Session 1 log-analysis

Abdelrahman Elkhateeb
Cairo University

12/08/2016
www.facebook.com/formation.evaluation
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*Petrophysics is the study of rock properties and their
interactions with fluids (gases, liquid hydrocarbons, and aqueous
solutions)
*The petrophysicist is the link between the subsurface disciplines
*Petrophysics is the science that integrates many valuable data
together to have quantitative formation evaluation based on
conventional/advanced logs and core analyses
*Log analysis vs. petrophysics

Log Analysis
Different Inputs CertainOutputs
GR Resistivity Porosity ɸ
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𝑆#𝑉%&

Borehole Sketch
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Schlumberger

What We Provide...
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Is it Complicated?!! J
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*A 24/7 job
*Presence of hydrocarbon is your call
*Wells/lithology could be very complicated
*Tough to learn in most companies
*Skill shortage in the industry
*Too much to learn
*What to start with?
*Where/How to start?
…........BUT, is it worth really?

A Shed of Light...
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*Everyone takes the petrophysicist outputs to use
*You are the link between the geologist and the RE
*You learn how to manage, work under stress and how to provide
only what is accepted
*You manage the LWD and wireline operations
*You are dealing with data no one literally see or use
*You are dealing with new technology, that makes you always
think
*Your output is seen at all management levels

Log Example
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This log is drawn by hand for presentation purposes only

What To Do (Simply)?
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*Estimate clay volume
*Calculate the porosity
*Read the resistivity
*Calculate water saturation
REMEMBER!!
The key for log interpretation is to understand how to read the log

How to Read the Log?
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Clay Volume Calculation
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*A simple equation using GR log is used to evaluate the clay
volume
*The neutron/density could be used to calculate the clay volume
with the same equation
*The softwares use different evaluation appraoches, a bit
complicated, depending on the inputs used
*Porosity tools or even resistivity could be used to evaluate the
clay volume
𝑮𝑹 𝐥𝐨𝐠 , 𝑮𝑹 𝒄𝒍
𝑮𝑹 𝒔𝒉 ,𝑮𝑹 𝒄𝒍
=shV

Density Porosity Calculation
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* The density log is the most robust porosity tool
* The porosity is calculated using simple equation:
∅ 𝑫 =
𝝆 𝒎𝒂 − 𝝆𝒍𝒐𝒈
𝝆 𝒎𝒂 − 𝝆 𝒇𝒍
* Matrix density differs by lithology
* L.ST: 2.71 g/cc
* S.ST: 2.65 g/cc
* Dol.: 2.847 g/cc
* The 𝝆 𝒇𝒍 is the density of the fluid saturating the rock immediately surrounding the
BH, (usually mud filtrate)....SLB
* The tool is affected by washouts, rugose holes and high tension

Density Porosity Chart
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27 p.u.
*Determine the porosity from the density log value of 2.4 g/cc for
a dolomite reservoir?

Sonic Porosity Calculation
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* The sonic log is the reflecting primary porosity only
* The porosity is calculated using simple equation:
* Matrix density differs by lithology
*S.ST: 55.5-51.3 µs/ft (18000-19500 ft/s)
*L.ST: 47.6-43.5 µs/ft (21000-23000 ft/s)
*Dol.: 43.5-38.5 µs/ft (23000-26000 ft/s)
* The tool is used for seismic tie, CMT job evaluation and secondary porosity
evaluation
∅< =
∆>?@A,∆>BC
∆>D?,∆>BC

Sonic Porosity Chart
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10.5 p.u.
*Determine the porosity from the sonic log value of 70 µs/ft for a
sandstone reservoir?
34 p.u.

Calculating Total Porosity
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* With 3 inputs, the porosity could be the average of them
* For Gas Intervals
ϕFGϕ<GϕH
I
= Tφ
φT =
FLMNOGHLMNO
P

Density-Neutron Crossplot
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g/cc2.38 = bΡ
0.165= NФ
0.20
L.St or S.S ??

Porosity-Resistivity Pick
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Water Resistivity (Rw)
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* This is a key parameter in saturation calculation
* Rw is evaluated through different methods:
* Reversed Archie Method
* Pickett Plot
* Formation Water Salinity
* The uncertainty in the Rw value affects the saturation profile
near the contact dramatically

Rw From Pickett Plot
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*Pickett plot is very important tool
to detect the water resistivity
*It should be used as a
confirmation for the reversed
Archie method with more
accuracy
*The m value is the slope of the
line
*(m) value is crucial in carbonates
*The core data is usually needed to
vary (m) parameter, otherwise the
default value is used

Salinity Value From Slb Chart
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*Easily used to detect the
Rw and salinity using the
formation temperature
*The salinity should not
change across different
locations/wells in the same
structure
*The Rw changes with
temperature/depth
*The formation salinity could
vary significantly from
formation to another

Calculating Water Saturation
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*There are several models for Sw calculations
*Two main reservoirs exist; clean and shaly reservoirs
*The clean formations are evaluated with Archie equation
*Shaly reservoirs use different models depend on the clay
distribution
*Schlumberger/Laminated sand-shale simplified model is one
simple model to evaluate the shaly sands
1
𝑅a
=
∅P
𝑆𝑤P
1 − 𝑉cde ∗ 𝑎𝑅𝑤
+
𝑉cde
𝑅%&
𝒂 ∗𝑹𝒘
Ф 𝒎
∗
𝑹𝒕
=n)W(S

Log Example
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The Effective Porosity (Oil)
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*In oil bearing reservoirs, the
effective porosity curve is
divided into three curves, the
bulk volume of water (BVW),
movable oil and residual oil.
Residual Oil
Movable Oil
Bulk Volume Water
0.5 0effФ

The Effective Porosity (Gas)
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*In gas bearing reservoirs, the
effective porosity curve is
divided into three curves, the
bulk volume of water, the
movable fluids and the gas.
0.5 0effФ
Residual Gas
Movable Hydrocarbon
Bulk Volume Water

Sw-BVW Relationship
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*Bulk Volume of Water is the volume of the water relative to the
total bulk volume of the rock.
*Water saturation is the amount of water in the pore spaces.
Sw= BVW/ ϕ
ϕ*wBVW = S

References
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*Schlumberger Chartbook 2009
*Schlumberger Log interpretation principles and applications 1989

Session 1 log-analysis

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