enhanced oil recovery : approaches and recovery...

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Enhanced Oil Recovery : Approaches

and Recovery Methods

King Saud University – Riyadh - 2007

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Webster defines - - 'to enhance' is to increase, to make or become larger, to advance, to elevate.

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• An Enhanced Oil Recovery (EOR) process aims at recovering oil over and above the amount produced by a traditional waterflood or by an immiscible gas injection project.

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Approach No. 1

• Increase contact between the Injection fluid and the remaining oil in the reservoir pore space.

• Lowering the Mobility ratio is the key!

• M = Mobility of Injection Fluid/ Mobility of Oil

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• Viscosity of oil can be lowered by Hot Water Injection, Steam Injection, In-Situ combustion and Solvents injection

• Viscosity of water can be increased by Addition of viscosity builders such as Polymers

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• Kw can be decreased by addition of permeability modifying polymers to the injection water.

• KC02 can be decreased by alternate injection of CO2 and water –WAG- Flooding.

• Ko can be increased by the addition of chemicals (such as Caustic Soda) to the injection water.

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Approach No. 2

• Minimize or Eliminate Capillary Force.

• Lowering Interfacial Tension is the key!

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Oil-Water interfacial tension can be lowered by the addition of Surfactants to the injection water (Surfactant Flooding).

Wettability can be altered by the addition of chemicals such as Caustic Soda (Caustic Flooding).

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Oil-Water interfacial tension can be totally eliminated by:

• Injection of solvents such as LPG and Alcohols that are miscible with oil -First-Contact Miscible Flooding.

• Injection of gases such as Natural gas, CO2, N2 that can become miscible with oil due to mass exchange –Multiple Contact Miscible Flooding.

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Development of a Recovery Method

• Development of a recovery method for a specific reservoir is a lengthy process and involves:

• Laboratory research (2-4 years)

• Field screening (1-2 years)

• Field pilot (4-6 years)

• Field results analysis (1-2 years)

• Semi-commercial project (5-10 years)

• Commercial development

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SURFACTANT AND MICELLARFLOODING

Significant reduction of capillary forces at the injected fluid / crude oil interface has given birth to many EOR processes. These aim at lowering the residual oil saturation and thereby improving displacement efficiency.

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POLYMER FLOODING

• Polymer is a chemical that is composed of a number of individual molecules that are attached in some manner.

• These units are usually associated in a pattern that repeats itself throughout the length of each polymer repeating units and are called MONOMERS.

Example: Polyacrylamide (PAM)

CH2 – CH – CH2 – CH – CH2 - CH –l l l

C = O C = O C = Ol l l

NH2 NH2 NH2

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• A polymer flooding is suited for reservoirs where normal waterfloods fail due to one of the two reasons: High Heterogeneity and High oil water mobility ratio.

• Polymer floods mainly target oil in areas of the reservoir that have not been contacted efficiently.

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• Polymers are used in many other EOR processes such as micellar floods to provide mobility control.

• In addition, polymers are also employed for treating conformance problems.

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The advantages of polymer flooding are two-fold:

1.a reduction in the quantity of waterrequired to reduce the oil saturation to its residual value in the swept portion of the reservoir.

2.an increase in the areal and vertical coverage in the reservoir due to a reduced water flood mobility ratio.

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Field Projects of some Polymer Flooding

No. of Projects Mean

•• Depth, ft 87 4005• T, ºF 88 117• Permeability, md 80 453• μo , cp 82 21.5• Polymer ppm 48 279• Recovery, %OOIP 20 3.85

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Caustic and Emulsion Flooding

The alkaline solution increase the capillary number value by reacting with the organic acids present in some crude oil to form emulsifying soaps, which reduces the interfacial tension by two or three orders of magnitude.

The alkaline agent changes the injection water pH and the rock wettability is reversed from oil-wet to water-wet.

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• The process starts with a water preflush injection followed by the injection of caustic solution slug of about 10 to 30%PV and by continuous injection of derive water.

