KING SAUD UNIVERSITY COLLEGE OF ENGINEERING

RESEARCH CENTER

Final Research Report No. 53/426

“ENHANCED OIL RECOVERY USING SOUND-WAVE STIMULATION”

By

Dr. Mohammed M. Amro Dr. Emad S. Al-Homadhi

Rajab 1427 H August 2006 G

i

TABLE OF CONTENTS

Page

Acknowledgment i

List of Figures ii

Abstract in Arabic 1

Abstract in English 2

1. Introduction 3

2. Stages of Oil Recovery Processes 4

2.1 Primary Recovery Methods 4

2.2 Secondary Recovery Methods 6

2.3 Tertiary Recovery Methods 6

3. Review of Relative Permeability 10

4. Experimental Work 12

4.1 Experimental Procedure 13

4.2 Materials Used 15

4.2.1 Crude Oil and Brine Solution 15

4.2.2 Core Samples 15

4.2.3 Acoustic Generator (PUNDIT) 16

4.2.4 High-frequency Ultrasound Source 17

5. Results and Discussion 18

5.1 Effect of Wave Stimulation on Oil/Water Relative Permeability 18

5.1.1 Horizontal Corefloods 19

5.1.2 Vertical Corefloods 25

5.2 Rate of Oil Recovery Using Wave Stimulation 28

5.3 Unconsolidated Corefloods 31

6. Conclusions and Recommendations 33

7. References 35

8. Abbreviations 42

i

ACKNOWLEDGMENT

This study was funded by the research center in the college of engineering

(Project No. 53/426). We gratefully acknowledge this support and we would like to

express our appreciation and thanks to the administration of the research center for their

financial support and cooperation.

ii

LIST OF FIGURES

Title Page

Fig.1: The growing gap between discovery and consumption and the curve showing exploration drilling 7

Fig. 2: Cross section in core holder cell including the transducers to transfer the acoustic from the generator to the core samples 12

Fig. 3: Amplitude of the waves produced from the low frequency generator (Pundit) 16

Fig. 4: Relative permeability curves to water and oil in absence of wave

stimulation. 19 Fig. 5: Behavior of water/oil relative permeability in absence and presence

of wave stimulation at OOIP. 20

Fig. 6: Fractional flow curves of reference and wave stimulated experiments 21

Fig. 7: Effect of wave stimulation on the water/oil relative permeability in high permeable sample 22

Fig. 8: Fractional flow curves generated for reference and wave stimulated experiments 23

Fig. 9: Water/oil relative permeability in cores with higher initial permeability. 24 Fig. 10: Comparison of fractional flow curves for water in presence

and absence of wave stimulation. 24 Fig. 11: Effect of wave stimulation on the water/oil relative

permeability in vertical corefloods. 26

Fig. 12: Comparison of fractional flow curves for water in presence and absence of wave stimulation in vertical corefloods. 26

Fig. 13: Effect of wave stimulation on water/oil relative permeability on low permeable core samples 27

Fig. 14: Fractional flow curves of low permeable core samples in presence and absence of wave stimulation. 28

Fig. 15: Effect of wave stimulation on the oil recovery in high and low permeable core tests 30

iii

Fig. 16: SEM photo showing the general texture of the core sample having a compressive strength of higher than 150 psi after wave stimulation 32

1

زيادة إنتاج الزيت باستخدام التنشيط بالموجات الصوتية

ملخص

ن ارب م ا يق ات % 60إن م د عملي ت بع ن الزي ي مكم ى ف ت تبق ن الزي لية م ة األص ن الكمي م

ة ة وجيولوجي ة ألسباب فيزيائي ة إضافية من . اإلستخالص األساسية والثانوي يمكن استخالص آمي

تخالص الزي طة طرق اس ي بواس ت المتبق سنة الزي ة(ت المح ق طرق ). الطرق الثالثي ن تطبي ولك

ي رة الت اطرة الكبي ة المخ ة ودرج ا العالي را لتكلفته ة نظ ست مغري سنة لي ت المح تخالص الزي إس

ل مخاطرة . يتضمنها تطبيقها ذه الطرق . ولذا فإن المنتجين يبحثون عن طرق أقل تكلفة وأق إحدى ه

ين الزيت المستحدثة إستخدام الموجات الصوتية او الفوق ا ب سطحي م صوتية للتغلب على الشد البين

الضغط الشعيري في المسامات ما ينتج عنه استخالص آمية أآبر من ضوالماء مما يؤدي إلى تخفي

ن ي المكم د ف ت المتواج د ا . الزي روف أن تولي ن المع ه م ي إن شارها ف وق صوتية وإنت ات الف لموج

.م حبيبات الصخر وآثافتهالوسط المسامي يعتمد على مدى مرونة الوسط وحج

ر الىتهدف هذه الدراسة المعملية اج اآب وق صوتية النت التحقق من مدى فاعلية تطبيق الموجات الف

ات الصخرية شبعة بالزيت -آمية من الزيت المتواجد في المكمن بواسطة تعريض العين ى -الم ال

الحالة الطبيعية للغمر المائي بدون وقد تمت المقارنة بين . الموجات الفوق صوتية أثناء الغمر المائي

ا اء في آلت استخدام الموجات والغمر المائي مع استخدام الموجات بقياس النفاذية النسبية للزيت والم

ة الزيت . الحالتين ادة آمي وق صوتية في زي ة استخدام الموجات الف وقد أظهرت النتائج مدى فاعلي

ة والتي المستخلص من العينات الصخرية بل وهناك الكث ذه الطريق ات المرتبطة به ير من الميكانيكي

ك أن استخدام الموجات الصو . تساهم في رفع آمية الزيت التي يمكن استخالصها ى ذل ية تأضف إل

. يمكن ان يطبق مع طرق استخالص الزيت المحسن مثل الغمر الكيميائي

2

ABSTRACT

In oil reservoirs about 60% of the original oil in place remains as residual oil

after primary and secondary oil recovery because of geological and physical factors.

