A Niche for Enhanced Oil Recovery in the 1990S- Disk 2

A Niche for Enhanced Oil Recovery in the 1990s

Aging fields and dwindling prospects for finding new, large reserves are turning attention to improving recovery from known oil fields. What is the status of enhanced oil recovery (EOR) and what role might it potentially play in the next few years?

Larry W. Lake Traditional primary and secondary produc- America. Reserves in the aging oil fields of University of Texas tion methods typically recover one third of the US and Canada are declining faster than Austin, Texas, USA oil in place, leaving two thirds behind. The new oil is being added by discoveries. In reasons for this are not difficult to under- the US, for example, 70% of the approxi- Raymond L. Schmidt stand. During the life of a well, there is mately 500 billion barrels of oil discovered Chevron Oil Field Research always a point at which the cost of produc- were found during the earliest 20% of La Habra, California, USA ing an additional barrel of oil is higher than drilling. About 130 billion barrels have been the price the market will pay for that barrel. produced to date and up to another 170 bil- Paul B. Venuto Production then halts. Under normal cir- lion barrels are considered a long-term tar- Mobil Research and Development Corp. cumstances, the well is abandoned, with get for advanced EOR technology. The situ- Princeton, New Jersey, USA 70% of the oil left in the ground. ation is similar in Canada. Given the Except for brief periods in which EOR was declining reserves and the low probability economical, or perceived to be so, there of locating significant new fields, producers were good economic reasons not to nurse sought additional oil in old reservoirs, mak- every drop of oil from a well. Oil was easy ing North America a proving ground for to find and another giant field was just EOR techniques. Today, it is estimated that around the corner—the cost of a newly North America produces more than half the found barrel of oil was far less than the cost world’s EOR production (below). of an EOR incremental barrel. This situation Research and development on many has begun to change, especially in North fronts indicate that the risks of EOR are

Estimated Annual Wordwide EOR Produced Oil, B/D (x1000) Country Thermal Miscible Chemical EOR Total % USA 454 191 11.9 656.9 42 Canada 8 127 17.2152.2 10 Europe 14 3 — 17.0 1 Venezuela 108 11 — 119.0 7 Other S. American 2NANA 17.0 1 USSR 20 90 50.0 160.0 10 Other (estimated) 171* 280** 1.5 452.5 29 Total 777 702 80.6 1574.6 100

*Mainly Duri field (Indonesia) **Mainly Hassi-Messaoud (Algeria) and Intisar (Libya) nEstimated annual worldwide production of oil by EOR. (USSR data from Simandoux P and Valentin E: “Improved Recovery, Strategic Option or Not?” pre- sented at the Offshore North Seas Conference, Stavanger, Norway, August 28-31, 1990; other data from Oil & Gas Journal 88, no. 17 (April 23, 1990): 62-67.)

January 1992 55 24 reservoir pressure, or pumping, until deple- •Fluid-flow simulations, based on a geo- 22 tion. Until the early 1940s, economics dic- logic reservoir model, can start with 20 tated when a well was to be plugged and assessment of primary and secondary abandoned, usually after recovery of 10 to recovery, matching the production history 18 Worldwide 25% of original oil in place (OOIP). to determine residual oil and waterflood oil production Secondary recovery methods are gener- recovery. Then EOR process-variable sen- 16 ally used to repressure the reservoir and sitivities can be calculated, followed by 14 drive out some of the remaining oil. predictions of EOR recovery, incremental Because water is usually readily available production rate and payout economics. 12 and inexpensive, the oldest secondary Reservoir geologic models are always 10 recovery method is waterflooding, pumping constrained by sparse data, simplified water through injection wells into the reser- concepts of reservoir structure and 8 voir. The water is forced from injection dynamics, inadequate data for history 6 wells through the rock pores, sweeping the matching and increasing computational