• The injection of a polymer slug behind the caustic solution for mobility control is desirable if it is cost effective.

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• Miscible oil displacement is the displacement of oil by fluids with which it mixes in all proportions without the presence of an interface, all mixtures remaining single phase.

Miscible oil displacement

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Miscible Agents

• Propane, LPG mixtures, and low molecular-weight alcohols:

• subjected to many limiting factors such as high costs, unfavorable mobility, and low volumetric sweep.

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• Natural gas, flue gas, nitrogen at high pressure, and enriched hydrocarbon gas:

• were found to achieve miscibility with reservoir oil.

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• Surfactant slugs: technically efficient.

• Carbon dioxide: miscible with reservoir fluids.

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Miscibility is achieved by one of the two methods

• First Contact Miscibility: the injection fluid mixes with the crude oil in all proportions and all their mixtures remain single phase.

• Multi-Contact Miscibility: the injected fluid and the crude oil are not miscible on first contact but through components transfer from one to the other fluid, they reach miscibility intime through multiple contacts.

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The following Injection fluids have been studied in the laboratory and applied in

many field projects.

1. A solvent such as LPG or NGL.

2. A rich hydrocarbon gas (containing C2 –C5)

3. Lean hydrocarbon gas (essentially C1)

4. Non-hydrocarbon gases such as CO2, N2. and Flue gas

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• The choice of the injection fluid depends upon the reservoir Pressureand Temperature conditions and compositions of the injection fluid and crude oil.

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• Continuous injection of any of the above fluids is often not economical. A small volume of the fluid ( less than one pore volume) is injected as a slug.

• The effectiveness of the slug demandsthat it maintains its integrity during its travel through the reservoir.

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Carbon Dioxide Flooding

• Factors that Make CO2 an EOR Agent

• Reduction in oil viscosity improves the mobility ratio

• Swelling of crude oil increases the recovery factor

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• Acid effect on carbonate and shaley rocks:

• CO2 in solution with water forms carbonic acid which increases the permeability of the carbonate rock.

• Miscibility effects:

• CO2 may develop miscibility through multiple contacts.

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CO2 FLOOD Performance

Predictions

• CO2 flood design and performance predictions differ from reservoir to reservoir and for different operation strategies.

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• Reservoir pressurization:

• is an initial phase of the CO2 miscible flood.

• CO2 requirements:

• are determined in many ways depending on the reservoir geometry and displacement direction and on the miscibility conditions and injection strategy.

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• CO2 injection pressure:

• Care must be taken to ensure that the injection pressures are always below formation parting pressure.

• The surface CO2 injection pressure is calculated to assure the required miscibility pressure in the reservoir.

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Preliminary Screening Procedure

• Assess Reservoir Suitability

• Criteria based upon theoretical and practical considerations have been identified which provide initial (go/no go) screening of a candidate reservoir for a CO2-flood project.

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• Determine Source of CO2

• Volume Requirement

• Natural or By-Product

• Supply - pipeline, truck, tank car.

• Assess Injection/Production Facilities

• Modification or New

• Metallurgy

• Produced gas re-injection or processing

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• Availability of Trained Manpower

• Project Economics

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Thermal Recovery Methods

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Steam Flooding• When steam is injected, a steam saturated zone

forms around the injection well, and further beyond there is a zone containing condensed steam.

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• Steam injection rate is an important factor, since a high rate can cause early steam breakthrough, while a low rate leads to heat loss.

• The temperature increase may cause an increase in the relative permeability to oil.

• Gravity override of steam becomes important in thick, permeable sands.

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Cyclic Steam Stimulation Is a single well process and involves:

• Injection of steam for several weeks (2 to 6 weeks) at the highest possible rates.

• The well is then shut-in for several days (3 to 6 days) to allow the steam to condense.

• Following the soaking phase, the well is put under production.

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• A long soaking period results in a loss of production, while a short period prevents adequate steam condensation.

