chapter 4 eor

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Chapter 4

Efficiency of a Displacement Process

EOR-Chapter 4

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IntroductionMicroscopic Displacement of Fluid in a ReservoirMacroscopic Displacement of Fluids in a Reservoir
EOR-Chapter 4

EOR-Chapter 2

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Trapped Oil

= (Microscopic Efficiency) (Volumetric Efficiency)

Production

Injection

EOR-Chapter 2

EOR-Chapter 2

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Overall Displacement Efficiency

Where;

E =overall hydrocarbon displacement efficiency ,the volume of hydrocarbon displaced divided by the volume of hydrocarbon in place at the start of the process measured at the same conditions of pressure and temperature

=Macroscopic (Volumetric) displacement efficiency

=Microscopic (Volumetric) hydrocarbon displacement efficiency.

EOR-Chapter 2

Chapter 2

Chapter 2

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EOR,R.Kharrat,chapter2,2004

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EOR,R.Kharrat,chapter2,2004

EOR-Chapter 2

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Microscopic & Macroscopic sweep efficiencies

EOR-Chapter 2

EOR-Chapter 2

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Macroscopic Displacement
Where;

= Overall displacement efficiency

= Macroscopic displacement efficiency

= Microscopic displacement efficiency

EOR-Chapter 2

EOR-Chapter 2

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However,
= Areal Sweep efficiency

= Lateral Sweep efficiency


EOR-Chapter 2

EOR-Chapter 2

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Therefore, using all these definitions, the oil recovery equation is

To use this equation we must have methods to evaluate the different efficiencies.

Estimates are available from:

Correlations

Scaled laboratory experiments

Numerical simulation

Oil Recovery Equation

EOR-Chapter 2

EOR-Chapter 2

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and is the volumetric sweep efficiency defined as

typical values of the overall recovery efficiency are:

Steam injection 30%-50%

Polymer injection 30%-55%

CO2 injection 30%-65%

Solvent injection 35%-63%

EOR-Chapter 2

EOR-Chapter 2

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Action on Sweep Efficiency at the Macroscopic Scale

By increasing water viscosity

Polymer flooding

By decreasing the oil viscosity

Steam drive

In-situ combustion

Carbon dioxide drive

Action on Displacement Efficiency at the Pore Scale

Miscible hydrocarbon gas flooding

Surfactant flooding

By using a miscible displacing fluid

By reducing the interfacial tension

By action on the rock wettability

Alkaline flooding

Action on Sweep & Displacement Efficiency

EOR-Chapter 2

EOR-Chapter 2

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Microscopic efficiency largely determines the success or failure of any EOR process. For crude oil it is reflected in the magnitude of Sor ( i.e., the residual oil saturation remaining in the reservoir rock at the end of the process).

Microscopic Displacement of Fluids

EOR-Chapter 2

EOR-Chapter 2

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This efficiency is measured directly from a coreflood (since =1). It can also be evaluated from the Buckley-Leveret (or fractional flow theory). For an immiscible displacement is bounded by a residual phase saturation of the displaced phase Sor. Miscible displacements eliminate - in principle -

Displacement Sweep Efficiency

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EOR-Chapter 2

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Example
Initial oil saturation, Soi, is 0.60 and Sor in the swept region for a typical water flood is 0.30ED = (Soi Sor) / SoiED= ( 0.60 0.30 ) / 0.60ED=0.50A typical waterflood sweep efficiency, Ev, at the economic limit is 0.70. Therefore, E =EDEV = 0.50 X 0.70 = 0.35
EOR-Chapter 2

EOR-Chapter 2

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Important factors relating to microscopic displacement behavior

Capillary Forces

Surface Tension and IFT

Solid Wettability

Capillary Pressure

Viscous Forces

EOR-Chapter 2

EOR-Chapter 2

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Important factors relating to microscopic displacement behavior

Capillary forces have a detrimental effect, being responsible for the trapping of oil within the pore.

Trapping is a function of the ratio of Viscous to Capillary forces.

The residual oil saturation decreases as the ratio (Viscous force/ Capillary force) increases.

