Post on 31-Mar-2015
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2
Tempelet (Svalbard)
chalk
Carbonates
Silicified Carbonates
Log
geli
nje
Midterhuken (Svalbard)
Log
geli
nje
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Cubic packing
= 47.6 %
Rombicpacking
= 26 %
Cubic packing,different grain size
= 12.5 %
1) PorosityPorosity:
%100%100
tot
matrixtot
tot
p
V
VV
V
V
Vp = pore volumeVmatrix = grain volumeVtot = bulk volume
Ideal
Porous sandstone
Sand grain
water
oiePore size:10 - 50 m
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Effective porosity eff is the porosity of interconneceted poresResidual porosity res is the porosity of the remaining pores
Typical effective porosity:
Sandstone: = 10 - 40 % depending on grain shapeLimestone and dolomite: = 5 - 25 % depending on fractures
Total porosity: tot = eff + res
How do we measure the porosity eff ?
1. In situ measurements in the reservoir (well logging) 2. Core analysis: drilling cores from the reservoir followed by laboratory analysis
• Drilled cores (d = 2,5 - 5´´)• Drilling sylindrical core plugs (d = 1,5´´, h = 3 ´´)• Clean and dry plugs
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2) SaturationA porous medium (reservoir or core plug) usually contains severalfluids: water, oil, gas
Saturation = the fraction of the total pore volume Vp which contains the actual fluid
Water saturation:p
ww V
VS Oil saturation:
p
oo V
VS Gas saturation:
p
gg V
VS
Normally, the entire pore volum will be filled be fluids, hence:
1 gow SSS
The porosity determines the amount of oil in the reservoir
V = the totale pore volume (PV)So = the oil saturationSwc = “connate water” – the original water saturation
Stock Tank Oil Originally In Place (STOOIP):
Bo = “oil formation volum factor” = the ratio between the oil volume in the reservoirand the oil volume in the stock tank at the surface (Rm3/Sm3). Often Bo > 1 because gas is released from the oil when brought to the surface.
Oil In Place (OIP): OIP = V So = V (1 - Swc) unit Rm3
STOOIP = OIP /Bo=V (1 - Swc)/Bo unit Sm3
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3) Miscible and immiscible fluids
a) water and oil are immiscible
The van der Waals force is largerbetween like molecules.
The molecules in a liquid is held together by electrostatical forces (van der Waals forces) acting between the molecules
b) water and alcohol (ethanol) are miscible
The van der Waals force is largerbetween unlike molecules.
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4) Wettability
= 0 - 30o strongly water-wet = 30o - 90o preferably water-wet = 90o – neutrally wet = 90o - 150o preferably oil-wet = 150o - 180o strongly oil-wet
Most oil reservoirs are water wet:
Wettability is the ability of one fluid to spread on a solid surface in the presence of other fluids
The wettability is defined by the wetting angle
Sandgrain
water
oilWettability may also be quantified by capillary pressure properties. We will return to this later.
solidwater
oil
= 0
Water wet
Vannoil
solid
water
= 90o
Neutrally wet
solid
water oil
180o
180o
oil wet
Pipette with water
Oil
Waterdrop
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5) ViscosityViscosity is internal fluid friction
Shear forces act between different fluid layers and between the fluid and container walls
y
Fv
Shear tension:A
F The velocity gradient in the y-direction:
y
v
dy
dv
Empirical studiesshows that formost fluids: dy
dv = the viscosity coefficient or simply the viscosity
Unit: 1 Pas = 1 Ns/m2 = 10 P (poise)This is a Newtonian fluid
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6) Darcy’s law og permeability
A pressure difference p is needed for a fluid to flow through a porous medium.
Henri Darcy (1856) discovered that the volume flow rate Q through a filter of cross section A:
pAaQ where the proportional constant a depends on both the fluid and the filter
The modern version av Darcy’s law for fluid flow in a porous medium (e.g a core plug):
L
ppKAQ AB
L
pBpA
Q A coreplug
Q = volume per unit time (volume flow rate)K = the absolute permeability of the medium = the fluid viscosity
Permeabilityunit
22
3
mPa
m
ms
msPa
AB pp
L
A
QK
This is a rather large unit. Therefore we define a new unit der:1 darcy (D)
2125
23
2
3
m 1098692.0Pa101.01325
s/cm 1sPa10
1atm
1cm
cm
/s1cm1cPdarcy 1
1 millidarcy (mD) = 10-3 D = 0.98692·10-15 m2
1010
7) Relative permeability
The flow possibility for one fluid may then depend on the saturation of the fluids present
Oil may flow more easily in this case: than here:
Sand grain
Sand grain
olje
water along thepore walls(water wet)
Sand grain
Sand grain
oil
Water willflow moreeasily thanthe oil
A sentral question arises: Does Darcy’s law and the permeability concept also apply when there are more than one fluid flowing in the porous medium?
