Review of Microbial Technology for Improving Oil Recovery

Abstract

Summary Microbial treatments are potentially cost-effective for increasing oil production, even in today's economic conditions. Field applications that use microorganisms can range from single-well to full-scale waterflooding, but both processes require some knowledge of the reservoir conditions. Coordinated laboratory and field testing is needed to develop microbial oil recovery technology further. Introduction Several literature reviews have been published on microbial enhanced oil recovery (MEOR). This paper updates the state of the art in MEOR technology and summarizes information about field projects. The most widely practiced technique for applying MEOR involves cyclic stimulation treatments of producing wells. In single-well stimulation treatments, improvement in oil production can result from removal of paraffinic or asphaltic deposits from the near-wellbore region or from mobilization of residual oil in the limited volume of the reservoir that is treated. Single-well stimulation treatments have been applied because they can be implemented with minor modifications to existing field facilities and are relatively inexpensive. An alternative method is to apply microbes in an ongoing waterflood to improve oil recovery. Although microbial-enhanced waterflooding has not been extensively tested in the field, some projects are being conducted to test its technical feasibility. In the laboratory, microorganisms have been shown to produce chemicals, such as surfactants, acids, solvents (alcohols and ketones), and gases (primarily CO2), that can be effective in mobilizing crude oil under reservoir conditions. Microbial growth and polymer production in porous media have been shown to improve sweep efficiency by permeability modification. Commercial fermentation has been used to produce such chemicals as polymers and surfactants that have been applied to EOR techniques. Xanthan gum, a biologically produced polymer, is used for mobility control applications in oil recovery. The production of surfactants by bioprocessing is attractive because the source of the chemical would not be directly linked to the price of petroleum, a difficulty with petroleum sulfonates and most of the commonly used synthetic surfactants. While aboveground production of EOR chemicals is important. this paper will focus on injection of microorganisms into subterranean formations for improving oil recovery in situ. As with any EOR process, MEOR offers both advantages and disadvantages. Table 1 lists several advantages and constraints to the application of MEOR technology. In general, MEOR methods are best applied in shallow sandstone reservoirs in mature producing fields. Mechanisms of Microbial Oil Recovery Laboratory research has demonstrated that microbial products can change the chemical and physical properties of oil, selectively plug, high-permeability zones to improve sweep efficiency, and increase wellhead pressure in single-well injections. Some microbial species can also significantly improve oil production by helping to remove suspend debris and paraffins from the near-wellbore region. Table 2 lists several scientific names of bacteria currently being used or considered for use in MEOR and the chemical products that they produce. Microorganisms produce surfactants that can reduce oil/water interfacial tension (IFT) and cause emulsification. Several types of surfactants are produced by microorganisms, including such anionic surfactants as carboxylic acids (fatty acids) and certain types of lipids. Cooper et al. isolated a neutral lipid biosurfactant from the anaerobe Clostridium pasteurianum, and anaerobic biosurfactants produced from Bacillus and other Clostridium spp. have been reported. Some biosurfactants can reduce the viscosity of a heavy oil by as much as 95%. Bacillus licheniformis also produces a surfactant under anaerobic conditions and has been investigated for in-situ use. Optimized surfactant systems formulated with biosurfactants have been found to lower oil/water IFT to as low as 5 × 10 to the -3 dyne/cm [5 × 10 to the -3 mN/m]. Micromodel studies with surfactant-producing microorganisms have shown that emulsification of crude oil can occur during flow in a porous medium. In addition, surfactants can alter the relative permeability of rock to oil by changing the wettability of the reservoir rock and thereby increasing oil recovery. The solvents that microorganisms produce are typically low-molecular-weight alcohols and ketones. These compounds are typical of those used as cosurfactants in microemulsion formulations. Under certain conditions, alcohols and ketones could also lower surface tensions and IFT'S, promote emulsification. and possibly help to stabilize microemulsions. Microbes also produce such gases as CO2, N2, H2, and CH4 that could improve oil recovery by increasing reservoir pressure and by reducing the viscosity and swelling of individual trapped droplets of crude oil caused by solubilization of gas. CO2 is usually the primary gaseous product of microbes under oilfield conditions. The increases in pressure that have been observed in field tests with microorganisms have been nominal. In laboratory tests with 10-in. [25-cm] cores, pressure increases of as much as 60 psi [414 kPa] have been observed; however, the observed increases in pressure are far below pressures required for miscibility. Sometimes, particularly with heavy crude oils, production of CO, may decrease the viscosity of the oil enough to lead to some improvement in oil production. Microorganisms that are most commonly used for MEOR field processes are species of Bacillus and Clostridium. These species have a greater potential for survival in petroleum reservoirs because they produce spores. Spores are dormant, resistant forms of the cells that can survive more stressful environmental conditions. Clostridium species produce surfactants, gases, alcohols, and solvents, while Bacillus species produce surfactants. acids, and some gases. There are also species of Bacillus that produce polymers. In carbonate formations or sandstone rocks with carbonaceous cementation, acid-producing microorganisms can increase permeability and thereby improve oil recovery. Although the organic acids typically produced by microbes are much weaker than strong mineral acids, the pH of aqueous solutions can be lowered by 2 to 3 pH units but usually no lower than pH 4. Another application for MEOR is for fluid diversion. Because many microbial strains do produce polymers, it has been postulated that some microbes could be used in situ to plug high-permeability streaks in reservoirs preferentially and thus improve sweep efficiency. Laboratory research has demonstrated that permeability alteration and fluid diversion can be achieved in parallel-core experiments by injection of nutrients into nonsterile Berea sandstone. Microbial Transport in Porous Media In MEOR applications, it is important that the microbes be capable of moving through the reservoir and producing chemical products to mobilize oil. SPERE P. 151^