EOR-Chapter 2

EOR-Chapter 2

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Capillary Forces: Surface Tension and IFT
Whenever immiscible phases coexist in a porous as in essentially all processes of interest, surface energy related to the fluid interfaces influences the saturations, distributions and displacement of the phases.
Sand Grain

Oil

Connate Water

Close up of oil water between grains of rock

EOR-Chapter 2

EOR-Chapter 2

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The surface force, which is a tensile force, is quantified in terms of surface tension
L

Liquid

Air or Vapor
The force per unit length required to create additional surface area is the surface tension, usually expressed in dynes/cm.
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EOR-Chapter 2

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The term surface tension usually is reserved for the specific case in which the surface is between a liquid and its vapor or air. If the surface is between two different liquids, or between liquid and solid, the term interfacial tension is used.The surface tension of water in contact with its vapor at room temperature is about 73 dynes/cm.IFTs between water and pure hydrocarbons are about 30 to 50 dynes/cm at room temperature.
EOR-Chapter 2

EOR-Chapter 2

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One of the simplest ways to measure the surface tension of liquid is to use a capillary tube.At the static condition the force owing to surface tension will be balanced by the force of gravity acting on the column of fluid.
EOR-Chapter 2

EOR-Chapter 2

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Capillary Forces- Solid Wettability
Fluid distribution in porous media are affected not only by the forces at fluid/fluid interfaces, but also by force of fluid/solid interfaces.Wettability is the tendency of one fluid to spread on or adhere to a solid surface in the presence of a second fluid.When two immiscible phases are placed in contact with a solid surface, one phase is usually attracted to the solid more strongly than the other phases. The more strongly attracted phase is called the wetting phase.
EOR-Chapter 2

EOR-Chapter 2

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Rock wettability affects the nature of fluid saturations and the general relative permeability characteristics of a fluid/rock system.The following figure shows residual oil saturations in a strongly water-wet and a strongly oil-wet rock.
Water-wet System

Oil-wet System

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EOR-Chapter 2

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Wettability can be quantitatively treated by examining the interfacial forces that exist when two immiscible fluid phases are in contact with a solid.
Water

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Wettability
Where , , = IFTs between water and oil, oil and solid, and water and solid respectively, dynes/cm. , contact angle, measured through the water
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Capillary Forces- Capillary Pressure
A pressure difference exists across the interface. This pressure, called Capillary pressure can be illustrated by fluid rise in capillary tube.The figure shows rise in a glass capillary. The fluid above the water is an oil, and because the water preferentially wets the glass of the capillary, there is a capillary rise.
EOR-Chapter 2

EOR-Chapter 2

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Capillary Pressure Equation
The difference pressure between oil water at the oil/water interface
EOR-Chapter 2

EOR-Chapter 2

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Capillary Forces- Capillary Pressure
Capillary pressure is related tothe fluid/ fluid IFTRelative permeability of fluids (through )Size of capillary (through r)The phase with the lower pressure will always be the phase that preferentially wets the capillary.Pc varies inversely as a function of the capillary radius and increases as the affinity of the wetting phase for the rock surface increases.
EOR-Chapter 2

EOR-Chapter 2

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Viscous Force
Viscose forces in a porous medium are reflected in the magnetude of the pressure drop that occurs as a result of fluid flow through porous medium.One of the simplest approximations used to calculate the viscous force is to consider a porous medium as a bundle of parallel capillary tubes.With this assumption, the pressure drop for laminar flow through a single tube is given by Poiseuilles law.
EOR-Chapter 2

EOR-Chapter 2

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Viscous Force
Capillary NumberWater floods typically operates at conditions where Nca < 10-6, and Nca values on the order of 10-7 are probably most common.
EOR-Chapter 2

EOR-Chapter 2

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Displacement Sweep Efficiency is a function of
Mobility ratiosThroughput or TransmissibilityWettabilityDip angleCapillary number
EOR-Chapter 2

EOR-Chapter 2

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All sweep efficiencies can be increased by decreasing the mobility ratio by either:

i.e. steam flooding

Oil recovery would still be limited by the residual or trapped oil saturation. Methods that target to reduce this saturation include solvent flooding.

Increasing

i.e. polymer flooding

Lowering

Displacement Sweep Efficiency

EOR-Chapter 2

EOR-Chapter 2

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Experimental evidence suggests that under most conditions the residual oil saturation

(usually a non-wetting phase) can be as large as the wetting phase saturation.

The relationship between trapping wetting or non-wetting phase and a local capillary

number indicates experimental evidence of trapping in a permeable media. This

relationship is called the capillary desaturation curve.

