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EOR Scenarios:

Different types of reservoirs exist on the earth depending upon their shape,

characteristics of rocks and formation etc. A review has been presented over

here concisely shedding light on different scenarios and suitability of EOR

processes relating to them.

Following picture shows a view of different types of off shore oil reservoirs

with different depths. Oil Platform is to be installed for offshore oil reservoirs

to house the workers as well as machinery to extract the oil. Different types

of oil platforms are used depending upon the depth of reservoir from sea

surface. Usually a floating platform is used for the reservoirs whose depth is

more than 200 meters from sea surface.

Different types of oil reservoirs exist on earth, we will discuss the scenarios

one by one.

1. The reservoirs which have oil intermixed with small amount of water or

no water at all, are suitable for using electric current as ohmic heating

element. But this would be favorable if oil is not lying under hugeamount of water (which is although a rare case) because in this case

initially water will be converted to steam before reaching the oil which

will increase the cost of production. Other limitation on this method is

depth of the reservoir, if the reservoir is very deep in that case huge

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amount of power will be required to heat the oil which will not be

feasible economically.

A flow diagram for this case will be as under:

Forward flow (I)

Return Path (I) Oil Flow

Production well

Current will be flowing up to the reservoir surface and returning back

through the return path. It will produce heat because it is passing through

the resistive lines (high resistive lines to be used) because of the power

dissipation phenomenon this heat will make the oil warm reducing its

viscosity and enabling it to flow towards production well. But limitation onthis method is that it needs a huge amount of energy to be dissipated so as

to raise the temperature of the reservoir. Following diagram shows a

prospective case for application of this method:

Oil

Reservoir,

(oil & water)

Electri

c

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The other phenomenon in this situation may be using the injection well as

anode and production well as cathode. Energy will be supplied through the

injection well and the current will flow through the conductive brine towards

production well which is cathode and provides a returning path for the

current. When the current flows through brine, it heats up the brine and oil

making steam as well which will provide additional heat and help in raising

the temperature in less time. As the temperature of oil increases, its

viscosity decreases and it starts flowing towards the production well.

(Anode) Production Well

Injection Well

Oil Reservoir with brine

(Current Flow through

brine)

Cathode

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Limitation on this method is that it requires the presence of a conductive

material inside the reservoir, otherwise current will not flow because the

path for current flow will not be completed.

2. If there are some inductive metal elements like ferrous present in the

reservoir, then Inductive heating process will be viable. Because the

inductors installed inside the reservoir will produce strong magnetic

field around themselves, the polarity of field will be subject to change

along with the frequency of applied power supply system. This

changing magnetic field will also link the inductive metal elements and

continuous changing of polarity will also produce along with the heat of

the coils themselves, which will help in raising the temperature of oilmaking it less viscous.

When an electric current is passed through an inductor, it produces a

magnetic field around the inductor whose direction is found by Right Hand

Rule. When the current alternates its polarity, direction of magnetic field is

also reversed. When an alternating current is passed through an inductor,

the polarity of magnetic field around the inductor changes at the rate of

applied alternating current frequency. This magnetic field is also linking theinductive material (Ferrous etc.) inside the well hence inducing an

alternating voltage in the inductive material as well. Rapid change of

frequency produces heat in the inductor as well as in the ferrous which heats

up the oil causing it to flow towards production well. A view of the proposed

method is as follows:

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The disadvantage of this method is the presence of magnetic material inside

the reservoir which should be in excess so as to make the process

economically and commercially viable. On the other hand, if magnetic

materials are not present in abundance then this mechanism of Inductive

heating would not be able to heat the reservoir up to the desired level for

EOR.

3. Some are having oil droplets trapped inside the rock pores but this is

not under the water surface. Water is there in the reservoir but it is in

the form of drops mixed with oil droplets and trapped inside the rocks.

For this type of reservoirs, using microwaves will be best suited

because when microwaves will be applied, the polar water molecules

will also be set to vibrations along with the frequency of applied

microwaves, the oscillations will convert these water molecules will

convert these water molecules into steam which will be in situ effect of

microwaves in addition to the heat which it will provide by friction of oil

molecules with each other. If hydrocarbons are mixed with sand, they

can absorb huge amount of microwaves and hence the temperature

Oil Reservoir

With the presence of Ferrous

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can be raised to 300-400 degree centigrade rapidly(Ellingsen 2002). A

good application area for microwaves is shown in the following figure:

