Basic Openhole Interpretation Walter

February 2007

FTW Global Training

Basic Open Hole Interpretation

6© 2006 Weatherford. All rights reserved.© 2006 Weatherford. All rights reserved.

Why run open hole logs?

• Does the wellbore penetrate formations that produce?

• Are hydrocarbons present?

• Are there commercial quantities present?

• Will the hydrocarbons flow to the wellbore?


Logging objectives

• Correlation: well to well of formation tops, major lithologies, unconformities

• Identifying porous and permeable reservoir rocks

• Identifying hydrocarbon intervals

• Reservoir engineering: pay thickness, average porosity, average water saturation

• Stratigraphic and structural studies

• Well completions

• Geophysical applications


Who uses logs?

Geologists Formation tops Reservoir rock Types of fluid Well economical? Offset opportunities?

Geophysicists Formation tops Integration of log derived sonigrams

into existing geophysical models

Drilling Engineers Calipers for annular and total hole

volumes Places to seat DST packers In deviated wells, where is TD

relative to surface location

Completion Engineers Perforation intervals Are zones of producible water close

to perforations?


Typical Log layout


Typical Log Header


Log Header Remarks Section


Adjacent Bed

Adjacent BedMudcake

Mud

Open Hole

Borehole

Environment(Immediately after

drilling)

Transition Zone

Transition Zone

Rmc

Uninvaded Zone

Uninvaded Zone

(Reservoir)

Rt

Rs

Sw

Rt - Formation Resistivity

Rw - Water Resistivity

Sw - Water Saturation

Rs

Rs

Flushed Zone

Flushed ZoneRxo - Flushed Zone Resistivity

Rmf - Mud Filtrate Resistivity

Sxo - Flushed Zone Water Saturation

Rxo

Rmf

Sxo

Rm


Common Open Hole Measurements

• SP

• Induction

• Laterolog

• Microlog

• Sonic

• Neutron

• Density


Spontaneous Potential (SP)

SP is a naturally occurring phenomena in the borehole environment.

First ever log (~1930)


Spontaneous Potential measurement

millivolts recorder

recording electrode

borehole

ground electrode

or

fish


RRttRRxoxo

RRss

RRssdi

RRmm

dh

hSP ElectrodeSP Electrode

SPSP

SSPSSP

Shale

Sand

Shale


Uses and applications of SP

• Correlation

• Bed boundaries

• Shale indicator Vshale = 1 – PSP/SSP

• Qualitative indicator of permeability

• Source for water resistivity (Rw)


sand

shale

-

SP curve

shale baseline


Factors affecting SP response

• borehole fluid must be conductive

• salinity contrast between formation fluid (Rw) and mud filtrate (Rmf) dictates magnitude of SP response

• formation and side bed resistivities

• bed thickness

• borehole diameter

• deep invasion


SPMV-140 10

DEPTHM

525

Typical SP Response


Formation Lithology Fracture Indication (Uranium Deposits) Volume Shale Indicator Effective Porosity Depth Correlation (Open hole & Cased hole)

Gamma Ray Tool Applications



Factors affecting GR response

• Any material between source and detector will reduce the GR response

• Hole size

• Formation density

• KCl mud

• GR emissions are statistical , hence curve variation and averaging of data

• GR indicative of K, related to shale content. GR response can be due to U and Th in clean zones


Does the wellbore penetrate producing formations?

Oil

Shale

Shale

Fresh Water

Hydrocarbon

Salt Water

Clean Sand

Dolomite

GR

Shaly Sand

Limestone

SP

Fluid Effects

Formation Effects


GRGAPI0 150

SPMV-140 10

525

Gamma Ray and SP Example


Resistivity

(ohm-meters)Resistance to the flow of

electrical current.

Hydrocarbons

High resistivity-low currentSalt Water

Low resistivity - high current.

0 1

10 1001000

Ohms

HydrocarbonSalt Water

Are hydrocarbons present?

© 2006 Weatherford. All rights reserved.© 2006 Weatherford. All rights reserved.

Open Hole Resistivity Tools

Dual Laterolog DLL/MDL

Micro Spherically Focused Log MSFL/MMR

Induction MAI/STI/ICAT

Focused Resistivity Log MFE/SFL

Microlog MML/MRT


Formation and Flushed Zone Resistivity Depth of Filtrate Invasion Estimates from tornado charts Water Saturations Fluid Contacts Permeability Indicator (SP) Hydrocarbon Indicators.

Resistivity Tool Applications


Rt Measurements

• Normally,Rt is directly read from the deep resistivity measurement

• In the case of thin beds or a formation with invasion, there should be a correction done for Rt

• This can be done from the chart books or can be software related (RtAP for Compact)


Tornado charts

• Used to correct for Rt in an invaded zone


Tornado charts


Bed Thickness Charts


Typical Resistivity Tool Response

Oil

Shale

Shale

Gas

Salt Water formations

Low Porosity

Fresh Water Formations

SP

GR

Deep

Shallow

Medium


Induction Log - Example


Induction Log - Example


Nuclear Tools

Spectral Photoelectric (Pe) SPeD or MPD

Density Tool

Compensated Neutron Survey CNS or MDN

Natural Gamma Ray Tool UGR or MCG/MGS

Spectral Gamma Ray SGR or SGS


Porosity (Density and Neutron) Formation Bulk Density Lithology (Density, Pe, Neutron, Gamma Ray) Mineral Analysis (Density, Pe, Neutron) Volume Shale Indicator (Neutron, Gamma Ray) Hydrocarbon Indicators (Density-Neutron Crossover) Borehole Volume (Caliper) Fluid Contacts Effective Porosity Correlation

Nuclear Tool Applications


Basic Density Measurement

Cs137 source emits Gamma Rays into the formation.

