Microbial Enhanced Oil Recovery: Diverse Successful Applications of Biotechnology in the Oil Field

The field history and performance of microbial culture products for the oil field is examined. For over 15 years, microbial culture products have been used for paraffin control, production enhancement, well bore treatments as well as for scale and corrosion problems. The wide-ranging capacity of microbes to effect positive changes in oil and water properties is described. The broad spectrum of oil types and formations that have been treated successfully is reported along with treatment protocols. Mechanistic considerations for modes of action are analyzed. Traditionally, these considerations involve the continuous production of biosurfactants, solvents and other oil mobilizing agents. Continuous advancement of microbial technology has led to more recent development of new applications that use unique metabolic capabilities of microorganisms to address specific well problems. Examples of applying these products to problems in oil field production systems are shown. The outlook for development of new technologies and the future application of these products to the oil field is discussed. Introduction Microbial culture products occupy an increasingly important and growing segments in oil field production operations. They are a truly environmentally benign treatment technology that can be used to replace and augment many conventional technologies, including many oil field chemicals. The extraordinary diversity of microorganisms with the concomitant likelihood for many more such products in the future suggests that their role in oil field operations will continue to expand and will supplant many conventional technologies in the next 100 years. It is therefore important to review the prior and current uses of this technology. Historical Applications of Microbial Culture Products Paraffin Control. Microbial culture products (MCPs) were first used in 1986 in the Austin Chalk formation in Texas to control paraffin deposition. The theory behind these products was that microorganisms can be isolated and combined in novel mixtures which will produce biochemicals that will mimic the action of classic oil field chemicals such as pour point depressants, crystal modifiers and wax dispersants. The advantage of using such biological products is the fact that the microorganisms will 1) produce these biochemicals continuously and 2) attach to surfaces where paraffin deposition is occurring and act directly at the site of deposition. The first successful application of these products began a pattern of expansion that continued throughout the 80s and 90s. Paraffin deposition results in a variety of problems for oil field operators, ranging from plugging of tubulars to occult formation deposition that reduces formation permeability. A continual increase in the number of products available to the industry allowed the expansion of the microbial technology for paraffin control into a variety of different oil types and formations. Conventional technologies to control paraffin deposition are thermal and chemical treatments. Both of these technologies have limitations that restrict their long-term effectiveness. In particular, hot oil or water treatments may lead to increased formation damage by forcing deposited high molecular weight paraffins into the formation where they can contribute to pore throat plugging and lead to production loss. Development of MCPs represents a successful alternative technology to remove paraffin deposits without causing lasting formation damage. Long term use of MCPs showed no damage to the oil field production system and their use increased throughout the mid continent region in the early 1990s. Examples of the successful application of this technology in the oil field have been previously documented in SPE papers. An example of the type of changes produced in paraffinic oils that were associated with control of wax deposition is SPE 72129 Microbial Enhanced Oil Recovery: Diverse Successful Applications of Biotechnology in the Oil Field S.A. Bailey, SPE, T.M. Kenney, and D.R. Schneider, SPE, Micro-Bac International, Inc. 2 S.A. BAILEY, T.M. KENNEY, D.R. SCHNEIDER SPE 72129 shown in Figures 1 and 2. In the first Figure, the viscosity of the oil has been reduced by microbial treatment. This reduction in viscosity is probably due to the production of small solvent molecules by the microbial population. These solvent molecules include alcohols, ketones and aldehydes and are functionally similar to oil field chemicals used as wax dispersants and pour point depressants. This "thinning" action can also be seen in Figure 2 where the metabolic capacity of the microorganisms to degrade high molecular weight paraffin molecules results in a change in the hydrocarbon profile of the oil as detected by gas chromatography. With a reduction in the average molecular weight in the hydrocarbon component, increases in API gravity and reductions in viscosity are frequently