Wettability data for quartz and quartz colonized by bacterial/fungal biofilms were collected using an environmental scanning electron microscope (ESEM).
An Environmental-Scanning-ElectronMicroscope Investigation Into the Effect of Biofilm on the Wettability of Quartz E. Jacqueline Polson and James O. Buckman, Heriot-Watt University; David Bowen, Core Laboratories; Adrian C. Todd, Heriot-Watt University; and Mary M. Gow and Simon J. Cuthbert, University of the West of Scotland
Summary The wettability relationships between oil, brine, gas, and rock are important in understanding reservoir dynamics. Chemical surfactants, scale inhibitors, and microbes introduced during exploration and production are all known to affect reservoir wettabilty. However, little thought has been given to the possibility of microbial contamination of cores during core preservation, handling, storage, or analysis and the effect that this may have on measuring parameters such as wettability. In an attempt to understand how wettability analysis of sandstone cores may be altered by the presence of microbial contamination, this paper examines the effect on wettability of bacterial/fungal biofilms on quartz. Wettability data for quartz and quartz colonized by bacterial/fungal biofilms were collected using an environmental scanning electron microscope (ESEM). The results illustrate that the introduction of bacteria and fungi to such systems can change wettability from hydrophilic to hydrophobic. These findings have important implications within the oil industry. Introduction Core Analysis and Biological Contamination. During core analysis studies of reservoir rocks, many parameters are recorded, such as porosity, permeability, mineralogy, and wettability (Anderson 1986a, 1986b). Wettability has important implications for factors such as relative water and oil permeabilities, resistivity, and capillary pressure (Anderson 1986a, 1986b, 1987; Wang et al. 1997). The development of microbiological growths within porous reservoir rock, such as sandstone, has many important implications for parameters such as porosity and permeability, for example by pore blocking (Udegbunan et al. 1991; Hayatdavoudi and Ghalambor 1996; Lappan and Fogler 1996; Brydie et al. 2001). New mineral phases may be biologically mediated and precipitated (Adams et al. 1992; Feldmann et al. 1997; Konhauser and Urrutia 1999; Neumeier 1999; Adamo and Violante 2000), and certain mineral phases may be particularly susceptible to chemical and physical alteration or destruction through biological action (Weed et al. 1969; Barker and Banfield 1996; Bennet et al. 1996; Paris et al. 1996; Ullman et al. 1996; Barker et al. 1998; Wakefield and Jones 1998; Welch and Ullman 1999). In addition, the coating of mineral grains by the growth of bacteria and fungi may potentially alter the overall wettability of the core sample. This paper’s focus is on this potential for alteration of wettability brought about by the presence of such microorganisms. Biological contamination of core materials can occur at many points during a core’s history, during core handling, preservation, or even while performing laboratory tests. It is also possible that microorganisms are present from the reservoir. Given those problems, it is of importance to minimize the degree of biofilm formation. For the purpose of this paper, a biofilm is defined as “a population of microorganisms concentrated at a solid/liquid interface.” No inference is made as to the thickness of the film, although, in the present study, biofilms were typically found to be 1 or 2 cells thick. The biofilm also includes extracellular products, such as proteins, that may be released by the microorganisms and coat surfaces. Copyright © 2010 Society of Petroleum Engineers Original SPE manuscript received for review 27 November 2007. Revised manuscript received for review 18 June 2009. Paper (SPE 114421) peer approved 19 June 2009.
