Download - Chapter-2 Environmental Corrections
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
9/21/2011 Chap-2 6
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
9/21/2011 Chap-2 19
Different Resistivity Profiles
9/21/2011 Chap-2 20
Invasion Profiles
9/21/2011 Chap-2 21
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