Dipmeter Data, Borehole Image Logs and Interpretation

Dipmeter data, borehole image logs and interpretation

Kris VickermanMay 25, 2016

Dipmeter refers to the bedding data (depth, dip, azimuth, quality, etc.). The small plot on top is a dipmeter plot.

Dipmeter also refers to an older tool with 4, 6 or 8 buttons Borehole image logs refer to any tool that samples an array

of measurements in the borehole: Resistivity – FMI, CMI, XRMI, etc. Ultrasonic images – UBI, CBIL, CAST LWD images – (GR, Density, Resistivity and so on.)

Introduction

Data comes from the logging truck typically via satellite or FTP transmission: File types such as DLIS, TIF, LIS, XTF, AFF, LAS, CSV Large files, often 100’s of MB

Data is also found in digital archives: Corporate archives as digital or paper well files Government archives (BCOGC), as scans, paper logs, and digital Service company archives (HEF for example has more than 10,000

wells in our Recall Database dating back to the early 90’s) Log data vendor archives as rasters, etc. Digitized data such as ASCII bed dip files from above sources

Data can also be sourced from physical media: Magnetic tapes, CD/DVD, scanning old paper prints and so on…

Introduction – input data sources

Outline

Basics of borehole image interpretation Bedding and structural dip analysis Natural fractures Stress features

Basics of borehole image logs

Wireline or MWD tool is positioned in the borehole (resistivity, sonic, density, gr)

Inclined surfaces intersect the measurement buttons at different depths, unrolling to a sinusoid in the standard display

Basics of borehole image logs

Wireline or MWD tool is positioned in the borehole (resistivity, sonic, density)

Inclined surfaces intersect the measurement buttons at different depths, unrolling to a sinusoid in the standard display

Typical Conductivity Image plot is shown as an unrolled view of the inside of the borehole

Conductive features are dark; resistive are light

Planes that intersect the borehole become sine waves in this view

Bedding (orange-yellow) and fractures (black) visible in this section

Borehole Image Example (FMI)

Image normalization

Image colour is statically normalized with conductive as black and resistive as white

To enhance local contrast, colours are renormalized in a sliding 1m window making a “Dynamically Normalized” image

Dynamically NormalizedStatic

Image logs and core Conductive shale is

black, resistive bitumen sand is white/yellow

We can often see resistivity contrast features that are hard to see in core

DynamicStatic

Oil-Based horizontal field imager Horizontal field

electric images see fractures better but also see bit marks

Acoustic images are lower resolution

Bedding is clear Some fracturing is

visible Some induced

features are visible

Borehole Image InterpretationStep 1: Processed Image

Borehole Image InterpretationStep 2: Beds

Borehole Image InterpretationStep 3: Large Fractures

Borehole Image InterpretationStep 4: Fine Fractures

Image interpretationDip “Tadpoles”

Hand-picked sinusoidsLithology zoning

“Basics” products Plot of the interpreted image at various scales

(Paper / PDF / TIFF) Output of the interpreted image in DLIS Output/backup of the interpreted image in DB

format like Recall or Geoframe, etc. Output of the interpreted features (Beds,

fractures, etc.) in LAS

Outline

Basics of borehole image interpretation Bedding and structural dip analysis Natural fractures Stress features

Basic Structural Dip Analysis

Tadpole Plots Stereonet Plots Stick Plots True Stratigraphic Thickness Plots

Tadpole Plot

Stereonet

Stick Plot (cross-section)

Interpreted Stick Plot

True Stratigraphic Thickness Plot

True Stratigraphic Thickness Plot

Interpreted Stick Plot

Example of structural interpretaion Each domain is taken to

have consistent average dip

The boundaries between the domains are oriented on the bisectors of the dip domains

Interpreted Stick Plot

Simple stereonetsUncluttered bed dips and subtle frac. den. curve

GR and tops markers

Depth tracks visible but not in the way

Projected bedding

Anything else you might like to add FDEN, tadpoles,

openhole data

Interpreted stick plot

Interpreted stick plot (Lithotect)

Stratigraphic beds

Describing bedforms and lithology Sand count and facies plots Vuggy porosity analysis

Sandy IHS Moderate GR,

moderate resistivity Inclined alternating

sand/mud beds Consistent bedding

dip direction towards channel centre

Vsh 10-40%

Trough crossbedded sand Very clean GR,

high resistivity >10° crossbeds Inclined truncations Vsh < 10% Dip Downstream

Trough crossbedded sand Very clean GR,

high resistivity >10° crossbeds Inclined truncations Vsh < 10% Dip Downstream

Planar-tabular crossbedded sand Clean GR, high

resistivity >10° flow

crossbeds, often alternating direction

Flat truncations Vsh < 10% Dip down-current

Mud Breccia Moderate to high

GR, low resistivity Often crossbedded Clast supported

conductive (dark) mud clasts

Petrophysically indistinguishable from laminated mud beds below

Vsh > 10%

Sand count plot

Sand count / facies plots can take many forms This one shows:

Openhole data on the right High-res resistivity curve for thin bed petrophysics (red, on the right) Facies track (Green/yellow/black) Sand count track (brown and yellow to the right of image) Sand bed thickness and percentage curves (yellow and grey to the right of image)

Secondary porosity plot

Image thresholding produces an estimate of irregular (secondary) porosity as a percentage of the whole