Oil production, billions of barells/yrOil production, oil ahead of it toward production wells. This uncertainty as calculations are extrapo- 4 is practical for light to medium crudes. Over lated into the future. Consequently, pre- Worldwide EOR 2 oil production time, the percentage of water in produced dictions that cover years of EOR perfor- fluids—the water cut—steadily increases. mance may be seriously in error. In 0 76 78 80 82 84 86 88 90 Some wells remain economical with a addition, small-scale heterogeneities, Year water cut as high as 99%. But at some which are difficult to define, are critical to point, the cost of removing and disposing of the success of EOR. nA slow, but steady increase in percent of world oil produced by EOR. Percent of water exceeds the income from oil produc- •Usually, a pilot test of the proposed EOR oil produced by EOR has more than dou- tion, and secondary recovery is then halted. process is carried out to investigate a bled since 1982, when EOR oil accounted Extensive waterflooding, which began in novel technique or to confirm expected for 0.9% of worldwide production. In 1990, the 1940s, within a few decades became performance before an expensive, full- EOR accounted for 450 million—about 2%—of the 22 billion barrels of oil pro- the established method for secondary oil scale implementation. Ideally, the pilot duced. (From Oil & Gas Journal data.) recovery, usually recovering about another test is performed in an area that is geolog- 15% of OOIP. On average, about one-third ically similar to the field and large enough of OOIP is recovered, leaving two-thirds, or to be statistically representative of overall being reduced and the potential for EOR twice as much oil as is produced, in the heterogeneity. Monitoring and data acqui- profitability increased (above). Computer- ground after secondary recovery.1 sition throughout pilot testing provide ized characterization of the reservoir, which Another recognized secondary recovery information needed to plan a full-scale quantifies the physical characteristics and technique is injection of a hydrocarbon- commercial operation. dynamic behavior of a field, is becoming based gas into an existing gas cap or •For commercial operations, important one of the most important tools for directly into the oil itself. Gas may be considerations are secure sources of water improved oil recovery (see “Reservoir Char- injected over a considerable period of and other injectants, storage and trans- acterization Using Expert Knowledge, Data time—up to a year—while producing wells portation facilities (like pipelines), surface and Statistics,” page 25). Success of oil are shut in, until reservoir pressure is processing, separation, recycling and recovery depends on applying the energy of restored and production resumed. Another upgrading facilities, and environmental injected fluids in the right place, in the right method is injection of gas to sustain pres- and safety requirements.3 amount and at the right time—a strategy sure during production. Gas injection The same principles of EOR engineering that a well-constructed reservoir simulator requires a nearby source of inexpensive gas may not apply to offshore oil fields. Because can help develop. in sufficient volume. offshore wells tend to be highly deviated or EOR is an imprecise term that historically While waterflooding is effective in nearly extended reach, the distance between them has been used to describe the third step (ter- all reservoirs, no single EOR technique is a is often greater than between onshore wells. tiary recovery) in oil and gas production. cure-all. Most reservoirs are complex, as are This extends the time between EOR initia- The term “improved oil recovery” (IOR) has most EOR processes. Efficient reservoir tion and meaningful results and flattens the come into use to describe all recovery management treats EOR as a high-cost, recovery response. These effects complicate methods other than natural (primary) pro- high-risk but critical component of a com- process control and limit the number of duction, reserving the designation EOR for prehensive plan that spans primary recovery EOR techniques that may be applicable. those processes beyond simple waterflood through abandonment.2 Greater spacing between wells also and gasflood—basically, recovery by injec- Once preliminary reservoir information increases the likelihood of undetectable het- tion of anything not originally in the reser- has been assembled and used to select EOR erogeneities between wells, impairing simu- voir (next page, top). The three major EOR options, engineering project design usually lations of well behavior. Because the num- methods are thermal (application of heat), follows several steps. ber of wells that can be drilled from a miscible (mixing of oil with a solvent) and •Laboratory studies test the proposed EOR platform is fixed, infill drilling, often an chemical (flooding with chemicals). processes in corefloods with samples of important strategy for both secondary recov- Primary recovery, in long accepted prac- reservoir rock and fluids. These small, ery and EOR, may not be possible. High tice, is defined as production by natural one-dimensional flow tests in relatively costs and extended time before EOR pro- homogeneous media do not always suc- duction begins mean that offshore EOR pro- cessfully scale up to reservoir dimensions. jects must be planned and started early But if the process fails in the laboratory, it will more than likely fail in the field. 