Dipmeter Data, Borehole Image Logs and Interpretation

Introduction to Borehole Image Logs presented to Geoscience Data Managers in 2016 by Kris Vickerman of HEF Petrophysical Consulting Inc.

1. Dipmeter data, borehole image logs and interpretation Kris Vickerman May 25, 2016

2. 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

3. 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

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

5. 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

6. 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

7. 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)

8. 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 Normalized Static

9. 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 Dynamic Static

10. 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

11. Borehole Image Interpretation Step 1: Processed Image

12. Borehole Image Interpretation Step 2: Beds

13. Borehole Image Interpretation Step 3: Large Fractures

14. Borehole Image Interpretation Step 4: Fine Fractures

15. Image interpretation Dip “Tadpoles” Hand-picked sinusoids Lithology zoning

16. “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

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

18. Basic Structural Dip Analysis Tadpole Plots Stereonet Plots Stick Plots True Stratigraphic Thickness Plots

19. Tadpole Plot

20. Stereonet

21. Stick Plot (cross-section)

22. Interpreted Stick Plot

23. True Stratigraphic Thickness Plot

24. True Stratigraphic Thickness Plot

25. Interpreted Stick Plot

26. 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

27. Interpreted Stick Plot Simple stereonets Uncluttered 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

28. Interpreted stick plot

29. Interpreted stick plot ( Lithotect )

30. Stratigraphic beds Describing bedforms and lithology Sand count and facies plots Vuggy porosity analysis

31. Sandy IHS Moderate GR, moderate resistivity Inclined alternating sand/mud beds Consistent bedding dip direction towards channel centre Vsh 10-40%

32. Trough crossbedded sand Very clean GR, high resistivity >10 ° crossbeds Inclined truncations Vsh < 10% Dip Downstream

33. Trough crossbedded sand Very clean GR, high resistivity >10 ° crossbeds Inclined truncations Vsh < 10% Dip Downstream

34. Planar-tabular crossbedded sand Clean GR, high resistivity >10 ° flow crossbeds , often alternating direction Flat truncations Vsh < 10% Dip down-current

35. 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%

36. Sand count plot Sand count / facies plots can take many forms This one shows: O penhole 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)

37. 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

38. 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

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

40. Natural fracture interpretation Fracture types (open, closed, shear) Fracture properties (geometry, density, aperture)

41. 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)

42. 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

43. Open Fractures

44. 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

45. 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

46. Healed Fractures

47. 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

48. Shear features

49. Natural fracture interpretation Fracture types, (open, closed, shear) Fracture properties (geometry, density, aperture)

50. 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

51. Fracture Density Comparison 2 metres of image

52. Fracture Density Comparison 9 m 2 /m 3 5 m 2 /m 3 2 metres of image

53. 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

54. Fracture aperture estimation 50 cm Open fractures are invaded by conductive drilling mud The amount of invaded mud is somehow proportional to aperture

55. 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

56. 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)

57. 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

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

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

60. Stress direction from breakout Measure s hmin by observing where breakouts occur in the wellbore Vertical and oriented in the plane of s hmin Borehole sloughs in when the drilling fluid pressure is less than formation pressure After: Mossop, Shetsen, 1994 Low Pf

61. Stress direction from breakout Breakout visible as paired vertical conductive smears Can pick the centre of the breakouts to get s hmin

62. Stress direction from breakout Breakout visible as paired vertical conductive smears Can pick the centre of the breakouts to get s hmin s hmin s hmin

63. Stress Magnitude from breakout Width of the breakout is proportional to the magnitude of s hmin Width of the breakout is also proportional to the rock strength Need a database of the strengths of various formations to measure s hmin Width

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

65. Stress direction - Induced fractures Measure s hmax by observing where drilling induced fractures occur Vertical and oriented in the plane of s hmax Borehole wall cracks when drilling fluid pressure is more than formation pressure High Pf

66. Stress direction – Induced fractures Induced fracs . visible as paired thin vertical conductive cracks Can pick the centre of the induced fractures to get s hmax

67. Stress direction – Induced fractures Induced fracs . visible as paired thin vertical conductive cracks Can pick the centre of the induced fractures to get s hmax s hmax s hmax

68. Stress direction – Both types s hmin s hmax s hmax s hmin

69. Stress direction – Both types s hmin s hmax s hmax s hmin

70. 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

71. 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

72. 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