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SPE 91787

Field-Scale Polymer Flooding: Lessons Learnt and Experiences GainedDuring Past 40 YearsY. Du, SPE, New Mexico Institute of Mining and Technology,and L. Guan, SPE, Texas A&M University

Copyright 2004, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 2004 SPE International Petroleum Conferencein Mexico held in Puebla, Mexico, 89 November 2004.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in a proposal submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper

for commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to a proposal of not more than 300words; illustrations may not be copied. The proposal must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abst ractEarly in 1964, Pye and Sandiford established the fact that

polymer flooding can result in greater oil recovery than the

conventional water flooding. Many additional papers

sustaining and extending this information have since appeared

in the literature. In the past forty years, many field-scale

polymer flooding projects have been put into production and

lots of information has been available from which to draw

conclusions regarding of lessons learnt and experiences gained

on field-scale polymer-flooding. The purpose of this paper isto examine the ranges of some important parameters within

which successful polymer flooding has been achieved and to

present lessons learnt and best practices on polymer flooding,

thus direct to design and further achieve a high-performance

polymer-flooding project.

IntroductionMechanisms of Polymer Flooding

In the reservoir, oil and water are immiscible fluids. As a

result, neither one can completely displace the other in the

subsurface condition. This is reflected by the non-zero

irreducible water (Swir) and residual oil saturation (Sor) on an

oil-water relative-permeability curve. In the lab, no matterhow large volume of water has been injected into a core, the

oil saturation will never be lower than Sor only by the

conventional water flooding.

However, it has been known for many years that the

efficiency of a water flooding can be greatly improved by

lowering the water-oil mobility ratio in the system. Such a

change may lead to better sweep efficiency and also to more

efficient oil displacement in the swept zone. By adding of

suitable polymer solutions to injected water, the water

mobility can be reduced and oil recovery increased as shown

in Figure 1.

Figure 1. Cluster Type Residual Oil by Polymer Flood ing andWater Flooding .

[1]

During polymer flooding, a water-soluble polymer is

added to the injected water in order to increase water

viscosity. Depending on the type of polymer used, the

effective permeability to water can be reduced in the swept

zones to different degrees. It is believed that polymer floodingcannot reduce the Sor, but it is still an efficient way to reach

the Sormore quickly or/and more economically.

According to Riley B. Needham [2], polymer solutions may

lead to an increase in oil recovery over that from a

conventional water flooding by three potential ways: (1

through the effects of polymers on fractional flow, (2) by

decreasing the water/oil mobility ratio, and (3) by diverting

the injected water from zones that have been swept. The above

three effects can make the polymer flooding process more

efficient.Early pilot studies on polymer flooding can be traced back

to 1944. Detling[3] (Shell Development Co.) obtained a U.S

patent covering the use of several additives for viscous waterflooding. His objective was to increase the viscosity of the

flooding water and then to improve water-oil mobility ratios

During the next two decades, many studies [4-13] have shown

up like mushrooms and many patents have been granted

covering specific water-soluble polymers or specific

conditions of viscous water flooding in the world.

In 1964, Pye and Sandiford [14]published the fact that the

mobility of the brine used in water flooding was greatly

reduced by the addition of very small amounts of hydrolyzed

polyacrylamide, a water-soluble polymer. This reduction in

brine mobility resulted in greater oil recovery than tha

attributable to conventional water flooding. Many additiona


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papers sustaining and extending this information have since

appeared in the literature [15-25].

All of these studies laid a solid theoretical foundation for

the polymer flooding in the field scale practices. However,

field scale practice of polymer flooding is a technically

sophisticated process and is usually muti-million dollar

investment. For this reason, a thorough knowledge of the

reservoir and the applicability of the polymer flooding areessential to the success of the project.

Reservoir rock and fluid properties determine the

mechanism and the effectiveness of a specific polymer

flooding process displacing the reservoir oil and water from

the formation. In addition, the project must indicate an

adequate rate of return on the investment. Oil recovery, price

of crude, cost of chemicals, and cost of wells and equipment

are important in making economic evaluations. We will

further discuss the reservoir characteristics favorable to

polymer flooding in detail later.

Polymer Types and Properties

Polymers that have been used in actual polymer flooding can

be classified into two general types: synthetic polymers and

biopolymers.

A synthetic polymer at most times means polyacrylamides.