The local capillary number is

Where

= interfacial tension between displacing and displaced fuid

u = displacing superficial velocity

=displacing fluid viscosity

Trapped Oil Saturation

EOR-Chapter 2

EOR-Chapter 2

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Typical capillary desaturation curve


EOR-Chapter 2

EOR-Chapter 2

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Note that it is required a substantial increase in the capillary number to reduce the residual oil saturation. The capillary number can be increased by either.

Lowering interfacial tension miscible/solvent methods

Increasing viscosity of displacing fluid polymer flooding.

There are physical, technical and economic limits of how much can the displacing fluid viscosity and velocity be increased, thus solvent methods are the natural choice to increase the capillary number and therefore lower the residual oil saturation

Capillary desaturation curves are also affected by wettability, and pore size distribution.


EOR-Chapter 2

EOR-Chapter 2

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Viscous Force
Viscous forces in a porous medium can be expressed in terms of Darcys law:
EOR-Chapter 2

EOR-Chapter 2

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Calculation of pressure gradient for viscous oil flow in a rock

EOR-Chapter 2

EOR-Chapter 2

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Example: Calculation of pressure gradient for viscous oil flow in a rock
Calculate the pressure gradient for flow of an oil with 10 cp viscosity at an interstitial flow rate of 1 ft/D. the rock permeability is 250 md and the porosity is 0.2. Solution:
EOR-Chapter 2

EOR-Chapter 2

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Example: pressure required to force an oil trap through a pore throat
Calculate the threshold pressure necessary to force an oil trap through a pore throat that has a forward radius of 6.2 micro meter and radius of 15 micro meter. Assume that the wetting contact angle is zero and IFT is 25 dynes/sec.PB-PA=2*25(1/0.00062-1/0.0015)= - 47300 dynes/cm2-47300*1.438*10^-5= - 0.68 psi
EOR-Chapter 2

EOR-Chapter 2

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Macroscopic Displacement of Fluids in Reservoir
Volumetric Displacement Efficiency & Material BalanceVolumetric Displacement Efficiency ExpressionDefinition & Discussion of Mobility RatioAreal Displacement EfficiencyCorrelationsVertical Displacement EfficiencyVolumetric Displacement Efficiency
EOR-Chapter 2

EOR-Chapter 2

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Macroscopic Displacement of Fluids In a Reservoir
Introduction

Oil recovery in any displacement process depends on the volume of reservoir contacted by the injected fluid. A quantitative measure of this contact is the volumetric displacement (sweep) efficiency defined as the fraction of reservoir (or project )PV that has been contacted or affected by the injected fluid. Clearly, is a function of time in a displacement process.

Overall displacement efficiency in a process can be viewed conceptually as a product of the volumetric sweep, ,and the microscopic efficiency,
EOR-Chapter 2

EOR-Chapter 2

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Volumetric Displacement Efficiency and Material Balance
Volumetric displacement ,or sweep efficiency, is often used to estimate oil recovery by use of material-balance concepts. for example, consider a displacement process that reduces the initial oil saturation to a residual saturation in the region contacted by the displacing fluid. If the process is assumed to be piston-like, the oil displaced is given by
Where ;

= oil displaced , = oil saturation at the beginning of the displacement process, = residual oil saturation at the end of the process in the volume of reservoir contacted by the displacing fluid, = FVF at initial conditions, = FVF at the end of the process, and = reservoir PV

EOR-Chapter 2

EOR-Chapter 2

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Where;

=OOIP at the beginning of the displacement process. if displacement performance data are available, above Eq. also can be used to estimate volumetric sweep. For example, if waterflood recovery data are available, the equation can be rearranged to solve for

EOR-Chapter 2

EOR-Chapter 2

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Where = oil produced in the waterflood.
EOR-Chapter 2

EOR-Chapter 2

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Volumetric Displacement Efficiency
Volumetric Displacement Efficiency Expressed as the product of Areal and Vertical Displacement Efficiencies

Volumetric sweep efficiency can be considered conceptually as the product of the areal and vertical sweep efficiencies. Consider a reservoir that has uniform porosity,thickness,and hydrocarbon saturation, but that consists of several layers. For a displacement process conducted in the reservoir, can be expressed as
EOR-Chapter 2

EOR-Chapter 2

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Where ;

All efficiencies are expressed as fractions. is the volumetric sweep efficiency of the region confined by the largest areal sweep efficiency in the system.

