Production Performance of Water Alternate Gas Injection Techniques for Enhanced Oil Recovery

8/10/2019 Production Performance of Water Alternate Gas Injection Techniques for Enhanced Oil Recovery

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132 Int. J. Oil, Gas and Coal Technology, Vol. 7, No. 2, 2014

Copyright 2014 Inderscience Enterprises Ltd.

Production performance of water alternate gasinjection techniques for enhanced oil recovery:effect of WAG ratio, number of WAG cycles andthe type of injection gas

Jigar Bhatia

School of Petroleum Technology,

Gandhinagar, 382007, India

E-mail: [email protected]

J.P. Srivastava

Institute of Reservoir Studies,

Oil and Natural Gas Corporation,

Ahmedabad, 380005, India


Abhay Sharma



and

Department of Mechanical Engineering,

Indian Institute of Technology,

Hyderabad, Medak, 502205, India


Jitendra S. Sangwai*



and

Department of Ocean Engineering,

Indian Institute of Technology (IIT),

Madras, Chennai, 600036, IndiaFax: +91-44-2257-4802


*Corresponding author

Abstract:Production performance of a water alternate gas injection (WAG)method has been reported for the effect of several operating parameters, suchas, WAG injection cycles, viz., single cycle WAG and five-cycle WAG and thetapered WAG at the reservoir conditions of 120C and 230 kg/cm2 forhydrocarbon gas and CO2gas. It is observed that the number of cycles in theWAG injection process affects the recovery of oil from the core sample. It is


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Production performance of water alternate gas injection techniques 133

observed that the tapering in the WAG injection process improves the recovery

of oil initially in place. The observations on the effect of gases revealed that theCO2gas with five-cycle WAG process gives higher incremental recovery thanthe five cycle WAG process using hydrocarbon gas. It is observed that thesaturation profile of CO2WAG injection shows the better gas saturation in thecore as against the hydrocarbon gas in the WAG process. [Received: April 30,2013; Accepted: October 7, 2013]

Keywords: enhanced oil recovery; EOR; water alternate gas; WAG; gastrapping; incremental oil recovery; hydrocarbon pore volume; HCPV.

Referenceto this paper should be made as follows: Bhatia, J., Srivastava, J.P.,Sharma, A. and Sangwai, J.S. (2014) Production performance of wateralternate gas injection techniques for enhanced oil recovery: effect of WAGratio, number of WAG cycles and the type of injection gas,Int. J. Oil, Gas andCoal Technology, Vol. 7, No. 2, pp.132151.

Biographical notes: Jigar Bhatia is currently working as an InstrumentationEngineer at ICAM Technologies Pvt. Ltd. Surat. It is basically the recognisedsystem integrator for Rockwell Automation. He completed his BTech inInstrumentation & Control Engineering from Nirma University, Ahmedabad in2008 and MTech. in Petroleum Engineering from PanditDeendayal PetroleumUniversity, Gandhinagar, India in 2010.

J.P. Srivastava is currently working as Reservoir Engineer in National OilCompany, ONGC at Mumbai, India. He worked in Institute of ReservoirStudies, Ahmedabad from 20002011 dealing with laboratory investigation andselection of gas-based EOR process to enhance recovery from mature fields ofONGC. He has published over six papers in conferences of internationalreputes. His research interest lies mainly in the field of gas-based EOR

techniques and reservoir characterisation through pressure transient analysis.

Abhay Sharma is currently working as Assistant Professor in the Department ofMechanical and Aerospace Engineering at Indian Institute of TechnologyHyderabad, Hyderabad, India. He obtained his MTech and PhD in MechanicalEngineering from IIT Roorkee in 2001 and 2008, respectively. His researchinterest lies mainly in modelling and optimisation manufacturing processes.

Jitendra S. Sangwai is currently working as Assistant Professor in thePetroleum Engineering Program, Department of Ocean Engineering at IndianInstitute of Technology Madras, Chennai, India. He obtained MTech (2001)and PhD (2007) in Chemical Engineering from IIT Kharagpur and IIT Kanpur,respectively. He worked with Schlumberger dealing with flow assurancesissues and on several commercial projects. He has published over 55 papers ininternational journals and conferences of international repute. He holds sevenpatents. His research interest lies mainly in the field of gas hydrates, enhancedoil recovery and flow assurance.