Polyacrylamide is a condensation polymer with an unusual

property. The structure of polyacrylamide is similar to that of

polyethylene, but have a hydrogen on every other carbon

replaced by an amide group, CONH2. The molecule is

composed of repeating CH2CH(CONH2) units. The amide

groups allow for linking between polymer strands. The

CONH2 group from one molecule can react with the same

group of another molecule, forming a link between them with

the structure CONHCO. This produces a network of

polymer chains, like a tiny sponge. The free, unlinked amide

groups, because they contain NH2groups, can form hydrogenbonds with water molecules. This gives the tiny cross-linked

sponges a great affinity for water. Polyacrylamide can absorb

many times of its mass in water. Ionic substances, such as salt,

cause polyacrylamide to release its absorbed water.

A variety of polyacrylamides are available from several

manufacturers. In general, the performance of a

polyacrylamide in a flooding situation will depend on its

molecular weight and its degree of hydrolysis. In a partially

hydrolyzed polyacrylamide, some of the acrylamide is

replaced by, or converted into, acrylic acid. This tends to

increase viscosity of fresh water, but to reduce viscosity of

hard waters.

Biopolymer is derived from a fermentation process, ratherthan by direct synthesis from their monomers in a chemical

reactor. The most commonly encountered biopolymer is

xanthan gum, which is produced by the bacterium

Xanthomonas campestris. In terms of molecular weight,

biopolymers fall toward the low end of the range encountered

with polyacrylamides. Their molecular structure gives the

molecule great stiffness. This characteristic gives biopolymer

excellent viscosifying power in high-salinity waters and makes

them very resistant to shear degradation. In very fresh waters,

however, they have less viscosifying power than

polyacrylamides.

Reservoir Condit ions Favorable to Polymer FloodingTo date, some field polymer flooding information has been

available from which to draw conclusions regarding the mos

suitable/favorable reservoir and fluid characteristics for

polymer flooding applications. The purpose of this paper is to

examine the ranges of some important parameters within

which successful polymer flooding has been achieved, and to

present lessons learnt and best practices on polymer floodingthus direct to design and further achieve a high-performance


While analyzing the applicability of polymer flooding to a

given reservoir, the importance of a complete understanding of

the reservoir and fluid characteristics cannot be

overemphasized. Such characteristics as the mobility ratios

permeability and its variation, porosity, the fluid saturation

the relative permeability, the formation temperature and

pressure, the formation type, the rock minerals and wate

properties can have a dramatic effect on the success or failure

of the flooding process. Each reservoir must be analyzed in

light of its own properties and characteristics. The following

are some critical factors to be considered while designing a


Mobility Ratio

Mobility ratio here means the brine mobility at residual oi

saturation to the oil mobility at irreducible water saturation

Published successful tests have occurred in the range from 0.1

to 42. In terms of oil viscosity, the highest record value is 126

cp for which success has been achieved.

Permeability

The level of reservoir permeability and permeability variation

can have great influence on the success of a polymer-flooding

project. Reservoir permeability dominates the water injection

rate, which will in turn control well spacing and project lifeThe well spacing and project life affects the economics of the

project. In other words, all else being equal, the projects of a

very low permeability reservoir developed on 2-acre spacing

definitely will not perform as good as a relative high

permeability reservoir developed on 5-acre spacing.

Polymer solutions used for flooding have lower

injectivities than the solvent brine because of their high

viscosity and reduced mobility. Usually this effect is

compensated for by the increased volumetric displacemen

efficiency of the polymer solutions so that flood life is no

extended. However, under pressure-limited conditions, as

often encountered in shallow, low-permeability reservoirs

decreased injectivity may be an economic problem.As a rule of thumb, cares should be taken if polymer

flooding is conducted with a very low average permeability

reservoir. The range of average permeabilities in which

successful floods have been conducted is from 20 md to 2,300

md. Permeability variation (Dykstra-Parsons V-factor) lies in

the range from 0.28 to 0.80.

Effective Porosity

Effective porosity here only refers to the porosity involving

connected void space, whereas total porosity involves tota

void space whether connected or not. Effective porosity can be


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SPE 91787 3

further classified as intercrystalline intergranular porosity

and fracture matrix porosity.

The type and nature of porosity may have considerable

influence on recovery efficiency by polymer flooding. For a

given oil saturation, porosity determines the oil in place and

the volume of recoverable oil present and thus directly affects

the economics of the process. In addition, porosity also

determines the total amount of polymer needed for a givenflooding operation.

In addition, the nature of the pore surfaces and space is

also very important in determining the flow and adsorption or

retention characteristics of the reservoir rock. The relative

absence or presence of clays in the pore spaces and in the pore

throats will have considerable effect on the flow behavior and

permeability of the reservoir rock.