For a real reservoir, in which porosity,thickness,and hydrocarbon saturation vary areally, is replaced by a pattern sweep efficiency ,


EOR-Chapter 2

EOR-Chapter 2

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Where ;

=pattern sweep (displacment)efficiency,hydrocarbon pore space enclosed behind the injected-fluid front divided by total hydrocarbon pore space in the pattern or reservoir a real reservoir.
In essence, is an ideal sweep efficiency that has been corrected for variations in thickness,porosity,and saturation. In either case, overall hydrocarbon recovery efficiency in a displacement process may be expressed as


EOR-Chapter 2

EOR-Chapter 2

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This figure illustrates the concept of the vertical and areal sweep efficiency

EOR-Chapter 2

EOR-Chapter 2

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The following figure illustrate the definition

of areal sweep efficiency

EOR-Chapter 2

EOR-Chapter 2

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These correlations are for piston like displacements in homogeneous, confined patterns. When the well patterns are unconfined, the total area can be much lager and smaller .

Areal Sweep Efficiency

The most common source of areal sweep efficiency data is from displacements in scaled physical models. Several correlations exist in the literature. Craig (1980) in his SPE monograph the reservoir engineering aspects of waterflooding discusses several of these methods.


EOR-Chapter 2

EOR-Chapter 2

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AREAL SWEEP EFFICIENCY
When oil is produced from patterns of injectors and producers, the flow is such that only part of the area is swept at breakthrough. the expansion of the water bank is initially radial from the injector but eventually is focused at the producer.
The pattern is illustrated for a direct line drive at a mobility ratio of unity.At breakthrough a considerable area of the reservoir is unswept.

EOR-Chapter 2

EOR-Chapter 2

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Parameters Affecting
The following definitions are needed to describe the effects of reservoir and fluid properties upon the efficiencies:
Mobility Ratio

Dimensionless Time

Viscous Fingering

Injection/Production well pattern

Reservoir permeability heterogeneity

Vertical Sweep Efficiency

Gravity Effect

Gravity/ Viscous Force Ratio

EOR-Chapter 2

EOR-Chapter 2

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Mobility Definition
The mechanics of displacement of one fluid with another are controlled by differences in the ratio of effective permeability and viscosity

The specific discharge (flow per unit cross sectional area) for each fluid phase depends on This is called the fluid mobility( ):
EOR-Chapter 2

EOR-Chapter 2

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Mobility Control

Mobility controls the relative ease with which fluids can flow through a porous medium.

= mobility of the displacing fluid phase

= mobility of the displaced fluid phase

EOR-Chapter 2

Mobility ratio
The mobility ratio is an extremly important parameter in any displacement process. It affects both areal and vertical sweep, with sweep decreasing as M increases for a given volume of fluid injected.M 1 then unfavorable displacement
EOR-Chapter 2

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EOR-Chapter 2

EOR-Chapter 2

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Dimensionless Time

This variable is used to scale-up between the laboratory and the field . The dimensionless time is defined as the

There are various definitions for the reference pore volume according to the application.

EOR-Chapter 2

EOR-Chapter 2

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Viscous Fingering
The mechanics of displacing one fluid with another are relatively simple if the displaced fluid (oil) has a tendency to flow faster than the displacing fluid (water).

Under these circumstances, there is no tendency for the displaced fluid to be overtaken by the displacing fluid and the fluid fluid (oil-water) interface is stable.

EOR-Chapter 2

EOR-Chapter 2

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Viscous Fingering
If the displacing fluid has a tendency to move faster than the displaced fluid, the fluid-fluid interface is unstable. tongues of displacing fluid propagate at the interface. This process is called viscous fingering.
EOR-Chapter 2

EOR-Chapter 2

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Viscous Fingering

- Decreases when the mobility ratio increases because the displacement front becomes unstable. This phenomena, known as viscous fingering results in an early breakthrough for the displacing fluid, or into a prolonged injection to achieve sweep-out. The next figure illustrates this phenomena, which is commonly observed in solvent flooding.