This paper is a revised and expanded version of a paper entitled Investigationson gas trapping phenomena for different EOR-water alternate gas injectionmethodologies presented at International Petroleum Technology Conference2012, IPTC 2012, Bangkok, Thailand, 79 February 2012.


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134 J. Bhatia et al.

1 Introduction

Enhanced oil recovery (EOR) methods, also referred to as tertiary oil recovery methods,

are employed when primary and secondary recovery methods do not improve the

production from brownfields. It is a well-known fact that the world average of oil

recovery factor is estimated to be 35% (Tayfun, 2007) thus almost more than 60% of the

oil initially in place (OIIP) remains in the reservoir after the primary and the secondary

recovery. There is, therefore, an enormous incentive for development of a field through

EOR methods aimed at recovering some portion of the remaining oil keeping in view of

increasing oil prices and the energy demand worldwide. There have been several kinds of

EOR methods that can be used and are shown in Figure 1, such, as, polymer-flooding,

alkaline-surfactant-polymer flooding, gas-injection, thermal techniques, such as, in-situ

combustion, steam injection, etc. The applicability of several of these techniques to a

given field depends on various factors. Out of these, gas-injection-based EOR methodsare one of the most preferred methods for low to medium API oil brownfields due to their

simplicity and economic advantages. One of the derivative methods of gas-injection

techniques is the water alternate gas (WAG) injection methods, wherein water and gas

are injected intermittently. Oil recovery by the WAG injection has been attributed to

contact of upswept zones, especially recovery of attic or cellar oil by exploiting

segregation of gas to the top or accumulation of water toward the bottom. The WAG

injection techniques has the potential for increased microscopic displacement efficiency

because the residual oil after gas flooding is normally lower than the residual oil after

water flooding, and three-phase zones thus obtained lowers the remaining oil saturation.

Thus, the WAG injection can lead to improved oil recovery by combining better mobility

control and contacting upswept zones, and by leading to improved microscopic

displacement.

Figure 1 Different methods of EOR

EOR METHODS

CHEMICAL GAS INJECTIONTHERMAL

BIOLOGICAL

Polymer flooding

Alkaline flooding

Surfactant flooding

Miscible gas injection

Immiscible gas injection

Stream flooding

In-situ combustion

Hot water

Microbial enhance oil

recovery

Source: Green and Willhite (2003)

1.1 Gas-injection method

Gas-injection-based EOR methods are one of the most frequently used methods for EOR

(Kulkarni and Rao, 2005). In this method hydrocarbon or inert gas is injected in to the

reservoir containing residual oil. The components of the gas get dissolved with the lighter

components of the oil which helps to reduce the viscosity and increase the sweep

efficiency in the presence of a chasing fluid such as water. The component exchange

processes between the injected gas and reservoir oil causes heavy and light compositions

in the reservoir which separately moves towards the production side. Different gases are

used in the gas-injection methods, such as, nitrogen, hydrocarbon gas (HC), flue gas and


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CO2gas. Some of the injectants such as, CO2, help to increase oil production by means of

oil viscosity reduction, oil swelling and solution gas drive (Green and Willhite, 2003).The use of specific gas depends on the availability of gas at the field. Previously liquefied

petroleum gas (LPG) and hydrocarbon gas were used for injection. But gradually as price

of natural gas increases, their priority got reduced. Gas-injection method can broadly be

classified as immiscible and miscible gas-injection, depending upon their miscibility with

the oil at reservoir condition. In immiscible gas-injection process the gas is injected at

lower pressure into the reservoir. It is further classified as dispersed gas-injection and

crestal gas-injection according to the injection region. In dispersed gas-injection, gas is

directly injected in to the oil bearing zone of the reservoir. This method is used in the thin

production zone. In crestal gas-injection method, gas is injected in to the gas cap above

the oil bearing zone. For this process, vertical permeability of the reservoir should be

high in order to push the oil towards the production end. Miscible gas-injection method

can be broadly classified as high pressure dry gas miscible displacement, enriched gasmiscible displacement and miscible slug flooding.

A large change in the mobility of gas and oil is observed in case of the gas-injection

methods due to difference in the viscosity of gas to the oil and water at the reservoir

conditions. This results in early breakthrough of the gas to the production side due to its

high sweep velocity. In order to control the sweep velocity of the gas, water and gas are

injected intermittently. This method is called as WAG injection method. Oil recovery by

WAG injection is due to the segregation of gas to the top and accumulation water at the

bottom resulting in the recovery of attic or cellar oil. As the residual oil after gas flooding

is typically lower than that of the water flooding, in addition to the formation of

three-phase zones, which may result in lowering the remaining oil saturation, therefore,

WAG injection shows the potential for increased microscopic displacement efficiency.

