Summary Oil recovery experiments using Bacillus licheniformis JF-2 (ATCC 39307) and a sucrose-based nutrient were performed with Berea sandstone cores (permeability 0.084 to 0.503 μm [85 to 510 md]). Oil recovery efficiencies for four different crude oils (0.9396 to 0.8343 g/cm3 [19.1 to 38.1°API]) varied from 2.8% to 42.6% of the waterflood residual oil. Microbial systems reduced interfacial tension (IFT) ˜20 mN/m [˜20 dyne/cm] for all oils tested. After the microbial flood experimentation, organisms were distributed throughout the core, with most cells near the outlet.
Introduction Zobell began research on microbial EOR (MEOR) in the 1940's. Updegraff described and reviewed the history of the early work, and Hitzman described the history of MEOR field testing throughout the world. In-situ MEOR processes involve injection of microorganisms represent a replenishable in-situ source of surfactant and other such beneficial metabolites as acid, gas, biopolymer, or solvent that can be supported and manipulated by addition of inexpensive nutrients, such as molasses. The most active areas of MEOR process research and testing include (1) single-well stimulation treatments for removal of near-wellbore formation damage from paraffin deposits or oil mobilization in the region around the wellbore; (2) use of microbial systems for permeability modification to improve waterflooding sweep efficiency; and (3) use of microorganisms to produce gas, surfactants, and alcohols useful for EOR.
Although the use of microorganisms for EOR has been studied and tested for many years, the specific mechanisms involved in oil recovery are not understood fully or known specifically for a wide variety of reservoir conditions, crude oils, and microbial and nutrient systems. If MEOR is to become an accepted alternative EOR process in the industry, microbial system oil recovery mechanisms in terms of such common petroleum industry variables as IFT, phase and volume diagrams, and adsorption need to be determined. The range of applicability by reservoir and crude oil type and the economic viability of MEOR processes compared with other EOR processes also must be determined. The appeal of MEOR technology is the potential it offers for more cost-effective EOR processes than existing chemical EOR methods, including cost-effective application combined with or following other EOR processes.
The purpose of this research was to obtain additional information concerning oil recovery mechanisms with a known surfactant-producing microorganism, to determine the effect of different oil characteristics on recovery, and to establish a reference for comparing the effectiveness of other organisms and conditions. Results reported in this paper were obtained by use of a single microorganism, B. licheniformis JF-2 (ATCC 39307), obtained from the American Type Culture Collection. Experiments were designed to meet the minimal nutritional requirements of the cells; therefore, oil recoveries reported in this paper are not necessarily indicative of oil recoveries obtainable with optimized, specifically designed microbial systems.
Experimental Procedures and Apparatus Oil Selection and Analysis.
Table 1 shows the four crude oils ranging from 0.9396 to 0.8343 g/cm [19.1 to 38.1°API] at 15.6°C [60°F] selected for experimentation. Suitable oils were found in a variety of geographic locations.
Analysis of the oils with respect to composition (aliphatic, aromatic, nonpentane-precipitable asphaltic, and pentane-precipitable asphaltic components), gas chromatographic profiles, elemental analysis (carbon, hydrogen, nitrogen, and sulfur), and viscosities have been reported previously.
Microbial System Preparation. B. licheniformis strain JF-2 was chosen for experimentation because it produces a surfactant that has been isolated and characterized by other laboratories. It has been used previously in laboratory MEOR studies and applied in the field, and genetically stable reposited cultures are available worldwide.
B. licheniformis JF-2 is characterized as a facultatively anaerobic, spore-forming, gram-positive rod approximately 0.7×2.0 μm (width by length). The organism has thermotolerance to 50°C [140°F], halotolerance (in the form of NaCl) to 10% (highest value tested in our laboratory), and a pH tolerance from 4.5 to at least 8.5 when grown on minimal 1502 Medium E for bacillus (Table 2) with 1% sucrose as the sole carbon source. Sucrose was chosen as the carbon source because it represents the main carbohydrate constituent of beet molasses (molasses historically has been the feedstock of choice for MEOR field applications). Sucrose represents 63.5% of total solids in beet molasses and 95.4% of all carbohydrate present. The organism does not reduce sulfate (indicated by the absence of H2S production on Triple Sugar Iron agar). Previously, B. licheniformis JF-2 has been demonstrated to have no metabolic capacity for crude oil degradation.