Plot shows limestone / dolostone flag on left, thresholded black and white image on right followed by secondary porosity curves in red, green and grey

Bed Interpretation products Stereonet, Tadpole, Stick, TST, etc. (Paper / PDF) Lithology zonation file (LAS) and plots Bed dip types on plots and in LAS / ASCII

Outline

Basics of borehole image interpretation Bedding and structural dip analysis Natural fractures Stress features

Natural fracture interpretation

Fracture types (open, closed, shear) Fracture properties (geometry, density, aperture)

Open Fractures Open fractures are filled

with conductive drilling mud (dark on borehole images)

Fractures are not infinite in length so partial intersections are common

Direct measurements include dip, azimuth, trace length, minimum radius, type (LAS)

Open Fracture Exaggeration

50 cm

This fracture is probably on the order of .5 mm, not 5 cm as it is seen here

Tool current “seeks” the conductive fracture before and after it, making it appear much larger *From Cheung, 1999

Open Fractures

Mineralized fractures might be filled with calcite, quartz or dolomite, all resistive

Often fracture traces are invisible

See artificial halo inside fracture plane

Healed Fractures

Healed Fracture Haloing

50 cm

*From Cheung, 1999

The resistive fracture itself is invisible, see halo instead

Tool current “piles up” inside of resistive fracture plane and is dispersed outside of it

Healed Fractures

Shear feature in Borehole Images Visible as a bedding offset Can be healed or open Can be mm-scale to km-

scale in throw Geologists would call these

faults but some managers might not be so keen

Shear features

Natural fracture interpretation

Fracture types, (open, closed, shear) Fracture properties (geometry, density, aperture)

Fracture Density

Fracture density can be calculated a few ways: As line-density 1-D As tracelength density 2-D As a modelled volumetric density 3D

Fracture Density Comparison

2 metres of image

Fracture Density Comparison

9 m2/m3 5 m2/m3

2 metres of image

Fracture Density Plot Gives an at-a-glance

curve to tell fracture intensity but no indication of aperture, permeability or connection to porosity

If drilling induced fractures or foliation is included, it gives false results

Fracture aperture estimation

50 cm

Open fractures are invaded by conductive drilling mud

The amount of invaded mud is somehow proportional to aperture

MUD

Fracture aperture estimation

*U.S. Patent No: 52435211

Aperture = A * Rt 0.1505 * Rm 0.8495

A = Excess conductance

Rt = Formation resistivity

Rm = Mud resistivity

Fracture aperture plot Apertures are calculated two

ways: As an average for each

fracture (red dots, second to right)

…And as a rolling mean (blue-red cuve on right)

Fracture Interpretation products Fracture types on tadpole, image and stereonet

plots and in LAS / ASCII Fracture density plot and LAS file Fracture aperture plot and LAS file Fracture statistics like trace length, minimum

radius, height and so on in LAS file

Outline

Basics of borehole image interpretation Bedding and structural dip analysis Natural fractures Stress features

Un-natural fractures Stress direction from borehole breakout Stress direction from induced fractures

Stress direction from breakout Measure shmin by

observing where breakouts occur in the wellbore

Vertical and oriented in the plane of shmin

Borehole sloughs in when the drilling fluid pressure is less than formation pressure

s hm ax

s hm in

After: Mossop, Shetsen, 1994

Low Pf

Stress direction from breakout

s hm a x

s h m in

Breakout visible as paired vertical conductive smears

Can pick the centre of the breakouts to get shmin

Stress direction from breakout

s hm a x

s h m in

Breakout visible as paired vertical conductive smears

Can pick the centre of the breakouts to get shmin

shmin shmin

Stress Magnitude from breakout Width of the breakout

is proportional to the magnitude of shmin

Width of the breakout is also proportional to the rock strength

Need a database of the strengths of various formations to measure shmin

Width

Un-natural fractures Stress direction from borehole breakout Stress direction from induced fractures

Stress direction - Induced fractures Measure shmax by

observing where drilling induced fractures occur

Vertical and oriented in the plane of shmax

Borehole wall cracks when drilling fluid pressure is more than formation pressure

s hm ax

s hm in

High Pf

Stress direction – Induced fractures Induced fracs. visible

as paired thin vertical conductive cracks

Can pick the centre of the induced fractures to get shmax

s hm ax

s hm in

Stress direction – Induced fractures Induced fracs. visible

as paired thin vertical conductive cracks

Can pick the centre of the induced fractures to get shmax

s hm ax

s hm in

shmaxshmax

Stress direction – Both types

shminshmaxshmax shmin

Stress direction – Both types

shminshmaxshmax shmin

Stress Interpretation products Horizontal maximum stress direction on stereonet Stress features on tadpole plots and in LAS files Further analysis can be done for more in depth

geomechanical understanding

Interpreted borehole image data should always be distributed as digital files (Downloaded via FTP/website or on DVD)

Can be printed on paper Can be supplied in a format that can be loaded into other

software packages (a DLIS array of the processed image) Should be stored by the interpreter and logging contractor (if

different) in some permanent database (Recall, etc.) Ideally should become part of government databases once

off confidential

Outtroduction – data outputs

The words Dipmeter and Borehole image log are pretty loaded and can mean a lot of things

Depending on the questions, these logs can provide a large suite of answers about the nature and textures of bedding and fracturing in the subsurface

The products come in a wide and challenging variety of plots, files and media

Conclusion