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Appendix D- Derivation of frontal theory relations for
applicationsrelations for applications
The fractional flow of water is defined as
ow
w
t
ww qq
qqqf +==
substituting the Darcys law owt qqq
( )c
woc
to
o Singg
xP
qAk
f
++
=1
o
w
rw
row
kkf
+
=1
orw
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3.1 . Fractional flow equation
This gradient can be expressed as
xS
SP
xP w
w
cc
=
although the value of can be determined from the appropriate (
given the knowledge whether it is
w
w
c
SP
pp p ( g gimbibition or drainage) oil water capillary pressure
curve, the values are not available. Sw
So in practice the capillary pressure term is neglected but not
forgotten
x
but not forgotten.
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3.1 . Fractional flow equation
As for the effect capillary pressure gradient, we know f h ill h
i lPfrom the capillary pressure curves that is always negative.
That is capillary pressure decreases with increasing water
saturation
w
c
SP
increasing water saturation.
Since the water saturation must decrease with distance in the
direction of flow, we can deduce that is always negative as
well.
xSw
Thus the capillary pressure gradient has a positive sign and its
effect is to increase the value of f
xPc
sign and its effect is to increase the value of fw.
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3.2 Frontal advance theory
Using the following assumptions:1 I ibl fl1. Incompressible
flow2. fw is a function of saturations only3 No mass transfer
between phases3. No mass transfer between phasesIn its differential
form; the frontal advance equation is
derived as:
S dfqdx Eq.3.4w
wS
w
wtSSw dS
dfAq
dtv == w
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3.2 Frontal advance theory
The frontal advance equation states that the rate of d ( l ) f l
f f dadvance (i.e. velocity) of a plane of a specified water
saturation is equal to the total fluid velocitythe total fluid
velocitymultiplied by the change in composition of the flowing
streamg p f f gcaused by a change in the saturation of the
displacing fluid.
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3.2 Frontal advance theory
In other words the frontal advance equation states that h i S l
h heach water saturation, Sw, travels through system at a
constant velocity that can be computed from the derivative of
the fractional flow curve with respect toderivative of the
fractional flow curve with respect to Sw.
It also states that as the total rate increases the velocity of
the plane of saturation increases correspondingly and vice
versa.
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3.2 Frontal advance theory
The location of a particular saturation is found by i i E 3 4 i
h i f llintegrating Eq.3.4 with respect to time as follows:
dftqww S
w
wtS dS
dfA
tqx =It is more convenient to work with dimensionless variables
hence dividing both sides by the total system length L;
ortS dftqx wdfQx =or
w
wS
w
wtS
dSdf
ALtq
L = wSwiD dSQx =
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3.2 Frontal advance theory
In the previous equation Qi ( or dimensionless time ) i d fi d l
i j itD ) is defined as pore volume injection.
Provided that we a formula for or we canwSw
dSdf
Provided that we a formula for or we can determine it accurately
from a plot of fw vs Sw; we can determine for each Sw, and hence
the
wwdS
Swx w,location of all saturations in the system (i.e the
saturation profile in the system at a particular time, )
Sw
We can also determine saturation history at a particular
distance as well.
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3.2 Frontal advance theory
Since the term is difficult to evaluate, we expect h bl i d i
i
xPc
dfto have problems in determining .
However whenever then f can be computed0Pc
wSw
w
dSdf
However, whenever , then fw can be computed directly from the
relative permeability data.
0x
The question is then, is there a water saturation range for
which that assumption holds ?and Can we determine that portion of
the saturation
values?values?
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3.2 Frontal advance theory
Examining the following figure and frontal advance equation
w
wS
wtSSw dS
dfAq
dtdx
v ==qreveals several important points.
wwdSAdt
78
1. Intermediate saturationsare faster than small saturations
123456
f
w
'
saturations2. Faster saturations must catch up small
t ti d t
Fig. x. Derivatives of the fractional flow curve for example 3.5
in textbook
01
0 0.2 0.4 0.6 0.8 1
Sw
saturations and must overcome them3. Result should be formation
of a saturation discontinuity
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3.2 Frontal advance theory
Is the previous conclusion supported by experiments?
Y ( t fi )Yes. ( see next figure )
The experimental observations of Terwilliger indicated that
lower range of saturations all moved at the same velocity. i.e. The
shape of the saturation distribution over this range of saturations
was constant with time This zone was calledof saturations was
constant with time. This zone was called STABILIZED ZONE.
