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Carbonate sediments can be deposited in many geological
environments. This variation is reflected in the fabric, structure,
texture and flow behaviour of the reservoir rock.
Advanced Formation Evaluation in Carbonates
Course Overview
It is impossible to accurately evaluate carbonate porosity, lithology, water saturation, permeability and rock type without taking into consideration the following:
• The porosity exponent m: This could vary over a wide range from 1.5 to over 6
• The saturation exponent n: Most carbonates tend to have mixed wettability. The exponent n can vary over a wide large from 1.7 to over 10 in oil wet zones.
• Volume of micro-porosity. This controls permeability and the residual oil saturation (Sor).
• Vugs volume: vugs are predominantly not isolated. They simply add storage but do not affect permeability or resistivity readings.
• High Technology Data Integration: The introduction of high technology tools such as dielectric and NMR, improved the quality and accuracy of carbonates interpretations.
The course deals in details on evaluating these parameters so that values of water saturation, permeability, pore radius and rock typing can be more accurately characterized.
Interpretations:
Carbonates interpretations is not about using values of m=n=2 and applying the Archie Equation. The pore geometry in Carbonates is very complex . Values of m in the range of 1.5 (in fractured reservoirs) to m>6 (in high vug bearing reservoirs) is not uncommon. Carbonates tend to have mixed wettability with value of n in the range 1.7< n < 8.
This course addresses variations of m and n in details and provides the technique of estimating both m and n at downhole conditions from log data. Note (fig-1) the large variation in estimated Sw for a range of variations of m and n.
Pore Typing:
Carbonates pore geometry is very complex. The throat radius could vary over a very wide range 0.01 μm < R < 200 μm. Since permeability K=f(r4) fig-2, this parameter has a large effect on the values of permeability and m.
This course dedicates a large part of the presentations to addressing pore typing. The NExT team developed a process of computing “r” and used that to provide pore carbonate classifications and flow units (fig-3). Note the large variation of “r” in the same carbonate reservoir.
This is the only commercial course that addresses this critical parameter in details, supported by field examples.
Computed Sw Values
fig-1
fig-2
High Technology Tools:
A new generation of scanner and other high technology tools are in commercial use now. This includes dielectric, pulsed neutron, acoustic, nuclear magnetic resonance, etc.. This new technology can provide an enhanced quality of interpretations and can be used to give measurements of m and n. This course presents the theory and applications of this technology to complex carbonates interpretations and uses field examples in the workshop sessions.
Vugs, Fractures and Microporosity:
These 3 factors play a major role in carbonate evaluations. Vugs add storage capacity but contribute very little to changes in permeability and resistivity. Microspores are essentially the “secret Shales” in carbonates but much more difficult to evaluate. Fractures can be quantified from borehole imaging. This course provides modelling for vugs and microspores and estimates both parameters from log data. Vugs are responsible for the observed high values of m (fig-4) and microspores contribute a large part to the concept of low resistivity pay zones (fig-5) whre dry oil is produced from zones with Sw>50%.
The NExT course on Advanced Carbonates is the industry’s only course
that addresses these four topics in great depth. This is supported by
workshops using 20 field examples.
fig-3
fig-5
fig-4
Data Quality Control:
Quick-Look to identify the following zones: water/ transition/oil/tar
zones.
Parameter Evaluations:
• Derive a variable m , the porosity exponent, as a function of
porosity from logs and cores.
• Derive a variable-n, the saturation exponent when sigma or a
dielectric log is available.
New High Technology Tools:
The applications of new family of high technology tools to
carbonate interpretations: Nuclear magnetic resonance, dielectric,
Pulsed Neutron and Neutron Spectroscopy and acoustic logging .
Vugs Characterization: Modelling of vugs in carbonates their
effects on m and on permeability estimation.
Micro-Porosity Evaluations: Modelling of Micro-porosity in
carbonates and quantifications in the oil zone or when Oil Base
Mud is used. Use the micro pores to estimate permeability using
Timur-Coats equation.
Permeability Estimations Pore Geometry: Pore models using
Winland and ADNOC equation. Estimating pore radius and hence
rock typing.
The Carbonate Challenge
Data Quality Control:
Quick-Look to identify the following zones: water/
transition/oil/tar zones.
Rxo shifted by the ratio of Rw/Rmf.
water
Tar
Tar
Interpretation in carbonates starts with a series of basic plots and correlations. This is important before diving –in at the deep end with computerized evaluation. The example above shows when we shift Rxo by the multiplier (Rw/Rmf), we essentially make Rmf=Rw in the water zone any other variations relates to the presence of Water/Oil/Tar zones. A dozen such plots can set the outline for computer applications.