56 Oilfield Review Oil Recovery Mechanisms enough so that production increases incre- mentally before primary and secondary pro- Conventional Primary duction begins to decline. Otherwise, Oil Recovery marginal costs may be too high to sustain profitability. However, this option must be balanced against other risks: insufficient Natural flow Artificial lift reservoir description at early stages of field Secondary production and lack of time to acquire pilot test results to evaluate the EOR process.4 Various mechanisms thwart recovery of 5 Waterflood Pressure much of OOIP after secondary recovery. Tertiary maintenance Reservoir geologic heterogeneities may cause a large volume of mobile oil to be bypassed and remain within a field. This is a result of poor sweep efficiency when EOR Chemical Thermal Miscible Other: microbial, electrical, injected displacement water moves prefer- chemical leaching, entially through higher permeability zones mechanical (vibrating, toward the production well. Even in regions horizontal drilling) that have been swept by large quantities of water, residual, immobile oil is held in the pore spaces by capillary forces. Many techniques have been tried in the Surfactant Polymer Caustic CO2 Miscible Inert gas solvent laboratory and field in hopes of recovering this additional oil. All employ one or more of three basic mechanisms for improving on Steam stimulation Steam or In-situ waterdrive alone: or cyclic steam hot water combustion injection •Increase the mobility of the displacement medium by increasing the viscosity of the water, decreasing the viscosity of the oil, or both. Foam •Extract the oil with a solvent. displacement •Reduce the interfacial tension between the oil and water. nOil recovery mechanisms. The three major EOR processes—thermal, (Adapted from Venuto PB, reference 2 and Donaldson EC et al, reference 5.) miscible and chemical—are each subdi- vided into several categories. Among the 60 nCost-perfor- three, thermal processes dominate, having mance compari- the greatest certainty of success and poten- son of major EOR tial application in about 70% of enhanced 50 methods. (Adapted from Simandoux P, oil recovery worldwide. Thermal methods Champlon D and also give the highest recoveries at the lowest 40 Velentin E: “Manag- costs (left). Surfactant ing the Cost of The term miscible means the mixing of Enhanced Oil Recov- two fluids—for instance, oil and a solvent ery,” Revue de L’Insti- 30 Thermal tut Français du Pét- such as carbon dioxide [CO2]—into a sin- role 45, no. 1 gle-phase fluid. It may also apply to a conti- CO injection (January-February 20 2 nuity between the oil and injected gas, due 1990): 131-139.) Polymer to a multiphase transition zone between the two. Use of miscible gasdrive has grown Incremental oil cost, $/barrel Incremental 10 rapidly in recent years, and today the Waterflooding method accounts for about 18% of EOR 0 applications worldwide. It has been suc- 01020304050607080 cessful at depths greater than 2000 ft [610 Total recovery, (% OOIP) m] for CO2 and greater than 3000 ft [915 m] for other gases. 1. Smith RV: “Enhanced Oil Recovery Update,” 4. Simandoux P and Valentin E: “Improved Recovery, a EOR chemical processes, such as surfac- Petroleum Engineer International 60, 61, four-part Strategic Option or Not?” Presented at the Offshore tant (detergent) flooding, have tantalized series (November 1988 to March 1989). North Seas Conference, Stavanger, Norway, August the industry with promises of significantly 2. Venuto PB: “Tailoring EOR Processes to Geologic Envi- 28-31, 1990. ronments,” World Oil 209 (November 1989): 61-68. 5. Smith RV, reference 1. improved recovery. As yet, cost and techni- 3. Venuto PB, reference 2. Donaldson EC, Chilingarian GV and Yen TF (eds): cal problems have precluded them from Schmidt RL: “Thermal Enhanced Oil Recovery—Cur- Enhanced Oil Recovery, II—Processes and mainstream application. Waiting in the rent Status and Future Needs,” Chemical Engineering Operations, Developments in Petroleum Science. Progress (January 1990): 47-59. Amsterdam, The Netherlands: Elsevier Science Pub- lishers, 1989. January 1992 Schmidt RL, reference 3. 57 Oil viscosity, centipoise, at reservoir conditions wings are processes like microbial EOR EOR method 0.11.0 10 100 1000 10,000 100,000 1,000,000 (MEOR) and some novel and exotic propos- als; these await confirmation by lab and Hydrocarbon miscible field experimentation and evaluation before taking their place as accepted practice. Nitrogen and flue gas Each EOR process is suited to a particular type of reservoir. Because unexpected or CO2 miscible unknown reservoir characteristics cause most EOR failures, EOR begins with thor- Surfactant/ polymer ough geologic study. Technical rule-of- thumb screening criteria are available to aid Polymer preliminary evaluation of a reservoir’s suit- ability for EOR (left). After these criteria are Alkaline applied to a prospect, stringent economic analysis follows, generally through repeated Fireflood reservoir simulations6 (see “Trends in Reser- voir Management,” page 8). Steamdrive Thermal Thermal methods are the main means of Permeability, md EOR method recovering heavy oils, those with gravity 10 100 1000 10,000 less than 20° API, representing viscosities of 200 to 2000 centipoise (cp). Such heavy oils Hydrocarbon miscible Not critical if uniform generally don’t respond significantly to pri- mary production or waterflooding so initial Nitrogen and flue gas Not critical if uniform oil saturation is typically high at the start of a thermal recovery project. The principle of CO2 miscible High enough for good injection rates thermal recovery is simple: increasing the oil’s temperature dramatically reduces its Surfactant/ polymer viscosity, improving the mobility ratio (next page, left). The two primary methods of Polymer heating reservoir oil are injection of fluid heated at the surface or production of heat Alkaline directly within the reservoir by burning some of the oil in place. Fireflood Although the idea of heating reservoirs dates back more than 100 years, large-scale Steamdrive steamdrive projects began in heavy oil fields in the US in the early 1950s and were fol- lowed shortly by projects in The Nether- Depth, ft EOR method lands and Venezuela. A relative of steam- 2000 4000 6000 8000 10,000 drive is cyclic steam injection, also called Hydrocarbon steam soak or “huff and puff.” It was miscible Deep enough for required pressure discovered accidentally in 1960 during a Nitrogen and Venezuelan recovery project. Cyclic steam flue gas Deep enough for required pressure recovery uses a single well for both injec- tion and production. Steam is injected into a CO miscible 2 Deep enough for optimum pressure well for several days or weeks, then the well Surfactant/ is shut in for several days to a month or polymer Limited by temperature more, the soak period. After this, the well is produced for up to six months, after which Polymer Limited by temperature the process is repeated. The steam heats the rock and fluids surrounding the wellbore Alkaline and also provides some drive pressure; by the time production resumes, the steam has Fireflood Deep enough for required pressure condensed and oil and water are produced.