Th l b d th t i th hi h f t tiThey also observed that in the
higher range of saturations called NONSTABILIZED ZONE, saturations
will move at different speeds and hence will continuously get
separated apart.
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3.2 Frontal advance theoryE i l b i f T illi FiExperimental
observations of Terwilliger Fig.
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3.2 Frontal advance theory
Supported by the above experimental observations, it wasti li d
th t th f t l drationalized that the frontal advance
solution is characterized by a saturation discontinuity at the
flood front where the water saturation jumps from Swi, initial
wisaturation to Swf, flood front saturation.
In conclusion we represented the stabilized zone whereIn
conclusion we represented the stabilized zone whereall saturations
moves at a constant speed as a line discontinuity i.e. the
front
Thus the expected saturation profile is as follows from frontal
advance equation.
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3.2 Frontal advance theory
In this figure, lets represent the saturation at the front as S
fSwf.
wfS
wS xfx
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3.2 Frontal advance theory
Remember that frontal advance equation assumes Sw is ti d diff
ti bl lcontinuous and differentiable along x.
Therefore, it is inappropriate to describe the situation at the
front itself. Now we have an half of the answer for the appropriate
portion of the fractional flow curve:
The saturation range from S i to S f of the fractionalThe
saturation range from Swi to Swf of the fractionalflow curve
constitutes the portion where frontal advanceequation does not
apply,
dandthe saturations from Swf to (1-Sor) constitute the region of
applicability for the frontal advance equation.
How to determine this Swf ?
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3.2 Frontal advance theory
The answer is much easier to describe mechanically but quite
involved to explain mathematically.q p yThe mechanical description
is that you have to draw a tangent to the fw curve through the
point (Swi, fwi) such as the one in the following figurethe one in
the following figure.
0.80.9
11fwf
0 30.40.50.60.7
f w
(Swi,fwi)
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.6500.10.20.3
0
S
Swf0.70.24 S w
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3.2 Frontal advance theory
An elegant method for the determination of the floodf i d i b d
h h i lfront saturation and is based on the
mathematicalconstruction of Welge.
The Welges equation for the average water saturation for the
interval x1
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3.2 Frontal advance theory
Welges equation is quite general and the point 2 can b di f h i
l 0 d Q i hbe at any distance from the inlet, x=0 and Qi2 is the
pore volume injection in the portion of the system from the the
inlet to the point 2from the the inlet to the point 2.
( )1t ftqSS ( )22
2 1 wtww fAxqSS +=
0222 fQSS iww +=
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3.2 Frontal advance theory
Th W l i l h f f iThe Welges equation relates three factors of
prime interest in waterflooding;
1. The the average water saturation and hence the oil
recovery,
2. Cumulative injected pore volume, 3. Oil cut and thus water
cut and producing WOR.
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3.2 Frontal advance theory
Coming back to determining the flood front equation, l id h f ll
i h ilets consider the following three equations.
1 a material balance from the inlet to flood front1. a material
balance from the inlet to flood frontIn-Out=Accumulation
Eq.3.9
SSfwiwftwit xASStqftq )( =
Eq.3.10wi
wiwf
f
t
fSS
xAtq
=
1 wif f
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3.2 Frontal advance theory
2. frontal advance solution written for flood front
positionposition
t tq = 1wfSw
wf
SfxA
3. Welges equation written for the flood front position
Eq 3 12Eq.3.12f
twfwfwf xA
tqfSS )1( +=
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3.2 Frontal advance theory
Rearranging Eq 3.12
)1(wfwft
fSS
xAtq =
Then comparing the above three equations we see;
)1( wff fxA p g q ;
f fff )1()1(wiwf
wi
wfwf
wf
Sw
w
SSf
SSf
Sf
wf==
)1()1(
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3.2 Frontal advance theory
Now, number 1:
wfwf
wf
Sw
w
SSf
Sf
=
)1(
Derivative means slope of the tangent line to the curve of the
function at the point where derivative
wfwfSw wf
curve of the function at the point where derivative is
calculated. The above relation dictates that the tangent line to
the fractional curve at the flood gfront values ( ) must pass
through the point wfwf fS ;( ); 1wf wS f =
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3.2 Frontal advance theory
Number 2:
if
wi
ff
wf
SSf
SSf
=
)1()1(
The above relation implies that the line connecting the
wiwfwfwf SSSS
p gpoints ( ) and ( ) will also have the same slope of the
tangent at ( )
wiwi fS ; 1;wfS
wfwf fS ;
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3.2 Frontal advance theory
Number 3: If two lines are required 1. to pass from the same
point and 2 t h th l2. to have the same slope
They have to be identical lines.
