Microbial Enhanced Oil Recovery

What Is Microbial Enhanced Oil Recovery?

Microbial enhanced oil recovery (also called MEOR or microbial EOR) is an enhanced oil recovery technique that harnesses indigenous reservoir microorganisms or introduces specially selected microbial cultures to produce biogenic gases, biosurfactants, biopolymers, and organic acids that collectively improve oil displacement efficiency. These metabolic products reduce oil viscosity, alter rock wettability toward more water-wet conditions, generate additional reservoir drive pressure, plug high-permeability streaks to improve sweep conformance, and break down heavy paraffin and asphaltene deposits that restrict flow near the wellbore.

How Microbial Enhanced Oil Recovery Works

The MEOR process begins with reservoir characterization to identify which microbial communities are present in the formation water and what metabolic pathways they are capable of performing under reservoir conditions. Modern molecular biology tools, particularly 16S ribosomal RNA gene sequencing, allow operators to census reservoir microbiomes without culturing, revealing the presence of biosurfactant-producing Bacillus species, biopolymer-producing Leuconostoc species, and hydrocarbon-degrading Pseudomonas and Arthrobacter strains. Once the microbial community is characterized, nutrient design targets stimulation of the most beneficial metabolic pathways. Injection of molasses or glucose as a carbon energy source, combined with ammonium nitrate or urea as nitrogen source and phosphate as a co-nutrient, stimulates anaerobic fermentation pathways that produce CO₂, methane, hydrogen, and organic acids within the reservoir. The generated CO₂ and methane dissolve into and pressurize the crude oil, reducing its viscosity and providing supplemental drive pressure that sweeps oil toward producing wells.

Biosurfactant production represents a second critical MEOR mechanism. Microbially produced glycolipids such as rhamnolipid from Pseudomonas aeruginosa and lipopeptides such as surfactin from Bacillus subtilis can reduce oil-water IFT from 20 to 30 mN/m down to 0.1 to 1 mN/m, which is substantially less than the ultra-low values achieved by synthetic surfactants but meaningful for mobilizing larger oil globules. These biosurfactants also alter the wettability of sandstone and carbonate pore surfaces from oil-wet toward water-wet, improving oil release from pore walls. Biopolymers produced by bacteria such as xanthan from Xanthomonas campestris and dextran from Leuconostoc increase the viscosity of the injected water phase, improving the mobility ratio between water and oil in a manner directly analogous to synthetic polymer flooding but at far lower reagent cost since the polymer is synthesized in situ. Selective plugging of thief zones by bacterial growth and biopolymer accumulation can redirect injection water into lower-permeability oil-bearing intervals that a conventional waterflood would bypass.

In-Situ Stimulation Versus Ex-Situ Microbial Injection

MEOR field applications follow two broad strategies. In-situ stimulation, the more widely practiced approach, relies entirely on the microorganisms already living in the reservoir. Nutrient slugs of carbon source, nitrogen, and phosphorus are pumped into injection wells, where indigenous bacteria metabolize them and produce the desired bioproducts. This approach avoids the logistical complexity and expense of growing and shipping large volumes of live microbial cultures, and the indigenous bacteria are already adapted to survive the reservoir temperature, pressure, salinity, and pH. The challenge is that operators have limited control over which metabolic pathways dominate: sulfate-reducing bacteria stimulated alongside biosurfactant producers can generate hydrogen sulfide, souring the reservoir and corroding equipment. Careful nutrient design using nitrate rather than sulfate as the terminal electron acceptor can suppress sulfate reducers while stimulating nitrate-reducing bacteria that are more likely to produce useful bioproducts.

Ex-situ microbial injection introduces laboratory-grown cultures of selected microorganisms, often Bacillus or Pseudomonas strains with demonstrated biosurfactant or biopolymer production, directly into the reservoir via injection wells. This gives operators greater control over the specific metabolic pathways activated, but the injected cells must survive filtration through reservoir pore throats, competition with indigenous microbial communities, and the hostile geochemical environment they encounter. Survival rates for injected organisms are often low, particularly in reservoirs with formation temperatures above 60 degrees Celsius or with highly saline brines. Several commercial MEOR service companies have developed spore-forming Bacillus strains that survive high-temperature sterilization and injection as dormant spores that germinate once they reach nutrient-rich zones within the reservoir, improving survival and distribution efficiency.