These Gamma Rays undergo Compton Scattering as they migrate toward the two detectors.

The Bulk Density of the formation is the main influence on the migration of the gamma rays and is the primary measurement of the

density tool.


Density PorosityEquation

corr - from tool

ma - must enter

fl - 1.00 g/cc

)Fl(

)Ma()( 1flma

corrma

ma = 2.71g/cc Limestone (CaCo3)

2.65 g/cc Sandstone(SiO2)

2.87 g/cc Dolomite (CaMgCo3)


Factors Affecting Density Response

• borehole size

• rugosity

• filter cake build up

• short axis orientation

• drilling mud

• hole deviation

• hydrocarbon effects

• Rollover


Neutron Tool Theory

Am241Be Neutron Source creates a population of fast neutrons in the formation near the borehole.

As the neutrons migrate towards the detectors, they are slowed by collisions with Hydrogen nuclei.

The neutron tool counts thermal (slow) neutrons. The more hydrogen available, the more neutrons that are slowed and counted.

Hydrogen is a large component of both water and hydrocarbons which are found in the pore space of the formation.

The ratio of the counts at each detector is then converted to porosity.

FARDETECTOR

NEARDETECTOR

Am241BeNEUTRONSOURCE

H

H

H


Factors Affecting Neutron Measurement

• Porosity

• Matrix (Elemental Makeup is different changing the capture cross-sections)

• Formation Fluid (water, gas, oil)

• Formation Fluid Salinity (Chlorine has a high capture ability)

• Formation Temperature

• Mud Type (density, salinity, temp, fluid type)

• Borehole Geometry (Diameter, Mudcake, Standoff, Shape)


Combined Open Hole Tool Response

Oil

Shale

Shale

Gas

Salt Water

Neutron PorosityGR

Low Porosity

Fresh Water

Density PorosityDeep Res.SP

Shallow Res.


Nuclear Tool Response

Shale

Shale

Gas

Water

Low Porosity

Neutron Porosity

Density Porosity

GR Pe

Limestone

Dolomite Sandstone

Oil


Log Examples


Matrix Conversions – Standard Neutron


Available Open Hole Acoustic Tools

Monopole Dipole Array MDA

Borehole Compensated Sonic HBC/MSS


tlog

Rock matrix

(1-)tma

Fluid in pore space

tfl

tma

tfl

tlog = tfl + tma ( 1- )

and solving for :

= (tlog - tma) / (tfl - tma )


Sonic Logging

Travel Time (T) =(T1R2 + T2R1) - (T1R1 + T2R2)

4

Wyllie Equation

Sonic Porosity () = (T log - T matrix)

(T fluid - T matrix)

T fluid - 189 sec / ft

T matrix

Dolomite - 43 sec / ft

Limestone – 47.5 sec / ft

Anhydrite - 50 sec / ft

Sandstone – 55.5 sec / ft

Shale - 60 to 170 sec / ft

T1

R1

R2

T2

T1R2

T2R1

T1R1

T2R2


Advantages of sonic logs

• Borehole salinity and mudcake have little effect on response

• Formation fluid has little effect unless it is gas

• Borehole size has little effect when diameter is constant

• Indicator of secondary porosity


Porosity from the Sonic log`


Factors affecting sonic logs

• Borehole size

• Residual gas saturations

• Compaction factors

• Ignores vugular porosity to certain degree

• Can NOT be run in air or gas filled boreholes


Micro Resistivity Tools

Micro Spherical Focused Log (MSFL or MMR)

• Range: 0.2 - 2,000 ohm-m

• Vertical resolution 0.25 ft (7.62 cm)

• Depth of investigation 1.0-4.0” (2.54 - 10.2 cm)

• Accuracy: 5% of reading, ± 1.0 ohm-m

Micro Electrical Log (MRT or MML)

• Range 0.2 - 500 ohm-m

• Vertical resolution 2.0” (5.08 cm)

• Depth of investigationMicro inverse 1.5” (3.8 cm)

Micro normal4” (10.2 cm)

• Accuracy 5% of reading, ± 0.05 ohm


Measurements

• Caliper and one of the following

• MSFL Resistivity (ohm-m)

• MEL Resistivity (ohm-m) 1.5” Micro-inverse 2.0” Micro-normal

Specifications

• Diameter (sonde): 3.375” (8.57 cm) MSFL or MEL Pad: 5” (12.7 cm)

• Length: 13.2 ft (4.01 m)

• Weight: 267 lbs (121.4 kg)

• Max Press: 20 kpsi (137 MPa)

• Max Temp: 350 F (177 C)

Micro Resistivity Tool


Combined Open Hole Tool Response in Fresh Mud Environments

Oil

Shale

Shale

Gas

Salt Water

Neutron PorosityGR

Low Porosity

Fresh Water

Density PorosityDeepSP

Shallow

Micro-Inverse

Micro-normal


Micro Resistivity Tool (MRT or MML)

MRT/MML Log Example



Archie Water Saturation Equation

Swn = a Rw / m Rt

Different versions of n, a, and m are available.

Most common are:

Archie: n=2, a=1, m = 2 (used for Carbonates ie limestones, dolomites)

Tixier n=2, a=0.81, m = 2 (used as an approximation for Humble or for Calcareous sands)

Humble: n=2, a=0.62, m = 2.15 (Used for unconsolidated sands)

Basic Openhole Interpretation Walter

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