By means of improved oil recovery (IOR) methods, additional oil can be mobilized. The

application of many IOR-methods is not always economical; therefore, efforts are made

to develop alternative tertiary methods with lower economical application risk. One of

these alternatives is the application of ultrasonic waves and pulse vibrations in the

reservoirs to overcome the interfacial tension between oil and water, resulting in

reduction of capillary pressure in the pores and therefore, a higher oil recovery can be

achieved. Ultrasonic waves are longitudinal mechanical waves, which are generated by

an infinitesimal compression of a medium. It is known that propagation of the sound

waves depends on the elasticity, grain size and density of the rock.

In this project, laboratory experiments on core samples were conducted to show

the applicability of ultrasonic waves as an effective method to transport energy through

the reservoir with the aim of oil mobilization and to determine the rate of oil

displacement by waterflooding in the presence of applied ultrasonic waves. Oil/water

relative permeabilities were determined with and without ultrasound. Results show that

the rate of oil displacement increases and the percentage of residual oil within the cores

decreases. Various mechanisms responsible for the increased oil recovery were

identified. Moreover, the effect of wave stimulation on unconsolidated core samples

was investigated.

3

1. INTRODUCTION

The world oil resources are limited and the exploitation of oil fields to a higher

degree is desirable, thus new methods are required to improve the recovery rates of oil

fields and to recover oil found in pores between rock particles.

Great efforts are being made to maximize the production of petroleum in place in

each reservoir. The production history of a petroleum reservoir may be divided into

different production stages. The first stage is the initial approach to produce oil and it

includes primary recovery process, in which the reservoir pressure causes the fluid to

flow into production wells and then to the surface. If the reservoir pressure is not

significant to maintain fluid flow to the surface due to pressure decline or flow

resistence, downhole pumps or gas lift are used to raise the oil to surface until the

energy available to drive the reservoir fluid through rock pores is not adequate to

maintain reservoir productivity. The average primary recovery rate is around 10-15% of

the original oil in place, depending on the oil and rock properties as well as drive

mechanism.

The second production stage known as secondary recovery methods includes fluid

injection, such as gas reinjection or water flooding, into the reservoir to improve oil

recovery. The injection of fluids is implemented to replace the produced reservoir fluid,

thus, the reservoir pressure can be maintained, or to displace the oil directly into the

production wells and then to the surface. The most common method involves flooding

the reservoir with water. The ultimate recovery factor can be increased to about 40% by

employing the secondary recovery method [1]-[4].

The main causes of the poor recovery of the first two production stages are the

existence of the interfacial tension between oil and water (capillary forces) and the

4

heterogeneities in the reservoir rock. Therefore, the oil left in the reservoir after the

primary and secondary methods is the potential target of the third production stage,

namely the tertiary recovery methods. The tertiary recovery method is often termed as

Enhanced Oil Recovery (EOR). In some cases, this term is synonymously used as

Improve Oil Recovery (IOR). It is to be noted that there might be considerable

confusion in the usage of the terms EOR and IOR. Theoretically, the term EOR is used

for the process in which recovery of oil is not possible by a secondary process. On the

other hand, IOR includes everything that could be done to increase the production.

Therefore, IOR could include EOR and the processes that aim to increase the

production, such as infill drilling, reservoir stimulation by hydraulic fracturing, use of

polymers for mobility control or improved conformance [1].

2. STAGES OF OIL RECOVERY PROCESSES

The oil recovery processes are categorized into three recovery stages as mentioned,

namely, primary, secondary and tertiary methods. In this section all of these methods

are summarized briefly.

2.1 PRIMARY RECOVERY METHODS

Primary methods refer to the natural drive mechanisms for the recovery of

reservoir fluid that is associated with the inherent source of energy in the reservoir. This

reservoir energy provides the driving force to push the reservoir fluid to the surface

depending on its pressure. There are different sources of energy for the primary

5

recovery that assist the fluid production, such as gas cap drive, solution gas drive, water

drive, gravity drainage or combination of these effects.

The gas cap drive maintains the oil recovery by expansion of the gas existing

above the oil. This will push the layer of oil downward and out of the well. The gas cap

drive can be supported by injection of hydrocarbon gas over considerable period of time

until reservoir pressure is restored and production resumed [2]. In this type of drive

mechanism the pressure drops slowly, while the gas-oil ratio (GOR) increases

continuously. A reservoir with gas cap drive may have long production life depending

on the size of the gas cap and oil properties.

The solution gas drive or depletion drive maintains the oil recovery by the

expansion of dissolved gas that is trapped in the oil as a result of the pressure drop

during production. However, above the bubble point pressure, only liquid expansion

occurs, while below the bubble point pressure, both liquid and gas expansion provides

the energy to drive the hydrocarbon to the surface. The solution gas drive is

characterized by rapid pressure drop. The GOR in such cases remains low until

reservoir pressure is equal to the bubble point pressure and then increases to maximum

and declines again. The production life in this scenario declines rapidly and installation

of downhole pumps is required to maintain productivity.

The water drive provides the energy to maintain reservoir production such that the

water layer is pushing against the oil layer due to water expansion. In water drive

mechanism, the pressure remains almost constant and the GOR remains also low, while

the water cut becomes in later production life excessive. However, the primary recovery

is highest with water drive approach.

