a stress sensitivity model for the permeability of porous media...

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A stress sensitivity model for the permeability of porous media based on bi-dispersed fractal theory X.-H. Tan * ,, C.-Y. Liu * , § , X.-P. Li * ,, H.-Q. Wang ,|| and H. Deng , ** * State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Southwest Petroleum University Chengdu 610500, P. R. China CNPC Southwest Oil and Gas Field Exploration and Development Research Institute 610041, P. R. China [email protected] § [email protected] [email protected] || [email protected] ** [email protected] Received 2 July 2017 Accepted 5 February 2018 Published 12 March 2018 A stress sensitivity model for the permeability of porous media based on bidispersed fractal theory is established, considering the change of the °ow path, the fractal geometry approach and the mechanics of porous media. It is noted that the two fractal parameters of the porous media construction perform di®erently when the stress changes. The tortuosity fractal dimension of solid cluster D cT' become bigger with an increase of stress. However, the pore fractal dimension of solid cluster D cf ' and capillary bundle D pf ' remains the same with an increase of stress. The de¯nition of normalized permeability is introduced for the analyzation of the impacts of stress sensitivity on permeability. The normalized permeability is related to solid cluster tortuosity dimension, pore fractal dimension, solid cluster maximum diameter, Young's modulus and Poisson's ratio. Every parameter has clear physical meaning without the use of empirical con- stants. Predictions of permeability of the model is accordant with the obtained experimental data. Thus, the proposed model can precisely depict the °ow of °uid in porous media under stress. Keywords: Fractal; porous media; permeability; stress; Darcy's law. PACS Nos.: 47.53.tn, 47.55.hb, 47.56.tr. Nomenclature D cf : solid cluster fractal dimension D cT : solid cluster fractal tortuosity dimension D cT' : solid cluster fractal tortuosity dimension at stress E : Young's modulus, Pa Corresponding author. International Journal of Modern Physics C Vol. 29, No. 1 (2018) 1850019 (12 pages) # . c World Scienti¯c Publishing Company DOI: 10.1142/S0129183118500195 1850019-1 Int. J. Mod. Phys. C Downloaded from www.worldscientific.com by UNIVERSITY OF MINNESOTA TWIN CITIES on 03/17/18. For personal use only.

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Page 1: A stress sensitivity model for the permeability of porous media …download.xuebalib.com/2knbIDXt0UuO.pdf · A stress sensitivity model for the permeability of porous media based

A stress sensitivity model for the permeability of porous

media based on bi-dispersed fractal theory

X.-H. Tan*,‡, C.-Y. Liu*,§, X.-P. Li*,¶, H.-Q. Wang†,|| and H. Deng†,**

*State Key Laboratory of Oil and Gas Reservoir Geology and

Exploitation Southwest Petroleum University

Chengdu 610500, P. R. China†CNPC Southwest Oil and Gas Field Exploration and

Development Research Institute 610041, P. R. China‡[email protected]

§[email protected][email protected]

||[email protected]**[email protected]

Received 2 July 2017Accepted 5 February 2018

Published 12 March 2018

A stress sensitivity model for the permeability of porous media based on bidispersed fractal

theory is established, considering the change of the °ow path, the fractal geometry approach and

the mechanics of porous media. It is noted that the two fractal parameters of the porous media

construction perform di®erently when the stress changes. The tortuosity fractal dimension ofsolid clusterDcT� become bigger with an increase of stress. However, the pore fractal dimension of

solid cluster Dcf� and capillary bundle Dpf� remains the same with an increase of stress. The

de¯nition of normalized permeability is introduced for the analyzation of the impacts of stress

sensitivity on permeability. The normalized permeability is related to solid cluster tortuositydimension, pore fractal dimension, solid cluster maximum diameter, Young's modulus and

Poisson's ratio. Every parameter has clear physical meaning without the use of empirical con-

stants. Predictions of permeability of the model is accordant with the obtained experimentaldata. Thus, the proposedmodel can precisely depict the °ow of °uid in porousmedia under stress.

Keywords: Fractal; porous media; permeability; stress; Darcy's law.

PACS Nos.: 47.53.tn, 47.55.hb, 47.56.tr.

Nomenclature

Dcf : solid cluster fractal dimension

DcT : solid cluster fractal tortuosity dimension

DcT� : solid cluster fractal tortuosity dimension at stress

E : Young's modulus, Pa

‡Corresponding author.