Organisms used in coreflood experimentation were grown routinely by inoculating 50 mL of Medium E (supplemented with 1% sucrose and 2.5% NaCl) with 100 μL of a fresh overnight culture. Incubation was conducted aerobically at 30°C [86°F] until the cultures reached an optical density of 1.0±0.15. The cells were harvested by centrifugation (3000 rev/min for 10 minutes at room temperature in a Sorvall rotor SS34) and resuspended in fresh Medium E with 1% sucrose and 2.5% NaCl added. This procedure was followed to allow differentiation between metabolic products produced outside the core and those metabolites produced in the core. Under this regimen, any oil displacement that occurred was the result of either in-situ production of metabolic products or the physical presence of bacterial cells.
Where noted, "cell-free" supernatants were used for core injection. The cells were removed by centrifugation as described above, and the supernatants were harvested. Table 3 gives analysis of cell-free supernatants used for all supernatant experiments.
Coreflooding Apparatus and Effluent Quantification. Fig. 1 shows the apparatus used for all coreflooding experiments, which was similar to that used by other researchers.
Oil Selection and Analysis. Table 1 shows the four crude oils ranging from 0.9396 to 0.8343 g/cm [19.1 to 38.1°API] at 15.6°C [60°F] selected for experimentation. Suitable oils were found in a variety of geographic locations.
Analysis of the oils with respect to composition (aliphatic, aromatic, nonpentane-precipitable asphaltic, and pentane-precipitable asphaltic components), gas chromatographic profiles, elemental analysis (carbon, hydrogen, nitrogen, and sulfur), and viscosities have been reported previously.
Microbial System Preparation. B. licheniformis strain JF-2 was chosen for experimentation because it produces a surfactant that has been isolated and characterized by other laboratories. It has been used previously in laboratory MEOR studies and applied in the field, and genetically stable reposited cultures are available worldwide.
B. licheniformis JF-2 is characterized as a facultatively anaerobic, spore-forming, gram-positive rod approximately 0.7×2.0 μm (width by length). The organism has thermotolerance to 50°C [140°F], halotolerance (in the form of NaCl) to 10% (highest value tested in our laboratory), and a pH tolerance from 4.5 to at least 8.5 when grown on minimal 1502 Medium E for bacillus (Table 2) with 1% sucrose as the sole carbon source. Sucrose was chosen as the carbon source because it represents the main carbohydrate constituent of beet molasses (molasses historically has been the feedstock of choice for MEOR field applications). Sucrose represents 63.5% of total solids in beet molasses and 95.4% of all carbohydrate present. The organism does not reduce sulfate (indicated by the absence of H2S production on Triple Sugar Iron agar). Previously, B. licheniformis JF-2 has been demonstrated to have no metabolic capacity for crude oil degradation.
Organisms used in coreflood experimentation were grown routinely by inoculating 50 mL of Medium E (supplemented with 1% sucrose and 2.5% NaCl) with 100 μL of a fresh overnight culture. Incubation was conducted aerobically at 30°C [86°F] until the cultures reached an optical density of 1.0±0.15. The cells were harvested by centrifugation (3000 rev/min for 10 minutes at room temperature in a Sorvall rotor SS34) and resuspended in fresh Medium E with 1% sucrose and 2.5% NaCl added. This procedure was followed to allow differentiation between metabolic products produced outside the core and those metabolites produced in the core. Under this regimen, any oil displacement that occurred was the result of either in-situ production of metabolic products or the physical presence of bacterial cells.
Where noted, "cell-free" supernatants were used for core injection. The cells were removed by centrifugation as described above, and the supernatants were harvested. Table 3 gives analysis of cell-free supernatants used for all supernatant experiments.
Coreflooding Apparatus and Effluent Quantification. Fig. 1 shows the apparatus used for all coreflooding experiments, which was similar to that used by other researchers.