6

2.2 SECONDARY RECOVERY METHODS

The secondary methods are applied to improve oil recovery by reinjection of

either associated gas or water to maintain reservoir pressure, or to displace the oil

remaining in the reservoir toward the producing well by water flooding which is

economically attractive. In this procedure, additional wells are drilled as injectors into

the reservoir. The secondary recovery techniques can contribute to a higher recovery

factor if these techniques are used simultaneously with the primary recovery. However,

waterflooding as secondary recovery method is a widely used method, in which water is

pumped through injection wells to displace part of the remaining oil in production

wells. This process is economically attractive and very effective in light to medium

crude oils. The estimated cost of waterflooding per additional recovered barrel of oil is

in the range of 2-8 $/bbl [2].

2.3 TERTIARY RECOVERY METHODS

The reserves, defined as the part of recoverable resources that has been located

but not yet used, declined since 1980, because the discovery is lower than production.

The ratio of discovery rate to consumption rate is one to four barrels of conventional oil,

thus, the gap between discovery and consumption is big and may grow [4,5].

The growing gap between discovery and consumption is shown in Fig. 1 from

year 1960 to year 2000. The bar represents the gap between discovery and consumption,

and the curve represents the corresponding number of wildcats wells. The maximum

number of wildcats wells is drilled in the year 1980. Even with this increased number of

wildcats wells in 1980, the deficit could not be covered [6].

7

Fig. 1: The growing gap between discovery and consumption and the curve showing exploration drilling [6].

Therefore, enhanced oil recovery methods (EOR) and other advanced methods

become more important to extract oil from known resources rather than from

exploration efforts to find new oil reserves. Thus, the remaining oil after primary and

secondary methods, which is about 60% of the total oil in a reservoir, is hard to recover

by the conventional methods due to physical phenomena such as wettability, capillary

pressure and oil viscosity or geological reasons such as heterogeneity. In order to

recover some of the oil left in the reservoir, EOR-methods have to be applied to

overcome the physical and geological effects. The main goal of the EOR methods is one

or more of the following [2], [7]-[10]:

• Reduction of the interfacial tensions between oil and water, which in turn

reduces capillary pressure.

• Decrease of the mobility ratio between oil and water by decreasing oil viscosity

or increasing water viscosity.

• Injection of chemical solvents.

8

However, efforts are made to develop new techniques with lower application

risk. One of these alternatives is the application of sound waves or ultrasound

stimulation. This technique is promising as new well stimulation technology to enhance

oil recovery and/or to remove formation damage around the wellbore. Sound waves can

be described as a disturbance that travels through a medium from one location to

another location and are longitudinal mechanical waves, which are generated by an

infinitesimal compression of a medium. It is known that propagation of the sound waves

depends on the elasticity, grain size and density of the rock [11].

Vibrating reservoir contents is thought to facilitate production by changing the

capillary forces, adhesion between rocks and fluids and causing oil coalescence

[12],[13]. Generating elastic waves in the reservoir can cause an acceleration of

gravitational segregation of gas, oil and water. International interest in developing

elastic wave stimulation as an effective enhanced oil recovery technology is growing. In

Russia, China, Canada, USA and Norway, laboratory investigations have focused on

elastic-wave vibration, pressure pulsing, vibro-seismic technology as new EOR

techniques, to study the effect of these technologies on improving oil recovery and

reducing water oil ratio [11]-[13]. Sound waves are generally used in the oil industry for

exploration and appraisal during seismic and logging surveys and they are used in many

other industrial applications to remove contaminants from other parts [13]-[16].

In this project, the potential of the sound waves to enhance oil recovery will be

investigated as an alternative method to the conventional EOR.

The advantages of this method compared to other conventional recovery

methods can be summarized as follows:

9

- Sound wave stimulation techniques would replace the need for chemical

stimulation (acid, solvents,…), which is in some cases not compatible with the

reservoir rock or fluid.

- Sound wave stimulation can be conducted at any defined interval allowing precise

wellbore stimulation.

- Sound wave stimulation might be conducted to remove the filter cake especially in

horizontal wells with long horizontal section, in which high amount of acid

solution might be required. Coiled-tubing attached with sound wave generator can

be run into the horizontal section to remove the filter cake along the section.

- Sound wave stimulation might be carried out while the well is producing.

However, laboratory experiments on core samples have been conducted under

reservoir conditions with the following objectives:

- to investigate the applicability of sound waves as an effective method to

transport energy through the reservoir with the aim of oil mobilization,

- to determine the rate of oil displacement by waterflooding in the presence of

applied sound waves,

- to identify the main causes of the additional recovery and

- to compare this approach with other conventional methods.

To describe and interpret the data obtained from laboratory displacement tests in this

study, determination of the relative permeability was essential for all flooding runs. The

following chapter presents a brief description of the oil/water relative permeability,

which was essential for the interpretation of the results obtained.

10

3. REVIEW OF RELATIVE PERMEABILITY

Relative permeability to a fluid is defined as the ratio of effective permeability at

a given saturation of that fluid to the absolute permeability at 100% saturation and

therefore it is expressed in percent or as a fraction. Water relative permeability (Krw) is

zero at residual water saturation (Swi). Therefore water can flow as long as the water

saturation exceeds the residual water saturation. Thus, relative permeability describes

the flow behavior in multiphase system. The relative permeability is required to

evaluate the performance of several reservoir engineering applications such as; reservoir

simulation, waterflooding and enhanced oil recovery methods and it is used in making

estimates of productivity, injectivity and ultimate recovery. It reflects the resistance to

flow due to an additional immiscible fluid. Relative permeability data are usually

obtained from displacement experiments with core samples in the laboratory and are

plotted as a function of water saturation in the core sample. It depends on fluid

saturation and on the saturation history. There are numerous proposed methods of

obtaining relative permeability, such as:

• Steady-state flow process, where both fluids are injected simultaneously

into the core,

• Unsteady-state displacement method, where only one fluid is injected

into the core to displace another fluid present in the core.

• Capillary method,

• Field performance data or,

• Relative permeability correlations.