Single phase flow: The absolute pemeabilitty K i Darcy’s equation is independent of the fluid, and depends only on the properties of the porous medium.
Multiphase flow: Several immiscible fluid phases (water, oil, gas) flow simultaneously through the porous medium
We see that the oil will flow more easily when more oil is present (large So)
oil
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Capillary pressure curves in capillary tubes
oil
water Rgg
ph owc 1cos2
One simplecapillary tube
oil
water
1.0Siw
water
oil
Fre water level(FWL)
Water/oilcontact (OWC)
Hei
ght;
C
ap.
pre
ssWater sat. (Sw)
0
Tube radius (R )small large
A battery of tubes with varying radius and therefore varying capillary pressure:
• There is a linear relation between capillary pressure pc and height h.• The total water saturation Sw below h in all tubes decrease when the tubes get thinner
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Darcy’s equations equations3 11
11 p
Kgu
equations 3 222
2 gu
pK
Equations of state equations 2 )( og )( 222111 pp
equations 2 )( og )( 222111 pp Viscosity
Continuity of equation equation 1 0)(
)( 1111
t
S
u
equation 1 0)(
)( 2222
t
S u
Saturation equation 1 121 SS
The capillary pressure equation 1 )( 112 Sppp c
Total 14 equations
13
x
Sf w
www
From the continuity equations we have:
0)(
t
S
x
fu
t
S
x
uf
t
S
x
u wwwwww
Hence 0
t
S
x
fu ww
Introduce
Finally we get: 0
t
S
x
S
xx
S
dS
du www
w
w
w
This is called thesaturation equation
2. order partiell differential equation for Sw(x,t); non-linear with coeffisients which are functions of the independent variable Sw . The equation must be solved numerically.
When Sw has been found, we may calculate fw and uw og uo, and finally pw the po,all as functions og x and t.
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Well logging
Goal: Petrophysics as function of depth in reservoir
A tool with instrumentslowered into the borehole.
The instruments in the probe measures the propertiesof formation and transmits data via mud to the surface
•lithological (rock type)•porosity •saturation
Reservoir
tool
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Tempelet (Svalbard)
chalk
Carbonates
Silicified Carbonates
Log
geli
nje
Midterhuken (Svalbard)
Log
geli
nje
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Øvretransmitter
Mottakerantenne
Øvretransmitter
Gamma-sensor
Logge-sonde
Kraft-kilde
Bore-krone
Slam-motor
Resisitivitets-sensor
a) Measurement While Drilling (MWD) Logging While Drilling (LWD)Tool at the bottom of the drill string. Signals transmitted as pressure waves through mud.
b) Wireline Logging
Drill string is pulled up and the probe is sent down with a wirethat transfer data to / from the logging instruments.Expensive, less common
Two methods
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Log tools must withstand:
high reservoir pressure, 1000 atm high reservoir temperatures, 120 ° C large mechanical stresses For time-efficient electronics
The tools measure into the formationoutside invasion zone for drilling fluids
Drilling Fluids (mudfiltratet) penetratesthe formation (invasion). This may give false results.
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1. Neutron-log Atom nuclei consist of positively charged protons and neutrons without chargeProtons and neutrons have roughly the same mass.
• Neutrons with energy 3-4 MeV sent into the formation from a source in the tool (1 MeV = 1.6 ° 10-13 Joules)
• Neutrons lose energy when colliding with atomic nuclei, hydrogen, in the formation•When the energy is reduced to a they may be “captured” by nuclei
• This excited nuclei emit gamma rays
• This radiation can be detected in a gamma- detector in the tool
nNeutronsource
Gamma-detector
Sond
e Formation
Clamp
Sikjerne
n n n
n
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Much hydrogen ….. Increased gamma radiation.
Most effective if the neutrons collide with protons (p),ie hydrogen nuclei
If the detector detects gamma radiationwe have a neutron-gamma log
Most probes simultaneously measure the epithermalneutrons (En> 1 eV). It is called the neutron-neutron log
The response from nøytronloggen is a measure of hydrogen-containing fluid (oil, water, gas) in the formationie, hydrogen index (HI)
Since these fluids are located in the pores, it is a measure of porosity.
Problem 1: Response from all hydrogen. Also from water bound to clay..