seen. This reduction in viscosity may lead to increased relative permeability and increased oil production, which will be discussed in later sections. Scale and Corrosion Control. Observations that some of the biochemicals produced by microorganisms had properties similar to scale and corrosion control chemicals lead to the development of new MCP product lines to address these oil field problems. The deposition of mineral scales in oil wells is a well-understood phenomenon. It is often related to the commingling of waters of different chemical types that produce a blend of ions that exceeds the solubility limit for compounds such as calcium carbonate, calcium sulfate or barium sulfate (to name the most commonly encountered oil field scales). Scale deposition can also be related to temperature and pressure changes occurring in the production string as the fluid column is brought to the surface. Conventional chemical technologies utilize compounds that control scale through either chelation or dispersant mechanisms. Microbial biochemicals such as organic acids are naturally occurring chelating agents that bind cations and thus restrict their capacity to form mineral deposits. Likewise, microbial biosurfactants such as rhamnose and trehalose glycolipids serve a similar function. Other microbial compounds may act as filming agents, coating surfaces and preventing nucleation sites for scale growth from forming. The ability of MCPs to prevent scale growth in vitro is shown in Figures 3 and 4. These photomicrographs show how the growth of typical oilfield scale crystals in brine can be controlled by treatment with MCPs. The mechanisms by which MCPs control scaling are chelation, crystal modification, and dispersion of scale nuclei. Filming activity of biosurfactants also prevents attachment of scale crystals to surfaces. Such filming agents may also act as passivating agents for controlling corrosion of metal surfaces. By coating surfaces, the interaction between corrosive compounds such as carbonic acids and sulfides is mitigated and corrosive processes reduced. Another effect associated with microbial culture product use is a decrease in the number and activity of sulfate reducing bacteria. Presumably due to competition for the usually limiting nutrients found in oil field production systems, decreases of two to three logs in sulfate reducing bacteria numbers are common. Waterflood Treatments. MCPs have found successful applications in waterfloods as well. In waterflood operations where injection of the drive fluid is restricted by scale and/or organic deposits, MCPs have been used to open up injector wells and improve injectivity. Deposits of mineral scale can form at any location in an injection system from surface equipment to downhole in the formation, and preformed scale from the surface may be carried downhole by the fluid flow. Skin and formation damage can also occur in injection wells by organic deposits such as paraffin or asphaltene precipitates, or stable emulsions formed from residual oil and grease carried over during re-injection of produced water. Buildup of scale and/or organic deposits in the near wellbore region blocks pore channels, which decreases permeability and can severely restrict fluid flow into the formation. These occlusions are a leading cause of reduced waterflood efficiency. MCP scale and corrosion control products can be used to inhibit and remove these occlusions from injection wells. Treatment of injection systems with MCPs has been shown to increase volumes of water injected as well as reduce injection pressures and energy costs. Application of MCPs for these types of treatments has been the subject of previous reports. In the reservoir, MCPs produce several enhanced oil recovery compounds that decrease capillary forces and increase oil mobility. Microbial metabolites such as surfactants, solvents, low-molecular-weight organic acids, and gases are well-known oil mobilizing agents. These products work by the same mechanism as traditional EOR chemicals to reduce interfacial tension, decrease oil viscosity, and improve the microscopic sweep efficiency of the waterflood. Application of MCPs in waterflooded reservoirs to improve sweep efficiency and increase recoverable reserves has been widely used. Well Stimulation for Increased Oil Production By far the largest application of microbial culture products, both in terms of volume of products used and number of wells treated, is well stimulation to increase the oil production rate. Mechanisms by which MCPs stimulate increased oil production range from removing skin and/or formation damage and opening pore channels, to improving oil flow properties. As oil flows through the reservoir toward the wellbore, high-molecular-weight fractions such as waxes, paraffins, and asphaltenes precipitate out of the oil, forming deposits on the rock matrix and occlusions in the pore channels. This deposition can become much more severe if the producing temperature is near or below the cloud point, or if