However, during displacement mechanism the saturation gradient in the reservoir is

changing with respect to time. Therefore, the unsteady-state displacement method was

11

used in all flooding tests in this project, because of its rapidity and reliability for oil-

water systems and it can be performed at different flow conditions within relatively

short time, but the evaluation of the data is a complex task [17]. Johnson, Bossler,

Nauman (JBN) method is used to interpret the obtained data. The JBN method presents

the calculation of relative permeability more accurately than other interpretation

methods [17],[18].

The variables required to calculate the oil/water relative permeability are rock

and fluid dependent variables, such as core's length and diameter, oil and water

viscosities, injection rates, injected and displaced fluids and pressure drop across the

core. For a given core sample and fluids, the core's length, diameter and fluid viscosities

are constant, while the injection rate, displaced fluids and pressure drop across the core

should be recorded.

The sum of the relative permeability to oil and water at any water saturation is

less than 1.0 because of the cross flow of the two phases during simultaneous flow

within the porous media.

12

4. EXPERIMENTAL PART

The purpose of sound/ultrasound technology is to provide continuous energy to

create hydrodynamic waves downhole for dislodging trapped oil at a distance from the

source. To evaluate the effect of sound/ultrasound wave stimulation for enhanced oil

recovery, flooding tests on core samples were carried out. Special design of flooding

apparatus has been set up to conduct the required experiments. The main components

of the flooding apparatus are fluid storage vessels, displacement pumps, Hock cell (core

holder), fraction collector and acoustic and ultrasound generator. The pressure drop

across the core was measured using pressure transducers. Fig. 2 shows schematic

diagram of the flooding apparatus including main components.

Fig.2: Cross section in core holder cell including the transducers to transfer the acoustic from the generator to the core samples.

13

4.1 EXPERIMENTAL PROCEDURE

The principal equipment used in this study consisted of flooding apparatus and

two types of sonic/ultrasonic generators. The effect of ultrasound waves on oil recovery

was investigated using core samples saturated with Saudi crude oil. Consolidated and

unconsolidated natural core samples from outcrops in Saudi Arabia and Berea sandstone

samples with different porosities and permeabilities are implemented. However, to

achieve the objectives of this study the following laboratory procedures were

established:

1. The petrophysical properties (Porosity and absolute Permeability) of the core

samples were determined using core flow apparatus (Fig. 2). The experimental

displacement procedure was established as follows for each sample. The core

sample was placed in the core holder with rubber sleeve around it. A confining

pressure was applied with hand pump (hydraulic oil up to 1500 psi). The core

sample was evacuated using vacuum pump and was saturated with brine. The

pore volume was measured and porosity was calculated. The core permeability

was then calculated at constant injection flow rate by recording the pressure drop

between the inlet and outlet sides using Darcy’s equation:

)1...(..........................................................................................*

**pALqk

Δ=

μ

where k represents the permeability in milli-Darcy; q is the flow rate in cc/sec; μ is

the viscosity in centi-poise; L is the length of the core in cm; A is cross sectional

area in cm2; and Δp denotes the pressure drop across the core sample in atm.

14

2. The core samples were then flooded with the crude oil and displaced with brine

at constant flow rate until the residual oil saturation is reached with continuous

recording of the pressure drop to determine the relative permeability. This group

of experiments will represent the waterflooding without applying sound waves as

reference experiment.

3. Wave stimulation while waterflooding were applied using ultrasonic wave

generator with special design at the residual oil saturation to mobilize additional

oil and to evaluate the oil/water relative permeabilities. The results of these

experiments are compared with those obtained in step 2.

4. Another dynamic displacement tests on core samples were conducted at the

original oil in place (OOIP) to investigate the effect of the waves under initial

reservoir saturations.

5. Two different crude oil samples (light and heavy) were used, to investigate the

influence of gravity differences between oil and water on the total recovery

(gravitational separation).

6. Steps 1 to 5 were be repeated using unconsolidated core samples to examine the

influence of the generated waves on oil recovery from poorly consolidated

sandstone formations.

7. Elastic waves are generated with frequencies ranging up to 50 kHz (Ultrasound)

at low power range of 0.3 KW.

15

4.2 MATERIALS USED

4.2.1 Crude Oil and Brine Solution

The major experiments were conducted using Arabian light crude oil, which has

a viscosity ranging from 13 to 16 cp and API gravity of 31.2° measured at ambient

conditions. However, other crude oils at different API gravities were used to study the

effect of oil gravity on the oil recovery.

Synthetic model brines at salt concentration of 5 % (w/w) were applied in all

runs, consisting of 83 % (w/w) sodium chloride (NaCl) and 17 % (w/w) calcium

chloride (CaCl2).

4.2.2 Core Samples

Two different types of core samples were used through this study, namely

consolidated and unconsolidated core samples. Berea sandstone as consolidated core

samples with a compressive strength of 5000 psi were selected, which consists mainly

of quartz. The homogenous and the wide range of available permeablities makes it

suitable porous medium for this project. The other type of core samples was cut from an

outcrop of sandstone rock as unconsolidated with a compressive strength of almost 150

psi. However, both rock samples reflect almost the mineral composition of some Middle

East reservoirs (Sandstone Formations). The core samples were washed and dried in an

oven at 120 °C for 24h. The dimensions of the core plugs range between 9.0 and 10.45

cm in length and 5.2 cm in diameter. A vacuum pump with vacuum gauge was used to

evacuate the core samples prior to each run. The cores were then saturated with brine

and the porosity was determined. Brine was injected with 5 pore volume (PV) and the

core permeability to brine was measured. The cores were left to stand for one day at this

16

stage. The cores were next flooded with oil to irreducible water saturation. The original

oil in place (OOIP) was determined.

