Production Performance of Water Alternate Gas Injection Techniques for Enhanced Oil Recovery
Transcript of 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
E-mail: [email protected]
Abhay Sharma
School of Petroleum Technology,
Gandhinagar, 382007, India
and
Department of Mechanical Engineering,
Indian Institute of Technology,
Hyderabad, Medak, 502205, India
E-mail: [email protected]
Jitendra S. Sangwai*
School of Petroleum Technology,
Gandhinagar, 382007, India
and
Department of Ocean Engineering,
Indian Institute of Technology (IIT),
Madras, Chennai, 600036, IndiaFax: +91-44-2257-4802
E-mail: [email protected]
*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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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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136 J. Bhatia et al.
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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Production performance of water alternate gas injection techniques 137
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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138 J. Bhatia et al.
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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140 J. Bhatia et al.
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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Production performance of water alternate gas injection techniques 143
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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144 J. Bhatia et al.
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)
Pore volume injected (cc)
WATERFLOODING
WATER
WATER
WATER
WATER
WATER
GAS G
AS
GAS
GAS
GAS CHASEWATER
WAG
CYCLE
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Production performance of water alternate gas injection techniques 145
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)
Pore volume injected (cc)
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)
Pore volume injected (cc)
WATER
FLOODING
GAS
CHASEWATER
GAS
GAS
WATER G
AS
GAS W
ATER
WATER
WATER
WATER
WAG
Cycle
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148 J. Bhatia et al.
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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Production performance of water alternate gas injection techniques 149
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
Tapered WAG(decreasingWAG ratio)
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
Tapered WAG(decreasingWAG ratio)
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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Production performance of water alternate gas injection techniques 151
References
Attanucci, V., Aslesen, K.S., Heji, K.A. and Wright, C.A. (1993) WAG process optimization inrangely CO2miscible flood, SPE 26622 presented at 68th Annual Technical Conference andExhibition, October 1993, Huston, Texas.
Bhatia, J. (2010) Comparative Studies of Gas-injection Methodologies for Enhanced Oil Recovery ,MTech thesis, PD Petroleum University, Gujrat, India.
Christensen, J.R., Stenby, E.H. and Skauge, A. (2001) Review of wag field experience, SPEReservoir Evaluation and Engineering, Vol. 4, No. 2, pp.97106.
Dong, M., Foraie, J., Huang, S. and Chatzis, I. (2005) Analysis of water-alternate-gas (WAG)injection using micromodel tests,Journal of Canadian Petroleum Technology, Vol. 44, No. 2,pp.19.
Ghomian, Y., Gary, P.A. and Kamy, S. (2008) Hysteresis and field scale optimization of WAGinjection for coupled CO2 EOR and sequestration, SPE 110639 presented at SPE/DOEImproved Oil Recovery Symposium, 1923 April 2008, Tulsa, Oklahama, USA.
Green, W.D. and Willhite, G.P. (2003) Enhanced Oil Recovery, 2nd ed., SPE Text Book Series,Richardson, TX.
Hadia, N., Chaudhari, L., Mitra, S.K., Vinjamur, M. and Singh, R. (2007) Experimentalinvestigation of use of horizontal wells in waterflooding, Journal of Petroleum Science andEngineering, Vol. 56, No. 4, pp.303310.
Kulkarni, M.M. and Rao, D.N. (2005) Experimental investigation of miscible and immisciblewater-alternating-gas (WAG) process performance, Journal of Petroleum Science andEngineering, Vol. 48, No. 1, pp.120.
Land, C.S. (1968) Calculation of imbibition relative permeability for two- and three-phase flowfrom rock properties, Society of Petroleum Engineers Journal, Vol. 8, No. 2, pp.149156.
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.
Tayfun, B. (2007) Development of mature oil fields a review, Journal of Petroleum Science andEngineering, Vol. 57, Nos. 34, pp.221246.
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.