Download - Session 1 log-analysis
Abdelrahman Elkhateeb
Cairo University
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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 Certain Outputs
GR Resistivity Porosity ษธ
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๐#๐%&
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?
โฆ........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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This log is drawn by hand for presentation purposes only
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
๐ฎ๐น ๐ฅ๐จ๐ ,๐ฎ๐น๐๐๐ฎ๐น๐๐,๐ฎ๐น๐๐
= shV
Density Porosity Calculation
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* The density log is the most robust porosity tool
* The porosity is calculated using simple equation:
โ ๐ซ =๐๐๐ โ ๐๐๐๐๐๐๐ โ ๐๐๐
* Matrix density differs by lithology* L.ST: 2.71 g/cc* S.ST: 2.65 g/cc* Dol.: 2.847 g/cc
* The ๐๐๐ 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 ยตs/ft (18000-19500 ft/s)*L.ST: 47.6-43.5 ยตs/ft (21000-23000 ft/s)*Dol.: 43.5-38.5 ยตs/ft (23000-26000 ft/s)
* The tool is used for seismic tie, CMT job evaluation and secondary porosity evaluation
โ < =โ>?@A,โ>BCโ>D?,โ>BC
Sonic Porosity Chart
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10.5 p.u.
*Determine the porosity from the sonic log value of 70 ยตs/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
ฯFGฯ<GฯH
I=Tฯ
ฯT =FLMNOGHLMNO
P
Density-Neutron Crossplot
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g/cc2.38=bฮก0.165=Nะค
0.20
L.St or S.S ??
Porosity-Resistivity Pick
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This log is drawn by hand for presentation purposes only
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
This log is drawn by hand for presentation purposes only
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
1๐ a=
โ P๐๐คP
1 โ ๐cde โ ๐๐ ๐ค+๐cde๐ %&
๐โ๐น๐ะค๐โ๐น๐= n )W(S
Log Example
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This log is drawn by hand for presentation purposes only
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 0effะค
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 0effะค
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/ ฯ
ฯ* wBVW = S
References
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*Schlumberger Chartbook 2009
*Schlumberger Log interpretation principles and applications 1989
Abdelrahman Elkhateebwww.facebook.com/formation.evaluation