4.2.3 Acoustic Generator (PUNDIT)

To generate low-frequency sound waves ‘PUNDIT’ apparatus was used. The

PUNDIT drives its name from the initial letters of the full title of ‘Portable Ultrasonic

Non-destructive Digital Tester’. It generates low frequency pulses and it couples the

generated waves to the rock samples using transducers, which are placed in contact with

the inlet and outlet faces of the rock samples inside the core holder. Also, it can measure

the time to pass from one transducer to another through the core interposed between

them. This setting allows the generated waves to be transferred through the pore fluids

or through the rock matrix between the transducers during water-flooding runs, thus the

effect of sound waves on the fluid flow through the porous media can be investigated.

Fig. 3 shows the amplitudes of the waves generated using PUNDIT .

-1.50E+02

-1.00E+02

-5.00E+01

0.00E+00

5.00E+01

1.00E+02

0 500 1000 1500 2000 2500

Time, ms

Am

plitu

de

Fig. 3: Amplitude of the waves produced from the low frequency generator (Pundit).

17

4.2.4 High-frequency Ultrasound Source

The high-frequency ultrasonic source is a Clifton ultrasonic bath MU-22. It

generates high-frequency waves at 50 kHz with a power output of 300 watts. The bath

is filled with water as carrying fluid to allow transmitting the waves to the core sample.

The core holder is placed in the center of the ultrasonic bath at a depth to ensure a

maximum immersion in the water and equal exposure to the ultrasonic waves. The

dimension of the ultrasonic bath allowed the experiments to be carried out in vertical

and in horizontal positions. The samples were exposed to ultrasonic stimulation using

two treatment procedures, namely intermittent treatment, which consists of generation

of ultrasonic energy in several cycles of short time (few minutes), and for continuous

wave stimulation treatment. Ultrasonic energy is used in this application to perform

some kind of mechanical work and the power required is the main important factor for

this method to be economical. However, the power may range from just few watts to

several kilowatts depending on the frequency.

18

5. RESULTS AND DISCUSSION

To study the effect of the generated waves on oil recovery, flooding experiments

on core samples were conducted. The core samples were mounted in the core holder and

the flooding experiments were carried out in two different procedures; horizontally and

vertically. However, the wave stimulation is started (1) at residual oil saturation after

waterflooding and (2) at original oil in place. Oil/water relative permeabilities were

determined in presence and absence of ultrasonic waves, additional oil recovery was

measured and water fractional flow was also obtained.

5.1 EFFECT OF WAVE STIMULATION ON OIL/WATER RELATIVE

PERMEABILITY

The displacement runs were made on Berea cores and other cores to investigate

the effect of wave stimulation upon the relative permeability at OOIP and at SOR after

waterflooding. The experiments were conducted in two series, horizontally and

vertically, to describe the flow of oil and water in the porous media. The relative

permeability of each phase is plotted against the water saturation to identify the

performance of wave stimulation. It is also worth mentioning that a reference flooding

test was conducted each time in case of any modification of flooding and/or rock

parameter occurred. Moreover, the excitation time, at which the core was exposed to the

wave stimulation, did not exceed 45 min. as per manufacturer recommendation. Several

repeat runs were made to ensure the reproducibility of the results obtained.

19

5.1.1 Horizontal Corefloods

Fig. 4 presents a typical behavior of relative permeability curves to water and oil

as a function of water saturation. The water/oil relative permeability in Fig. 4 is

generated in absence of wave stimulation as a reference experiment for the horizontal

flooding tests.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1Sw, Water Saturation, % PV

Rel

ativ

e Pe

rmea

bilit

y, K

r

Kro

Krw

Fig. 4: Relative permeability curves to water and oil in absence of wave stimulation.

As shown in Fig. 4, the recoverable oil by waterflooding after water breakthrough lies

between the irreducible water saturation (Swi = 25.7 % PV) and the water saturation at

residual oil saturation, which is Swor =1-Sor = 65.9 % PV. Moreover, the relative

permeability to oil is close to one at Swi and decreases rapidly as oil is displaced by

water while the relative permeability to water rises gradually reaching a value close to

one at Swor.

Another flooding experiment was run in presence of wave stimulation at original oil in

place (OOIP) to compare the water/oil relative permeability with that presented in the

previous run in Fig. 4. Using the displacement data, water/oil relative permeability was

20

determined. Fig. 5 compares the water/oil relative permeability given in Fig. 4 with that

obtained in presence of wave stimulation at OOIP.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1Sw, Water Saturation, % PV

Rel

ativ

e Pe

rmea

bilit

y, K

r Wave stimulated

Reference experiment

Fig. 5: Behavior of water/oil relative permeability in absence and presence of wave stimulation at OOIP. As shown in Fig. 5, the rate of oil recovery by water in presence of wave stimulation,

which started at OOIP, increases considerably. In the presence of wave stimulation, the

oil recovery is at a level of about 59 % of OOIP, or the residual oil saturation after

waterflood of 2 PV is at 31.25% of PV, while in the absence of wave stimulation, the oil

recovery was at 54.1% of OOIP, or the residual oil saturation is at 34.11% of PV.

Fig. 5 shows also that the relative permeability to oil and water of the wave stimulated

experiment is very close to that obtained in the reference experiment. However, the

improvement of relative permeability to oil in wave stimulated run appears at water

saturation higher than 60% of PV. This is an indication that the wave stimulation might

be beneficial in the reservoirs at advanced stages and not at early stages. Therefore, it

was decided to study the effect of wave stimulation in a series of corefloods at residual

oil saturation, which will be presented later in this chapter. In addition, the water

fractional flow in porous media was considered to determine the water saturation after

21

breakthrough. Fig. 6 presents the fractional flow curves of the core samples given in

Fig. 5.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Sw, Water Saturation, % PV

Fw

Wave stimulated

Reference experiment

Fig. 6: Fractional flow curves of reference and wave stimulated experiments.