Steamdrive Normal range 6. Taber JJ and Martin FD: “Technical Screening Guides for the Enhanced Recovery of Oil,” paper SPE 12069, Crudes up to 20° API presented at the 58th SPE Annual Technical Confer- ence and Exhibition, San Francisco, California, USA, ° October 5-8, 1983. Lighter crudes, 30 API Venuto PB, reference 2. 7. Taber JJ and Martin FD, reference 6. Good Possible Fair Difficult Not feasible Smith RV, reference 1. Donaldson et al, reference 5. nSelection of EOR techniques by oil viscosity, permeability and depth. (Adapted from Venuto PB, reference 2. Taber JJ and Martin FD, reference 6.) Schmidt RL, reference 3. The advantage of “huff and puff” is the rela- be insulated, as well as downhole tubing attempts to recover oil by igniting a portion tively short “huff” time so that the well is below 1500 ft [460 m]. Thermal expansion of the in-place crude by injecting air or oxy- producing most of the time. The disadvan- may damage downhole equipment and gen or by chemical or electrical means tage is that only the reservoir near the well- cause cement failure. Steam is very reactive, (below). Fireflooding appears attractive: it is bore is stimulated. This method therefore causing pipe corrosion and scaling, miner- thermally more efficient than steam, has no often has to be followed by continuous alogical dissolution or reprecipitation, clay depth restriction and is well suited to rela- steam injection to drive oil toward a sepa- swelling and changes in permeabilities. tively thin (less than 25 ft [8 m]) reservoir rate production well. Reservoir pressure, Steam is more mobile than oil, overriding sands. But in practice it is not so simple. Cap- dictated by depth, imposes a limit on steam- the oil and channeling through thief zones; ital costs are high, the process is extremely flooding: higher pressures require more fuel no satisfactory methods have been found to complicated and difficult to predict or con- to generate higher temperatures on the sur- improve sweep efficiency. Steam is usually trol, and operational problems from the high face to produce saturated steam needed for generated by burning natural gas, but if temperatures include cement failures, sand- efficient steamflood. The higher temperature lease crude is used—typically at about one ing/erosion, corrosion at both injection and also leads to greater heat losses. barrel of crude for three or four barrels of oil production wells because of oxygen and The recovery mechanism of steamflood- recovered—the air pollution byproduct may moisture, and high gas production rate. ing is complex. Besides viscosity reduction, be costly to control. Despite some successes and more than 30 the second most important recovery mecha- The other significant thermal recovery years of laboratory and field trials, in-situ nism is steam distillation of lighter compo- process, in-situ combustion or fireflooding, combustion applications have not increased.7