Thus, WAG injection can lead to improved oil recovery by combining better mobility

control and contacting upswept zones, and by leading to improved microscopic

displacement. Some factors such as wettability, interfacial tension, connate/fossil water

saturation and gravity segregation increases the complexity to the design of a successful

WAG flood. The WAG injection methods can be classified as miscible WAG, immiscible

WAG, hybrid WAG and simultaneous water alternate gas (SWAG) methods (Christensen

et al., 2001). Several screening criterion are to be considered before the application of

WAG technique for any particular field operation. These are mainly, reservoir pay

thickness, vertical permeability of the reservoir, availability of the gas, type of formation,

mobility ratio, etc.

The aim of the current work is to evaluate the performance of the different

gas-injection methodologies for a given brownfield in India. It includes comparative

studies on different WAG injection methods and to verify their effects on the production

enhancement from the given field. Core flooding experiments are performed at close tothe reservoir conditions of the pressure and temperature to identify:

a the effect of WAG injection method for various WAG cycles

b the recovery efficiency for different methods using different gases like hydrocarbon

gas and CO2gas at reservoir condition

c the effect of tapering on the WAG performance.


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WAG processes which have been studied and discussed in this work (on the basis of

WAG cycles) are,

1 single cycle WAG using HC gas

2 five cycles WAG using HC gas

3 tapered WAG (with increasing and decreasing WAG ratio) using HC gas

4 five cycles WAG using CO2gas.

The following Section 2 provides the experimental details of the present investigation

followed by the outcomes of the experimental work and discussion thereon.

2 Experimental details

The experiments were performed using the in-situ core sample obtained from the

reservoir and fitted in the core pack, which was then kept horizontally during all the

experiments. The gas and oil samples were collected from the separator and recombined

in the laboratory with given gas-oil ratio (GOR) so as to become representative of the

in-situ reservoir fluid. The recombination process is discussed in detail elsewhere

(Bhatia, 2010). The experiments were performed using the recombined separator fluid as

a reservoir fluid in the core sample and the hydrocarbon or CO2gas with water as a mean

for injection in the core sample during WAG process. The water was injected at 20 cc/hr

and gas was injected at 10 cc/hr, which remained same for all the experiments asmentioned above. The basis to choose these injection rates for water and gas are

purely based on our experience of several laboratory studies done in-house to mimic the

scaled-up water and gas injection rate that are possible in real field applications. The

water and gas ratio remained same except for the experiments where the effect of

tapering was studied. The details on the ratio of water and gas used are described later in

experimental procedure Section 2.3.

2.1 Properties of the experimental fluids and the reservoir

The composition of the hydrocarbon gas used for injection is given in Table 1, which was

obtained by using gas chromatographic technique. The major component of the injection

gas was methane (about 90%) of the total concentration. The gas contains around 2%CO2. The gas gravity was observed to be 0.8351 gm/cc. Another gas used for the WAG

process was pure CO2. The basic reservoir data and rock properties are given in Table 2.

The given reservoir is a sandstone reservoir and is under depletion. API gravity of the oil

was about 42 which indicates the light oil reservoir.


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Table 1 Composition of the injected gas in mole fraction obtained by gas chromatography

Component Mole fraction

N2 0.00000

CO2 0.02400

C1 0.90739

C2 0.05237

C3 0.01310

i-C4 0.00089

n-C4 0.00094

i-C5 0.00040

n-C5 0.00048

C6 0.00020

C7 0.00014

C8 0.00005

C9 0.00001

C10 0.00000

Total 1.00000

Table 2 Basic data for the reservoir and the core sample experiments

Details on the reservoir and the core sample

Sr. no. Parameters

1 Reservoir rock type Sandstone

2 Initial reservoir pressure (kg/cm2) 292.7

3 Current reservoir pressure (kg/cm2) 230

4 Bubble point pressure (kg/cm2) 269.6

5 Reservoir temperature (C) 128

6 Density of oil (gm/cc) at 128C 0.5142

7 Stock tank oil density at 15.5C 0.8161

8 API gravity of oil 41.5

9 Oil FVF (v/v) 1.84

10 Specific gravity of gas 0.8364

11 Solution GOR (v/v) 222

12 Core length (cm) 2013 Core diameter (cm) 3.8

15 Avg. permeability (mD) 323.23

2.2 Experimental set-up

High pressure apparatus was selected for the core flooding experiments. All the flooding

experiments were performed at the reservoir pressure of 230 Kg/cm2and temperature of