The intercept of the tangent line drawn from the initial water saturation with the line,

fw=1, is the actual value of average water saturation after breakthrough. The fractional

flow for water (fw) varies between zero, which means 100% oil is leaving the core

sample and one, which means that 100% water is leaving the core sample. However, the

average water saturation after breakthrough is at 61% in the reference experiment, while

it is at 58.2% in the wave stimulated core. Thus, a slightly early breakthrough is formed

in the treated core sample, which would affect the flooding performance negatively and

therefore, more oil is left behind in the wave stimulated core at the time of

breakthrough.

22

To clarify the result of the previous runs, a series of corefloods were conducted using

core samples having higher initial permeability (± 3000 mD). Fig. 7 presents the relative

permeability curves in core samples having high initial permeability.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1Sw, Water Saturation, % PV

Rel

ativ

e Pe

rmea

bilit

y, K

r

Wave stimulated

Reference experiment

Fig. 7: Effect of wave stimulation on the water/oil relative permeability in high permeable sample.

Fig. 7 compares the relative permeability to oil and water in presence and absence of

wave stimulation. The oil recovery in presence of wave stimulation is at a level of

59.3% of OOIP and therefore is higher than that obtained in the reference experiment

(54.1% of OOIP). Although the final oil recovery of the wave stimulated sample is

higher than that of the reference sample, an early water breakthrough was observed in

wave stimulated core. Fig. 8 shows the fractional flow curves of water for the runs

presented in Fig. 7.

23

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Sw, Water Saturation, % PV

Fw

Wave stimulated

Reference experiment

Fig. 8: Fractional flow curves generated for reference and wave stimulated experiments.

This experimental run shows clearly that the average water saturation after

breakthrough in case of wave stimulated core sample is significantly lower than that of

the reference experiment in absence of wave stimulation. The average water saturation

of the wave stimulated core sample is at a level of 42.2%, while it is 61% in the

reference experiment. Moreover, similar results were obtained in higher initial

permeability cores. Fig. 9 presents the performance of waterflood in the high

permeability cores. It shows an overall match of the relative permeability curves in the

presented cases. However, the water breakthrough in the simulated experiment occurred

earlier than that in the reference experiment (Fig. 10), which affected negatively the oil

recovery and lead to oil recovery of 59.3% OOIP compared to 59.1% OOIP obtained in

the reference experiment.

24

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1Sw, Water Saturation, % PV

Rel

ativ

e Pe

rmea

bilit

y, K

r Wave stimulated

Reference experiment

Fig. 9: Water/oil relative permeability in cores with higher initial permeability.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Sw, Water Saturation, % PV

Fw Wave stimulated

Reference experiment

Fig. 10: Comparison of fractional flow curves for water in presence and absence of wave stimulation.

Fig. 10 stressed again the effect of wave stimulation on the early breakthrough that

occurred in presence of wave stimulation. The average water saturation of the

25

stimulated core sample is at 42.2%, while it is 59.9% in the reference experiment.

Although the presence of wave stimulation leads to higher oil recovery, it accelerates

the water breakthrough especially in cores having high permeability.

The observation of early water breakthrough, especially in the experiments of high

permeability corefloods in the presence of wave stimulation, guided us to study the

effect of gravitational separation, which might be the reason behind this phenomenon.

Therefore, a series of core floods in vertical positions were performed.

5.1.2 Vertical Corefloods

A series of corefloods were conducted vertically with the same flooding parameters and

conditions used in horizontal corefloods to evaluate the effect of wave stimulation on

the oil/water segregation. The wave stimulation was applied at original oil in place

(OOIP) and at residual oil saturation after water flooding. Also, reference experiments

were necessary to compare the recovery performance in presence and absence of wave

stimulation. Water/oil relative permeability curves and fractional flow curves for water

were generated taking into consideration the gravitational contribution due to pressure

head in vertical position.

Fig. 11 shows the flood performance in presence and absence of wave stimulation. The

figure exhibits a difference in the relative permeability and in the recovery performance

between the reference and stimulated cores. The presence of wave stimulation causes an

improvement of relative permeability to oil at any water saturation taking into

consideration the difference of the initial water saturation in the core samples, and

higher oil recovery occurred. In presence of wave stimulation 58% of OOIP was

obtained, while it was about 49.9% of OOIP in absence of wave stimulation.

26

00.10.20.30.40.50.60.70.80.9

1

0 0.2 0.4 0.6 0.8 1Sw, Water Saturation, % PV

Rel

ativ

e Pe

rmea

bilit

y, K

r

Wave stimulatedReferenceexperiment

Fig. 11: Effect of wave stimulation on the water/oil relative permeability in vertical corefloods.

On the other hand, the water breakthrough in the stimulated core appears later than that

in the reference core. Therefore, fractional flow curves of water for the same cores were

calculated and presented in Fig. 12.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Sw, Water Saturation, % PV

Fw

Wave stimulated

Reference experiment

Fig. 12: Comparison of fractional flow curves for water in presence and absence of wave stimulation in vertical corefloods.

27

The results indicate that the average water saturation at breakthrough is higher in wave

stimulated core (55.2%) than that obtained in the reference experiment (50.4%). This

means that the oil recovery at breakthrough is higher in presence of wave stimulation.

This result matches with the conclusion drawn in the horizontal corefloods, where early

breakthrough was observed due to water/oil separation. Therefore, wave stimulation

would be more encouraging in horizontal wells, which are placed in the upper layers of

the reservoir to benefit from the gravitational separation.

Additional vertical coreflood tests were conducted on core samples having lower initial

permeability (176 mD). Fig. 13 shows the relative permeability curves in the stimulated

and reference core samples.

