Bacteria–SPB and Non-surfactant Producing Bacteria–NSPB as a surrounding phase. Contact angle of the dodecane drop placed on the bottom of a quartz plate submerged in the same fluids (Brine, SPB and NSPB) was measured to determine wettability. Both IFT and contact angle experiments were conducted in a static condition with a constant volume of the surrounding phase. The temperature was kept at 30 °C in all cases. Each experiment has been repeated at least once. Table 3 gives the stabilized IFT and contact angle values. 6. Results and discussion Crescente et al. (2005) have reported positive effects on the final recovery from conducting numerous core flood experiments using bacterial solutions as displacing fluid. The subsequent laboratory studies are performed to investigate the active mechanism(s) that increase additional oil recovery. One of the main suspended mechanisms by which the bacteria enhance the oil recovery may be wettability changes. Therefore, in the present work, Rhodococcus sp. 094 has been assessed and examined both as surfactant producing bacteria (SPB) and non-surfactant producing bacteria (NSPB) for altering wettability. The effects of the bacteria on wettability alteration are assessed on initial different wettability properties. For that purpose, both neutral wet and water wet Berea sandstone cores were employed because the flow behavior and the distribution of fluids in the two systems are different (Fig. 2). In water wet sandstone, water is in contact with the sand grains and occupies the narrow cracks and small spaces between sand grains while oil is out in the center of the pore space, and flows more freely. On the other hand, in oil/neutral wet sandstone, oil is in the small spaces in contact with sand grains and can remain continuous to much lower saturations. The results of the wettability tests in two systems, water wet and neutral wet are presented in Tables 4 and 5. All fluid volumes were measured and hereby the displacement by water ratio (Iwater) and the displacement by oil ratio (Ioil) or in general the wettability indices (WI) were calculated. Figs. 3 and 4 show the wettability index versus residual oil saturation in water wet and neutral wet cores, respectively. The residual oil saturation was calculated from the initial oil volume minus the sum of the oil volume produced during spontaneous uptake of imbibition and the oil volume from forced displacement. The results obtained in our investigations coincide and are complementary with the results from Crescente et al. (2005, 2006) and this makes it possible to explain the different ability of NSPB and SPB to modify the
Table 3 Interfacial tension and contact angle results. System
Stabilized interfacial tension Drop size
Fig. 1. Imbibition cell with oil saturated core plug surrounded by water/bacteria suspension in brine (a) and water saturated core plug surrounded by oil (b).
Brine–quartz–C12 NSPB–quartz–C12 SPB–quartz–C12
IFT
(μl)
(mN/m)
9 9 9
18.3 13.6 10.4
Stabilized contact angle–water wet quartz Time
3h 5.5 h 6h
Drop size
Contact angle
Time
o
(μl)
( )
12 12 12
35.5 39.5 47.8
2.5 h 6h 6h
258
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
Fig. 2. Schematic of a pore cross-section in a porous medium. (a) Water wet rock grains are surrounded by a thin film of brine and are not contacted by oil. (b) Neutral wet pore has adjacent oil wet and water wet regions.
wettability. The results show that NSPB and SPB have different behaviors regarding the change of wettability. The wettability in the case of brine flooding is the base line. It is clear that NSPB tends to give the rock even more water wet characteristics. This trend is similar for all permeabilities and for both initially water wet and initially neutral wet samples. The trend for the SPB goes to less water wet characteristics both for the initially water wet cores and the initially neutral wet core samples. However the change in wettability is less significant in the case of initially neutral wet core samples. The general observable trend is that there are stronger changes in wettability in the water wet cases than in the neutral wet cases, but the changes are not very significant for any of the experiments. The different behavior of NSPB and SPB to change the wettability may have several reasons. The first reason may be due to bacteria characteristics. From the work by Bredholt et al. (1998) it can be seen
that SPB is hydrophobic and will tend to form clumps so that each cell has the smallest possible part of its surface in contact with water and the bacteria travel through the porous space in larger pieces than the NSPB. On the other hand, NSPB cells are hydrophilic and travel as loose particles. These results are consistent with the results obtained by contact angle measurements. The stabilized contact angles given in Table 3 indicate wettability properties resulting from interaction of the forces existing between three oil/brine or culture/quartz interfaces. The contact angles measured with the bacteria are compared with the results obtained with the brine. The contact angle with brine indicates the initial wettability status of the plates. It is clear that the NSPB in comparison to the brine indicates more water wet characteristics while the SPB shows less water wet characteristics. Consequently the results show that NSPB and SPB compared to brine have hydrophilic and hydrophobic properties. The hydrophobicity of the SPB means that
Table 4 Wettability test results in water wet (untreated) Berea sandstone cores. Core number
UHA
UMA
ULA
Displaced fluid
Dodecane
Displacing fluid
NSPB
Brine
SPB
NSPB
Brine
SPB
NSPB
Brine
SPB
Vo1 (cm3) Vo2 (cm3) Residual oil saturation after centrifugation in water Vw1 (cm3) Vw2 (cm3) Water index, Iw Oil index, Io WI = Iw − Io Wettability type
6.60 0.70 0.29 0.00 5.6 0.90 0.00 0.904 SWW⁎
5.90 0.90 0.34 0.00 4.9 0.87 0.00 0.868 SWW
5.80 1.20 0.32 0.00 4.7 0.83 0.00 0.829 SWW
5.30 0.50 0.34 0.00 3.3 0.91 0.00 0.914 SWW
4.50 0.60 0.42 0.00 3.2 0.88 0.00 0.882 SWW
4.60 0.90 0.38 0.00 3.1 0.84 0.00 0.836 SWW
4.30 0.30 0.40 0.00 2.2 0.93 0.00 0.935 SWW
3.70 0.40 0.47 0.00 2.3 0.90 0.00 0.902 SWW
3.70 0.60 0.44 0.00 2.0 0.86 0.00 0.860 SWW
Dodecane
Dodecane
SWW⁎: strongly water wet.