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128C. The schematic of the core flooding experiment is shown in Figure 2. The heart of

the set-up is the core pack which holds the actual core sample at reservoir conditions. Thecore pack is placed in the oven which is maintained at reservoir temperature. Pressure

gauges are used to indicate the pressures at inlet and outlet of the core pack. The pressure

in the core pack is maintained at reservoir conditions by using positive displacement

pump (Ruska) which injects the fluid (gas, oil, water) at different flow rates in the core

pack. The inlet pressure is regulated by same positive displacement pump through which

kerosene has been used as a displacing fluid to displace any gas or liquid from the gas

cell/buffer cell/rocking cell into the core pack. The gas cellcontains gas (HC or CO2) to

be injected during the WAG process. The buffer cellcontains water (2% KCl) which is

used as a buffer to displace oil or water. The rocking cellis used to prepare the live oil

(recombined fluid) from the oil and gas samples collected from the separator. A

backpressure regulator regulates the flow from the core outlet by maintaining constant

pressure difference at the input and the output side. The produced fluid (water, gas andoil) collected in the separator flask at the outlet of the core pack indicates the quantity of

the produced fluid and one end of the flask connected to the gas meter indicates the

quantity of the produced gas during the WAG process. Steel pipe of 1/8" diameter is used

for fluid transportation within the experimental set-up. The experimental setup described

here was same for all the experiments carried in this work. As the current experiment

set-up consist of a horizontal core flood reactor having a core diameter of about 3.8 cm,

which is sufficiently small, we assume that the flow of the fluid in the core sample is

predominantly unidirectional. In the current set-up, the control over vertical sweep may

not possible due to the small core diameter. This may need better set-up which can

quantify and control the vertical sweep of the injected fluid (Hadia et al., 2007). Before

carrying our actual experiments on the WAG process, initial preparation is done on the

recombination of reservoir fluids using the separator sample of the oil and gas and the

determination of GOR and formation volume factor (FVF) of the recombined reservoir

fluid. The GOR and FVF of recombined fluid are then matched within the accepted limit

with the reservoir GOR and FVF in order to check the reliability of the recombination

process.

Figure 2 Schematic diagram of the experimental set-up used for displacement studies (see onlineversion for colours)

Porous medium

BufferBuffer

CellCell

PumpPump--II

HighHighpressurepressure experimentsexperiments

GasGas

CellCell

RockingRockingCellCell

Hot AirHot Air OvenOven

LiquidLiquid((oiloilandandbrinebrine))

volumevolume measurementmeasurement

GasometerGasometer

GasGasvolumevolumemeasurementmeasurement

Back PressureBack Pressure

RegulatorRegulator

PumpPump--IIII

Water

Cell


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2.3 Experimental procedure

The experimental procedure for all the WAG cases studied mainly consists of the

preparation of the core pack, cleaning and drying of the core pack, evacuation of the core

pack, determination of the pore volume (PV) with saturation of the brine solution,

determination of the hydrocarbon pore volume (HCPV) by displacing the brine solution

with heavy and light paraffin oil. The obtained value of the PV (60 cc) and HCPV gives

the connate/fossil water saturation inside the core. The core pack is then cleaned using

kerosene for studies with recombined fluid. Subsequently, the cleaned core pack is then

saturated with the recombined oil prepared in the laboratory. This is followed by the

secondary water flood until oil saturation in the core reaches to the residual oil saturation.

After completion of the water flooding, to produce the residual oil from the core, WAG

injection is started. The overall process resembles to the actual recovery process a

reservoir may undergo during its production phase. The details on other experimental

procedures related to core pack preparation and cleaning, absolute permeability

determination, PV and HCPV determination, oil saturation and water flooding procedure

can be found elsewhere (Bhatia, 2010) and not discussed here. In the subsequent

Section 2.3.1, a brief discussion on the process of tertiary gas-injection of WAG method

is presented.