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Wettability. Wettability is a characteristic that defines the interaction and relative behavior between fluids and solids. In the oil industry, wettability is a major controlling factor in oil production (see Alveskog et al. 1998; Tweheyo et al. 1999). Wettability can be affected by many variables, such as pH, the polarity of the molecules involved, redox status, surface charge, surface energy, surface roughness, temperature, and salinity (Wang and Guidry 1994; Buckley 1996; Morrow et al. 1998; Lawrence and Li 2000; Somasundaran et al. 2000). However, the effect of microbes on wettability is regularly overlooked. It is assumed or speculated in some industries that microbes affect wettability; however, their role is left relatively unexamined and neglected. For example, in tertiary oil recovery, microbially enhanced oil recovery (MEOR) has been used for many years. Slugs of microbial consortiums are injected into reservoir formations that are not as productive as they previously had been. When successful, the action of the microbes releases oil from the formation returning it to a financially viable operation. One reason that this works is thought possibly to be a change in wettability; this paper demonstrates that certain microbes can do exactly this. For the purposes of this paper, wettability is defined as “the relative affinity of a liquid for a solid substrate, as expressed by the extent to which the liquid will preferentially wet the surface it rests on.” The angle of contact between a fluid and a solid is an index of the wettability relationship between the solid and the liquid. Conventionally the contact angle () is measured through the denser phase, as indicated in Fig. 1. In the case of water droplets measured in a water/air/solid system, if the contact angle is below 90°, the surface is regarded as hydrophilic (water wet); if the contact angle is 90°, the surface is regarded as intermediate; and if the contact angle is greater than 90°, the surface is regarded as hydrophobic (water repellent) (Fig. 1). The research covered in this paper will show that, when quartz chips are coated in microbes, wettability can change from more hydrophilic to hydrophobic. Materials and Methods A Philips XL30 ESEM was used to determine the wettability of quartz chip surfaces with and without the presence of bacterial/ fungal colonization (biofilm). This method of recording wettability has been covered in a number of recent publications (Buckman et al. 2000; Stemashenko et al. 2001; Guan et al. 2003; Wei et al. 2003; Al-Shafei and Okasha 2007), allowing the determination of wettability on surfaces at a micron scale. The ESEM was also used to determine the character of the biofilms in their natural state and, through the use of energy dispersive X-ray analysis (EDX), to ensure that pure quartz chips were being used. Initial Quartz Wettability. The ESEM was used to determine the initial wettability of 20 quartz samples. The quartz chips used were dry, freshly broken samples (2–3 mm in size) from display-quality quartz crystals. The ESEM was operated in wet mode, with a 500 micron gaseous secondary electron detector (GSED) using water vapor as the imaging gas. The quartz chips were precooled to approximately 5°C then placed on a Peltier cooling stage within the ESEM chamber and held at a temperature of 5°C. The chamber was pumped to 5 Torr then passed through five flooding cycles from 223
θ (a)
θ (b)
θ (c)
Fig. 1— Contact angles of water droplets measured in a water-vapor atmosphere on a solid surface. (a) Low contact angle (< 90°), forming on hydrophilic surfaces. (b) Intermediate contact angle (≈ 90°), forming on surfaces of intermediate wetness (neither hydrophilic nor hydrophobic). (c) High contact angle (> 90°), forming on hydrophobic surfaces.
5 to 10 Torr. The latter provides a suitable atmosphere for charge suppression, facilitates image amplification, maintains sample hydration, and supplies the wetting medium (water) used during wettability studies. While maintaining the sample temperature at 5°C, the chamber pressure was increased to 6.5 Torr (relative humidity = 100%), at which point water starts to condense on to the surface of the quartz chips. Observations of droplet morphology and contact angle relative to the quartz substrate were made. Images were acquired under the following conditions: temperature, 5°C; pressure, 6.5 Torr (or slightly higher); working distance, approximately 7.5 mm; operating voltage, 20 kV; and a spot size of between 4 and 6. Biofilm Development. Bacterial and fungal samples were collected and cultured from swabs taken at an active core-handling facility located in the northern hemisphere. A loop full (0.1 mL) of microbial culture was taken from each of eight pure isolates of bacteria and one pure isolate of fungi. These were transferred into a conical flask containing 250 mL of medium X (growth medium composed of 50% nutrient broth and 50% malt extract broth). The microbial mixture was then incubated at 25°C for 2 days to facilitate the development of a microbial consortium. The quartz chips were individually transferred into 20 100 mL conical flasks. Half of the flasks contained 25 mL of distilled water and 10 mL of medium X, the remainder contained only 25 mL of distilled water. The flasks were sealed and sterilized for 15 min at 121°C. Once the flasks had cooled to room temperature, 10 mL of the bacterial/fungal inoculum was added to the 10 flasks containing only distilled water. The remaining 10 flasks were left intact to provide a control, allowing any effects caused by the water and the
10 mL of medium X to be observed. All flasks were then incubated at 25°C for a further 2 days, encouraging biofilm development on the 10 inoculated quartz chips. Quartz/Biofilm and Quartz Control Wettability. Using the same method of assessment as initial wettability, changes in wettability were checked for both the inoculated and the control quartz chips. Images were taken using the same method as for initial wettability. Results All 20 quartz chips examined during the initial wettability assessment exhibited a hydrophilic character, typified by low dome-shaped droplets of water with contact angles of less than 90° (Fig. 2). The 10 control samples (which had not been exposed to microbes) retained their initial hydrophilic nature (Fig. 3). All chips exposed to the bacterial/fungal consortium successfully developed a biofilm. This biofilm was variably developed over the quartz chip surfaces (Fig. 4), with biofilms typically being one or two cells thick. Quartz chips coated in biofilm displayed a change in wettability from hydrophilic to hydrophobic, typified by spherical droplets of water, with contact angles of greater than 90° (Figs. 5 and 6). Discussion The initial wettabilities of the 20 quartz chips were hydrophilic, which was the expected result, as quartz from sedimentary reservoir rocks is widely recognized as water-wet (Barclay and Worden 2000). As all of the samples that had bacterial/fungal development showed a change in wettability from hydrophilic to hydrophobic, it is likely that this was because of the presence of the bacterial/fungal
Fig. 2 — ESEM image of untreated quartz-chip surface displaying low-contact-angle hydrophilic droplets. Gaseous secondary electron (GSE) image, horizontal field of view = 325 , accelerating voltage = 20 kV, pressure = 6.9 torr, working distance = 6.7 mm. 224
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Fig. 3 — ESEM image of quartz control chip, also displaying low-contact-angle hydrophilic droplets. GSE image, horizontal field of view = 433 , accelerating voltage = 20 kV, spot size = 3.8, pressure = 6.5 torr, working distance = 7.4 mm.