Table 5 Wettability test results in neutral wet (treated) Berea sandstone cores. Core number
THA
TMA
TLA
Displaced fluid
Dodecane
Dodecane
Dodecane
Displacing fluid
NSPB
Brine
SPB
NSPB
Brine
SPB
NSPB
Brine
SPB
Vo1 (cm3) Vo2 (cm3) Residual oil saturation after centrifugation in water Vw1 (cm3) Vw2 (cm3) Water index, Iw Oil index, Io WI = Iw − Io Wettability type
0.00 8.60 0.17 0.00 6.2 0.00 0.00 0.000 NW⁎
0.10 8.30 0.19 0.10 5.6 0.01 0.02 − 0.006 NW
0.20 8.30 0.18 0.30 6.3 0.02 0.05 − 0.022 NW
0.10 7.00 0.20 0.00 6.0 0.01 0.00 0.014 NW
0.10 6.70 0.23 0.00 5.7 0.01 0.00 0.015 NW
0.30 6.80 0.20 0.30 6.0 0.04 0.05 − 0.005 NW
0.20 5.80 0.21 0.00 3.5 0.03 0.00 0.033 NW
0.20 5.50 0.25 0.00 3.8 0.04 0.00 0.035 NW
0.30 5.60 0.23 0.10 3.4 0.05 0.03 0.022 NW
NW⁎: neutral wet.
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
259
Fig. 3. Residual oil saturation vs. wettability index for water wet cores.
when a group of SPB comes in contact with a drop of dodecane, each cell will try to have as much of its hydrophobic surface as possible. Another reason may be related to interfacial tension between dodecane and bacterial solutions. Looking at the IFT measurements from Table 3 it is clear that there is a significant change in the interfacial tension between two variants of the bacteria for the observation period. The reduction of interfacial tension is more significant with SPB than with NSPB. The result for NSPB cells is close to 13.6 mN/m and for SPB it seems to be around 10.4 mN/m. The reason is related to the fact that the bacteria are aerobic, so they use oil as nutrients and oxygen during their metabolic activities that lead to reduction of interfacial tension. Another observation is that the residual oil saturations are reduced when the NSPB and SPB were used in the flooding experiments compared to pure brine flooding. Both variants of bacteria have demonstrated that after the spontaneous and forced imbibition processes, the residual oil saturations are less than when brine solution is used. The
reduction of residual oil saturation is more significant in water wet cores than in neutral wet cores. NSPB has reduced more of the residual oil saturation than SPB in both water wet and neutral wet cores. The reason could be that SPB surrounds the drops of oil it comes in contact with much faster than NSPB, thereby increasing the chance of the oil drop being trapped and if that happens the bacteria and oil drop will not move throughout the rest of the experiment. On the other hand NSPB will spread slower on the surface of the dodecane, giving more chance of interfacial tension reduction, allowing more of the oil to be released. It is obvious that biosurfactants are produced as a byproduct of metabolism. These chemicals lower the IFT at the oil–water interface. Capillary trapped residual oil is reduced by lowering IFT and this leads to displacement of oil that cannot be displaced by water alone. NSPB cells need more time than SPB cells to produce the biomass which leads to the creation of an emulsification phase between oil and water. Therefore the SPB cells are quickly adapted to metabolize on
Fig. 4. Residual oil saturation vs. wettability index for neutral wet cores.
260
M.S. Afrapoli et al. / Journal of Petroleum Science and Engineering 69 (2009) 255–260
dodecane, while NSPB needs some time to attach to the dodecane and to be able to produce surfactant. It is interesting that there is almost no spontaneous imbibition of brine or bacterial solution in the neutral wet cores, while there is significant production from the spontaneous imbibition in the water wet cores. However, the forced imbibition process in the neutral wet cores results in lower final residual oil saturation than the water wet cores. The final residual oil saturation in neutral wet cores is less than that for water wet cores for both brine and bacterial flooding. It is also obvious that with increased permeability the residual oil is decreasing. The drainage/imbibition process depends on the capillary pressure curve which is a function of the wettability properties. Hence the changes in wettability will change the drainage/imbibition processes and the recovery of oil. Wettability change occurs mostly by bacterial mass attaching to the grain surfaces and adsorption of ingredients from the bacterial solution on the surface of the rock. Further research will hopefully reveal the mechanisms that result in the wettability modifications observed in the present work. 7. Conclusion The most important conclusions from this work are the following: (1) Mixture of SurfaSilTM and Pentane solution changed the original wettability of Berea sandstone to neutral wet properties. (2) Changes in wettability were obtained with both SPB and NSPB. This indicates that wettability alteration is a possible recovery mechanism. (3) The magnitude of the wettability changes of originally water wet cores was larger than for the neutral wet cores. For the neutral wet cores, the change in wettability was insignificant. (4) NSPB changed the wettability of the originally water wet cores to be even more water wet. SPB changed the originally water wet cores to a state of wettability that was slightly less water wet than the initial wettability. (5) Reduction in residual oil saturation was observed for both water wet and neutral wet cores when exposed to SPB and NSPB. This means that wettability changes have effects on the overall recovery.