2.3.1 Tertiary gas-injection

The tertiary gas-injection is carried out mainly by WAG process using hydrocarbon gas

and CO2gas. Different WAG methods applied for EOR were single cycle WAG, five

cycle WAG (with HC gas and CO2 separately), and tapered WAG (with increasing

and decreasing WAG ratio). For single cycle WAG and five cycle WAG total 1 PV

(1 PV = 60 cc with 0.5 cc) of gas and water was injected intermittently with the WAGratio of 1:1 at the end of water flooding experiment. For tapered WAG injection method a

total of 1.5 PV gas and water was injected intermittently at the end of water flooding

experiment. In tapered WAG (with increasing and decreasing WAG ratio) WAG ratios,

as given in Table 3,were selected and used for the experimental study.

Table 3 Injection WAG ratio for different cycle of tapered WAG methods

WAG ratio for tapered WAG (water :gas)Cycles

Increasing WAG ratio Decreasing WAG ratio

1 3:5 3:1

2 3:4 3:2

3 1:1 1:1

4 3:2 3:4

5 3:1 3:5

Total of five experiments, namely, single cycle WAG (with hydrocarbon gas), five cycle

WAG (with hydrocarbon gas), five cycle WAG (with CO2gas), and tapered WAG using

HC gas (with decreasing and increasing gas tapering) were investigated. In core-flooding

experiments, total PV (about 60 cc) of the core was divided according to the number of

cycles. In a single cycle WAG process, PV (about 60 cc) was divided as 0.5 PV (about

30 cc) gas and 0.5 PV (about 30 cc) water and were injected accordingly. Similarly, for


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five cycles WAG process, 0.1 PV of gas and 0.1 PV of water were injected in five cycles

sequentially so as to make total injection equal to 1 PV (about 60 cc). Attanucci et al.(1993) observed that in case of tapered WAG process an injection of total 1.5 PV gives

better results. In the present study, the experiments for tapering WAG were carried using

1.5 PV of gas and water as total injections. In case of tapered WAG (with decreasing

WAG ratio) more amount of gas was injected in the first cycle and was gradually

decreased in the subsequent cycles. Amount of water to be injected during each cycle

remained constant. In case of tapered WAG (with increasing WAG ratio) similar

procedure was followed but in reverse direction. The details on the quantity of gas and

water used for each of the above processes are given in the Table 4.

Table 4 Details on the amount of water and HC gas used for tapered WAG process

Tapered WAG(increasing WAG ratio)

Tapered WAG(decreasing WAG ratio)

Amountof (cc)

Amountof (cc)

Type ofprocess/numberof cycles

Water Gas

Fractionof total PVper cycle

WAG ratiofor each

cycle Gas Water

Fraction oftotal PV

per cycle

WAG ratiofor each

cycle

1 9 15 0.40 3:5 3 9 0.20 3:1

2 9 12 0.35 3:4 6 9 0.25 3:2

3 9 9 0.30 1:1 9 9 0.30 1:1

4 9 6 0.25 3:2 12 9 0.35 3:4

5 9 3 0.20 3:1 15 9 0.40 3:5

Total 45 45 1.5 --- 45 45 1.5 ---

The hydrocarbon gas collected from the adjacent field and pure CO2obtained from other

sources were used as injection gas. The pressure and temperature conditions of the corepack were kept at reservoir condition and the injection rate for water was maintained at

20 cc/hr and for gas was maintained at 10 cc/hr to avoid the early breakthrough of the

gas. The brine, oil and the gas volumes produced at the end of the experiment were

measured from the separator (flask) and gas meter readings and tabulated as a function of

time. Material balance procedure was used to calculate the saturations of oil, gas and

water components.

2.3.2 Chasing water post WAG process

The chasing water was injected to get the additional recovery of HCPV after the WAG

injection process. Chase water helps to push the trapped gas and water in the core pack,

with that combined oil also gets produced at the production side. In this experimentalstudy maximum of 0.5 PV (around 30 cc) chasing water was injected after the completion

of the WAG injection process. The results tabulated during process are discussed in the

following Section 3.

3 Results and discussion

The core-flooding experiments are carried out to verify the effect of different parameters

of the WAG injection methods. The main objectives of this work are to study the


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efficiency of different WAG processes and the parameters affecting the production

enhancement, viz., tapering of gas, WAG cycles, and type of the injected gas. The resultsare discussed in the subsequent section with respect to the incremental oil recovery over

water flooding.