EOR-Chapter 2

EOR-Chapter 2

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Flooding Patterns

EOR-Chapter 2

EOR-Chapter 2

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Permeability Heterogeneity
It is often has a marked effect on areal sweep. This effect may be quite different from reservoir to reservoir, however, and thus it is difficult to develop generalized correlations. Anisotropy in permeability has great effect on the efficiency.
EOR-Chapter 2

EOR-Chapter 2

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Effect of Mobility Ratio
The following figures show fluid fronts at different points in a flood for different mobility ratios. These results are based on photographs taken during displacements of one colored liquid by second, miscible colored liquid in a scaled model.
EOR-Chapter 2

EOR-Chapter 2

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Correlations Based on Miscible Fluids, Five-Spot Pattern. Figure 1 shows fluid fronts at different points in a flood for different mobility Ratios. The Viscosity Ratio varied in different floods and, because only one phase was present, M is given by Equation.

Correlations Based on .

EOR-Chapter 2

EOR-Chapter 2

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Figure-1: Miscible displacement in a quarter of

a five-spot pattern at mobility ratios

EOR-Chapter 2

*

M=2.40

PV

BT

BT

M=4.58

0.3

0.2

0.1

0.06

0.2

0.3

0.1

PV
PRODUCING WELL PV=PORE VOLUME INJECTED
X INJECTION WELL BT=BREAKTHROUGH

Figure 2: Miscible displacement in a quarter of a five-spot pattern at mobility ratios>1.0,viscous fingering (from Habermann)

EOR-Chapter 2

EOR-Chapter 2

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M=17.3

M=71.5

BT

BT

0.15

0.05
PRODUCING WELL PV=PORE VOLUME INJECTED
X INJECTION WELL BT=BREAKTHROUGH

Figure-3: Miscible displacement in a quarter of a five-spot pattern at mobility ratios>1.0,viscous fingering (from Habermann)

EOR-Chapter 2

EOR-Chapter 2

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Habermann presented values of EA as a function of dimensionless PVs injected,Vi/Vp,after breakthrough, as shown in Figure 4 Results are given for M=0.216 (favorable) to 71.5 (unfavorable).

Correlations Based on Miscible Fluids, Other Patterns

Numerous modeling studies for patterns other than a five-spot have been reported. Craig gives a summery listing of references. As an example of such studies, Figure 5 shows one reported result of areal sweep as a function of mobility ratio for one-eighth of a nine-spot pattern.

Correlations Based on .

EOR-Chapter 2

EOR-Chapter 2

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Pore Volume Injected, Vi/ Vp

Figure-4: Areal sweep efficiency after breakthrough as a function of mobility ratio and PVs injected

Areal Sweep Efficiency, EA%

EOR-Chapter 2

EOR-Chapter 2

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Correlation Based on Miscible Fluids
Numerous modeling studies for patterns other than a five-spot have been reported.One-eight of a nine-spot pattern is shown as an example.This study was conducted with miscible liquids and the X-ray shadowgraph method
EOR-Chapter 2

EOR-Chapter 2

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Figure-5:Areal sweep efficiency as a function of mobility ratio;

EOR-Chapter 2

EOR-Chapter 2

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Correlations Based on Immiscible Fluids, Five Spot Pattern
Craig et al. conducted an experimental study of areal displacement efficiency for immiscible fluids consisting of oil, gas, and water.The study was conducted in consolidated sandstone cores, and fronts were monitored with the X-ray shadowgraph technique.
Figure 6 compares areal sweep efficiency at breakthrough as a function of mobility ratio to the data of Dyes et al., which were obtained with miscible fluids.
EOR-Chapter 2

EOR-Chapter 2

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Figure-6: Areal sweep efficiency at breakthrough as a function of mobility ratio( immiscible fluid displacement);

Water-Gas

Miscible

Gas-Oil

Areal Sweep Efficiency at Breakthrough ,EAbt%

Mobility Ratio,M

EOR-Chapter 2

EOR-Chapter 2

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Prediction of Areal Displacement Performance on the Basis of Modeling Studies
Prediction based on Piston-Like DisplacementCaudle & Witte correlationClaridge correlation (viscous fingering)Mahaffey et. Al model (dispersion )Parallel plate glass modelMathematical Modeling-Numerical
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Prediction of Areal Displacement Performance
on the Basis of Modeling Studies
Prediction Based on Piston Like Displacement.
Caudle and Witte published results from laboratory models of a five-spot pattern in which displacements were conducted with miscible liquids.