In other words you cannot pass two different lines from the same
point and require them to have f p qdifferent slopes.
Therefore the line originating from ( ) and tangent to the
fractional curve fixes the location of the front This is the
mathematical proof
wiwi fS ;
the front. This is the mathematical proof.
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3.2 Frontal advance theory
As a result we can state that we can determine the fl d f l ( )
b dflood front values ( ) by drawing a tangent line to the
fractional curve that originates from the point ( )
wfwf fS ;
fS ;point ( ).
In addition intersection of that tangent line with
wiwi fS ;
f gaxis will give us a very valuable parameter
namely, (i.e the average saturation behind the 1=wff
wfSfront).
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3.3 Estimating the displacement performance for a linear
waterflood at constant rate
I i l i h di l fIn simulating the displacement performance we
are interested in three quantities;
1. Volume oil displaced 2. The rate of oil production,p ,3. The
volume of water that must be handled per
volume of oil after water production begins;4. And we want to
determine them as a function of
time.
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3.3.1 Cumulative oil displaced, Np
By definition,)( wiwpp SSVN =
and again by definition the pore volume injection for h l l h
ithe total length is
LAtqQ ti =
Lets revisit Welges equation expressed for the interval x=0 to
x2 which is applicable at all times to 2 ppcalculate the average
water saturation.
0222 fQSS i+= 0222 fQSS iww +
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3.3.1 Cumulative oil displaced, Np
R i W l iRearranging Welges equation
22 ww SSQ
Also from the frontal advance equation2
2 1 wi f
Q =
q
12i f
Q =
2wSw
w
Sf
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3.3.1 Cumulative oil displaced, Np
Equating the above two equations, one obtains
Eq.3.1922
21
2 ww
w
Sw
w
SSf
Sf
w
=
Lets also see the graphical form of the derivative
2w
g pIn the following figure
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Fig (3.11) showstangent drawn to the fractional flow
curvefractional flow curve at a saturation Sw2.
From (fig 3.11)
21 ww ff =
Eq 3 20
22 weSwSSS
w=
Eq.3.20
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3.3.1 Cumulative oil displaced, Np
Comparing Eq 3.19 and 3.20, one can see thatSS
Hence, average water saturation in the region from
2we SS =
x=0 to x=xSw2 can be obtained by finding the intersection of the
tangent to the fractional flow
ith li01fcurve with line.
Note XS 2 is the position of the saturation S 2
0.1=wf
Note XSw2 is the position of the saturation Sw2.
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3.3.1 Cumulative oil displaced, Np
The above relation has an implication of great value.When
XSw2=L, it allows us to determine:When XSw2 L, it allows us to
determine:
the average water saturation, after the breakthrough.
wS
In other words, finding the intersection of the tangent t th f
ti l fl t S lto the fractional flow curve at any Sw2 value between
(Swf and (1-Sor) with line will give us the average saturation in
the system when
0.1=wfgive us the average saturation in the system when that
production end saturation is equal to that particular saturation
where we draw the tangent. p g
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3.3.1 Cumulative oil displaced, Np
Remember that Sw2 can also be Swf and for that special case we
have arrived at the same intercept conclusion earlier about the
intersection of tangent with 0.1=wf
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3.3.1 Cumulative oil displaced, Np
Since the fluids are considered to be incompressible, Np is
equal to change in the volume of water in the system; and the
cumulative displacement is calculated as:the cumulative
displacement is calculated as:
or)( 2 wiwpp SSVN = )( 2 wiwp
p SSVN =
Where the average saturation value is obtained from the
intersection of the tangent line to the fractional
pV
curve with3.3.2 Production ratesThe fractional flow of water is
determined from the frontal
0.1=wf
The fractional flow of water is determined from the frontal
advance solution for every value of Sw2. Thus qw2 and qo2 are given
by andtww B
qfq 22 = two Bqfq )1( 22
=wB oB
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3.3.3 WOR the water oil ratio
WOR is a measure of the efficiency of the displacement process
at a point in the process. It represents, the volume of water that
must be handled to produce a unit volume of oil and is expressed
as:
w
o
o
wwo B
BffF
2
2=
3.3.4 Time required for displacement
Si th i j ti t i d t t th ti i dSince the injection rate is
assumed constant, the time required to inject Qi PVs of fluid
is
LAqQt i =qt