3.1 Oil recovery

The oil recovery from the above experimental data can be expressed as the displacement

efficiency or recovery in percentage of the total HCPV and can be calculated as,

( )0Displacement efficiency (%HCPV) 100

LHCPV Q FVF V

HCPV

= (1)

where Q0is the flow rate of oil and VLis the total line volume. VLis actually a kind ofdead volume of oil remained in the production tubing of the core flood apparatus and

which need not to be accounted as oil recovered. The results on the displacement

efficiency (in % of total HCPV) with respect to the total PV of fluid (water/gas-water)

injected for different WAG processes are shown in Figures 3 to 7. It is to be noted that

for all the experiments, a water flooding is carried out prior to each WAG process to

represent the secondary oil recovery using water flooding. The water flooding process

required a total of about 1.251.5 PV of the water to be injected in the core sample

(refer to Table 5). Percentage of recovery obtained using the water flooding prior to

WAG process is also given in Table 5 and observed to be in the range of 47 to 57%.

Actual WAG process starts after the end of the water flooding. The WAG processes

consume the fluids (gas and water) in different ranges of PV and have been shown in

Table 4 and are cumulative on the x-axis of Figures 3 to 7 after the pre-water floodingsection. Recoveries in percentage of HCPV have been shown for each of these processes

in Figures 3 to 7. At the conclusion of each WAG process, chasing water is flown

through the core pack to see any incremental recovery. The recovery obtained for

different phases of each process visible in Figures 3 to 7 are tabulated in Table 5.

The maximum recovery is noticed in CO2 five cycle injections (about 97.86% of

HCPV), and next maximum recovery is in tapered WAG injection (decreasing WAG

ratio) (about 72.48% of HCPV). The maximum incremental recovery over the water

flood is seen with CO2gas with five cycle WAG injection (about 40.2% of HCPV), and

the next is noticed with tapered WAG HC gas-injection with increasing WAG ratio

(about 23.92% of HCPV). The maximum recovery with CO2is obtained probably due to

its better miscibility with the crude oil (in the core pack) at reservoir conditions as

compared with the HC gas used in WAG processes. Better recovery is obtained in case of

tapered WAG injection (decreasing WAG ratio) as against all WAG processes using HC

gas is due to an increased sweep efficiency governed by an initial dissolution of

maximum amount of gas with the crude oil in the first cycle, thus helping better mobility

in the pore of the core sample. This results in an increased relative permeability of oil in

the core sample which is enhanced by the subsequent water cycle in the WAG process.

The recovery is affected by different parameters like WAG cycles, type of the injected

gas, tapering, etc. The effects of these parameters are discussed in the following

Section 3.2.


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Table 5 Summary of results for WAG injection methods

Recovery (%HCPV)

WAG injectionpattern

Typeofinjectiongas

AmountofWaterinjected

pre-WAGprocess(PV)

Chasingwaterinjectedafter

WAGprocess(PV)

TotalPVinjectedincluding

forWAGprocess

Recoverywith

waterflooding

Incrementalrecovery

overwaterflood

Totalrecovery

Incrementalrecovery

duringchasewater

Residualoilsaturation

(%HCPV)

Single cycle WAG HC gas 1.246 0.25 2.378 51.57 12.75 64.32 0 30

Five cycle WAG HC gas 1.028 0.5 2.478 52.05 19.30 71.30 2.1 30.06Tapered WAG(increasingWAG ratio)

HC gas 1.255 0.35 3.127 48.44 23.92 72.36 0 29.9

Tapered WAG(decreasingWAG ratio)

HC gas 1.232 0.33 3.063 51.33 16.91 72.48 4.23 16

Five cycle WAG CO2gas 1.452 0.28 2.723 57.67 40.2 97.86 0 29

3.2 Effects of various operating parameters

3.2.1 Number of WAG cycle

Zhang et al. (2010) observed that by increasing the number of the WAG cycles in

gas-injection methods helps to get more recovery of the oil from the reservoir. The

effects of WAG cycle are also studied in this work to see the applicability for the given

reservoir. The results obtained are shown in Figures 3 and 4 for single cycle WAG and

five cycle WAG process using HC gas. The single cycle WAG process using HC gas

shows 12.74% incremental recovery (recovery obtained after the initial water flooding)

and five cycle WAG process using HC gas (no tapering) shows about 17.16% of HCPV

incremental recovery over the water flooding. This indicates that the number of cycle

affects the recovery of HCPV. Increment in the number of WAG cycle improves the

recovery for the same amount of gas utilisation. However, in some of the studies it is

observed that the recovery does not improve significantly even increasing the number of

WAG cycles, probably due to increased water saturation and reduced discontinuity of the

oil phase (Dong et al., 2005).