Scanning Electron Microscope (SEM) studies are an

invaluable tool in the study of porosity in sandstone reservoir

rocks.

Mobile Oil Saturation

In general, low mobile oil saturation is an adverse factor for

polymer flooding as well as for water flooding.

Simulation results of polymer floods by Needham [2]

indicate that mobile oil saturation is a key variable to

determine whether a polymer flood can be successful.

Heterogeneous reservoirs containing oil, which could be

produced at high WOR, have significant volume of remaining

mobile oil. They are good candidates for polymer flooding.

However, successful polymer floods have been observed

in the mobile oil saturation range from 0.15 to 0.46, an

extremely wide range.

Initial Water Saturation

It hasbeen stated in some literature that high initial water

saturations can be deleterious to polymer flooding. However,some projects were successful in spite of their high initial

water saturations, even as high as 0.47.

Depth Temperature and Pressure

Reservoir depth usually controls the temperature and initial

pressure (in normal pressure system) of a reservoir. Favorable

temperature may keep polymer stable without degradation.

The deepest and hottest successful flood was operated at

6,500 ft and 229oF. There seems limited reason to believe that

greater depths and higher temperatures cannot be polymer-

flooded successfully, provided that the usual precaution is

observed to maintain an absolutely oxygen-free system (0.0

ppm) by chemical means. However, reservoirs withtemperatures above 300oF should be avoided because of

polymer decomposition above that point, even in the absence

of oxygen.

Depletion Stage

Economic and technical successes have been reported for

polymer floods in both secondary and primary applications.

On the basis of published results to date, secondary floods

recover substantially more oil with less polymer usage than

tertiary floods. Polymer flooding is therefore best to be

applied in the early life of a water flood. The average

preference of floods initiated at WOR > 10 appears to be

significantly lower.

Projects started near the end of primary depletion tended to

be more successful than that started during the secondary

recovery stage. The earlier polymer flooding is initiated in the

flood life, the more likely it will be successful.

Formation Type

Successful floods have been conducted in both sandstone and

oolitic limestone formations. Grossly vugular limestones have

been avoided because laboratory evidence indicates that no

appreciable resistance effect can be generated in these rocks.

Economic and technical successes have been reported for

polymer floods in both sandstones and carbonates.

Rock MineralsThe presence of different minerals can affect the efficiency of

the process. Certain clays swell when contacted with non-

equilibrium waters and can have drastic effects on water and

polymer injectivity. In addition, in the case of a preflush, ion

exchange with the clays can increase the concentration of

multivalent ions seen by the micellar solution.

Gypsum (CaS04*2H20) is a slightly water-soluble minera

present in some reservoirs. However, the volubility of calcium

can possibly be high enough to cause precipitation of

petroleum sulfonate and to react with polyacrylamide, which

reduces the viscosity of the polymer solution and reduces the

efficiency of the flooding.

Similarly, other clays can reduce the effectiveness of a

miceller-polymer flood by adsorbing surfactant, by adding

calcium to the flooding solution, and by adsorbing polymer

all of which have a negative effect on the flooding process

The presence of clay minerals is very important.In the consideration of micellar-polymer flooding, a high

concentration of clay minerals can increase the ion exchange

capacity of the rock and thus affect both the micellar and

polymer slug behavior.

It is imperative that a thorough mineralogy study be

conducted on the reservoir prepared for polymer flooding.

Water Salinity

The salinity of reservoir brines can either be a favorable or

unfavorable effect on some polymers and micellar solutions

depending on the total salt concentration and the concentration

and type of monovalent and divalent salts in the reservoir

brine.The degradation of micellar solutions can be accelerated

by the precipitation of petroleum suifonates in the slug as they

contact reservoir brines containing multivalent ions such as

calcium and magnesium. Micellar solutions can be designed to

be compatible with reservoir brines. However, if care is no

taken in the design, multivalent ions in the brine can cause the

micellar solution to break up into a water phase and oil rich

phase or may cause the precipitation of surfactants.


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4 SPE 91787

Figure 2. Effect of salini ty on the vi scosit y of 0.05 percentpolymer solution

[27].

The viscosity of partially hydrolyzed polyacrylamide

polymers is quite sensitive to both the brine and the presence

of multivalent ions. Figure 2 illustrates the effect of salinity

on the viscosity of Dow Pusher 500 and 700 polyacrylamide

solutions [26]. The loss in viscosity, when a polymer contacts

the high concentration reservoir brine or divalent ions, results

in increasing mobility of the buffer solution, which may result

in fingering and other displacement inefficiencies. As stated

previously, a preslug is often injected to displace the reservoir

brine.