The performance calculations are restricted to those floods in which piston-like displacement is a reasonable assumption; i.e., the displacing phase flows only in the swept region and the displaced phase flows in the upswept region. No production of displaced phase occurs from the region behind the front.

EOR-Chapter 2

EOR-Chapter 2

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Figure 8 gives EA as a function of M for different values of the fractional flow of the displacing phase ,fD, at the producing well.

Figure 7 through 9 show data from the experiments. In Figure 7, EA is given as a function of M for various values of injected PVs.

The ratio Vi/Vpd is a dimensionless injection volume defined as injected volume divided by displaceable PV, Vpd. For a waterflood, Vpd is given by

Prediction Based on Piston Like Displacement

EOR-Chapter 2

EOR-Chapter 2

*
Figure 9 presents the conductance ratio, , as a function of M for various values of EA, but only for values of M between 0.1 and 10. Conductance is defined as injection rate divided by the pressure drop across the pattern,
At any mobility ratio other than M=1.0,conductance will change as the displacement process proceeds. For a favorable mobility ratio, conductance will decrease as the area swept, EA, increases. The opposite will occur for unfavorable M values. The conductance ratio, shown in Figure 9 is the conductance at any point of progress in the flood divided by the conductance at that same point for a displacement in which the mobility ratio is unity (referenced to the displaced phase).


EOR-Chapter 2

EOR-Chapter 2

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By combining Figures 7 through 9 , performance calculations can be performed. Areal sweep, as a function of volume injected, is available from Figure 7.Fractional production of either phase can be determined with Figure 8.Rate of injection may be determined as a function of EA from Figure 9. To apply Figure 9, however , it is also necessary to use the appropriate expression for initial injection rate. This is given by Craig for a five-spot pattern using parameters for the displaced phase:

EOR-Chapter 2

EOR-Chapter 2

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Where i=injection rate at start of a displacement process, B/D; k=absolute rock permeability ,md;Krd=relative permeability of displaced phase, h=reservoir thickness ,ft; =pressure drop, psi; =viscosity of displaced phase, cp;d=distance measured between injection and production wells ,ft; and rw= wellbore radius, ft.

At any point in the flood, the flow rate is given by


EOR-Chapter 2

EOR-Chapter 2

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Areal Sweep Efficiency,EA

Figure-7: Areal Sweep efficiency as a function of mobility ratio and injected volume.

EOR-Chapter 2

EOR-Chapter 2

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Mobility Ratio,M

Areal Sweep Efficiency,EA

Figure-8:Areal sweep efficiency as a function of mobility ratio and fractional flow at displacing phase

EOR-Chapter 2

EOR-Chapter 2

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Mobility Ratio,M

Conductance Ratio,

Figure-9:Conductance ratio as a function of mobility ratio and areal sweep.

EOR-Chapter 2

EOR-Chapter 2

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Example: Performance Calculations Based on Physical Modeling Results
A waterflood is conducted in a five-spot pattern in which the pattern area is 20 acres. Reservoir properties are:
EOR-Chapter 2

EOR-Chapter 2

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Required

Use the method of Caudle and Witte to calculate:

(1) the barrels of oil recovered at the point in time

at which the producing WOR=20 ,

(2) the volume of water injected at the same point

(3) the rate of water injection at the same point in time

(4) the initial rate of water injection at the start of waterflood

EOR-Chapter 2

EOR-Chapter 2

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Solution
Apply the correlations in Figs 7 through 91. Calculate oil recoveredM=2.9, fD=20/21=.95 From Fig 8, EA=.94Np=321000 STB2. Calculate total water injected. From Fig 7, Vi/Vpd=2.5 (at EA=.94)Vpd = Vp (Soi Sor) = 341300 bblVi = Vpd x 2.5 = 853300 bbl3. Calculate water injection rate at the same point in time. From i=63.4 B/DFrom Fig. 9, =2.7, from = 63.4x2.7 = 171 B/D4. Calculate initial water injection ratei=63.4 B/D
EOR-Chapter 2

EOR-Chapter 2

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Calculation of EA with Mathematical Modeling
Models are based on Numerical analysis methods and digital computersDouglas et al-2D immiscible displacement. This method is based on the numerical solution of the PDEs that describe the flow of two immiscible phases in two dimensionsHiggins and Leighton mathematical model is based on frontal advance theory
EOR-Chapter 2

EOR-Chapter 2

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Comparison of calculated and experimental results, 5 spot pattern (Douglas et al.)