3.2.2 Effect of tapering

The increase or decrease in the water to gas ratio during the WAG cycles is known as

tapering phenomena in the WAG process. It is also known as the hybrid WAG method

(Christensen et al., 2001). In the tapered WAG process, gas-injection after the initial


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water flood pushes the oil to the production well in case the oil saturation is high, and if it

is low then it will displace the oil to the higher water saturated channels and some ofthe gas stays in the small channels and resist the water mobility (Dong et al.,

2005). Increasing the tapering form a WAG ratio (water: gas) of 1:1 to 1:2 to 1:3

followed by chase water increases the efficiency of the oil recovery (Attanucci et al.,

1993). Results due to the present experimental study on tapering with both increasing

and decreasing WAG are shown in Figures 5 and 6. It is evident that increasing

WAG ratio shows more recovery than decreasing WAG ratio during each cycle. The

large quantity of gas injected during the first cycle (refer to Table 5) helps to get

connected with more residual oil and makes it move towards the high permeable area.

Subsequently, the water cycle flowing through this highly permeable area (channels)

helps to push the oil-gas system towards the production well. The experimental results

for tapered WAG with increasing WAG ratio show that there is no further oil

production after three cycles of WAG injection, which indicate that tapered WAGinjection method can be efficiently used for lower WAG cycles for the equivalent oil

recovery. It is also observed that the chase water also plays an important role during

tapering of gas. The oil recovery obtained in case of tapered WAG with decreasing WAG

ratio is about 68.25% of the HCPV before the chase water flooding. An additional

recovery of 4.2% is achieved after the chase water flooding at the end of WAG process

resulting in a total 72.48% recovery from the given experiment. The chasing water is a

concluding process and acts similar to water cycles in a WAG process only except that

sufficient quantity of water is injected in the core in order to get the maximum possible

recovery from the reservoir. The current study may not be sufficient enough to deduce on

the optimum WAG ratio required for field application. This may depend upon the

reservoir fluid and rock properties, in addition to the type of gas injection being used. In

addition, economics may play a vital role in deciding an optimum value for the WAG

ratio. WAG process may help to reduce the viscous fingering effect associated with

sample gas injection techniques, thanks to the better mobility ratio provided by the

alternate water injection. However, these phenomena may depend on the reservoir fluid

properties, such as, viscosity and density, which may have significant impact on the

optimum WAG ratio.

3.2.3 Effect of injecting gas

The results for five cycle WAG using HC gas and CO2gas are shown in Figures 4 and 7,

respectively, and tabulated in Table 5. The results show that CO2injection in five cycles

WAG gives recovery of about 97.86% of HCPV, which is very high compared to the

recovery of five cycle injection of hydrocarbon gas (about 71.3% of HCPV). This is due

to the fact that compared to the hydrocarbon gas CO2gas is having better miscibility with

the crude oil at the reservoir condition that helps in increasing the solution GOR of the oil

and also helps in reducing viscosity of the oil. This probably results in the solution gas

driven production and increasing the relative permeability of the oil phase. The reservoir

condition of pressure and temperature of 230 kg/cm2and 120C shows that the CO2gas

may be at near supercritical state at the reservoir condition resulting in better miscibility

with the reservoir fluid.


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Figure 3 Displacement efficiency vs. PV injected for single cycle WAG injection using HC gas

as injectant (see online version for colours)

0

10

20

30

40

50

60

70

80

90

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Displacem

entEfficiency(%HCPV)

Pore volume injected (cc)

WATER FLOODINGGASINJECTION WATER CHASE

WATER

WAG

CYCLE

Figure 4 Displacement efficiency vs. PV injected for five cycle WAG injection using HC gas asinjectant (see online version for colours)

0

10

20

30

40

50

60

70

80

90

0.0 0.5 1.0 1.5 2.0 2.5 3.0

DisplacementEfficiency(%HCPV)


WATERFLOODING

WATER

WATER

WATER

WATER

WATER

GAS G

AS

GAS

GAS

GAS CHASEWATER

WAG

CYCLE


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Figure 5 Displacement efficiency vs. PV injected for tapered WAG injection (with increasing

WAG ratio) using HC gas as injectant (see online version for colours)

