Article pubs.acs.org/EF
Optimization of a Low-Concentration Bacillus subtilis Strain Biosurfactant toward Microbial Enhanced Oil Recovery M. Souayeh,† Y. Al-Wahaibi,*,† S. Al-Bahry,‡ A. Elshafie,‡ A. Al-Bemani,† S. Joshi,‡ A. Al-Hashmi,† and M. Al-Mandhari§ †
Department of Petroleum and Chemical Engineering, and ‡Department of Biology, Sultan Qaboos University, Muscat 123, Oman Petroleum Development Oman, Muscat 100, Oman
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ABSTRACT: The lipopeptide biosurfactant produced by Bacillus subtilis strain W19 was investigated for the potential to maintain additional oil recovery at different dilutions and concentrations and in combination with synthetic chemical surfactant or alkali. The effect of salinity on the biosurfactant performance and the effect of biosurfactant on permeability reduction were also studied, at reservoir conditions. Core-flooding experiments were conducted to quantify the biosurfactant dosage for optimized enhanced oil recovery. Berea sandstone cores with respective average gas permeability and porosity of 223 mD and 21.5%, crude oil of American Petroleum Institute (API) gravity of 32°, and formation brine with salinities ranging from 7 to 9% were used. Biosurfactant reduced the interfacial tension (IFT) between the aqueous phase and crude oil from 20.9 to 1.8 mN/m. In core flood tests with cell-free biosurfactant broth at different dilutions (undiluted, 2.5×, 5×, 10×, and 20× diluted) and using crude biosurfactant powder (1 and 0.4 g/L), we observed additional 15 and 13% oil recovery over residual water-flood oil saturation, respectively. These results confirmed that a minimal biosurfactant concentration required for effective oil recovery was 0.4−0.5 g/L. Because biosurfactant broth is more economical then extracting biosurfactant, we have used it for further experiments. Salinity effect studies on oil recovery showed that this biosurfactant can maintain an additional recovery of 20% even at up to 20% (w/v) salinity. A mixture of 10× diluted biosurfactant with chemical surfactant to the ratio of “75:25”, respectively, resulted in 28% additional recovery, which was better than using either alone. Mixing of biosurfactant with alkaline (Na2CO3 at 0.5 and 1.0% concentrations) resulted in further reduction of IFT by a factor of 10, but no further improvement of oil recovery was observed. Diluted biosurfactant also showed very minimal reduction in permeability of sandstone cores. This study showed that the biosurfactant would produce an appreciable amount of additional oil after water-flooded residual oil at a low concentration, without much formation damage, and its performance can be improved even further by mixing it with chemical surfactants.
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INTRODUCTION Current production technologies recover only one-third to onehalf of the oil that is originally present in an oil reservoir. Twothirds of the oil ever found still remains unproduced in the reservoir because it is trapped by capillary forces and wettability effects after primary and secondary production. 1 The exploitation of oil resources in existing reservoirs will be essential in the future because the discovery of new oil resources is not likely.2 Microbial enhanced oil recovery (MEOR) techniques are considered to be cheap, economically effective, and environmentally friendly.3 The MEOR processes produce trapped oil by improving sweep efficiency via alteration of oil/water/rock interfacial properties, changing flow behavior by permeability profile modification, permeability, and porosity stimulation or any combination of these mechanisms.4−6 Several research papers and patents have been published since the first pioneering work on MEOR, but still the idea of MEOR as an effective recovery mechanism is not widely accepted by the oil industry. This skepticism of the petroleum industry toward widespread applications of MEOR is mainly because of the fact that much data are still not available for many successful MEOR field trials. However, this trend has been recently changed from “chemically synthesized” to “biologically produced compounds”, mainly because of the environmental issues associated with the applications of © 2014 American Chemical Society
synthetic compounds as a result of their non-biodegradability, which, in turn, presented a challenge to dispose of them safely, especially in off-shore environment. The use of bioproducts that have comparable EOR performance is preferable. Sen7 reported that injecting ex-situ-produced biosurfactants with other chemicals would facilitate the emulsification of oil with the water and result in better recovery. An early work by Sabatini et al.8 reported that mixtures of biosurfactants and synthetic surfactants resulted in higher recovery of toluene and hexane, up to 85%, more than using individual biosurfactants or synthetic surfactants. Their approach was to formulate biosurfactant/surfactant mixtures that provide the appropriate hydrophobic/hydrophilic conditions to generate ultralow interfacial tension (IFT).8 In this study, the performance of ex-situ-produced lipopeptide biosurfactant in enhancing the oil recovery is investigated at different dilutions and concentrations. Moreover, the effect of salinity on enhanced oil recovery (EOR) potential of the biosurfactant and mixing the biosurfactant with a synthetic surfactant or alkali at different ratios are studied. Received: April 28, 2014 Revised: August 9, 2014 Published: August 12, 2014 5606
dx.doi.org/10.1021/ef500954u | Energy Fuels 2014, 28, 5606−5611
Energy & Fuels
Article
All measurements were performed in triplicate at ambient temperature (25 ± 2 °C) and atmospheric pressure (1 atm), and the average values are reported. Mineral Analysis of the Berea Core Plugs. The Berea core plugs were analyzed for their mineral compositions, using X-ray diffraction (XRD) at Central Analytical and Applied Research Unit (CAARU), Sultan Qaboos University, Oman, using the X’Pert Pro XRD system (Pan Analytical, Netherlands). The XRD is normally used to study the mineralogy of the core material and provide detailed information on rock mineralogy and percent composition of different minerals, particularly the clay (