In this method of heating the reservoir, although ultrasonic waves may also

be used for EOR but microwaves are more productive because microwavescan produce high temperatures instantly because of in situ heat

phenomenon as compared to sound waves which are providing stimulations

to the water and oil droplets instead of heating them electrically. In addition,

porous media of the oil well is a big challenge for ultrasound waves

propagation; they can penetrate through up to just a few millimeters in the

porous media. And hence the oil and water trapped inside the porous rocks

in more depth (up to a few centimeters will not feel the effect of these

ultrasonic vibrations) but Microwaves can penetrate in more depth into the

porous rocks as compared to ultrasonic waves. Microwaves cable can be

employed inside the injection well for maximizing the energy input to the

polar molecules (water molecules).A pictorial view will be as follows:

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(Inside Injection Well)

Microwaves Cable

Water and oil droplets caught in the rock pores will be heated by the

application of microwaves which will cause the water droplets to be

converted to steam and oil droplets to flow (because of a reduction in

viscosity). Oil droplets will be collected from different pores and start flowing

towards the production well.

All the heating methods discussed until now have a common disadvantage

that they require some measurement and adjustment of temperature plan

inside the reservoir. Some mechanism should be installed which can

measure the temperature of inside of reservoir and send a command to

generator to increase or decrease the energy supply as per requirement.

One proposal in this aspect is to measure the temperature of the oil being

extracted and estimate the temperature of inside reservoir considering the

depth of reservoir and then decide whether to increase or decrease the

temperature inside the reservoir. This can be achieved by employing a

comparator to compare the estimated temperature and a threshold, if the

difference is negative then send a command to generator to increase the

output otherwise decreases the energy supply from generator.

4. In some cases we have much viscous and dense oil as compared to

water which sinks underneath the water. In this case Ultrasound waves

can be preferred because they need a medium for propagation so they

will travel through the water and reach up to the crude oil which will be

warmed by ultrasonic energy. Following figure shows a case of

application of ultrasound waves:

Oil Reservoir

MW

Generat

Flow of oil

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Ultrasonic waves can also be applied to the reservoirs which have oiland water molecules trapped inside the rock pores and it has high

water content. Application of ultrasonic waves will change the capillary

forces and the adhesive forces between rocks and fluids. Interaction of

sound waves with the fluid in pores change the relative permeability

of rock to oil and water, and therefore increase the oil recovery(Amro,

Al Mobarky et al. 2007).A cable can be employed for transferring

maximum energy from the Ultrasound Generator to the intermixed oil

and water of this situation. A good advantage of Ultrasonic waves on

other heating methods is that there is no danger of losing oil properties

in case of heating beyond threshold limit. A pictorial view of this

process is provided below:

Injection Well

Production Well

Ultrasound Cable

Oil Reservoir

(Oil intermixed with water and lying beneath water)

Ultrasound

Generator

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In the above proposed method, ultrasonic energy is supplied to the water

which is carrying heavy oil at its depth. As ultrasound waves are supplied to

the water surface through a conducting medium therefore the maximum

energy will be available at the water surface which in turn will vibrate the oil

lying at the bottom of the reservoir. Ultrasonic waves can easily propagate

through water without losing much energy and reach up to the layers of oil

at the bottom of water. When this heavy oil is vibrated mechanically by the

applied ultrasonic waves, it starts flowing towards the production well.

Microwaves cannot be applied in this scenario as they will not be able topass through the water and unless the water is not removed heat cannot be

reached up to the oil lying in the bottom. But this is a rare case, in fact in

majority of the cases oil lies at the surface of water because it is less dense

as compared to water. In this case microwaves may also be applied causing

the surface water molecules to set into vibrations at the frequency of applied

microwaves and as a result raising the temperature of the water surface. But

since it may be a deep reservoir of water with oil at the surface, so this

increase in temperature will not be viable because microwaves cannot

penetrate into the depth of water(hence cannot change the temperature of

water a few centimeters down from the surface).As a result, the process of

movement of oil will be very slow. This process will be feasible in case the

water is not much in quantity .But microwaves are best to use in the case

when oil droplets are intermixed with water droplets and trapped inside the

rock pores (Amro, Al Mobarky et al. 2007).

5. In case of vertical reservoirs in which permeability decreases along

with the increase of depth of reservoir, ultrasonic waves may also be

applied. Because the permeability of oil increases along with the

decrease in interfacial tension by application of ultrasonic waves which

results in enhanced oil recovery. The proposed method of employing

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ultrasonic waves would be same as discussed above. As the

penetration of ultrasonic waves in porous media is a big issue and the

intensity of waves reduces as they penetrate through porous media of

the oil well therefore, use of a cable for propagation of ultrasonic

waves will be feasible in this case.