0

10

20

30

40

50

60

70

80

90

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Displace

mentEfficiency(%HCPV)


WATER

FLOODING G

AS

WATER

CHASE

WATERGAS

GAS

GAS

GAS

WATER

WATER

WATER

WATER

WAG

CYCLE

Figure 6 Displacement efficiency vs. PV injected for five cycle WAG injection (with decreasingWAG ratio) using HC gas as injectant (see online version for colours)

0

10

20

30

40

50

60

70

80

90

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Displaceme

ntEfficiency(%H

CPV)


WATER

FLOODING

GAS

CHASEWATER

GAS

GAS

WATER G

AS

GAS W

ATER

WATER

WATER

WATER

WAG

Cycle


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17/20


efficiency of 70%, while those with five cycle WAG was found to have areal sweep

efficiency of about 80%. The five cycle CO2WAG was observed to have highest arealsweep efficiency of 90%. It is to be noted here that, as the core sample used in this study

are relatively small and homogeneous, the vertical sweep may be completely absent. This

may result in decrease of overall efficiency of the process at reservoir scale. We expect

the reduction in the efficiency may be of the order of 20 to 30% of the measured

efficiency at the laboratory scale, which may largely depend upon the local reservoir

conditions and may vary from field to field. The general conclusion from this study is

that the CO2-WAG gives better relative displacement efficiency as compared to other

processes studied in this work.

Figure 9 Saturation of phases during (a) five cycle WAG injection, (b) tapered WAG injection(increasing WAG ratio), (c) tapered WAG injection (decreasing WAG ratio) and(d) five cycle CO2WAG injection (see online version for colours)

(a) (b)

(c) (d)

Notes: WF: water flooding, G: gas-injection, W: water injection (WAG process);Sw,g,o= saturation of water, gas and oil.


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Table 6 Summary of saturation profile results and Lands trapping constant

Saturation (%PV)

Initial FinalWAG injectionpattern

Injecting gas

Sw Sg So Sw Sg So

Lands gastrapping

constant C

Single cycleWAG

Hydrocarbon gas 28.43 0.00 71.57 50.46 24.00 25.54 1.39

Five cycleWAG

Hydrocarbon gas 29.32 0.00 70.68 37.61 42.1 20.25 1.80

Five cycleWAG

Carbon dioxide gas 28.33 0.00 71.66 60.8 37.7 1.53 1.6

Tapered WAG(increasing

WAG ratio)

Hydrocarbon gas 28.78 0.00 71.21 59.11 21.21 19.68 2.7


Hydrocarbon gas 27.5 0.00 72.5 56.22 23.83 19.95 2.2

Table 7 Gas utilisation factor for different WAG cycle

WAG injectionpattern

Type of theinjection gas

Volume of gasinjected during

WAG process (cc)Gas utilisation factor

Single cycle WAG HC gas 30 2.353261

Five cycle WAG HC gas 30 1.748252

Five cycle WAG CO2 30 0.746269

Tapered WAG(increasingWAG ratio)

HC gas 45 1.881271


HC gas 45 2.659647

4 Conclusions

The implementation of any EOR process should intend experimental verification of

process parameters and cost effectiveness of the process. The experimental study,

followed by simulation and a pilot project implementation provides the better estimation

of the process parameters at the field scale. This study consists of comparative study ofdifferent WAG injection process for the core sample collected from the brown field and

the live oil prepared in the laboratory, from sample of oil and gas collected from the field

separator. The experimental work is done at the reservoir temperature at 120C and

pressure at 230 kg/cm2. The single cycle WAG (with HC gas), five cycle WAG (with HC

gas and CO2 gas) and tapered WAG using HC gas (with increasing/decreasing WAG

ratio) have been investigated experimentally. Based on the experimental results some

important conclusions are derived for implementation of WAG process.


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References

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Rogers, D.J. and Grigg, B.R. (2000) A literature analysis of the wag injectivity abnormalities inthe CO2 process, SPE 73830, was revised for publication from paper SPE 59329, firstpresented at the SPE/DOE Improved Oil Recovery Symposium, 35 April 2000, Tulsa.

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Zhang, Y.P., Sayegh, S.G., Luo, P. and Huang, S. (2010) Experimental investigation of immisciblegas process performance for medium oil, Journal of Canadian Petroleum Technology,Vol. 49, No. 2, pp.3239.

Documents

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