Comparison of pore fabric based on high-resolution X-ray computed tomography and magnetic pore

fabric in sedimentary rocks Yi Zhou*, Michele Pugnetti*, Anneleen Foubert**, Christoph Neururer** & Andrea R. Biedermann* *Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland

**Department of Geosciences, University of Fribourg, Chemin du Musée 6, CH-1700 Fribourg, Switzerland

• The pore size and geometry, as well as their connectivity determine the permeability and preferred permeability direction.

• X-ray micro-tomography (XRCT) is a widely used technique to generate digital rock models, visualize and quantify pore shape and size distributions, and the connectivity of pores with a spatial resolution on the order of 10 microns.

• Magnetic pore fabrics (MPF), obtained by impregnating ferrofluid prior to measuring anisotropy of magnetic susceptibility could be complementary to existing techniques and capture smaller pores.

• This study is aimed at quantitatively analyzing relationships between pore fabric or permeability anisotropy and MPF.

• Plio-Pleistocene calcarenite (Apulia, Italy) with ~50% porosity, ~300 μm average equivalent diameter of pores and complex pore structure.

• Upper Marine Molasse sandstone (OMM, Belpberg, Switzerland) with 5-20% porosity, ~150 μm average equivalent diameter of pores and relatively homogeneous pore structure.

1. Introduction

2.1 Samples

2.3 Magnetic pore fabric method

• Impregnation by vacuum and fifteen-directions measurement scheme to determine the full susceptibility tensor.

2. Methods

2.2 XRCT• XRCT scanning and full-tensor permeability simulations in 6 directions

4. Summary

3.2 Sample: molasseAvizo 2019.4 pore network modellingUser Guide from www.fei.com

15-position measuring

3. Results

3.1 Sample: calcarenite• MI-1-Z3 (calcarenite)

Total pore shape ellipsoidMinimum pore dimensionMaximum pore dimension

Max

Min

Intermediate

• MI-1-Z3 (calcarenite)

• D1112Y (molasse)

Pore

s ≥4

×4×4

voxe

ls

• Orientation density functions of maximum and minimum pore axes are affected by resolution artefacts unless smaller pores are filtered out.

• Total shape ellipsoid reflects preferred orientation of pore shape, and is largely unaffected by artefacts related to unresolved pores.

• Permeabilities can be simulated from the XRCT data, but can be still improved in accuracy and need verification against measurements.

• The orientation of the MPF maximum axis correlates with the maximum dimension of the mean shape ellipsoid at 95% confidence.

• The MPF anisotropy degree is generally smaller than that of the total shape ellipsoid.

simulations in 6 directions

Impregnation

Orientations and orientation density functions of pore axes

All

mea

sure

d p

ore

sPo

res

≥4×4

×4vo

xels

Pore

s ≥1

6×1

6×1

6vo

xels

Minimum pore dimensionMaximum pore dimensionOrientations and orientation density functions of pore axes

Mu

ltiples o

f rand

om

d

istribu

tion

Max

Min

Intermediate

Upper Upper

Upper

Upper Upper

Upper

Upper Upper Upper

UpperUpper

Upper Upper Upper Upper

Mu

ltiples o

f rand

om

d

istribu

tion

Min

Intermediate

Total pore shape ellipsoid

Upper

0.5

Mu

ltiples o

f rand

om

d

istribu

tion

90

70

50

30

10

4.5

3.5

2.5

1.5

Mu

ltiples o

f rand

om

d

istribu

tion

Upper4.5

3.5

2.5

1.5

0.5

Upper

Max

Int

Min

90 (Y)

0 (X)• Max• Int• Min

90 (Z)

Upper

MeanIndividual data95% confidence

3.5

3

2.5

2

1.5

1

0.5

Max

Max

Min

Intermediate

Upper

Simulated permeability

Max

Int

Min90 (Y)

0 (X)

• Max• Int• Min

Upper

90 (Z)

Samples drilled in different directions

MI-1-Z3

Z

X

Y

MI-2-Y3

0 (X)

90 (Y)

• Max• Int• Min

90 (Z)

Upper

Max

IntMin

• MI-1-Z3• MI-2-Y3• MI-3-X15

Max

Min

Intermediate

Max

Min

Intermediate

Max

Min

Intermediate

Comparison ontotal shape ellipsoid

Max

Int

Min

0 (X) • Max• Int• Min

90 (Z)

Upper

90 (Y)

k1=max eigenvalue; k2=intermediate eigenvalue; k3=min eigenvalue;P = k1/k3, anisotropy degree;U = (2*k2-k1-k3)/(k1-k3), shape

• Checking for consistency of pore shape in three perpendicular cores

Simulated permeabilityMPF

P-U

Upper

Bootstrapped total shape ellipsoid

Max

Int

Min

90 (Y)

0 (X)

• Max• Int• Min

Upper

90 (Z)

MeanIndividual data95% confidence

Bootstrapped total shape ellipsoid

P-U

Upper