eor screening

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EOR Screening


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Learning Objectives

Describe the main methods which can be usedto improve reservoir recovery efficiency.

For each method, state whether it can improve

displacement, vertical or areal sweep efficiencyand explain how it works.

Describe screening criteria for enhanced oilrecovery methods.

Use a systematic decision analysis approach forselecting an alternative to improve reservoirrecovery efficiency.


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1. The goal of any enhanced oil recovery process is to

mobilize "remaining" oil. This is achieved by enhancing oildisplacement and volumetric sweep efficiencies. Oil displacement efficiency is improved by reducing oil

viscosity (e.g., thermal floods) or by reducing capillary

forces or interfacial tension (e.g., miscible floods). Volumetric sweep efficiency is improved by developing

a more favorable mobility ratio between the injectantand the remaining oil-in-place (e.g., polymer floods,water-alternating-gas processes).

2. It is important to identify remaining oil and the mechanismsthat are necessary to improve recovery prior toimplementing an EOR process.

Goal of EOR Techniques


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Fig.1-2. EOR methods

EOR Methods


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CLASSIFICATION OF ENHANCED RECOVERY BY THEMACN MECHANISM OF OIL DISPLACEMENT

Solvent Extraction or Miscible-Type Processes Hydrocarbon Miscible Methods

Carbon Dioxide Flooding

Nitrogen and Flue Gas

Alcohol Flooding or other Liqufied Solvent Flooding

Solvent Extraction of Mined, Oil-Bearing Ore

Interfacial Tension Reduction Processes Surfactant Flooding

Surfactant/Polymer (Micellar) Flooding (sometime including miscible-typeflooding above)

Alkaline Flooding

Viscosity reduction (of oil) or viscosity increase (of driving fluid) Steam Flooding

Fire Flooding

Polymer Flooding


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Chemical EOR Methods


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Polymer Flooding

Description

Polymer augmented waterflooding consists of adding watersoluble polymers to the water before it is injected into thereservoir.

Mechanisms That Improve Recovery Efficiency:

Increasing the viscosity of water.Decreasing the mobility of water.

Contacting a larger volume of the reservoir.


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Polymer Flooding Limitations:

High oil viscosities require a higher polymer concentration. Results are normally better if the polymer flood is started before the

water-oil ratio becomes excessively high.

Clays increase polymer adsorption.

Some heterogeneity is acceptable, but avoid extensive fractures. If

fractures are present, the crosslinked or gelled polymer techniquesmay be applicable.

Challenges: Lower injectivity than with water can adversely affect oil production

rates in early stages of polymer flood Acrylamide-type polymers loose viscosity due to sheer degradation,

or it increases in salinity & divalent ions

Xanthan gum polymers cost more, are subject to microbialdegradation, & have greater potential for wellbore plugging


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Polymer Flooding Screening parameters


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DescriptionSurfactant / polymer flooding consists of injecting a slug that

contains water, surfactant, electrolyte (salt), usually a co-solvent

(alcohol), and possibly a hydrocarbon (oil), followed by polymer-

thickened water.

Mechanisms That Improve Recovery Efficiency

Lowering the Interfacial tension between oil and water.

Solubilization of oil.

Emulsification of oil and water.

Mobility enhancement.

Surfactant/Polymer Flooding


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Surfactant/Polymer Flooding

Challenges Complex and expensive system.

Possibility of chromatographic separation of chemicals.

High adsorption of surfactant.

Interactions between surfactant and polymer.

Degradation of chemicals at high temperature.

Limitations: An areal sweep of more than 5O% for waterflood is desired.

Relatively homogeneous formation is preferred.

High amounts of anhydrite, gypsum, or clays are undesirable.Available systems provide optimum behavior within a narrow setof conditions.

With commercially available surfactants, formation water chloridesshould be < 20,000 ppm and divalent ions (Ca++ and Mg++) < 500ppm.


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Surfactant/Polymer Flooding Screening parameters


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Alkaline Flooding

Description Best result are obtained if the alkaline material reacts with the crude oil; the oil

should have an acid number of more that 0.2 mg KOH/g of oil. The interfacial tension between the alkaline solution and the crude oil should be

less than 0.001 dyne/cm. At high temperatures and in some chemicals environments, excessive amounts

of alkaline chemicals may be consumed by reaction with clays, mineral or silicain the sandstone reservoir

Carbonates are ussualy avoided because they often contain anhydrite orgypsum, which interact adversely with the caustic chemical.

Mechanisms That Improve Recovery Efficiency A reduction of interfacial tension resulting from produced surfactants. Changing wettability from oil-wet to water-wet.

Changing wettability from water wet to oil-wet. Emulsification and entrainment of oil. Emulsification and entrapment of oil to aid mobility control. Solubilization of rigid oil films at oil-water interfaces


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Alkaline Flooding

Limitations

Alkaline or caustic flooding involves the injection ofchemical such as sodium hydroxide, sodium silicate,

or sodium carbonate. These chemicals react withorganic petroleum acids in certain crudes to createsurfactants in situ and also react with reservoir rock tochange wettability.

Challenges Scaling and plugging in the producing wells. High caustic consumption.


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Alkaline Flooding Screening parameters


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Gas Floodings


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Miscible Gas Flooding (CO2 Injection)


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Description

CO2 flooding consists of injecting large quantities ofCO2 (15% or more hydrocarbon pore volumes) in thereservoir to form a miscible flood.