00.10.20.30.40.50.60.70.80.9

1

0 0.2 0.4 0.6 0.8 1Sw, Water Saturation, % PV

Rel

ativ

e Pe

rmea

bilit

y, K

r Wave stimulatedReferenceexperiment

Fig. 13: Effect of wave stimulation on water/oil relative permeability on low permeable core samples

The flooding performance in both core samples shows a uniform displacement, which

may be an indication that more excitation time of wave stimulation should be applied in

cores of lower permeability to mobilize additional oil from the core samples. However,

28

taking again the fractional flow curves of both core samples into consideration (Fig. 14),

the water breakthrough in the reference run (47.4 %) occurred slightly earlier than that

in the stimulated experiment (51%).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Sw, Water Saturation, % PV

Fw

Wave stimulated

Reference experiment

Fig. 14: Fractional flow curves of low permeable core samples in presence and absence of wave stimulation.

5.2 RATE OF OIL RECOVERY USING WAVE STIMULATION

To evaluate the effect of wave stimulation on oil recovery at residual oil saturation,

core samples were water-flooded until the residual oil saturation was reached. Generally

about 4 PV of brine was injected before the residual oil saturation was reached. The

cores were further water-flooded in presence of wave stimulation. The effluents were

collected using fraction collector and the oil recovery was calculated. Different values

were used to calculate the oil recovery; at the end of initial waterflooding in percent of

pore volume (PV) and at the end of wave stimulation (Sors) in percent of the residual oil

saturation attained by initial water flooding and in percent of original oil in place. All

29

core samples stimulated at residual oil saturation showed additional oil recovery varing

between 2.5% and 5% of OOIP or 5% to 10% of residual oil saturation, respectively.

However, the additional oil recovery occurs in all runs after an excitation time of 15-20

min. However, two procedures of wave stimulation were applied namely; intermittent

and continuous wave stimulation. Fig. 15 presents the oil recovery performance of two

core samples with different initial permeability 300 mD and 1500 mD as a function of

the cumulative volume injected. The wave stimulation was applied after waterflooding

at residual oil saturation as indicated in Fig. 15.

The performance of the initial waterflooding of the high permeability core shows

lower oil recovery than that obtained from the low permeability core and therefore the

residual oil saturation, which is exposed to the wave stimulation, is 39 % PV (50%

OOIP) and 31 % PV (41% OOIP), respectively. Thus, the presence of wave stimulation

in the high permeable core leads to higher oil recovery than that of low permeable core.

Nevertheless, the potential of additional oil recovery is available in both cores in case of

longer excitation time.

30

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Injected water volume, PV

Res

idua

l Oil

Satu

artio

n, P

V

High permeable core (1500 mD)

Low permeable core (300 mD)

Wave stimulation

Fig. 15: Effect of wave stimulation on the oil recovery in high and low permeable core tests.

The figure exhibits a clear difference in the recovery performance of both core

samples. The remaining oil saturation in case of high permeable core dropped from 39%

to 35% PV (50% to 45% OOIP) and in case of low permeable core from 31% to 30%

PV (41% to 39% OOIP). However, the low recovery is mainly due to the time limitation

of the manufacturer on operation of sound/ultrasonic wave generators. It was not

recommended to exceed a running time of 45 min. Therefore, the ultimate recovery

would increase in case of increasing the excitation time. Moreover, it must be

emphasized that the maximum increase in the oil recovery was associated with the

cycled intermittent excitation. This is probably due to the fact that water/oil separation

did not take place within the porous media during the very short period of excitation. In

contrast, the continuous wave stimulation of 30 to 45 min. contributes to additional oil

31

recovery but at the same time water/oil separation would occur resulting in low

recovery and early water breakthrough.

However, based on the results obtained in this study, the main factors leading to

additional oil recovery are: the reduction of the interfacial tension due to the mechanical

work created by the ultrasonic energy, and the coalescence of oil droplets and lenses of

mobile part and immobile part of oil phase. Also, the wave stimulation can improve the

oil recovery by the intensity of coalescence of water droplets and their sticking to the

solid surface. Langnes et al. [19] describes the effect of coalescence of water droplets

on the attachment of water to the solid surface, which can cause an alteration of

wettability. Additionally, the formulation of emulsion was observed in all runs

involving the wave stimulation, which is also one of the factors leading to additional oil

recovery.

5.3 UNCONSOLIDATED COREFLOODS

Artificial unconsolidated core samples were prepared with a compressive

strength ranging from 50 to 80 psi to study the effect of generated waves on the stability

of the consolidation during the wave stimulation. The unconsolidated samples were

flooded with brine (oil flooding was not performed) and the pressure drop was observed

and recorded. The measurement showed stabilized pressure drop at constant flow rates.

However, waterflooding was applied in presence of wave stimulation, which resulted in

sand production and the flow resistance of brine in porous media increases gradually,

which is indicated by the increase of pressure drop across the core. This phenomenon is

due to the motion of the grains within the porous media caused by the mechanical

energy created by sound/ultrasonic generators.

32

Several attempts were made on other core samples with higher compressive strengths.

Difficulties were still experienced in applying wave stimulation simultaneously with

water flooding in all runs on porous media below a compressive strength of 150 psi.

Fig. 16 shows the general texture of core sample having a compressive strength higher

than 150 psi. The SEM photo shows that the texture of the core was not affected by the

wave stimulation. Therefore, it is concluded that the application of wave stimulation is

not recommended in oil reservoirs with rock compressive strength lower than 150 psi

(unconsolidated rocks).

Fig. 16: SEM photo showing the general texture of the core sample having a compressive strength of higher than 150 psi after wave stimulation.

Finally, it is worth mentioning, that among the several features observed through this

study, no negative impact on the rock and fluid properties was noticed due to the wave

stimulation. Therefore, this method can be applied in wide ranges of types of reservoirs.