EOR-Chapter 2

EOR-Chapter 2

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Vertical sweep ( displacement) efficiency, pore space invaded by the injected fluid divided by the pore space enclosed in all layers behind the location of the leading edge (leading areal location) of the front.

Areal sweep efficiency, must be combined in an appropriate manner with vertical sweep to determine overall volumetric displacement efficiency. It is useful, however, to examine the factors that affect vertical sweep in the absence of areal displacement factors.

Vertical Displacement Efficiency

EOR-Chapter 2

EOR-Chapter 2

*


EOR-Chapter 2

EOR-Chapter 2

*

Vertical Displacement Efficiency is controlled primarily by four factors:
Heterogeneity

Gravity effect

Gravity segregation caused by differences in density

Mobility ratio

Vertical to horizontal permeability variation

Capillary forces

EOR-Chapter 2

EOR-Chapter 2

*

Observation of thre figure indicates a stratified reservoir with layers of different permeability. The displacement of the fluid is an idealized piston-flow type. Due to the permeability contrast the displacing fluid will break through earlier in the first layer, while the entire cross-section will achieve sweep-out at a later time, when layer #4 breaks through.

Heterogeneity

EOR-Chapter 2

EOR-Chapter 2

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Heterogeneity:Location of the water front at different Location

EOR-Chapter 2

EOR-Chapter 2

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Heterogeneity:Dykstra-Persons model

EOR-Chapter 2

EOR-Chapter 2

*

Gravity Segregation in Horizontal Bed
Water tongue
Water
Gas umbrella
Gas

EOR-Chapter 2

EOR-Chapter 2

*

Gravity is a factor that affects the vertical efficiency not only in heterogeneous reservoirs but in homogenous as well.

Gravity effects will be important when: (1) vertical communication is good. This is satisfied when is large.

(2) When gravity forces are strong compared to viscous forces. This is satisfied when the gravity number Ng is large.

Gravity Effect

EOR-Chapter 2

EOR-Chapter 2

*

Where:

= relative mobility of displacing fluid

= density difference (displaced - displacing)

u = superficial velocity

Both numbers are dimensionless.

The following figures indicate gravity effects for two different situations

1- Density of displacing fluid lower that density of displaced fluid

The displacing fluid will tend to flow to the top of the reservoir and bypass the fluid in the lower region (tongue over).

Gravity Effect

EOR-Chapter 2

EOR-Chapter 2

*

Tonguing will occur when M < 1 as long as and Ng are large. The effect of heterogeneity and gravity can be mitigated by a favorable mobility ratio.

Gravity tonguing does not require a dipping reservoir (although dipping can be used as an advantage when gravity is important). Gravity tonguing is important in steam flooding applications.

Density of displacing fluid lower that density of displaced fluid

Gravity Effect

EOR-Chapter 2

EOR-Chapter 2

*

Density of displacing fluid higher than density of displaced fluid

Gravity Effect

EOR-Chapter 2

EOR-Chapter 2

*

Gravity segregation occurs when the injected fluid is less dense than the displaced fluid, Figure10a.
Gravity override is observed in steam displacement, in-situ combustion, CO2 flooding, and solvent flooding processes.

Gravity segregation also occurs when the injected fluid is more dense than the displaced fluid, as Figure10b shows for a waterflood.
Gravity segregation leads to early breakthrough of the injected fluid and reduced vertical sweep efficiency.

Effect of Gravity Segregation and Mobility Ratio on Vertical Displacement Efficiency

EOR-Chapter 2

EOR-Chapter 2

*

Gravity Segregation in displacement processes

Figure-10: Gravity Segregation in displacement processes.