10,000

10 ° API 100

20 ° API

10

Viscosity, centipoise Viscosity, 30 ° API

2 100°F 200°F 300°F 38°C 93°C 149°C Crude oil temperature

nThe empirical relationship of crude oil viscosity and temperature depends on oil composition. (Adapted from Lake LW, refer- ence 10.)

nents of the oil which form a solvent bank ahead of the steam. Other factors are ther- mal expansion, solution gasdrive and misci- ble and emulsion drive. Steamflooding has been tested in light oil reservoirs where the viscosity mechanism plays a minor role and distillation to solvents predominates. Some nChanges in a 3D seismic section for “preburn,” “mid- field tests of light oil steamfloods have burn” and “postburn.” The bright spot—an increase in shown virtually 100% recovery of oil from envelope amplitude, marked by arrowheads—appeared zones reached by the steam. at midburn time and expanded by postburn time. Below the bright spot is a dim spot, caused by a decrease in All is not roses with steamflooding, as envelope amplitude. The bright spot is caused by numerous technical problems may com- increased gas saturation along the top of the reservoir bine to make a recovery project uneco- boundary. The dim spot shows good correlation with the nomical, inefficient or even dangerous. burn volume in distribution and direction, as deter- Heat losses in surface equipment, in the mined by core analysis. From studies like these, seismic data can be used to map an estimated burn thickness. wellbore, in rocks around the reservoir, in (Courtesy of the Society of Exploration Geophysicists. From connate water and in the gas cap may Greaves RJ and Fulp TJ: “Three-dimensional Seismic Monitoring defeat the process. Surface equipment must of an Enhanced Oil Recovery Process,” Geophysics 52, no. 9 (September 1987): 1175-1187.) 59 Miscible Injector Producer The fastest growing EOR process, miscible nPhase behavior flooding, uses a solvent that mixes fully with and flow dynamics residual oil to overcome capillary forces and in miscible flooding increase oil mobility. Displacement effi- CO2 Residual oil (a) for ideal perfor- ciency nears 100% where the solvent con- mance, (b) during Oil bank the influence of tacts the oil and miscibility occurs. The Miscible zone fluid density and numerous successful solvents include liqui- a (c) in the setting of viscosity contrasts, fied petroleum gas (LPG), nitrogen, CO2, flue gas (mainly nitrogen and CO ) and alcohol. which produce fin- 2 gering of the EOR A sufficient supply of a particular solvent CO gas into oil. and the value of injected hydrocarbons, 2 Residual oil (Adapted from Venuto like natural gas, have considerable impact PB, reference 2.) on the choice and economics of miscible b flood projects. In Canada, for example, the abundance of natural gas makes that the choice, while the US has huge reserves of CO in the western states that have been 2 CO tapped for EOR in the Permian Basin fields 2 Residual oil of west Texas. Miscible displacement EOR can be subdi- c vided into three significant processes: misci- ble slug, enriched gas and high-pressure lean gas, including CO2. For each of these •Similarly, in vaporizing gasdrive, lean gas orders of magnitude. The result is an unsta- processes, there is a range of pressures, or is injected at high pressure, 3000 to 6000 ble front between the gas and oil which

depths, temperatures and oil gravities neces- pounds per square inch gauge, and C2–C6 allows viscous fingers to form and propa- sary to achieve and maintain miscibility. components from the oil are vaporized gate through the displaced fluid, leaving Solvents have much lower viscosity than oil, and mutually exchanged between the oil much of the hydrocarbon uncontacted so reservoir stratification—vertical and hori- and gas during multiple contacts, eventu- (above). Today, the primary means of attack- zontal permeability contrast—strongly ally forming a miscible slug. ing this problem is the water-alternating-gas affects sweep efficiency. Early breakthrough, Carbon dioxide is a special case of high- (WAG) technique. In this process, water- bypassing substantial amounts of oil, and pressure miscible recovery. This gas is flood and gasflood are alternated, with the viscous fingering problems have plagued highly soluble in crude oil, swelling the oil design parameters being timing and the many field projects involving the four misci- and reducing its viscosity, while simultane- ratio of water to gas. WAG claims the ble displacement processes: ously extracting lighter hydrocarbons by virtues of decreasing the mobility of the gas, •In the miscible slug process, a slug of liq- vaporization. The displacing gas front, maintaining pressure and saving operating uid hydrocarbons, about half the reservoir enriched by vaporized hydrocarbons costs by substituting inexpensive water for pore volume (PV), is injected and mixes through multiple contacts, forms a miscible relatively expensive gas. Ideally, the gas pro- with the oil on contact. This is followed slug as long as minimum miscibility pres- vides miscibility and the water improves