International Journal of Modern Physics C

Vol. 29, No. 1 (2018) 1850019 (12 pages)

#.c World Scienti¯c Publishing CompanyDOI: 10.1142/S0129183118500195

1850019-1

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Fc : the shape factor of solid cluster

Fp : the shape factor of capillary bundle

K : the permeability of porous media at the e®ective pressure

K� : the permeability of porous media at the e®ective pressure at stress

K0 : the permeability of porous media at zero stress, m2

Kþ : normalized permeability

k : geometry correction factor

L : representative length

Lc : solid cluster length

Lc� : solid cluster length at stress

Lc0 : original solid cluster length at zero stress

Lp : capillary bundle length

Lp� : capillary bundle length at stress

Lp0 : capillary bundle length at zero stress

L0 : original representative length

Nc : the number of the solid clusters

q : the °ow rate of a capillary in porous media

q� : the °ow rate of a capillary in porous media at stress

Q : the total °ow of porous media

Q� : the total °ow of porous media at stress

" : aspect ratio

� : the porosity of porous media at the e®ective pressure

�c : solid cluster diameter

�cmax : maximum solid cluster diameter

�cmax0 : original maximum solid cluster diameter at zero stress

�cmin0 : original minimum solid cluster diameter at zero stress

�c� : original solid cluster diameter at stress

�c0 : original solid cluster diameter at zero stress

�p� : capillary bundle diameter at stress

� : viscosity

� : Poisson's ratio

� : the stress in cross-sectional area

�eff : e®ective pressure

�p : pressure di®erence

Superscript

þ : normalized

Subscripts

c : solid cluster

e® : e®ective

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p : capillary bundle

T : tortuosity

max : maximum

min : minimum

1. Introduction

The phenomenon of °ow through porous media under stress attracts lots of

researchers' attentions. Such phenomena of °ow universally occurs both in natural

and arti¯cial materials.1–3 The research of the °uid behavior in porous media under

stress crosses the disciplines of science and engineering, such as hydraulics,4,5 phys-

ics,6–9 and chemical engineering,10 etc. Great concerns of the methods, theories and

progresses in this ¯eld have also been arised.

It is the numerical methods that have exerted vital function in analyzing the °ow

through porous media.11,12 Kupkov�a and Kupka13 derived an expression for veloci-

ties of elastic waves in a medium with su±ciently high number of pores with power-

law distribution of their sizes. Schreyer-Bennethum14 described a phenomenon of

°owing through a swelling porous material on the assumption that this media swelled

when whetted and shrunk when dried. A novel comprehensively coupled °ow de-

formation method was presented by Khoshghalb and Khalili15 and the validity of the

meshfree arithmetic of analyzing °ow-deformation feature in porous media full of

°uid was con¯rmed. Sabetamal et al.16 outlined a numerical procedure in order to

analyze dynamic coupled problems in geomechanics. However, the application of

numerical methods to delineate this phenomenon is constrained for these methods

require tedious calculations and necessitates precise model.17

The extensive investigation of °uid °owing through porous material, using fractal

theory, has lasted for more than two decades, such as the seepage characters of

°ow,18–20 thermal conductivity,21,22 non-Darcy °ow,23,24 capillary driven °ow25–27

and multiphase °ow,28–30 as well as electrical conduction.31 Xiaoet al.32 developed a

model on the °uid penetration behavior of ¯brous GDL in PEMFCs. Validating the

experimental data very well, this con¯rmed model indicated that the water and gas

relative permeabilities were merely in terms of water saturation as the porosity is

irrelevant. Utilizing both the fractal theory and Monte Carlo technique Xu et al.33

constructed a probabilistic model to represent the °uid behavior of radial direction

through fracture system of porous material. In comparison with usual numeric

model, this one excelled and can possibly be applied to forecast other motion features

of fracture system.

Consequently, it is noted that the not well-developed researches on the perme-

ability of porous media under stress should be attributed to the di±culty of calcu-

lating the permeability of porous media under stress, because of the disorder and

exceeding complication of microstructures of porous media.34,35 Hence, fractal the-

ory, to characterize the microstructures of porous media, is introduced. Sukop et al.36

BDFT stress sensitivity model for permeability

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thought that highly complex and rich fractal models can be produced by structured

geometries. Based on the fractal model, water retention is researched considering the

connections between pores and atmosphere. The fractal model can readily determine

the relations between fractal parameters utilized to describe the porous media and

the water retention behavior of the media. According to the assumption of fractal

pore-space geometry and the Kozeny–Carman approach, a model to represent how

the permeability of porous media was related to the porosity of that, was detected by

Costa.37 It was simple for the equation to depict the permeability of porous media.

