Chapter-2Environment where logs are run

By

Dr. Jorge Salgado Gomes

1Chap-2 Duration of this chapter: 4 classes (180’)9/21/2011

The simplified radial profile: CasedHole

Open Hole

Educational Outcomes

• Review the environment where we run logs

• Borehole corrections to be applied to the measurements

• Logs more sensitive to the environment corrections

• How to detect borehole anomalies

Chap-2 29/21/2011

Environmental corrections

• Tool responses are affected by:– Mud Invasion (resistivity)– Borehole shape (sonic)– Borehole deviation (resistivity, sonic)– Borehole diameter (resistivity, density, neutron)– Mud salinity (resistivity)– Mud properties (density, neutron)– Bed thickness (resistivity)– Bed resistivities (resistivity)– Borehole Temperature (neutron)– Mud cake thickness (resistivity, neutron)– Tool design– Tool position in hole - standoff

3Chap-29/21/2011

The Borehole and its environment

9/21/2011 Chap-2 4

Mud (m)

Mud cake

(mc)

Invasion

(flushed) zone

Virgin zone

The simplified radial profile:

also models with a transition zone are used

Use of Mud System

• Pressure control

• Formation control

• Bit life extension

• Hole cleaning

• Hole maintenance

• Drilling power (torque)

• Telemetry (MWD/LWD)

9/21/2011 Chap-2 5

Mud Damages and Invades Formation

Formation Damage• Shale swelling & alteration

• Pore Blockage

• Clay swelling

• Channel Blockage

• Mechanical damage

Invasion Process• PISTON DISPLACEMENT

• MIGRATION– Type and characteristics of

filtrate

– Rate of continuing invasion

– Formation permeability

– Permeability distribution

– Formation fluid type

– Formation fluid properties

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Review: Types of Logging Effects

9/21/2011 7Chap-2

by Lecturer

Borehole cased/completed

Mud

Casing/Tubing

Cement

Open hole

Mud

Caliper

Invasion

limitations for

some methods

non

homogeneous

situation

CasedHole

Open Hole

Some Considerations

• The well itself originates an inhomogeneity effect. Therefore caliper and mud properties influence the measured property. This effect must be “corrected” if we will determine formation properties.

• Invasion creates additional inhomogeneity in radial direction.

• The vertical inhomogeneity is (depending on the vertical resolution) originated by the thickness of layers.

9/21/2011 Chap-2 8

Additional Information

• Tools „average“ over a volume of the well + formation – this effect depends on the response of the individual tool

• Tools have a specific depth or radius of investigation - it describes the contribution of sections with different distance from the tool axis to the measured value.

• Tools have a specific vertical resolution - it describes the ability to detect and separate thin layers individually.

9/21/2011 Chap-2 9

Invasion with Different Resistivity Profiles

9/21/2011Chap-2

10

Rw<Rmf Rw<<Rmf

Water in well/reservoir

Oil in well/reservoir

Resistivity over time as a function of invasion

9/21/2011 Chap-2 11

Borehole environment and the invasion

9/21/2011 Chap-2 12

1 10 100 1000

1 … 8

8 … 20

20 … 30

> 30

Po

rosi

ty in

%

depth of infiltration in cm

8.5 inch

12.25 inch

17.5 inch

borehole

diameter

mud mud cake rock

Invasion stops if mud cake is impermeable

depth of invasion is controlled by porosity

1 inch = 2.54 cm 1 cm = 0.39 inch

Depth of Invasion

9/21/2011 Chap-2 13

Asquith and Krygowky, 2004 give the following rules:

High porosity dj/dh = 2

Intermediate porosity dj/dh = 5

Low porosity dj/dh = 10

where

dj = diameter of invaded zone (outer boundary)

dh = borehole diameter

Environmental Problems and Solutions

9/21/2011 Chap-2 14

by Lecturer

The Problem:

In most cases the non-invaded, virgin zone is of interest,

Most logs are influenced by the mud, mud cake, flushed zone,

shoulder beds, thickness of layer, ...

Solutions:

Tool design (hardware) Dual spacing tools

Focusing tools

Pad tools

Data processing (software) Correction charts (Tornado charts)

Data inversion

Log Quality Control (LQC)

9/21/2011 Chap-2 15

„LQC“ is a set of methods that identifies and analyzes data deviations from

established standards and allows the design of remedy“ (Ph. Theys, 1999)

Quality evidences such as:

repeat sections, relogged intervals

quality control curves

calibration tails

confirm the validity of the formation-related data,

but - in most cases – do not add directly the information about the formation.

Log analyst interpret the logs, keeping in mind the performance and

limitations of the tool and the log quality control reports originating from the

wellsite or the field location (Theys, 1999).

Example of a Log Quality Form

9/21/2011 Chap-2 16

Source: Theys, 1999)

9/21/2011 Chap-2 17

Logging – The General Workflow

1- Logging measured data from

various tools/methods

2- Data processing, corrections, inversion,

....

3- Radial/spatial distribution of

corresponding physical parameters,

e.g. resistivity

Rxo Rt

4- Interpretation Additional

information, models

etc.

5 - Distribution of properties (reservoir

properties, e.g. saturation, porosity)

Sxo Sw

The workflow for a proper QA/QC and interpretation

9/21/2011 Chap-2 18

Step 1

Measurement

physical data

corresponding to

individual methods

Step 2

Processing, corrections,

filtering, inversion etc.

corrected physical data

in geometrical

distribution

corresponding to

individual methods

Step 3

Interpretation, joint

inversion etc.

reservoir information in

geometrical distribution

the result

Example:

resistivity log resistivity profile saturation

porosity and density log caliper corrected log porosity

BACKUP MATERIAL

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Different Resistivity Profiles

9/21/2011 Chap-2 20

Invasion Profiles

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Step profile

Idealized, abrupt boundary between

invaded and virgin zone

Transition profile

More realistic with transition (mixture of

mud filtrate and formation water

(+residual hydrocarbon)

Annulus profile

Temporary fluid distribution; formation

water is pushed ahead by the mud

filtrate.

(Asquith and Krygovski, 2004)

Depth of Investigation and vertical resolution

9/21/2011 Chap-2 22

rSource

detector

dr

0

0,5

1,0

r

source

detector

dz

0

0,5

1,0

z

a b

c d

r

G(z)

g(z)

G(r)

g(r)

z