Shaly sand reservoirs- Impact on Porosity and Water Saturation; …€¦ · Porosity & Saturation...
Transcript of Shaly sand reservoirs- Impact on Porosity and Water Saturation; …€¦ · Porosity & Saturation...
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Shaly sand reservoirs-Impact on Porosity and Water Saturation;The Dual Water Model
Rubi Rodriguez
Sr. Petrophysicist
07th March 2019
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Introduction
Shale definition
Shale Effects on Logs
Porosity & Saturation
Dual Water (examples)
Summary
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Evolution of shaly sand models
1927
1942
19501953
19681984
Dual Water model by Clavier
Waxman & Smit model
“Double Layer” concept by W.M.McCardell
Archie’s law doesn’t apply to shaly sand by M.R. Wyllie
First electric Log by Conrad & Marcel
Schlumberger
Archie’s law published
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Archie’s equation
• First quantitative log analysis methodology
• Relating resistivity, porosity and saturation
The shaly sand effect
• Formation factor (F) was not intrinsic rock property, but instead was seen to vary with water salinity due to clay
• X called extra conductivity
The problem
“The most important problem that has received thus far no satisfactory
solution is that of shaly sands” - Henri Doll 1953
Water ConductivityR
ock
Con
duct
ivity
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How much hydrocarbon is there?
STB
RB=B
B
)hASφc=OOIP w
0
0
1 1( −OOIP=original oil in place
c1=7758 units conversion
h=height
=porositySw=water saturation
A=area
B0=oil formation volume factor
RB=reservoir barrels
STB=stock tank barrels
Shale presence in hydrocarbon reservoirs has a large impact on estimates of reserves
and producibility
The clay minerals present in the shale complicate the determination of and Sw
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Introduction
Shale definition
Shale Effects on Logs
Porosity & Saturation
Dual Water (examples)
Summary
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Type of Rocks
Earth
IGNEOUS
SEDIMENTATY
METAMORPHIC
SedimentaryClastic
Chemical
Conglomerate - Coarse-grained,
mixture of rounded pebbles and sand
ranging widely in size; well rounded
pebbles imply some transport in a
high energy system
Sandstone- commonly quartz,feldspar, or rock fragments;deposited in many environments
Shale, very fine grained; composedprimarily of clay; deposited in low-energy environments such as lakes,bays, lagoons, of deep marinesettings
Sedimentary Rocks- Most important for the oil industry as
they contain most reservoirs, seals and source rocks.
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Rock Classification
Clastic Rock Type Particle diameter
Conglomerate Pebbles 2 - 64mm
Sandstone Sand .06 - 2mm
Siltstone Silt .003 - .06mm
Shale Clay <.003mm
b N (thermal) Pe
Kaolinite 2.54 59.6 1.85
Illite 2.52 47.9 3.97
Smectite 2.02 87 1.70
Chlorite 2.73 59.6 4.07
Four main clay minerals, Kaolinite,Illite,Smectite as the most
common, with widely varying different log responses
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Introduction
Shale definition
Shale Effects on Logs
Porosity & Saturation
Dual Water (examples)
Summary
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They are often radioactive
They have porosity and it is filled with salted water
Resistivity logs exhibit shales as low resistivity
They show high Gamma Ray value
They exhibit high porosity in the Neutron porosity
logs
Density and sonic logs react to the porosity and the variation in the grains
Shale and Logs
Shales have properties that have important influences on log
readings:
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Lithology in shaly sands
The volume of shale (Vsh) in a sand
is used for the evaluation of shaly
sand reservoirs
GR - GRclean
GRshale - GRclean
Vshale =
Vsh can be computed from several
methods:▪ Gamma Ray
▪ Neutron-Density
▪ Spontaneous Potential
Most traditional method
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Lithology in shaly sands
Geochemical Logging tool is essential
in the evaluation of complex shaly
sand reservoirs
Elemental Capture Spectroscopy
(ECS)
*The figure compares elemental weight fractions measured by the
tool (black lines) to those derived from core analysis (red points).
*Radtke, Lorente, et al- A NEW CAPTURE AND INELASTIC SPECTROSCOPY TOOL TAKES
GEOCHEMICAL LOGGING TO THE NEXT LEVEL-SPWLA 2012
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Introduction
Shale definition
Shale Effects on Logs
Porosity & Saturation
Dual Water (examples)
Summary
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Porosity in shaly sands
▪ Porosity is defined as the ratio of pore volume to its total volume
Fluid
Matrix
(1-)
1 - = amount of rock (matrix) per unit volume
= amount of fluid per unit volume
𝜑 =Fluids volume
Total volume=𝑉𝐹𝑉𝑇
▪ Effective porosity (PHIE) is the pore space in the formation that is
neither in the shale nor in the clay
▪ Total Porosity (PHIT)
Where is the total porosity of shale/clay fraction
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Porosity in shaly sands
▪ The way shale is distributed through the reservoir rock
will have an impact on porosity
▪ Ignoring distribution leads to reservoir potential being
overestimated
▪ Shale distribution types: Laminar, Structural, Dispersed
▪ Quantification of shale distribution can be done using
*Thomas-Stieber model (among other models)
*THE DISTRIBUTION OF SHALE IN SANDSTONES AND ITS EFFECT UPON
POROSITY,
E. C. Thomas and S. J. Stieber, SPWLA SIXTEENTH ANNUAL LOGGING
SYMPOSIUM, JUNE 4-7, 1975
.