References Alveskog, P.L., Holt, T., Torsaeter, O., 1998. The effect of surfactant concentration on the Amott wettability index and residual oil saturation. J. Pet. Sci. Eng. Amott, E., 1959. Observation relating to the wettability of porous rock. Trans. AIME 216, 156–162. Anderson, W.G., 1986a. Wettability literature survey: part 2 — wettability measurement. J. Pet. Technol. Nov. Anderson, W.G., 1986b. Wettability literature survey: part 6 — the effect of wettability on waterflooding. J. Pet. Technol. Dec. Bredholt, H., Josefsen, K., Vatland, A., Bruheim, P., Eimhjellen, K., 1998. Emulsification of crude oil by alkane oxidizing Rhodococcus species isolated from seawater. Can. J. Microbial. 44, 330–340. Bryant, R.S., 1989. Review of microbial technology for improving oil recovery, SPE paper 16646. SPE Reserv. Eng. J. 4 (2) May. Crescente, C., Rasmussen, K., Torsaeter, O., Stroem, A., Kowalewski, E., 2005. An experimental study of microbial improved oil recovery by using Rhodococcus sp. 094, SCA2005-45. SCA2005-45, presented in the SCA Annual Conference in Toronto, Canada, pp. 21–25. Sep. Crescente, C., Torsaeter, O., Hultmann, L., Stroem, A., Rasmussen, K., Kowalewski, E., 2006. An experimental study of driving mechanisms in MIOR processes by using Rhodococcus sp. 094. SPE paper 100033, presented at the International Symposium on Improved Oil Recovery, Tulsa, Oklahoma, 22–26 April. Cuiec, L.E., 1984. Rock/crude oil interaction and wettability: an attempt to understand their interrelation. Paper SPE 13211 presented at the SPE Annual Technical Conference and Exhibition, Houston, Sep. 16–19. Cuiec, L.E., 1990. Evaluation of reservoir wettability and its effect on recovery. In: Morrow, N.R. (Ed.), Interfacial Phenomena in Oil Recovery. Marcel Decker, New York, pp. 375–391. Donaldson, E.C., Chilingarian, G.V., Yen, T.F., 1989. Microbial Enhanced Oil Recovery, Developments in Petroleum Science 22. Elsevier Science Publications, Amsterdam. p. 220. Gregory, A.T., 1984. Fundamental of the microbial enhanced hydrocarbon recovery. SPE 12947. Kowalewski, E., Holt, T., Torsaeter, O., 2002. Wettability alterations due to an oil soluble additive. J. Pet. Sci. Eng. 33. Morrow, N.R., 1990. Wettability and its effect on oil recovery. JPT. Dec. Morrow, N.R., Lim, H.T., Ward, J.S., 1986. Effect of crude oil induced wettability changes on oil recovery. SPE Form. Eval. Feb. Mu, B., Wu, Z., Chen, Z., Wang, X., Ni, F., Zhou, J., 2002. Wetting behavior on quartz surfaces by the microbial metabolism and metabolic products. Paper presented at the 7th International Symposium on Wettability and its Effect on Oil recovery, Tasmania, Australia, March 12–14. Polson, E.J., Buckman, J.O., Bowen, D., Todd, A.C., Gow, M.M., Cuthbert, S.J., 2002. An ESEM investigation in to the effect of microbial biofilms on the wettability of the quartz. Paper presented at the 7th International Symposium on Wettability and its Effect on Oil recovery, Tasmania, Australia, March 12–14. Tweheyo, M.T., Holt, T., Torsaeter, O., 1999. An experimental study of the relationship between wettability and oil characteristics. J. Pet. Sci. Eng. 24.
Glossary Acknowledgments We would like to thank Mona Myren at the Laboratory of Biotechnology at NTNU for providing us with the bacterial strain studied here. Thanks also to Post-Doc Hassan Karimaie and Laboratory Engineer Roger Overaa for laboratory support.
VO1: oil volume produced during spontaneous water imbibition VO2: oil volume produced during water flooding (forced imbibition) VW1: water volume produced during spontaneous uptake of oil VW2: water volume produced during oil flooding (forced drainage) Iwater: displacement-with-water ratio Ioil: displacement-with-oil ratio WI: wettability index