Where compatibility presents no problem, the use of a

fresh water source rather than more saline brine can lead to

lower concentration requirements, hence lower polymer costs,for the same mobility effect.

Polymer Selection

Polymer type selection

All else being equal, a high-molecular-weight polymer will

produce higher viscosities and resistance factors than a low-

molecular-weight polymer for a given concentration. These

potential advantages may be offset by a greater tendency for

shear degradation, which reduces molecular weight, and by a

reduced injectivity, which can be significant in low-

permeability formations. For large-scale applications,

polyacrylamides are available in powder form (90% + active),

in the form of a pumpable inverse emulsion (33 to 55%

active), or can be manufactured on site in a concentrated

solution form.

Each polymer type has advantages and disadvantages.

Polyacrylamides have a relatively low price, develop good

viscosities in fresh waters, and adsorb on the rock surface to

produce a long-lasting permeability reduction (the residual

resistance effect). Their primary disadvantages are a tendency

to shear degradation at high flow rates and poor performance

in high-salinity water (low viscosity and frequently excessive

retention). The primary advantages of biopolymer are their

excellent viscosifying power in high-salinity waters and their

resistance to shear degradation. Biopolymers are not retained

on rock surfaces and thus propagate more readily into a

formation than polyacrylamides. This can reduce the amount

of polymer required for a flood but sometimes it also means

that there is limited residual resistance effect.

Both polymer types are restricted in the range of reservoir

condition where they can be effective. Biopolymer thermally

degrades too fast at temperatures above 200oF (93oC). A

temperatures above 170o

F (77o

C), polyacrylamides mayprecipitate in waters containing too much calcium. In

principle, this does not prevent their being used successfully in

fresh water, but makes control of the salinity of the floodwater

much more critical.

The results from polymer core flooding have indicated tha

the polymer molecular weight is a very important parameter in

increasing the viscosity of the polymer solution and reducing

the water permeability. The higher the polymer molecular

weight, the higher the viscosity of the polymer solution, the

more the permeability is reduced, and the higher the oi

recovery that will be achieved. But if the polymer molecular

weight is too high, the polymer may plug the formation pore

space as it flows through it.

In order to find the optimal polymer weight, which is

suitable for a certain formation pore space, the matching

relation between the polymer molecular weight and the

reservoir permeability must first be studied. A rule of thumb is

that when five times the gyration radius of the polymer

molecule is smaller than the median size of the pore space o

the reservoir, the polymer molecule will not plug the

formation porespace.A goodpractice is 1). Analyze the data

of the core taken from the polymer flood area and find out the

lower limit value of the permeability in which 75% of the net

thickness is swept out by the polymer flood. 2). According to

mercury injection data, the median pore space radius is

determined, which corresponds to the lower limit

permeability. 3). The suitable polymer molecular weight isdetermined from the relation between the molecular weigh

and permeability.

All of the tests included in the tables used an essentially

linear, highly soluble, partially hydrolyzed polyacrylamide as

the mobility control agent. Considerable variation in the

properties of this material is possible, particularly in the higher

molecular weight. In reservoirs with high permeability, the

polymers with higher molecular weights are often preferred in

order to achieve an adequate resistance factor. In other

reservoirs, conversion from the existing polymer type to a

recently available polymer of higher molecular weight has

allowed reduction in concentration to achieve the same

resistance effect with a considerable cost reduction.Practical considerations for the polymer solution are tha

[28]: (1) it must be injectable into the reservoir, (2) it must

survive, and (3) it must be able move through the reservoir

and provide the required viscosity.

Concentration of the polymer slugsOn condition of the same amount of polymer injected, the

more heterogeneous the reservoir is, the better the

displacement results with a polymer slug of high concentration

compared to that of low concentration [29]. With an increase o

the injected slug concentration, cumulative fluid injection for

the entire period of polymer flooding decreases and the


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SPE 91787 5

amplitude of water cut reduction of the produced fluid

increases. For this case, the oil recovery increases 0.4 and the

lowest water cut decreases 10% when the concentration of the

polymer solution injected increases from 400 mg/L to 800

mg/L; while only 0.l% for oil recovery and 7.8% for water cut

with a concentration increase from 800 mg/L to 1,200 mg/L.

The infectivity of the polymer solution decreases with a rise of

injection pressure caused by the high concentration of thepolymer slug. There are some special cases. Given the

reservoir rock and fluid properties prevalent in the Hale and

Mable leases, a lowconcentration polymer flood is just as a

higher-concentration flood as long as the total pounds of

polymer injected is the same by Hovendlck, M. [30].