Cyclic steam stimulation process is widely used in Canada and Venezuela because of its applicability in very viscous oil formations, and quick payout.

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• The total oil recoveryby steam stimulation averages about 10 to 15% of the oil-in-place.

• In Cold Lake, Alberta, it is over 25% or higher.

• In Venezuela, recovery from this process as high as 40% have been noted.

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In Situ Combustion

• This is a pattern flood process.

• A small portion of the oil in place is burned establishing heat to the rock and its fluids.

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• A burning front and combustion zone is propagated to the producing well by air injection into a well (forward combustion).

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• In situ combustion process is applicable for a wide range of oil gravities (8 to 36ºAPI), but commercial success has been possible only in oils that are sufficiently mobile at reservoir conditions.

• Oil viscosity should be less than 5000 cp.

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Bio-Chemical Recovery Method

• During the last fifteen years scientific and engineering efforts in the laboratories of King Saud University (Saudi Arabia) and Cairo University (Egypt) has established the basic start for Microbial Enhanced Oil Research technology in the Arab World.

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• It is expected that Microbial Enhanced Oil Recovery may recover up to 30% of the residual oil under the Arab reservoir conditions.

• The actual recovery, however can only be determined through laboratory and pilot tests under field conditions.

• A new technology should be developed to apply MEOR successfully.

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• Microbial enhanced oil recovery (MEOR) technology is the process of introducing or stimulating microorganisms in an oil reservoir for the purpose of enhancing oil recovery.

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• Some microorganisms produce gases that could improve oil recovery.

• Some other species produce acids that can improve permeability of the reservoir rocks thus improve recovery.

• Microorganisms produce bio-surfactant can decrease surface, and interfacial tension between oil and water, which causes emulsification.

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• Several research studies in our Laboratory have shown, that MEOR is a potentially effective technology for increasing oil recovery through the improvements in interfacial forces, wettability characteristics, displacement tests and modeling of the process.

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• There are different forms of microbial oil recovery:

• Cyclic well stimulation treatments.

• Microbial enhanced water flooding.

• Permeability modifications.

• Well-bore cleanup.

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• In cyclic microbial well stimulation treatments, improvements in heavy oil production can result from removal of asphalted deposits from the near-wellbore region or from mobilization of residual oil.

• Microbial well stimulationprocess can be considered successful not only by improving oil production rate but also decreasing the cost of maintenance and operation of a well.

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• For a microbial water flooding, It is important that bacteria be capable of moving through the reservoir and producing chemicalproducts to mobilize oil.

• It has been suggested that some bacteria producing polymerscould be used in situ to plug high-permeability zones.

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• In our laboratories at Cairo University the experimental results reported that some species of bacteria producing polymers and bio-surfactant that can be used in field applications of MEOR processes.

• Currently, our research studies indicated that some species of bacteria can resist high temperature effects. These results have not reported before.

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Environmental Effects of MEOR

• The environmental control of MEOR is of great importance.

• It is necessary to prevent any adverse effects on the environment when applying this recovery method.

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• Great effort is being expanded by investigators to understand the complex subsurface environment of a petroleum reservoir in relation to bacterial metabolism.

• One of the possible effects is the stimulation of indigenous sulfate-reducing bacteria which causes bio-corrosion in oil fields.

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• The effect of microorganisms, used in MEOR laboratory tests, on the corrosion of surface and subsurface equipment in oil fields was investigated recently.

• The possible contaminations of surface, ground water and agriculture land during bacterial transport are of major environmental concern associated with MEOR field application.

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• Sometimes the mineral content of the initial water in the oil formation may inhibit the growth of the selected bacteria.

• Injected and connate water salinitiesequal or less than 100,000 ppm is required for the application of the MEOR process.

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• Some types of microorganisms, however, can live in higher salinity environment, although great efforts will be needed to identify such organisms that resist high salinity conditions. Some of these efforts are now being done by the investigators at Cairo University.

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