Gravity Override (a)

Gravity Underride (b)

Displaced Phase

Displacing Phase

Displacing Phase

Displaced Phase

Displaced Phase

Displacing Phase

EOR-Chapter 2

EOR-Chapter 2

*

Experimental Result
Craig et al. studied vertical sweep efficiency by conducting a set of scaled experiments in linear systems and five-spot models. Both consolidated & unconsolidated sands were used.The linear models used were from 10 to 66 in. long with length/height ratios ranging from 4.1 to 66. Experiments were conducted with miscible and immiscible liquids having mobility ratios from 0.057 to 200. Immiscible water floods were conducted at M

EOR-Chapter 2

*

Craig et al. Results
Results of the linear displacements are shown in the next Figure, where EI at breakthrough is given as a function of dimensionless group called a viscous/gravity ratio.
EOR-Chapter 2

EOR-Chapter 2

*

Vertical sweep efficiency at breakthrough as a function of the ratios of viscous/gravity forces, Linear system (from Craig et al.)

EOR-Chapter 2

EOR-Chapter 2

*

Example: Relative Importance of Gravity Segregation in a Displacement Process
A miscible displacement process will be used to displace oil from a linear reservoir having the following properties:
EOR-Chapter 2

EOR-Chapter 2

*

Solution

EOR-Chapter 2

EOR-Chapter 2

*

Mathematical Model
Spivak used a 2D and 3D numerical model to study gravity effects during water flooding and gas flooding
EOR-Chapter 2

EOR-Chapter 2

*

Gravity Segregation in two-phase flow

EOR-Chapter 2

EOR-Chapter 2

*

The correlations of Craig et al. and Spivak on gravity segregation
Gravity segregation increases with increasing horizontal and vertical permeability.Gravity segregation increases with increasing density difference between the displacing and displaced fluids.Gravity segregation increases with increasing mobility ratioGravity segregation increases with increasing rate. This effect can be reduced by viscous fingeringGravity segregation decreases with increasing level of viscosity for a fixed viscosity ratio.
The correlations of Craig et al. and Spivak indicate the following effects of various parameters on gravity segregation, as summarized by Spivak

EOR-Chapter 2

EOR-Chapter 2

*

Flow Regions in Miscible Displacement at Unfavorable Mobility Ratios

(A) REGIONS I AND II

Oil

SOLVENT

Flow experiments in a vertical cross section in horizontal porous media have shown that four flow regions, are possible when the mobility ratio is unfavorable.

Region I occurs at very low values and is characterized by a single gravity tongue, with the displacing liquid either underriding or overriding the displaced liquid. Vertical sweep is a strong function of .At larger values, in region II, a single gravity tongue still exists, but vertical sweep is relatively insensitive to the value of the viscous/gravity ratio.

EOR-Chapter 2

EOR-Chapter 2

*

Flow Regions in Miscible Displacement at Unfavorable Mobility Ratios

(B) REGION III

(C) REGION IV

SOLVENT

SOLVENT

Oil

Oil

The transition to region III occurs at a particular critical value.

In region III, viscous fingers are formed along the primary gravity tongue and appear as secondary fingers along the primary gravity tongue. Vertical sweep is improved by the formation of the viscous fingers in this region.

In region IV ,flow is dominated by the viscous forces and by viscous fingering. A gravity tongue does not form because of the strong viscous fingering. The vertical sweep in this region is relatively insensitive to

EOR-Chapter 2

EOR-Chapter 2

*

Flow Regimes in Miscible Displacement

EOR-Chapter 2

EOR-Chapter 2

*

Volumetric Efficiency
Methods of estimating volumetric displacement efficiency in a 3D reservoir fall into two classifications.Direct application of 3D modelsPhysicalmathematicalLayered reservoir model. The reservoir is divided into a number of no communicating layers. Displacement performance is calculated in each layer with correlations of 2D. Performance in individual layers are summed to obtain volumetric efficiency
EOR-Chapter 2

EOR-Chapter 2

*


EOR-Chapter 2

EOR-Chapter 2

*

Calculation of volumetric sweep with Numerical Simulators

EOR-Chapter 2

EOR-Chapter 2

*

EOR-Chapter 2

Action on Sweep Efficiency at the Macroscopic Scale

By increasing water viscosity

Polymer flooding

By decreasing the oil viscosity

Steam drive

In-situ combustion

Carbon dioxide drive

Action on Displacement Efficiency at the Microscopic/Pore Scale

Miscible hydrocarbon gas flooding

Surfactant flooding

By using a miscible displacing fluid

By reducing the interfacial tension

By action on the rock wettability

Alkaline flooding

Action on Sweep & Displacement Efficiency

M

E

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V

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/

chapter 4 eor

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