by water or chase gas to push the slug sure (MMP) is maintained. Since CO2 can sweep efficiency. However, one study of through the reservoir. extract heavier components, it is miscible WAG in 15 CO2 flood projects showed •For the enriched, or condensing, gas pro- with crude oils having fewer C2–C6 compo- lower recoveries than for cases using a sin- cess, 10 to 20% PV of natural gas, nents. Carbon dioxide has a lower MMP gle injection of CO2 followed by a water- enriched with intermediate molecular than natural gas, nitrogen or flue gas, and flood. Gravity segregation between water weight hydrocarbons, is injected, fol- therefore can be applied in shallower (lower and gas is thought to compromise the effec- lowed by lean gas and, in some cases, pressure) wells. tiveness of the WAG process.8 water. At the proper reservoir pressure, A major problem with miscible gasflood Performance of many EOR techniques

the C2–C6 components transfer from the EOR is the adverse mobility ratio caused by suffers from differences in mobility between enriched gas to the oil, forming a miscible the low viscosity of the typical injectant gas the EOR product and the oil it is supposed solvent bank. compared to oil, perhaps by one or two to recover. One possible solution to this is foam, which is dispersed gas bubbles in a 8. Taber JJ and Martin FD, reference 6. 11. Bondor PL: “Dilute Surfactant Flooding for North Sea liquid. Foam can reduce reservoir gas Smith RV, reference 1. Applications—Technical and Economics Considera- phase permeability to less than 1% of its Donaldson et al, reference 5. tions,” Proceedings of the 6th European Symposium on Improved Oil Recovery, Stavanger, Norway, May original value. Typical surfactant-based Stosur G, Singer M, Luhning R and Yurk W: 21-23, 1991, volume 2: 749-758. “Enhanced Oil Recovery in North America: Status foams can last indefinitely. Such foams and Prospects,” Energy Sources 12, no. 4 (1990): 12. Taber JJ and Martin FD, reference 6. have been used for mobility control misci- 429-437. Smith RV, reference 1. ble gas injection and steamfloods with 9. Taber JJ and Martin FD, reference 6. Venuto PB, reference 2. mixed results. Problems include abnormally Smith RV, reference 1. Lake LW, reference 10. high injector-to-producer pressure differen- 10. Taber JJ and Martin FD, reference 6. 13. Donaldson et al, reference 5. Lake LW: Enhanced Oil Recovery. Englewood Cliffs, Simkin EM and Surguchev ML: “Advanced Vibro- tials required for propagation, rapid New Jersey, USA: Prentice-Hall, Inc., 1989. seismic Technique for Water Flooded Reservoir changes in foam stability and quality as it Stimulation, Mechanism and Field Tests Results,” migrates away from the injection well and Proceedings of the 6th European Symposium on Improved Oil Recovery, Stavanger, Norway, May foam breakdown in small pores. 60 21-23, 1991, volume 1: 233-241. Chemical Injector Producer Chemicals used in EOR include polymers, surfactants and alkalis. All are mixed with water and, occasionally, other chemicals before injection. Broadly speaking, targets for chemical recovery are crudes in the Surfactant range between the heavy oils recovered by slug thermal processes and light oils recovered Polymer- Flood- thickened Oil-water Residual oil and by miscible gas injection. water fresh bank resident brine Polymer flooding is simply an accompani- water ment to waterflooding. It is the most com- monly used chemical enhancement process since it is easy to apply and requires rela- tively small investment. Although polymer flooding increases recovery by a modest nSurfactant-polymer flooding, showing the surfactant slug on its way toward the pro- amount, on the order of 5%, it can yield ducer, pushed by polymer-thickened fresh water and floodwater from the injector well. (Adapted from Donaldson EC et al, reference 5.) solid profits under the right circumstances. Adding high molecular weight polymers tant performance is optimal over a narrow sification, which lowers viscosity. Alkalies increases the viscosity of water and, with salinity range and is subject to adsorption such as sodium or potassium hydroxide are some polymers, reduces the aqueous phase and retention through ionic exchange with used. Caustics can react strongly with min- permeability without changing the relative reservoir rocks. To avoid these problems, erals in the connate water and with the permeability to oil. This can greatly improve the first slug may be a preflush water solu- reservoir rocks to the detriment of the pro- waterflood volumetric sweep efficiency. tion. However, it is often ineffective. The cess. This complex process is poorly under- Polymer concentrations are 100 to 