Ultrasonic Generators:

Different types of Ultrasonic Generators are available in the market; I found

two of them to be suitable for applications in EOR.

1. Elmasonic X-tra 250 LSM (made by Elma-Hans Schmidbauer,

Germany). Its frequency range is 25/45 KHz and 300 watts power of

ultrasound waves.

2. Sonic Digital LC Ultrasonic Generators made by Weber Ultrasonics

The most suitable is Sonic Digital LC Ultrasonic Generator because it

can create digital ultrasonic frequencies of 25, 30, 40, 80 to 150 KHz

and power of up to 2000 watts.

Dr. Mohammed M. Amro and Dr. Emad S. Al-Homadhi of King Saud Universityused PUNDIT, a low frequency ultrasonic generator in their final research

report No.53/426.

PUNDIT stands for Portable Ultrasonic Non-Destructive Digital Tester (Amro

and Al-Homadhi). It can generate the low frequency ultrasound pulses and

couples the generated waves to rocks through transducers which are placed

in contact with inlet and outlet faces of the rock samples. Following is a view

of the amplitude of generated waves by PUNDIT versus time.

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On the other hand, for high frequency Ultrasonic waves, they used Clifton

Ultrasonic Bath MU-22 which is also used by other researchers as well. Its

output frequency is 50 KHz with a power output of 300Watts. This bath can

be used for the case where oil is intermixed with water or lying underneath

the water surface. In experimental phase core samples can be placed inside

the bath in the centre of the bath by filling it with water so as to create a

practical scenario. Ultrasonic power required for this experiment can be

calculated basing upon the frequency of operation. Low frequency ultrasonic

waves have greater depth of penetration but are less focused which means

there intensity (ultrasonic energy per unit area) will be less. Similarly high

frequency ultrasonic waves are highly focused but have less penetration

depth. Focusing of ultrasonic beam also depends upon the diameter of the

transducer, the larger the diameter, the focused the ultrasonic beam will be.

But the energy distribution in this case is uniform and the minimum energy

will be close to the transducer (Amro and Al-Homadhi).

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Conventional EOR:

The conventional enhanced oil recovery methods for example CO2

Injection, Water and Steam Injection etc. have two types of Injections.

1. Miscible Injection, in which the injected fluid is mixed up with the oil

and water inside the reservoir and the underlying oil starts flowing

towards the production well as a result of a decrease in the viscosity

of oil.

2. In case of Immiscible Injection, on the other hand injected fluid does

not mix up with the oil and water completely because of the

characterization of oil. It is just absorbed inside the oil and water

and starts building pressure which in turn tends to the movement ofoil blobs lying at different places. These oil blobs are mixed with

each other and start flowing towards the production well.

The main consideration in Miscible and Immiscible injection is that

miscible injection is carried out for large fracture pressure reservoirs

while immiscible injection is done for limited fracture pressure

reservoir. Say, for example, if a reservoir has 1000 PSI fracture

pressure reservoir then we cannot proceed with the Miscible Injectionof 1200 PSI because it will fracture the reservoir and immiscible fluid

will be better in this case.

As far as the recovery time is concerned, it depends upon the

heterogeneity of the reservoir. If it is permeable and not very heavy oil

then it will take less time as compared to the reservoir having less

permeability and high viscosity.

ADCO carried out the first pilot for CO2 injection in Rumaitha field in

November 2009. Co2, for this purpose, was supplied by Masdar. It was

the very first CO2 injection EOR Pilot in a complex onshore reservoir in

Abu Dhabi, UAE. Most of the EOR reservoirs lie in high API gravity

(more than 40) and resident depths of 8000 to 10,000 feet.

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Fig. ADCO CO2 EOR Single well Pilot configuration

Three wells were used for this pilot; CO2 Injector well, Observer well

and Oil Producer well as shown in above fig. The wells were perforatedand acid stimulated before co2 injection. They observed the fluid

properties after CO2 injection and any fracturing because of CO2

injection near well bore especially Thermal fracturing and they found it

economical to employ CO2 injection in other reservoirs (Al-Hajeri,

Negahban et al.).

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References

1.Al-Hajeri, S., S. Negahban, et al. Design and Implementation of the first

CO2-EOR Pilot in Abu Dhabi, UAE.

2.Amro, M., M. Al Mobarky, et al. (2007). Improved Oil Recovery by Application ofSound Waves to

Water Flooding.3.Amro, M. M. and E. S. Al-Homadhi "ENHANCED OIL RECOVERY USING SOUND-

WAVE STIMULATION."4.Ellingsen, O. (2002). Method to increase the oil production from an oil reservoir,

Google Patents.