Microbial Enhanced Oil Recovery Synonyms and Related Terminology

• MEOR: the universal abbreviation used in technical literature and regulatory submissions
• microbial EOR: informal full-form version of the abbreviation, used interchangeably with MEOR
• biostimulation: specifically refers to the in-situ nutrient injection approach that stimulates indigenous reservoir microbes
• bioaugmentation: refers to the introduction of exogenous microbial cultures into the reservoir to supplement indigenous populations

Related terms: enhanced oil recovery, polymer flooding, biosurfactant, conformance control, reservoir souring

Frequently Asked Questions About Microbial Enhanced Oil Recovery

Why hasn't MEOR been adopted more widely despite its low cost?

The primary barrier is unpredictability. Microbial communities in reservoirs are complex ecosystems that respond to nutrient injection in ways that are difficult to forecast from surface laboratory tests alone. Field pilots have produced highly variable results even in geologically similar reservoirs with comparable indigenous microbiomes, because subtle differences in pore water chemistry, pH, redox conditions, and competing microbial metabolic pathways produce very different outcomes. Operators and reservoir engineers, who are trained to manage risk through deterministic models, have been reluctant to commit capital to a process whose performance is inherently probabilistic and dependent on biology rather than physics and chemistry. Advances in metagenomics and reservoir biogeochemistry modeling are slowly improving predictability, which should support broader adoption over the coming decade.

Does MEOR work in carbonate reservoirs as well as sandstones?

MEOR has been piloted in both sandstone and carbonate reservoirs, with some notable successes in carbonates. The wettability alteration mechanism may actually be more impactful in carbonates, which are often naturally oil-wet and hold larger volumes of residual oil on mineral surfaces than water-wet sandstones. Biosurfactant flooding of carbonate cores in laboratory studies has demonstrated significant incremental oil release through wettability reversal. The acid-producing metabolic pathway, where organic acids from bacterial fermentation react with carbonate rock to dissolve pore-filling calcite and improve permeability, is unique to carbonate reservoirs and represents an additional MEOR mechanism not available in siliciclastic systems. However, carbonate reservoirs often have larger fracture networks that create early breakthrough of nutrients and bacteria to producing wells, reducing contact time with the oil-bearing matrix.

How is MEOR performance monitored during a field pilot?

Monitoring a MEOR pilot involves tracking multiple indicators simultaneously. Produced water samples from offset producers are analyzed for microbial population changes using culture-based and molecular methods to confirm that the nutrient injection is stimulating the target metabolic pathways. Gas composition of produced gas is measured for increases in CO₂ and methane that indicate active fermentation. Produced oil is analyzed for changes in API gravity, viscosity, and paraffin content that reflect biodegradation activity. IFT measurements on produced water confirm biosurfactant breakthrough. Production data from the pilot pattern, including oil rate, water cut, and injection pressure, are tracked against a pre-pilot decline model to quantify incremental oil attributed to MEOR. This multi-parameter monitoring approach is essential because no single indicator conclusively proves that observed production improvements are caused by MEOR rather than natural reservoir variability.

Why Microbial Enhanced Oil Recovery Matters in Oil and Gas

MEOR represents one of the most cost-effective EOR options available to operators of mature, low-pressure fields that cannot support the capital intensity of CO2 floods or chemical EOR programs. Its primary appeal is economic: nutrient costs are orders of magnitude lower than synthetic surfactant or polymer costs, and the process does not require expensive mixing and injection facilities beyond what is already in place for waterflooding. For small independent operators and national oil companies managing large inventories of aging fields in developing economies, MEOR offers a realistic path to extending field life and improving ultimate recovery without the infrastructure investment that more sophisticated EOR methods demand. As global energy policy increasingly emphasizes maximizing recovery from existing fields before opening new frontier areas, MEOR is drawing renewed interest from research institutions, EOR service companies, and operators looking for economically viable solutions to the challenge of recovering the estimated two trillion barrels of oil currently classified as residual and unrecoverable by conventional methods.