A novel model, to describe the penetration behavior of porous media, was proposed

by Tan et al.38 in consideration of stress sensitivity. But the experimental data

showed that the samples in porous media reservoirs may obey Hook law stress–strain

diagram. Because the assumption of the extended length of a solid cluster under

stress may not accord with physical fact,3 an improved model for the permeability

under stress needs to be researched, considering the constant length and the

changing tortuosity fractal dimension of a solid cluster under stress.

2. Theoretical Model

The assumption of the porous media considered in present work is that a set of

crooked solid clusters comprises the porous media and the interspaces of solid clusters

are capillary bundles. What is featured in Fig. 1 is the porous media that consists of a

large number of solid clusters. There is a transversal shrinkage of solid clusters when

exerting the stress on a set of solid clusters. The diameters of capillary bundles and

solid clusters in porous media diminish, but the shapes of those are the same, as is

shown in Fig. 2. The fractal model for permeability of porous media will be derived

on the basis of the mechanics of solid clusters.

The cumulative distributions of solid clusters in porous media with a greater

diameter or an equal one as diameter �c obey the fractal scaling law:39

Ncðl � �cÞ ¼�cmax

�c

� �Dcf

; ð1Þ

Fig. 1. (Color online) A porous medium formed by all kinds of solid clusters with di®erent diameters.

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where Dcf , Nc, �cmax and �c are the fractal dimension, the number, the maximum

diameter and the diameter of solid cluster, respectively.

The di®erential form of Eq. (1) with regard to �c obeys the quantity of solid

clusters with the sizes varying in the extremely small domain of �c to �c þ d�c on

intersection surface area.

�dNc ¼ Dcf�Dcfcmax�

�ðDcfþ1Þc d�c: ð2Þ

In Eq. (2), �dNc > 0, which suggests a decrease of number of solid clusters occuring

as the solid cluster diameter increases.

The fractal scaling law for the solid clusters in porous media can be

expressed as:40,41

Lc ¼ � 1�DcTc LDcT ; ð3Þ

where Lc, L, DcT is the length, straight distance and tortuosity fractal dimension of

solid cluster, respectively. It is noteworthy that Eq. (3), based on the e®ort of

Wheatcraft and Tyler,42 is a totally empirical relationship.

According to experimental data, before the solid cluster of the crude porous

material becomes invalid, it yields little or even not at all. Moreover, one of the

concrete, of which the value of tensile strength is low, is put in a category of

fragile material. So Hook43 described the stress–strain illustration for solid cluster in

porous media. The diameter and length of a solid cluster in porous media can be

expressed as:3

�c� ¼ �c0 1� ��

E

� �; ð4Þ

and

Lc� ¼ Lc0 1þ 1

��

E

� �; ð5Þ

where �� is the stress, E is Young's modulus, � is the ratio Poisson, �c� is the solid

cluster diameter under stress, �c� ¼ �c0 as the value of stress is 0, Lco is solid cluster

length under stress, Lco is solid cluster length when the value of stress in transversal

surface is 0.

Fig. 2. (Color online) Comparisons of solid cluster diameters under di®erent stress.

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Rewriting Eq. (3) yields

Lco ¼ �1�DcT0

c0 LDcT0 ; ð6Þ

and

Lc� ¼ �1�DcT�c� LDcT� ; ð7Þ

where DcT� represents the fractal tortuosity dimension of solid cluster of porous

media under stress.

When a radial stress is exerted on a solid cluster, the latter wrinkles because of

the axial stress contracts laterally (Fig. 3). It is notable that the length Lc� and the

fractal tortuosity dimension DcT0 of solid cluster will have an increase while the solid

cluster straight distance L gets no change, as the solid cluster is compressed.

Inserting Eqs. (4), (5) and (6) into Eq. (7), the solid cluster fractal tortuosity

dimension of porous media under stress can be presented.