Problem 2: The gas has a low HI, - underestimation of porosity. - Detect gas layer.
nNeutron-source
Gamma-detector
Sond
e Formation
Clamp
Sikjerne
n n
p
n
n
p
p p
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Radioactive isotopes:
• Occur in the earth (crust)• Type and rate of radioactivity depends on the mineral type• Depends on rock type, occurs particularly in shale• Radioactivity is a "finger print" of great interest to the lithologic and stratigraphic description of the reservoir
2. Gamma-log
It measures naturally occurring radioactivity in the formation.Only gamma-rays have sufficient penetration ability in the formation ofreaching the detector in the logging tool
Gamma-detector
Sond
e
Formatjon
40Knuclei
238Unuclei
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Important isotopes
1) 40K ; T1/2 = 1.3·1010 year
40K
40Ar
40CaE=1.46 MeV
+ -
2) 232Th ; T1/2 = 1.4·1010 year
232Th + 228Ra 208PbThorium-series:
3) 238U ; T1/2 = 4.5·109 year
Uranium-series: 238U + 234Th 8 6 206Pb
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The most important minerals that may contain radioactivity are:
1) Quartz [SiO2] (sandstone) – Clean regular lattice – little room to accommodate radioactive isotopes
2) Carbonates (chalk) [ CaCO3 ] – Deposits of living organisms - clean
3) Dolomitt [ CaMg(CO3)2 ] – Traces of Uranium
4) Feltspat [KAlSi3O8] and mica clay and shale - Crystalline, containingAl, K, Na, Ca, Ba – silicates poor crystal structure, ie foreign atoms (eg. radioactive) can take place: thus much radioactivity
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Petrophysics from gamma-log:
1) Lithology (rock type) – Identify shale and clean sand (in addition to mud log)
2) Clay content. Gamma-index:
min
minlog
GRGR
GRGRI
maxGR
GRmin = intensity of the clean zone (without clay / shale)GRmax = intensity of the assumed pure clay zoneGRlog = intensity of the current zone
3) The turning points in the IGR-curve defines the transition between the layers.
4) Depth Reference. Can be used to determine casing need.
0 < IGR < 1
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3. Density log (gamma-gamma log)
A radioactive source (60Co, 137Cs) - gamma radiation.
Principal:
mud
clamp
Radioactivsource
Gamma-detectors
Bore-hole
Formationto
ol
led
Gamma radiation (photons) scattered from electronsof atoms in the formation. Photons lose energy.Those who lose the most energy are those scattered backthe probe.
This decreases the number of electrons with the originalenergy recorded in the detectors.
Absoprbsjonskoeffisienten is proportional to the numberelectrons in the Z atom (molecule) which in turn dependsmass density b .
Gamma-gamma log measures density in formation
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mfb )1(
Bulk Mass of formation is the sum mass of pore volume (liquid)and matrix (rock) :
Porosity:
fm
bm
The matrix density m and the fluid density f til reservoir fluids is known, porositymay be found vi by measuring b with the density log.
We must expect that the density-log records:
•High density of shale and cemented layers•Greater density in the oil-bearing sandstone layers than the layer of gas•Greater density in lower porosity layers•Slightly greater density in the water zones than in the oil zones
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Gas
Oil
Oil
Van
n
Neutron-log
Shale
Sand-stone
Semented
Semented
Semented
Sand-stone
Sand-stone
Sand-stone
Coal
Shale
Tak-berg- art
High
Low
Gamma-log Density-log Shale: high radio activity,A lot of water (bound in clay), high density
Sandstone: low natural gammaLow neutron pga gas (low HI)
Cemented sandstone: high density
Sandstone with oil: HI high,high neutron-log,high densityCemented sandstone: high density
Shale: high gamma, much water
Sandstone with oil: density log and neutron log depends on porosity
Increasing effect on the density log the transition to the water zone
Cementert sandstone: high density
Coal: high water content, inc. neutron
Sandstones with low porosity, Increased density, less water
gamma-log:• high for shale• fingerprint for minerals• identify layers
Neutronlog: • high for oil/water• low for gas• high for shale
Density-log (gamma-gamma log):• high in shale and cemented layers• higher in oil/water compared to gas
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Phase Coherency
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CPMG Sequence
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Fast Relaxation
Slow Relaxation
T2 is a measure of Poresize
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Poresize Distribution -NMR
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NMR
1H
1H
isibi TV
S
TV
S
T
111
1
i = 1,2
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PTEK100 H2011 - Boreteknologi
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Azimuthal Deep Resistivity (ADR) tool
MWD forts …
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BAT Sonic tool