Chapter-2 Environmental Corrections

Cased Hole

The simplified radial profile:

Open Hole

Chapter-2 Environment where logs are run By

Dr. Jorge Salgado Gomes 9/21/2011

Chap-2

Duration of this chapter: 4 classes1(180’)

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

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Environmental corrections • Tool responses are affected by: – – – – – – – – – – – – 9/21/2011

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 Chap-2

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The Borehole and its environment The simplified radial profile:

Mud (m) Mud cake (mc) Invasion (flushed) zone Virgin zone

also models with a transition zone are used 9/21/2011

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Use of Mud System • • • • • • •

Pressure control Formation control Bit life extension Hole cleaning Hole maintenance Drilling power (torque) Telemetry (MWD/LWD)

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Mud Damages and Invades Formation Formation Damage

Invasion Process

• • • • •

• PISTON DISPLACEMENT • MIGRATION

Shale swelling & alteration Pore Blockage Clay swelling Channel Blockage Mechanical damage

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– 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 Cased Hole Borehole cased/completed

Mud Casing/Tubing Cement

limitations for some methods

Open hole

Mud Caliper Invasion

non homogeneous situation

Open Hole

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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

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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

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Invasion with Different Resistivity Profiles

Rw
Rw<
Water in well/reservoir Oil in well/reservoir 9/21/2011

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Resistivity over time as a function of invasion

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Borehole environment and the invasion mud

mud cake

rock

Invasion stops if mud cake is impermeable

1 inch = 2.54 cm

1 cm = 0.39 inch

depth of infiltration in cm 1

10

100

1000

Porosity in %

1…8 borehole diameter

8 … 20 20 … 30

8.5 inch 12.25 inch

> 30

17.5 inch

depth of invasion is controlled by porosity 9/21/2011

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Depth of Invasion 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

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Environmental Problems and Solutions 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

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Log Quality Control (LQC) „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). 9/21/2011

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Example of a Log Quality Form

Source: Theys, 1999) 9/21/2011

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Logging – The General Workflow 1- Logging measured data from various tools/methods

Rxo Rt Sxo Sw

2- Data processing, corrections, inversion, .... 3- Radial/spatial distribution of corresponding physical parameters, e.g. resistivity 4- Interpretation

Additional information, models etc.

5 - Distribution of properties (reservoir properties, e.g. saturation, porosity) 9/21/2011

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The workflow for a proper QA/QC and interpretation Step 1

Step 2

Step 3

Measurement

Processing, corrections, filtering, inversion etc.

Interpretation, joint inversion etc.

corrected physical data in geometrical distribution corresponding to individual methods

reservoir information in geometrical distribution

physical data corresponding to individual methods

the result

Example:

resistivity log

resistivity profile

saturation

porosity and density log

caliper corrected log

porosity

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BACKUP MATERIAL

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Different Resistivity Profiles

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Invasion Profiles 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) 9/21/2011

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Depth of Investigation and vertical resolution a

b

1,0

G(r) detector

0,5 Source

g(r)

r dr

0

r

c

d 1,0

G(z) z detector

0,5

g(z) dz source

0

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zr Chap-2

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