Clean formation Structural shale
Porosity
Porosity
Matrix
Matrix
Porosity
Matrix
Porosity Shale
Shale
Matrix
Porosity
Matrix
Laminar shale Dispersed shale
Sh
ale
Sh
ale
Total porosity vs Shale
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Lithology & Porosity
ECS ELAN+ECS
Integration of geochemical log (ECS) with multimineral solver (ELAN)
provides a better lithology evaluation hence more accurate porosity
▪ Elemental Log Analysis (**ELAN)
provides advanced multimineral log
analysis capabilities
▪ The mineral solver doesn’t rely on a
model to derive the clay volume
▪ It is a simultaneous equation solver
that combines log measurements and
parameters in response equations to
compute volumetric results
*Example shows a complex shaly sand formation (with calcite-
cemented zones) from Nile Delta.
*Van Steene, SPE 127965. ** Quirein et al,SPWLA 1986
Eeffective
Porosity
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◼ Any measurement that displays a contrast between water and
hydrocarbon properties can be used to estimate saturation
◼ Resistivity is dominant method for saturation evaluation from logs
◼ Archie’s Equation- works well for clean environments
◼ In shaly environments the basic assumption of non-conductive
formation does not apply
◼ Other methods needed to be found
Saturation in shaly sand
w
n
S =a
mRw
Rt
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Saturation in shaly sand
Which saturation equation?
◼ Many equations have been
developed to account for the
electrical properties of clays:
◼ Most of them are area specific
◼ They require Vsh
+
=
Sw
QvBRwRt
RwSw
m
n
1
1*
*
*
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Introduction
Shale definition
Shale Effects on Logs
Porosity & Saturation
Dual Water (examples)
Summary
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Dual Water Model
◼ Dual water model attempts to correct for
excess clay conductivity in the sands
especially at low formation water salinity
◼ It splits the conductivity of a water filled
rock into water conductivity and clay
conductivity
◼ Dual Water model refines Waxman-
Smits model by removing a fraction of
the pore volume water with clay water
◼ It divides the clay into dry clay and its
associated water, called bound water.
This avoids the necessity of determining
Qv.
Fo = F*(1 – vqQv)
Fo = Dual Water formation resistivity factor Vq = volume of water per counterion (cc / meq)Qv = charge concentration per unit pore volume, (meq / cc P.V)
Clavier, Coates and Dumanoir (1984)
SPE paper 6859
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Dual Water Model Concept
◼ The model divides the formation into
solids and liquids
◼ It proposes two distinct waters to be
found in the pore space; Free &
Bound water
◼ It suggests that the amount of bound
water is directly related to the clay
content
◼ In hydrocarbon bearing zones the
model leads to the following
relationship (in bound-water terms)
as:
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Saturation in shaly sand (example)
◼ Shaly sand zoneArchie (blue) shows the highest Sw
as Vsh increases (black), Swt from
D-W (green) reflects the shale
contribution and Swe from D-W (red)
shows the lowest Sw in the shaly
zone
◼ Clean sand zoneArchie and Dual Water give about the
same Sw value when Vsh is zero.
Swt=Swe because Swb is zero in
clean formations.
Vsh
Swe DW
Swt DW
Archie
Vsh increase
Shaly sand
Clean
sand
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Saturation in shaly sand (example)
Water Saturation (Sw) HPV=PHI*So*H
Zone Vsh Av PHI Archie Simandoux Dual WaterHPV
Archie HPV Sim HPV DW
% v/v v/v v/v v/v m m m
A 10 0.23 0.24 0.22 0.18 0.52 0.54 0.57
B 2 0.25 0.33 0.32 0.33 0.50 0.51 0.50
C 20 0.2 0.32 0.24 0.21 0.41 0.46 0.47
D 30 0.18 0.41 0.26 0.23 0.32 0.40 0.42
All 1.75 1.90 1.96
+ 12% reserves index
Dual Water model provides an increase of 12% in reserves compared
to Archie’s equation (in this specific case)
The difference in Hydrocarbon Pore volume (HPV) computed using
water saturation from Archie, Simandoux and Dual Water equations in
shaly sand reservoirs is not negligible
Where H=3m
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Introduction
Shale definition
Shale Effects on Logs
Porosity & Saturation
Dual Water (examples)
Summary
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Saturation in shaly sand- Model Validation
◼ All saturation models should be validated with core data, well test
data (DST) and production information
◼ The integration of conventional logs with more sophisticated
measurements like NMR, Dielectric and Spectroscopy will reduce
the uncertainty of the petrophysical model
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Summary
◼ Most sandstone reservoirs contain some clay or shale, which
reduces the effective porosity, lowers the permeability, and
alters the resistivity predicted by Archie’s equation
◼ Shaly sand evaluation is a complex task and a good geological
and local knowledge is important
◼ Integration of all available data is key in assessing the most
reliable petrophysical models in shaly sands evaluation
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