reservoir petrophysics - · pdf file4 capillarity: definition capillarity ..... is the...

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These powerpoint files were produced for the Earth History class at the Free University Berlin, Department of Geological Sciences
The copyright for texts, graphical elements, and images lies with C. Heubeck, unless otherwise specified.
Download, reproduction and redistribution of theses pages in any form is hereby permitted for private, personal, non-commercial, and class-related purposes use as long as the source is identified.
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Prof. Christoph HeubeckInstitut fr Geologische WissenschaftenFreie Universitt BerlinMalteserstr. 74-10012249 BerlinGERMANY
ph: ++49-(0)30-83870695 fax: ++49-(0)[email protected] http://userpage.fu-berlin.de/~cheubeck/
Reservoir Petrophysics
Todays Lecture
Pressure Distribution in a ReservoirBuoyancy / DisplacementOil and water distribution in a reservoirPorosimetryCapillarity

2
Pressure, arbitrary units
Hei
g ht b
elo w
wa t
er s
urfa
c e, f
t
40 50 60 70 80 90
-15
-10
-5
0
5
-20
Wood density = 0.6Water density = 1.00
The longer the wooden beam, the greater the buoyant force at the top of the board
In general:p = h*
p
Free water surface
Buoyancy
Hei
g ht a
bove
fre e
wat
e r, f
t
40 50 60 70 80 90
0
5
10
15
20
- 5
gas = 0.3
water = 1.00
p = 10
light oil = 0.7
heavy oil = 0.9
contact
contact
contact
shale seal
sandstonereservoir
Reservoir Pressure Gradients
Hei
g ht a
bove
fre e
wat
e r, f
t
40 50 60 70 80 90
0
5
10
15
20
- 5
ga
s =
0.3
water = 1.00
lig
ht o
il =
0.7
he
avy
oil =
0.9
shale seal
sandstonereservoir
Reservoir Pressure Gradients Repeat Formation Tester
Pressure, arbitrary unitsde
pth
40 50 60 70 80 90
80
70
60
50
40
90
water = 1.00
sandstonereservoir
shale seal
heavy oil = 0.9
contact

3
The Concept of Displacement Pressure
how to get the oil in the rock !
The Concept of Displacement Pressure
Capillary water
Types of Water in the Reservoir
Structural water (chemically bound)
Hydration water (chemically bound)
Bound water, immobile water, irreducible water
Bound to the grain by capillary force
Capillary water
Water and what to do about it !
Structural water (chemically bound)
Hydration water (chemically bound)
Bound water, immobile water, irreducible water(Haftwasser)
Cant do much about them !
Displace as much as possible !

4
Capillarity: Definition
Capillarity ...
... is the tendency of wettingliquids to ascend minute openings(< 0.5 mm diameter) through theagency of a molecular surfaceforce, and (possibly) actingagainst the force of gravity.
Observations
sponge sucking up liquid sponge on kitchen counter staying wet water rising through plants
The Concept of Capillary Pressure
Hydrocarbon
The capillary pressure of a rock is a function of 3 variables :
water
rock rock
hydrocarbon-water interfacialtension ,
wettability (expressed as thecontact angle ), and
radius of of the pore throat r
Hydrocarbon
water
rock rock
r
radius of of the pore throat r
Hydrocarbon
water
rock rock
r
radius of of the pore throat
wherePc = displacement pressure = oil-water interfacial tension
(surface tension) = contact angle of wetting fluid
against the solid (wettability)r = radius of the pore throat
2 cos r Pc =

5
Extreme Example of VERY LOW Capillary Pressure
water
rock rock
2 cos r Pc =
As , PcAs , PcAs r , Pc
Hydrocarbon
r
Extreme Example of VERY HIGH Capillary Pressure
water
rock rock
2 cos r Pc =
As , PcAs , PcAs r , Pc
r
Hydro-carbon
A closer look at these three factors
Interfacial tension(surface tension)
Wettability
Radius of the porethroat
1. Interfacial Tension (Surface Tension)
The surface tension of a fluid is a measure of the cohesion of the molecules at a fluids surface a function of density r and area of cross section
... is an experimentally determined constant:
All values against air; x 10-3 Nm-1
Pure water, 20 deg C 72.25 Brines higherLight crude oils 20-30 Heavy crudes 35 Mercury 500
Surface tension declines with increasing temperature (and shows a complex behavior with pressure)