For the same volume of the polymer injected, we can use

high-concentration small slug or the low-concentration large

slug. Evaluation of the high-concentration small slug vs. the

low-concentration large slug was done by simulating a single-

pattern consisting of 20 layers with crossflow only at the

wellbore [31]. Oil displacement was by fractional flow, and

areal sweep was imposed according to mobility ratio

correlations. Polymer viscosity was treated in terms of

resistance factor polymer retention was successfully included.

Slug SizeSuccessful projects have used slug sizes varying from 7

percent PV to 33 percent PV. Smaller slug sizes have been

tested, thus far without success.

Combination of the Polymer SlugsBecause a small amount of polymer injected results in a

small size polymer slug in the reservoir, it is easy for the post

water slug to breakthrough the polymer slug. Thus a sufficient

amount of polymer injected as a mobility control is needed.

However, under condition of a large amount of polymer

injected, it is difficult for the post water slug to breakthroughthe polymer slug. Therefore, the effect of mobility control is

not as obvious as that for small amounts of polymer injected.

Quality ControlA good program for quality control is helpful and necessary in

the field to minimize the chance for formation plugging and to

ensure that the injected fluids meet the design specifications.

Fortunately, a good quality control program requires only

relatively simple tests. Important quality control parameters

can be held to reasonable tolerances throughout the life of a

polymer project

Viscosity control is critical to a successful polymer project.

The viscosity test insures that the polymer is properly mixedand that its viscosity falls within the specified range.

These quality control tests are run frequently during the

start-up phases. After operating procedure was worked out and

the mixing procedures become routine, one or two quality

checks per day should normally be sufficient.

Unsuccessful Floods ObservationThe following summarized some possible published reasons

for the failure of polymer flooding.

Tertiary stage. The unsuccessful floods were undertaken

in reservoirs that had been extensively flooded by other

processes. When the polymer flooding initiated, the

hydrocarbon resource in place was limited. Hence, resulted in

poor performance.

High oil viscosity. Oil viscosities are high. As indicated

under the discussion of successful floods, the highest Oi

viscosity in which success has been achieved to date is 126 cpExtremely small polymer slug. The polymer slug is too

small to improve the flooding efficiency. The conclusion to bedrawn from former studies is that slug sizes smaller than 7

percent PV have not been successful.

Injectivity problems. Projects suffered from low

injectivity. Especially for the shallow reservoirs with low

average permeability, the water injectivity is low. If polymer

were added to the water, the injectivity will be very low. The

low injectivity makes it harder to maintain the reservoir

pressure by limited number of injectors.

Best PracticesSeveral key steps may be taken during the designing and

implementation of the field scale project to increase the

probability of a successful polymer flood.

1.Reservoir characteristics. Reservoir characteristics were

studied in detail before polymer flooding was identified as a

potential method of improving flood performance and

recovery efficiencies. Adverse reservoir characteristics were

identified early during the planning of the project [2, 27, 28 and 32].

2.Laboratory tests.Laboratory tests were conducted to (1

identify polymers, (2) optimize polymer concentration, (3

quantify polymer degradation and retention, (4) help to design

polymer slug, optimize the does of biocides and oxygen

scavengers[29, 32 and 33].

3. Fractional flow calculations. Fractional flow

calculations were useful screening guides to estimate polymer

flooding potential.

4. Simulation. Computer simulation was used to designthe optimal polymer concentration and slug size [32, 33].

5. Tests. Pressure transient tests may be used to improve

reservoir description [32]. Polymer injection tests were

conducted to: (1) determine sustained rates and pressures, (2)

measure in-situ polymer viscosity, and (3) evaluate the

physical handling of flake and liquid polymers. Field

injectivity tests were essential to determine polymer injectivity

and provided evidence about the polymer molecular weigh

and viscosity. These tests may support laboratory and

computer observations.

6. Quality control. Four quality and performance contro

measures were instituted [32]: a polymer quality contro

laboratory was built at the delivery point, a productionevaluation laboratory were constructed at the field to monito

injected and produced fluids, well test data were frequently

obtained with computer-controlled test satellites, and

maximum field withdrawal was assured with computer

controlled pumpoff controls. Bacterial control in polymer

solutions sometimes may appear attainable according to

laboratory results but could not be sustained in the field.

7. Continues efforts and close field monitoring

Successful field implementation requires continuous efforts

and close field monitoring to improve the efficiency and

effectiveness of the polymer EOR techniques.


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6 SPE 91787

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