Download - BWLA - Water Saturation and Resistivity
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GEO4250Reservoir Geology
Basic Well Log Analysis
Determination of Saturation
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Reminder
hArSN hcep φ=hcw
n
t
on
t
ww
S1SRR
RFRS
−=
==
( ) PorosityEffectiveV1logfrom
PorosityDensity
PorositySonictttt
shte
n
fmatrix
bmatrixden
matrixf
matrixlog
s
−×φ=φφ
ρ−ρρ−ρ
=φ
⎟⎟⎠
⎞⎜⎜⎝
⎛Δ−ΔΔ−Δ
=φ
mw
0 aRRF
φ==
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Reminder
• Determined Rw from the SpontaneousPotential
• Determined φt and φe from porosity and gamma ray logs
• The only parameter we lack now is thetrue resistivity of the formation
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Resistivity
• The resistivity (specific resistance) of a substance is the resistance between opposite faces of a unit cube ofthat substance at a specific temperature
• Symbol R, measured in Ω·m (ohm·m2/m)
• With r = resistance in ohms; A = area in square meters; L = length in meters
• Resistivity is a basic measurement of a reservoir’s fluid saturation and is a function of porosity, type of fluid, amount of fluid and type of rock
• Usually between 0.2 and 1000 ohm·m
LArR ×
=
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Resistivity
• Dependent on:– Presence of Formation water / Hydrocarbons– Salinity of Formation water– Temperature of Formation water– Volume of water-saturated pore space– Geometry of the pore space– Morphology and species of clay minerals
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Tools
• Conventional Electrical logsFirst developed before 1950– Normal devices
• Short Normal (SN)• Long Normal (LN)
– Lateral devices• Laterologs (LL, e.g. LL3, LL7, LL8, LLD, LLS, SFL)
• Induction loggingFirst developed after 1950– Induction devices
• 6FF40 (combined tool: Induction, Normal and SP, 1960’s)• DIL-LL8 (improved induction-normal combination)• Induction SFL• DIL-SFL• Phasor Induction SFL• Other abbreviations: ILD, ILM, ILS, RD, RM, RS, RT
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Conventional Electrical Logging
• Current of constantintensity between A and B
• Resultant potentialdifference measuredbetween M and N
• The larger the’spacing’, the deeperthe measurement
NormalDevice
LateralDevice
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Conventional Electrical Logging Normal and Lateral Curves
Normal Device• A: Beds more resistive
than surrounding beds• B: Beds less resistive
than surrounding beds
Lateral Device• C: Beds more resistive
than surrounding beds• D: Beds less resistive
than surrounding beds
A B C D
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Induction Logging
• High frequency, alternating current of constant intensity is sent through a transmitter coil (A)
• The alternating magnetic field (B) created, induces currents in the formationsurrounding the borehole (C)
• These currents flow in circular ground loops coaxial with the transitter coil and create, is turn, a magnetic field (D) that induces a current in the receiver coil (E)
AB
CD
E
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Resistivity LoggingFocusing
• Minimization of borehole and adjacent formationaffects
• Focusing currents to control the path taken by the measure current
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Resistivity LoggingInfluence of well bore variables and log correction
• Resistivity measurements are influenced by:– Borehole mud– Adjacent beds– Invaded zone
• Readings must be corrected, always in thefollowing way:– Borehole effect– Adjacent bed effect– Invasion correction
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Resistivity LoggingCorrections - Borehole Effect
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Resistivity LoggingCorrections - Adjacent Bed Effect
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Resistivity LoggingInvasion Correction
’Tornado’ or ’Butterfly’ Chart
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Laterolog vs Induction
Induction Log• Recommended in holes drilled
with moderately conductive drilling muds, non-codunctive muds and in empty or air-drilled holes
• Most accurate in low- to medium-resistivity formations
• Rxo > Rt
Induction Log• Recommended in holes drilled
with moderately conductive drilling muds, non-codunctive muds and in empty or air-drilled holes
• Most accurate in low- to medium-resistivity formations
• Rxo > Rt
Laterolog• Recommended in holes drilled
with conductive muds (salt muds)• Most accurate in medium- to high-
resistivity formations• Rxo < Rt
Laterolog• Recommended in holes drilled
with conductive muds (salt muds)• Most accurate in medium- to high-
resistivity formations• Rxo < Rt
Both have unique characteristicsthat favor their use in specific, and often different, situations and application
Both have unique characteristicsthat favor their use in specific, and often different, situations and application
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ResistivityScaling
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Determination of Saturation
mw
0 aRRF
φ==n
t
ww R
FRS =
tw
w
t
ww
1a,2mn
tm
wnw
R1
SR
RRS
RaRS
=φ→
=φ→φ
====
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Determination of Saturation
EXAMPLE• 100% water saturated formation• Sw = 1; Rt = R0• R0 vs φ is a straight line;• If Sw ≠ 1, but constant, all points also
on a straight line• From plotted points, assume some
from 100% water saturated rock line through f=0, R=∞ and through the most westerly plotted points
• Slope of this line defines the value ofRw
• For φ=10%, R0 = 6.5 ohm·m• For compacted formations:
– F = 1 / φ2
• So, in this case F = 100• We also now that:
– Rw = R0/F• So, Rw = 6.5/100 = 0.065 ohm·m
tw RR=φ
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Determination of Saturation
• Only possible for formations with constant matrixand for constant Formation Water Resistivities