DcT� ¼ log��; ð8Þ

where � ¼ L�c0 1þ��

Eð Þ and � ¼ �E����Eþ���

L�c0

� �DcT0

.

In Eq. (8), DcT� is a function of the stress ��, the mechanics parameters � and E.

The DcT� increase with an increase of the stress ��. When the �� equals 0, the DcT�

equals theDcT0. As the values of the Poisson's ratio � and the Young's modulusE get

higher, the DcT� is smaller. It is the responsible phenomenon because the resistance

which wrinkles solid clusters is bigger when the Poisson's ratio � is bigger. In addi-

tion, the contract of solid clusters is larger with the higher Young's modulus E.

This validates the physical situation.

Approximating the length of the solid clusters, comprised of capillary bundle,

the Capillary bundle's length Lp� is

Lp� ¼ �1�DcT�

c0 LDcT� 1� ��

E

� �1�DcT�

: ð9Þ

The capillary bundle diameter of porous media can be expressed as38

�p� ¼ �c�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3

p � 2Fc

4Fp

s¼ �c0

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3

p � 2Fc

4Fp

s1� ��

E

� �: ð10Þ

Fig. 3. (Color online) Comparisons of solid cluster lengths under di®erent stress.

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The following is the expression of the °ow rate of a capillary, whose aperture shapes

are various, in porous media:44

q� ¼ k��4

p��p

128�Lp�

; ð11Þ

where q�, �p, � and k is the °ow rate of a capillary in porous media, the pressure

di®erence, the viscosity and a geometry factor, respectively. When the intersection

surface of a capillary bundle is circular, square and regular triangular, the value of

k is separately 1, 1.43 and 1.98.45

Substituting Eqs. (9) and (10) into Eq. (11), the °ow rate of a capillary in porous

media is acquired.

q� ¼�k�p� 3þDcT�

c0

ffiffi3

p �2Fc

4Fp

� �21� ��

E

� �3þDcT�

128�LDcT�: ð12Þ

Integrating Eq. (12) from the minimum value to maximum one of solid cluster

diameter, the total °ow Q� can be acquired, when the stress is considered.

Q� ¼ �kDcf�p�3þDcT�

cmax0

128�ð3þDcT� �DcfÞLDcT�

ffiffiffi3

p � 2Fc

4Fp

� �2

1� ��

E

� �3þDcT�

: ð13Þ

By making an integration of the transversal surface area of the regular triangular

unit cell. Au� from the minimum value to the maximum one of solid cluster diameter,

the transversal surface area of the porous media A� can be calculated.

A� ¼ �Z �cmax�

�cmin�

Au�dNc ¼ffiffiffi3

pDcf�

2cmax0

4ð2�DcfÞ1� ��

E

� �2ð1� 2�Dcf Þ; ð14Þ

where ¼ �cmin0

�cmax0.

The permeability of porous media can be expressed by Darcy.

K� ¼ Q��L

A��p: ð15Þ

Substituting Eqs. (13) and (14) into Eq. (15), the permeability of porous media is

acquired.

K� ¼ ð2�DcfÞ�kL1�DcT��1þDcT�

cmax0

32ffiffiffi3

p�ð3þDcT� �DcfÞ

ffiffiffi3

p � 2Fc

4Fp

� �2

1� ��

E

� �1þDcT�

: ð16Þ

The permeability of porous media K� tends to 0 with the solid cluster fractal di-

mension Dcf approximating 2 in Eq. (16), which indicates the porous media is ¯lled

up with solid clusters. However, as Dcf approaches 0, K� approximates maximum,

which suggests the porous media is ¯lled up with pores. This validates the physical

situation.

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In Eq. (16), when the value of stress is 0, the original permeability of porous media

K0 can be expressed as

K0 ¼ð2�DcfÞ�kL1�DcT0�

1þDcT0

cmax0

32ffiffiffi3

p�ð3þDcT0 �DcfÞ

ffiffiffi3

p � 2Fc

4Fp

� �2

: ð17Þ

The de¯nition of the normalized permeability is the permeability ratio of K to K0,

which is obtained.

Kþ ¼ K�

K0

¼ ð3þDcT0 �DcfÞð3þDcT� �DcfÞ

�cmax0

L

� �DcT��DcT0

1� ��

E

� �1þDcT�

; ð18Þ

where DcT� can be calculated by Eq. (8).

