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.