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2. Wettability
calcite
oil
quartz
oil
water
water Wetting Liquid
Non-wetting LiquidContact
angle
A water-wet system
Water-wet vs. oil-wet
less mobile
mobile
So
100
0
High initial SoRapid declineHigh recovery rate
Time
An oil-wet system
Water-wet vs. oil-wet
Time
So
100
0
Low initial SoLong slow declineLow recovery
less mobile
mobile
Reservoir Wettability
Initially, all reservoirs are thought to be water-wet
Only after migration, reservoirs may change to oil-wet why ?Complex chemical and physical interactions of HC with mineral surfaces
Rule of thumb:Carbonate reservoirs are generally oil-wet;Siliciclastic reservoirs are generally water-wet

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Proportions of oil and water in a reservoir
After finding a reservoir, need to estimate the volumeof oil in it
To what degree has oil beencapable of entering thereservoir pore space, displacingthe capillary water ?
Displacement Pressure vs. Buoyancy Pressure
Capillary Pressure and Buoyancy Pressure: Migration
Capillary pressure measures the forcenecessary to displace capillary water from apore space
Buoyancy pressure is the additional force by which water is displaced by lighter oil from a given volume
For a given reservoir and fluid, it is afunction of reservoir height
For a given reservoir and fluid, it is afunction of pore size
Pw
Pnw
Remember ? Pressure Distribution in a Reservoir
4050 4060 4070 4080 4090
Hei
ght a
bove
fre e
wat
e r, f
t
4040
0
50
100
150
200
-50
Oil density = 0.77Water density = 1.00
p = 100*(0.433-0.333)=10 psi
p = 150*(0.433-0.333)=15 psi
In general:p = dh*(brine-hc)
Buoyancy pressureOil pressure gradient
slope = 0.333 psi / ftWater pressure gradient
slope = 0.433 psi / ft
Pressure
Pressure Distribution in a Reservoir
4050 4060 4070 4080 4090
Hei
ght a
bove
fre e
wat
e r, f
t
4040
0
50
100
150
200
-50
Pressure
Available buoyancy pressure
Necessary capillary pressure

8
Force Balance in a Reservoir: Saturation Sw, So
Pressure
4050 4060 4070 4080 4090
Heig
ht
ab
ove f
ree w
at e
r , f
t
4040
0
50
100
150
200
-50
Available buoyancy pressure
Necessary capillary pressure
No HC entry into pore space
Beginning HC entry into pore space
Moderate entry into pore space
Strong entry into pore space
SaturationSo
Oil saturation(% of pore volume)
100 80 60 40 20 0
10
50
100
200
500
0
20
Oil-water contact
Seal
Sandstone reservoir
Saturation Sw, So as a function of Pressure
Theoretical curve for perfectly sorted pore
space
Seal
Oil-
wat
er c
apill
a ry
P res
sure
(o
il co
lum
n i n
feet
)Oil saturation
(% of pore volume)
Oil-
wat
er c
apill
a ry
P res
sure
(o
il co
lum
n i n
feet
)
100 80 60 40 20 0
10
50
100
200
500
0
20
Oil-water contact
Sandstone reservoir
Saturation Sw, So as a function of Pressure
Actual curve for perfectly sorted pore
space
Seal
Seal
Irreducible Sw
Entry Pressure
Pore space geometry
1 2
3
3 Shuaiba ls; f=11.9%; k=0.163mD Source: Core Lab
2 Sierra Chata ss; f=7.9%;

reservoir petrophysics - · pdf file4 capillarity: definition capillarity ..... is the...

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