cellular or extracellular material (e.g., proteins). The fact that the wettability of the control chips remained unchanged indicates that the water plus medium X had no effect. The noted differences in wettability were, therefore, wholly caused by biofilm development. The biomatter responsible for these observed changes in wettability and the mechanisms involved were not investigated in this research but may include changes in surface texture, surface chemistry, surface energies, or pH. However, it is well known that hydrophobin, a fungal protein, has hydrophobic properties (Wosten et al. 1995). Nevertheless, whatever the mechanisms are that alter wettability, this work clearly indicates that, in the case of quartz (the most dominant component of sandstones), biofilm developed on the mineral surface can significantly shift wettability in a more hydrophobic direction.
It is worth noting that not all biofilms will produce a hydrophobic surface but may develop a hydrophilic microenvironment (Schaumann et al. 2007). Therefore, the likely effect on wettability of each biofilm or consortium of microorganisms has to be considered separately because all will not necessarily have the same consequences observed during the present study. If not recognized, biofilm development could severely compromise data sets on parameters collected during core analysis. As indicated, this is particularly a problem for wettability analysis. However, as wettability affects the assesment of relative permeability, capillary pressure, and resistivity, this will have major implications because such parameters are critical variables in reservoir analysis.
Fig. 4 — ESEM image showing detail of typical bacterial biofilm developed on quartz surface after 2 days of growth. Note patchy growth of biofilm. GSE image, horizontal field of view = 60 , accelerating voltage = 20 kV, spot size = 5.2, pressure = 6.4 torr, working distance = 7.7 mm. March 2010 SPE Journal
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Fig. 5 —ESEM image showing high-contact-angle water droplets (hydrophobic) on a bacterial biofilm. Biofilm developed over 2 days. GSE image, horizontal field of view = 70 , accelerating voltage = 20 kV, spot size = 4.4, pressure = 6.8 torr, working distance = 7.5 mm.
Fig. 6 — ESEM image of water droplets on a mixed bacterial /fungal biofilm surface. Note that high-sphericity droplets occur on the fungal hyphae, indicating that the fungal surfaces are hydrophobic. Biofilm developed over 2 days. GSE image, horizontal field of view = 150 , accelerating voltage = 20 kV, spot size = 5.3, pressure = 6.5 torr, working distance = 7.4 mm.
Conclusions • Wettability is an important parameter recorded during core-analysis studies and has important implications for other recorded data, such as relative permeabilities, capillary pressure, and resistivity. • The wettability of quartz-dominated sandstones is typically water-wet (hydrophilic), whereas, within the present study, bacterial/fungal biofilms have a hydrophobic character (thus promoting a change from more hydrophilic to hydrophobic). • Bacterial/fungal biofilms can be easily grown on quartz surfaces, with well-developed biofilm recorded after only 2 days growth. • In light of the previous conclusions, it is important that biofilm development be recognized and taken into account. • New strategies may be required to prevent biofilm formation rather than trying to remove established biofilm (as such films 226
could prove difficult to remove without further altering wettability or other core parameters). Acknowledgments This work was carried out on a Philips XL30 ESEM acquired through a Scottish Higher Education Funding Council grant. E.J. Polson acknowledges the receipt of an Engineering and Physical Sciences Research Council studentship. References Adamo, P. and Violante, P. 2000. Weathering of rocks and neogenesis of minerals associated with lichen activity. Applied Clay Science 16 (5): 229–256. doi: 10.1016/S0169-1317(99)00056-3. March 2010 SPE Journal
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