This makes the wave stimulation more attractive than the other conventional EOR

33

processes. In the conventional EOR methods, there is no universal method which can be

implemented in any reservoir. The selection of an EOR process is based on different

reservoir parameters, such as; temperature, salinity, types of formation, depth,

permeability, etc. However, a selected EOR method, which is not suitable for the given

reservoir, would have a negative impact on the reservoir associated with high

restoration cost.

6. CONCLUSIONS AND RECOMMENDATIONS

It should be emphasized that the ultimate aim of this research is to investigate the

effect of wave stimulation as new proposed method to improve oil recovery and to

identify the recovery mechanisms. Thus, extensive coreflood tests were conducted

horizontally and vertically. The wave excitation was carried out at OOIP and at residual

oil saturation. Relative permeability curves were created to evaluate the flooding

performance in presence and absence of wave stimulation. However, based on this

research the following conclusions can be drawn:

- The proposed wave stimulation shows an increase in the oil recovery in case of

vertical and horizontal corefloods.

- The interaction of the generated waves with the fluids in pores causes changes in

relative permeabilities of the rock to oil and water, and improves the rate of oil

production and reduces water cut.

- The relative permeability to oil in case of horizontal corefloods was less than that

obtained in the reference experiments till certain water saturations and early water

breakthrough was observed in such configuration. Those phenomena are due to

water/oil separation in the core.

34

- Gravitational separation leads to earlier water breakthrough in horizontal

corefloods while it appears later in the vertical corefloods.

- To overcome the problem of gravitational separation, horizontal wells are

especially of benefit, whereby wells in the upper layer of the reservoir should be

utilized.

- Wave stimulation at residual oil saturation shows in the corefloods an increase in

the oil recovery rate and therefore, this method can be recommended in reservoirs

having high water saturation or in depleted oil reservoirs.

- Wave stimulation is not recommended in unconsolidated formations with a

compressive strength of lower than 150 psi.

35

7. REFERENCES

[1] J. Roger Hite, S.M. Avasti, and L. B. Paul, “ Planning EOR project”, SPE 92006, 2004 SPE International Petroleum Conference, Puebla, Maxico, November 8 – 9, 2004.

[2] L.W. Lake, R.L. Schmidt, and P.B. Venuto, “A Niche for Enhanced Oil Recovery in the 1990s,” Oilfield Review, January 1992.

[3] G.L. Chierici, “Principles of Petroleum Reservoir Engineering,” Translated from Italian by Westaway, P.J., Springer-Verlag, 1994.

[4] J. Laherrere, “Estimates of Oil Reserves,” paper presented at the EMF/IEA/IEW meeting, IIASA, Luxemburg, Austria, June 19, 2001.

[5] R.W. Bentley, “Global oil & gas depletion: an overview,” Energy policy 30, Elsevier, 2002, pp. 189-205.

[6] C.J. Campbell, “Peak Oil,” presented at the Technical University of Clausthal, Germany, Dec. 2000.

[7] Littmann, Wolfgang, “Application of Surface-Active Agents in Petroleum Production,” Handbook of Surface and Colloid Chemistry, CRC Press LLC, 1997, pp. 689-694.

[8] B. Williams, “Progress in IOR technology, economics deemed critical to staving off world’s oil production peak,” Oil & Gas Journal, Vol. 101, Issue 30, August 4, 2003.

[9] M.M. Amro, “Microbial Enhanced Oil Recovery,” PhD Thesis, Technical University of Clausthal, ISBN 3-930697-45-9, Germany, (1994).

[10] R. Gharabi, "Application of an expert system to optimize reservoir performance," Journal of Petroleum Science and Engineering 49, 2005, 261-273.

[11] J.R. Frederick, “Ultrasonic Engineering,” John Wiley and Sons, Inc., USA, (1965), pp. 12-49.

[12] O.L. Kouznetsov, et. al., “Improved oil recovery by application of vibro-energy to waterflooded sandstones,” Journal of Petroleum Science and Engineering 19, (1998), pp. 191-200.

[13] T. Hamida, and T. Babadagli, "Effect of ultrasonic waves on the capillary-imbibition recovery of oil," SPE-paper Nr. 92124, 2005.

[14] R.V. Westermark, et. al., “Enhanced Oil Recovery with Downhole Vibration Stimulation,” Paper SPE 67303 presented at the production and operations symposium, Oklahoma City, March 24-27, (2001)

[15] V.N., Nikolaevskiy, et.al., “Residual Oil Reservoir Recovery with Seismic Vibrations,” SPE Production and Facilities, May (1996), pp. 98.

[16] E. S., Al-Homadhi, and G.M., Hamada, “Determination of Petrophysical and Mechanical Property Interrelationships for Simulated Sandstone Rocks,” The 6th

36

Nordic Symposium on Petrophysics, NTNU, Trondheim, Norway, 15-16 May (2001)

[17] J. Toth, T. Bodi, P. Szucs, and F. Civan, "Direct determination of relative permeability from nonsteady-state constant pressure and rate displacements," SPE Paper No. 67318, 2001.

[18] S.M. Al-Fattah, "Equations for water/oil relative permeability in Saudi Arabian sandstone reservoirs," Saudi Aramco Journal of Technology, summer 2004, pp. 48-58.

[19] G.L. Langnes, et. al. "Secondary recovery and carbonate reservoirs," New York: Am. Elsevier Publ. Co., 1972.

37

8. ABBREVIATIONS

EOR Enhanced oil recovery

Fw Water fractional flow

IOR Improved oil recovery

Kro Relative permeability to oil, %

Krw Relative permeability to water, %

OOIP Original oil in place, %PV

PV Pore volume, cc

SEM Scanning electronic microscopy

Sor Residual oil saturation, %PV

Sw Water saturation, %PV

Swi Initial water saturation, %PV

Swor Water saturation at residual oil saturation, %PV