Basic Well Log InterpretationFormation Evaluation

SESSION 1

M. Wahdanadi Haidar

KSO Pertamina EP – Patina Group Ltd

Mochamad Wahdanadi Haidar

Recent Activity:

2006-2007 Asisten Tetap UPP-IPD

2008-2009 Support Engineer (Geotech System) and Junior GeoPhysicist @ ETRL

2009-Now Geophysicist in KSO Pertamina EP – Patina Group Ltd

Few Years Study:

SMUN 70 angkatan 1999-2002

Bachelor degree majoring in Physics University of Indonesia 2002-2006 (Instrumentasi Elektronika’02)

Master degree majoring in Reservoir Geophysics University of Indonesia 2007-2009

(email, YM, FB: adiui70@gmail.com)

Date/Place of Birth: Surabaya 22 November ’84

Home Address: Pasar Minggu, Jakarta Selatan

Well Logs

• What?

• Why?

• When?

• Where?

• How?

Well Hole ConditionOpen hole

loggingCased hole

logging

Perforation & Production

logging

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Tool string is moving at a certain logging speed, and data are recorded at certain intervals called sampling rate.

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And we get a group of wiggly lines called a “log”

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And we get a group of wiggly lines called a “log”

Purposes of Well Logs

• Lithology (reservoir rock?)• Resistivity (HC,water,both?)• Porosity (how much HC?)• What type of HC

• Formation mech. properties• Permeability / cap pressure• Shape of the structure• Geological information• Geothermal• Unconventional applications

Ways of well logging

Well log measurement Reference

Well Logs DataUsually Well logs digital data provided in LAS format. Divide into 2 main information:

1.Well Header contains some drilling parameters and well information (Hardcopy).

2.LAS Value

Borehole Environment

Borehole Environment

Pada waktu mengebor:- drillstring berputar pada 50 -- 150rpm- mata bor menahan beban sebesar

10 000 sampai 40 000 lb.- cuttings yang dihasilkan diangkat oleh lumpur

pemboran (drilling mud) kepermukaan.Selama proses pengeboran:

- bisa terjadi erosi pada formasi sehingga menimbulkan goa yang diameternya lebih besar dari diameter mata bor.

- cairan lumpur pemboran masuk meng-invasi zona-zona yang permeable.

- kerak lumpur (mud cakes) menumpuk dimuka zona-zona yang permeable.

Type of Basic Well Logs

• Spontaneous Potential

• Gamma Ray

• Resistivity

• Neutron

• Density

• Sonic

Permeable and porous lithology

Predict and calculate Porosity and fluid content

Predict reservoir characteristic

Lithology

Spontaneous PotentialSP results from electric currents flowing in the drilling mud.

3 sources of the currents:•Membrane potential - largest. - electrochemical•Liquid - junction potential - electrochemical•Streaming potential – smallest - electrokinetic

Membrane and Liquid PotentialThese two effects are the main components of the SP. They are caused because the mud flitrate and the formation waters contain NaCl in different proportions. Firstly, shales are permeable to the Sodium ions but not the Chlorine. Hence there is a movement of charged particles through the shale creating a current and thus a potential. The ions Na+ and Cl- have different mobilities at the junction ofthe invaded and virgin zones. The movement of the ions across this boundary creates another current and hence a potential.

Streaming PotentialThis is generated by flow of the mud filtrate through the mud cake. As this does not normally occur this effect is small. It will only become important if there are high differential pressures across the formations.

Spontaneous Potential

Measure different potential (mV) between moving electrode in well bore with surface electrode as result of mud filtrate invasion

SP logs can be used for:

•Identify porous and permeable zone by its deflection.

•Predict bed boundary and shaliness of reservoir zone.

•Predict Rw (water resistivity) in permeable zone.

SSP = −k log (Rmf/Rw)

k = (61+0.133*T) T in F degree

Spontaneous PotentialWhat we can analysis from this sp logs?

1. Deflection means there is a permeable zone.

2. Resistivity of fluid contain can be predicted from direction of deflection and sp value.

3. Resisitivity mud filtrate must be known to prevent pitfalls in fluid contain resistivity in formation.

Spontaneous Potential

Spontaneous PotentialWhat is the Analysis if we used oil base mud filtrate with resistivity 50 ohm.m?

What is the Analysis if we used salt water base mud filtrate with resistivity 2 ohm.m?

Assumes:

oil zone resistivity 30 ohm.m

Water formation resistivity 1 ohm.m

Can you identify wheter prospect or water zone?

Gamma RayTheory of Gamma Ray Log tools is recording radioactive material in rocks (Uranium (U), Thorium (Th), and Potassium (K). Radioactive material are usually deposited in shale (no permeability).

SGT (Standar Gamma Ray Tools) measure total of radioactivity from 3 elements in GAPI unit. While NGT (Spectroscopy Gamma Ray Tools) measure total of each radioactivity from 3 elements.

Gamma RayApplications of Gamma Ray log are:•Lithology identification.•Study of depositional environments.•Investigation of shale types.•Correction of the GR for clay content evaluation.•Identification of organic material and source rocks.•Fracture identification.•Geochemical logging.•Study of a rock's diagenetic history.

Gamma RayCan you analyze lithology and reservoir potential???

Resistivity

There are 2 principal theory in measuring resistivity:

1. Laterolog = sends ac current to formation.

2. Induction = induction electric current to formation. This induction tool usually known as conductivity tools because it’s measuring conductivity and converted into resistivity.

Resistivity Measurements

Resistivity

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SonicIn This Session we only limited to sonic logging tools for measuring slowness of compressional wave (primary wave). Sonic Tools measuring slowness in uS/ft. a source gives a signal to formation and will be received by near and far receiver. Two receiver will be calculated time between first break on near receiver and far receiver. This measured time will be change to slowness by dividing with distance between near and far receiver.

Velocity is reciprocal of slowness (1/slowness)

DensityThe Litho Density Tools (LDT) use a chemical gamma ray source and two or three gamma ray detectors.There are 3 interactions that can be happened if gamma ray with high energy shot to a material; Photoelectric, Compton Scattering, Twin Production.

If initial Energy (E) <100 keV If 75 keV<E<2 MeV If E>1.2 MeV

LDT is designed for high sensitivity in 2 interaction by choosing radioactive material as required like Cesium – 137 which maximum gamma ray energy at 662 keV.

DensityGamma Ray can be a electromagnetic wave and foton particle, known as dualism particle. When GR shot to rock there will be compton scaterring, foton will be losing its energy and will be scattered to different direction. Energy is loosed by foton will be absorbed by electron. Electron will become free from its previous state. This process will cycle untill foton energy become weak and will be totally absorbed. LDT measures electron density that scaterred as result of gamma ray shot.

Neutron

Compensated Neutron Tools (CNT) can be used for predicting porosity of reservoir.

CNT spreads neutron to formation, fast neutron will be slower when hit hydrogen atom (elastic collision). This happens because size of neutron and hydrogen are looks like similar. Detector in CNT will measure population of neutron in thermal region.

Gas and water from density neutron cross over

Oil and water from density neutron cross over

Density Neutron CrossPlot

Coal Interpretation from well Logs

Cross Plot Interpretation

Example of logs responses

Can you find the “beauty” reservoir?

Well Logs Correlation to find potential reservoir related to structure condition

A B C

Well log correlationA B C