New High Technology Tools:
The applications of new family of high technology tools to
carbonate interpretations: Nuclear magnetic resonance, dielectric,
Pulsed Neutron and Neutron Spectroscopy and acoustic logging .
107
108
109
10
20
30
40
50
60
Frequency [Hz]
Rel
ati
ve
Per
mit
tiv
ity
m=1.9
m=2.15
CRIM
107
108
109
0.2
0.3
0.4
0.5
0.6
0.7
Frequency [Hz]
Co
nd
uct
ivit
y [
S/m
]
m=1.9
m=2.15
CRIM
Magnetic ResonanceDielectric Log
Spectroscopy for complex lithology
A new generation of upgraded high technology logging tools improved the quality of carbonates interpretations. This helped to define the texture (dielectric) , complex lithology (spectroscopy) and hydrocarbon type and effect (NMR).
Parameter Evaluations:
• Derive a variable m , the porosity exponent, as a function of
porosity from logs and cores.
• Derive a variable-n, the saturation exponent when sigma or a
dielectric log is available.
The Archie equation can best be described by a variable(m) and not an empirical values of (a, and m). The variable m value has a physical meaning defining tortuosity. The addition of “a” and fictitious “m” will disguise some of the carbonate features.
)(
)}./(.{
)}./(.{log)(log.
)./(.
SxoLog
RxoRmfaLogn
RxoRmfaSxon
RxoRmfaSxo
m
m
mn
Parameter Evaluations:
• Derive a variable m , the porosity exponent, as a function of
porosity from logs and cores.
• Derive a variable-n, the saturation exponent when sigma or a
dielectric log is available.
Exponent (n)
The saturation exponent (n) can only be
measured at downhole conditions. Core
analysis are on the whole not representative
as the core is subject to hysteresis.
Most modern Open Hole logging tools, run
on wireline or LWD, will contain one or both
of a dielectric log and a neutron capture
sigma, which when combined with Rxo can
estimate the in-situ values of n.
Vugs Characterization: Modelling of vugs in carbonates their
effects on m and on permeability estimation.
Vugs are the most misunderstood parameter in carbonates. Mathematical modelling demonstrates that they add storage, but have negligible effects on permeability and tortuosity. Since the porosity increases without an effective increase in “Rt”, then the net result is a large increase in “m”.
The extreme effects of vugs, giving values of m >6
The effects of vugs, gives an increase in m
Form
atio
n F
acto
r (F
F)
Porosity
Micro-Porosity Evaluations: Modelling of Micro-porosity in
carbonates and quantifications in the oil zone or when Oil Base
Mud is used. Use the micro pores to estimate permeability using
Timur-Coats equation.
.
Microporosity is equivalent to a “secret
shale” in carbonates. This appears as
high Sw values in the virgin zone
before water flood. This can be
quantified from NMR or from Rt in the
oil zone or Sigma and dielectric in the
invaded zone when OBM is used.
Mic
ro-p
oro
sity
o
bta
ined
fro
m S
w
Mic
ro-p
oro
sity
can
be
ob
tain
ed f
rom
NM
RKTC = a . ϕ4 ( Free Fluid Volume / Bonded Fluid Volume )2
High Sw in the oil
zone reflect the
presence of micro-
pores
Permeability Estimations Pore Geometry: Pore models using
Winland and ADNOC equation. Estimating pore radius and hence
rock typing.
Carbonates can best be characterized by their pore geometry. Winland correlated porosity to permeability through core analysis. Others (ADNOC Model) correlated that using theory and made the first attempt to compensate for vugs porosity. Carbonates ‘s rock typing can then be segmented in various categories based on the texture variations as the defined by the pore radius.
Course Agenda
Day-1:
Carbonate Geology and deposition
Dolomitization
Nuclear magnetic resonance and NMR-Scanner
Acoustic measurements and the acoustic scanner
Day-2:
Borehole imaging using micro-resistivity and ultrasonic imaging
Physics of neutron logging using pulsed neutron to give a sigma log
The physics of dielectric logging and the dielectric scanner
Variable-m and the formation factor applications
Day-3:
Wettability and the variable-n
Effect of Vugs and fractures on resistivity measurements
Dual Porosity: Macro-Porosity and Micro-Porosity: Quantitative evaluation of
the dual porosity in carbonates
Day-4:
Permeability estimations in carbonates
Connectivity Theory: a new approach for interpretations in carbonates
without the use of the Archie Equation.
Flow Units and the Lorenz plots
Day-5:
Capillary Pressure from core analysis
Capillary pressure from the NMT T2 conversion
The J-Function
Rock types: Winland and the ADNOC Pore Model
There will be daily practical workshops on each of the topics covered using field examples.