Enhanced Oil Recovery Backgrounder

Enhanced oil recovery (EOR) refers to the recovery of oil that is left behind after primary and secondary recovery methods are either exhausted or no longer economical. Before proceeding with a discussion of EOR, it is important to have a basic understanding of primary and secondary recovery.

Primary production is the first oil out, the “easy” oil. Once a well has been drilled and completed in a hydrocarbon–bearing zone, the natural pressures at that depth will cause the oil to flow through the rock or sand formation toward the lower pressure wellbore, where it is lifted to the surface. While primary recovery is the least expensive method of extraction, since it uses natural forces to “move” the oil, it typically recovers only 10 to 15 percent of the original oil in place (OOIP).

Secondary recovery methods are used when there is insufficient underground pressure to move the remaining oil. The most common technique, waterflooding, utilizes injector wells to introduce large volumes of water under pressure into the hydrocarbon–bearing zone. As the water flows through the formation toward the producing wellbore, it sweeps some of the oil it encounters along with it. Upon reaching the surface, the oil is separated out for sale and the water is reinjected. While somewhat more expensive than primary production, waterflooding can recover an additional 10 to 30 percent of OOIP.

When waterflooding for secondary recovery reaches a point when production is no longer cost–effective, a decision must be made whether to transition the field to a tertiary recovery phase. An EOR program utilizing surfactant–polymer (SP) flooding may be indicated, depending on the characteristics of the formation being produced and the economics involved. The chemical components of the SP process, used alone or in combination, are mixed with water which is injected into the formation as in a traditional waterflood. Surfactant cleans the oil off the rock – much like dish soap cuts the grease in a frying pan; Polymer spreads the flow through more of the rock.

EOR is not a “cookie cutter” approach; rather, it is a highly–individualized process, specific to each field’s characteristics. An evaluation is performed to determine first whether a chosen field is a suitable candidate for SP flooding; and, if so, extensive laboratory testing is conducted to determine what combination of chemicals is necessary to most efficiently sweep the field. To supplement its own in house analysis, Cano has teamed with consultants that are recognized experts in surfactant–polymer flooding techniques.

Although surfactants and polymers have been in use since the 1950’s, technological advances in the 1980’s by major oil and service companies have resulted in a reliable and cost–effective technology. SP flooding has been used extensively worldwide and has been successful in recovering an additional 15 to 25 percent of OOIP. Although relatively more expensive than primary or secondary production, with oil prices above $25 bbl, SP flooding is a cost’effective solution that eliminates the exposure to exploration and drilling risk and will allow Cano Petroleum to more effectively exploit the potential of its oil fields.

Chemical Methods

Chemical methods focus mainly on alkaline–surfactant–polymer (ASP) processes that involve the injection of micellar–polymers into the reservoir. Chemical flooding reduces the interfacial tension between the in–place crude oil and the injected water, allowing the oil to be produced. Micellar fluids are composed largely of surfactants mixed with water. Goals of polymer floods are to shut off excess water in producing wells, and to improve sweep efficiency to produce more oil. Chemical field trials by industry indicate that surfactants can recover up to an additional 28% of reservoir oil; however the economics have not been favorable when the price of oil is factored against the cost of surfactants and polymers. Chemical flooding technologies are subdivided into alkaline–surfactant–polymer processes, polymer flooding, profile modification, and water shut off methods.

Gas Flooding

Gas flooding technologies primarily use carbon dioxide flooding as a method to produce more oil from the reservoir by channeling gas into previously-bypassed areas. CO2 flooding technologies experiment with a number of foams, gels, and thickening agents to improve sweep efficiency. CO2 floods are extensively used in some regions of the U.S., particularly in West Texas and the southern Rocky Mountains. CO2 flooding currently produces about 190,000 BOPD. In the past decade flooding with nitrogen gas, flue gas, and enriched natural gas have also shown some beneficial results by increasing recovery when used to re–pressure reservoirs. Nitrogen and flue gas may be useful in areas where CO2 is not economically available for use.

Microbial Processes

Microbial Enhanced Oil Recovery (MEOR) relies on microbes to ferment hydrocarbons and produce a by–product that is useful in the recovery of oil. MEOR functions by channeling oil through preferred pathways in the reservoir rock by closing/ plugging off small channels and forcing the oil to migrate through the larger pore spaces. Nutrients such as sugars, phosphates, or nitrates frequently must be injected to stimulate the growth of the microbes and aid their performance. The microbes generate surfactants and carbon dioxide that help to displace the oil.