EOR Mechanisms

CO2 extracts the light-to-intermediate components fromthe oil, and, if the pressure is high enough, developsmiscibility to displace oil from the reservoir (vaporizinggas drive).

Viscosity reduction / oil swelling.


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Limitations

Very low Viscosity of CO2 results in poor mobilitycontrol.

Availability of CO2 Challenges

Early breakthrough of CO2 causes problems.

Corrosion in producing wells.

The necessity of separating CO2 from saleablehydrocarbons. Repressuring of CO2 for recycling.

A large requirement of CO2 per incremental barrelproduced.


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Screening Parameters

Gravity > 27 API

Viscosity 30% PV Formation type sandstone/carbonate

Net thickness relatively thin

Average permeability not critical

Transmissibility not critical

Depth > 2,300 feet


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HYDROCARBON MISCIBLE FLOODING


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Limitations

Minimum depth is set by the pressure needed tomaintain the generated miscibility. The

required pressure ranges from about 1,200 psi for theLPG process to 3,000-5,000 psi for the High PressureGas Drive, depending on the oil.

Challenges

Viscous fingering results in poor vertical andhorizontal sweep efficiency.

Large quantities of expensive products are required.

Solvent may be trapped and not recovered.


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Gravity >27 API


Net thickness relatively thin


Transmissibility not critical Depth >2,000 feet (LPG)

>5,000 feet (lean gas)

Temperature


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NITROGEN ANO FLUE GAS FLOODING

Description Nitrogen or flue gas injection consists of injecting large quantities of

gas that may be miscible or immiscible depending on the pressure

and oil composition.

Large volumes may be injected, because of the low cost. Nitrogen or flue gas are also considered for use as chase gases in

hydrocarbonmiscibl and CO2 floods.

Mechanisms that Improve Recovery Efficiency

Vaporizes the lighter components of the crude oil and generatesmiscibility if the pressure is high enough.

Provides a gas drive where a significant portion of the reservoir

volume is filled with lowcos gases.


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Limitations

Miscibility can only be achieved with light oils at high pressures;therefore, deep

reservoirs are needed.

A steeply dipping reservoir is desired to permit gravitystabilization of the displacement,

which has a very unfavorable mobility ratio.

Challenges

Viscous fingering results in poor vertical and horizontal sweepefficiency.

Flue gas injection can cause corrosion.

Nonhydrocarbon gases must be separated from saleable gas.


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Screening Parameters Gravity >24 API (35 for nitrogen)


Net thickness relatively thin (not critical for pressure

maintenance)


Transmissibility not critical

Depth >4,500 feet

Temperature not critical


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Thermal (Steamflooding)

Description

Steamflooding consists of injecting 80% qualitysteam to displace oil.

Normal practice is to precede and accompany thesteam drive by a cyclic steam stimulation of theproducing wells (called huff and puff).

EOR Mechanisms

Viscosity reduction / steam distillation.

Supplies pressure to drive oil to the producing well.


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Thermal (Steamflooding) Limitations

Applicable to viscous oils in massive, high permeability sandstones or

unconsolidated sands.

Oil saturations must be high, and pay zones should be > 20 feet thick tominimize heat losses to adjacent formations.

Less viscous crude oils can be steamflooded if they don't respond towater.

Steamflooded reservoirs should be as shallow as possible, because ofexcessive

wellbore heat losses.

Steamflooding is not normally done in carbonate reservoirs.

Since about 1/3 of the additional oil recovered is consumed to generate

the required steam, the cost per incremental barrel of oil is high.

A low percentage of water-sensitive clays is desired for good injectivity.

Challenges

Adverse mobility ratio and channeling of steam.


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Thermal (Steamflooding)


Gravity 20 cp (10-5,000 cp)

Composition not critical Oil saturation >500 bbl/acre-ft (>40-50% PV)

Formation type sandstone

Net thickness >20 feet

Average permeability >200 md Transmissibility >100 md ft / cp

Depth >200-5,000 feet

Temperature not critical


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Cost of EOR


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Cost of Chemicals

As the oil prices rise, so does the cost of chemicals,but not in the same proportion

Typical Costs:

Polymer - $3/lb Surfactant - $1.20/lb

Caustic - $0.60/lb

Isopropanol - $20/gallon

Micellar slug - $25/bbl


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EOR Recovery Processes

Typical


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EOR Recovery Processes


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Estimated Cost of a Barrel of EOR Injectant

Taber, 1990

EOR S i C it i


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EOR Screening Criteria

EOR S i


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EOR Screening


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Chemical Floods Worldwide


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Incremental Oil

Recovery Evaluation


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Incremental Oil Recovery (IOR)

Oil (HC) produced in excess ofexisting (conventional) operations

Difficulties.

Comingled productionOil from outside project

Inaccurate decline estimates

IOR recovery efficiency = 100 IOROOIP


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Decline Curve Analysis

a 1qdqdt[]time1Decline rate:

Np q()d0tCumulativeoil produced:


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Decline rate:

Decline rate

types:

a ai qqi

b

b 0 Exponential0 b 1 Hyperbolic1 Harmonic


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Exponential decline:

Rate-time

q qieat

q qi aNp Rate-cumulative


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Rate-Time...


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Rate-Cumulative...


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Accelerated Production...


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Increased Mobile Oil...


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Deaccelerated Production...


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Accelerated Production...


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IOR Example

eor screening

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