1000 second slug, perhaps 10 to 30% PV, con- stood; it has had some technical successes parts per million (ppm) and treatment may tains the surface active agent (5 to 10% by but no great financial successes.12 require injection of 15 to 25% PV over sev- volume), hydrocarbons, electrolyte and Besides the mainstream techniques eral years followed by a typical waterflood.9 cosolvent, usually alcohol. This is followed discussed, many interesting ideas have Cross linking, or gelling, polymers in situ by a slug of polymer-thickened water for been proposed and tried. Today, most of with metallic ions can augment perfor- mobility control and, finally, typical water- these are not economical or have some mance in sweep profile control—helping to flood (above). technical flaw: plug high conductivity zones or minor frac- Koninklijke-Shell Exploratie en Produktie •Microbial EOR injects bacteria and nutri- tures that degrade sweep efficiency. The Laboratorium in Rijswijk, The Netherlands, ents into the reservoir where the bacteria main drawbacks to the use of polymers are developed a surfactant capable of being multiply and biochemically manufacture their high cost and the low injection rate injected in low concentrations (less than 1% polymers and surfactants; this technique is caused by high viscosity (which impacts by volume) in seawater in an offshore envi- still unproved, although some successes economic rate of return), degradation at ronment during late secondary recovery have been reported. higher temperatures, intolerance to high waterfloods.11 Shell’s analysis of the eco- •Thermal enhancements include downhole salinity, polymer deterioration from shear nomics of implementing this technique in a steam generation, mixing gas and solvents stress imparted by pumping, flow through moderately sized North Sea field—100 mil- with steam, downhole radio frequency tubulars and perforations, and long-term lion stock-tank barrels of OOIP—is reveal- (induction) heating, nuclear steam genera- instability in the reservoir environment. ing. Shell found that high front-end capital tion and jet leaching by high-pressure hot Surfactant flooding, also known as deter- costs would include building a chemical water with additives. gent, micellar-polymer or microemulsion plant to manufacture the product and addi- •Other ideas: injection of direct current for flooding, uses low concentrations of surfac- tional dedicated floating facilities to handle electro-osmotic effect, chemical alteration tants in water to reduce the interfacial ten- injection and production. The surfactant of micellar configuration of in-situ sion between oil and water. Although the cost alone was $18 per barrel of incremen- petroleum, earthquake simulation using idea has been around since the 1920s, seri- tal oil recovered, undiscounted at an esti- high-powered surface vibrators (successful ous research and field trials did not start mated 9 kilograms (kg) [20 lb] of product in the USSR13). until 20 years ago, and uniformly profitable injected for each barrel of oil. Projections EOR technology does not seem to be on field performance is still elusive. Of all EOR showed that front-end investment of $750 the threshold of any dramatic technological processes, surfactant floods may be riskiest, million would be incurred in front-end capi- breakthroughs. Instead progress will proba- involving the most difficult design decisions, tal costs and chemicals injected prior to the bly come through gradual evolution, stimu- requiring large capital investments, and production of significant amounts of incre- lated by growing motivation to recover being strongly affected by reservoir hetero- mental oil. The analyzed technical cost more oil from known fields. The major con- geneities. For these reasons, and as a result came to $90 per barrel assuming a 15% dis- tribution to EOR is likely to be from the con- of marginal field performance, interest in count rate (cost of capital). stantly improving art of reservoir characteri- surfactant flooding has declined. The caustic or alkali flooding process zation for predicting EOR response and A surfactant flood must be designed for a relies on a chemical reaction between the from horizontal drilling. —SM specific crude oil in a specific reservoir tak- caustic and organic acids in the crude oil to ing into account such factors as salinity, produce in-situ surfactants that lower inter- temperature, pressure and clay content.10 facial tension between water and oil. Other Generally, multiple slugs are used. Surfac- mechanisms that may enhance recovery are changing rock from oil-wet to water-wet, January 1992 which lowers interfacial tension, and emul- 61