Microbial growth can be either within the oil reservoir (in situ) or on the surface where the byproducts from microbes grown in vats are selectively removed from the nutrient media and then injected into the reservoir. For in situ MEOR processes, the microorganisms must not only survive in the reservoir environment, but must also produce the chemicals necessary for oil mobilization.

DOE has been a leader in the development of microbial technology. In addition to funding research in the United States, DOE has participated in cooperative technology exchange internationally. DOE and the Venezuelan Ministry of Energy and Mines conducted cooperative research on MEOR; a final report titled “Microbial Enhanced Oil Recovery” was published in April, 1997 (DOE/BC-97/3/SP OSTI ID:14278). This report contains research results and documentation/references in the field of MEOR. The report references many of the spin–off technologies from MEOR including waste remediation and refining (upgrading of crude oil). MEOR processes continue to be evaluated for the following different applications:

Microbial Well Stimulation. This process is being applied on a commercial basis throughout the world. The major applications have been in the heavier oil reservoirs dealing with problems associated with paraffin and asphaltene deposits. The major areas of application include the United States, Venezuela, China, and Indonesia.

Microbial Enhanced Waterflooding. This process, which requires the transport of nutrients over a long distance within the reservoir, is still in the development phase. DOE has field–tested this process in several fields in Oklahoma.

Profile Control and Sweep Improvement. This process uses microbes that produce polymers, biomass, and slimes that selectively plug the more permeable zones. This process is still in the development phase. DOE has field-tested this process in an oil field in Alabama.

MEOR has two distinct advantages: (1) microbes do not consume large amounts of energy, and (2) the use of microbes is not dependent on the price of crude oil, as compared to many of the other EOR processes. In some reservoirs, beneficial microbes are indigenous and only need nutrients to stimulate growth. Because microbial growth occurs at exponential rates, it should be possible to produce large amounts of useful products rapidly from inexpensive and/or renewable resources. Thus, MEOR has the potential to be more cost-effective than other EOR processes. Studies have shown that several microbially-produced bio-surfactants compare very favorably with chemically synthesized surfactants. The ability to produce effective surfactants at a low price may make it possible to recover substantial amounts of residual oil.

Thermal Processes

Heavy oil is recovered by introducing heat into the reservoir through thermally controlled processes. Steam flooding and in situ combustion or air injection are the most frequently-used thermal recovery methods. Steam flooding is used extensively in the heavy oil reservoirs in California. Experiments with cold production and sand injection and horizontal well production of heavy oils have been conducted mainly in Canada and Venezuela, which have extensive heavy oil reservoirs. Steam flooding is conducted by injecting steam into reservoirs that are relatively shallow, permeable, and thick, and contain moderately viscous oil. The dominant mechanism in thermal recovery by steam is the reduction in the viscosity of the oil, allowing flow to the wellbore. Problems with reservoir heterogeneity and steam distribution are being overcome. Steam flooding production in the U. S. averages nearly 500,000 BOPD. In situ combustion introduces heat in the reservoir by a process of injection air and downhole ignition to burn portions of the oil to displace additional oil. The combustion front is sustained and propagated through continuous injection of air into the reservoir. Premature breakthrough of the combustion front contributes to operational problems. Both steam flooding and in situ combustion have high surface facility costs and require special safety measures.

Novel Methods

Novel methods include downhole electric heating, microwave heating, seismic wave stimulation, and wettability reversal. Of these, seismic stimulation has met with success in Russia and is currently being tested in the U.S. Wettability studies to influence oil-wet and water-wet conditions and to design a brine to reverse wettability show promise for future EOR recovery.

Computer Simulation

Reservoir simulation is advancing rapidly with improved computing capabilities. Reservoir simulators are useful in the design and prediction of performance in EOR projects. Improved hardware and software programs are becoming available that include EOR applications. The development of computer clusters allows high speed data processing at relatively low cost. Current goals are to develop software and user guides that predict reservoir properties suitable to independent operators. Reservoir simulation should be considered as a tool in any enhanced oil recovery project. One such model, TORIS (Total Oil Recovery Information System), is an integrated suite of databases and analytical computer models focused on the U.S. oil resource. It is designed to facilitate the decision-making process integral to formulating government policy affecting the upstream petroleum sector. TORIS is capable of evaluating the entire domestic oil resource (conventional, immobile, unrecovered mobile, and undiscovered) by modeling state-of-the-art recovery processes. The program evaluates the technical and economic viability of these processes for individual reservoirs, and selects the most desirable recovery technology.