3. Results and Discussion

Compared to experimental data, the validity of the model which is used to charac-

terize the permeability under stress of porous media is discussed. The mechanics

parameters of the porous media are E ¼ 2:79� 4:76� 109 Pa, � ¼ 0:08� 0:22,46 so

E ¼ 2:79� 109 Pa, � ¼ 0:08 and E ¼ 4:76� 109 Pa, � ¼ 0:22 are decided based on

the properties of porous media, and � ¼ 0:1, k ¼ 1, Fp ¼ �=4, Fc ¼ �=4; �eff ¼ �� are

used in this study. As is shown in Fig. 4, the predictions of the presented fractal

model (Eq. (18)) with extremum values of Young's modulus and Poisson's ratio are

compared with the available experimental data.47–49 The dots, of which the e®ective

stress domain varies from 0 to 120 �106 Pa, can represent the experimental data of

normalized permeabilities. The samples 1–6 are from Chierici et al.'s data,48 which is

marked as solid points. The samples 7–12 are chosen from Yale's paper,49 marked as

void points. The samples 13–15 are selected because of Hsu's contribution47 with half

solid points. The two solid lines represent the predicted normalized permeabilities of

Fig. 4. (Color online) Comparisons of the permeability of porous media between the prediction value and

experimental data.47–49

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the proposed model with extremum values of Young's modulus and Poisson's ratio.

It is noticed that almost all the experimental data locate between the two lines,

the vast majority of the experimental data distributed in the intermediate

region which is between the maximum values and minimum ones of the predictive

data. That means the model predictions validate the available experimental

data very well.

Figures 5 and 6 show the normalized permeabilityKþ of porous media a®ected by

mechanics parameters, the Young's modulus E and Poisson's ratio �, respectively.

Fig. 5. (Color online) The permeability of the porous media versus e®ective stress with di®erent Young's

modulus, E:

Table 1. The details of samples.

Sample no.

Minimum

pressure (MPa)

Maximum

pressure (MPa)

Permeabilitywith minimum

pressure (mD)

Permeabilitywith maximum

pressure (mD) Reference

1 0.1 45 67.7 58.97 Ref. 48

2 0.1 45 72.3 62.613 0.1 45 400 369.2

4 0.1 45 23 20.59

5 0.1 45 31.9 29.226 0.1 45 352 329.47

7 1 49 30.3 27.61 Ref. 49

8 1 49 129.6 116.12

9 1 48 49.0 45.04

10 1 49 901.9 842.2811 1 49 152.2 131.27

12 1 49 494.6 442.22

13 4.6 120 0.58 0.48 Ref. 47

14 4.7 120 0.34 0.24

15 4.9 120 0.64 0.48

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When the e®ective stress �eff increases, the normalized permeability Kþ reduces

gently. Meanwhile, as the e®ective stress �eff increases, the pore space decreases and

the °owing path increases as well as the solid cluster fractal tortuosity dimension

DcT�, which indicates the decrease of the cross-sectional area of capillary bundles.

Then, the °ow resistance increases with the increase of the e®ective stress �eff , re-

ducing the permeability under stress, so the normalized permeability Kþ of porous

media is always less than 1.0. Figure 5 reveals that the permeability of porous media

K� is smaller as the Young's modulus E gets higher. Two factors contribute to this

phenomenon. One is the higher resistance that contracts solid clusters with the

higher Young's modulus E and the other is the pore space of porous media which gets

harder to compress with the increase of Young's modulus E. From Fig. 6, we can also

see that the permeability of porous media K� increases as the Poisson's ratio �

increases. This can be explained by the fact that the solid cluster fractal tortuosity

dimension DcT� of porous media becomes less easy to increase when the Poisson's

ratio � increases. So, the permeability of porous mediaK� increases with the increase

of the Young's modulus E and the Poisson's ratio �.

4. Conclusion

The previous research for permeability of porous media is reviewed ¯rst, and then

based on the stress e®ect, the predicted permeability method of porous media is

proposed considering the mechanics of porous media and using the fractal statistics

method. The predictions of the present model, as tested by experimental data, val-

idate the experimental data very well. Thus, the validity of the model for the pre-

diction of permeability in porous media is veri¯ed. Consequently, the fractal

characters of porous media have an e®ect on the permeability of porous media.

Considering the fractal properties of microstructure of porous media and based on

Fig. 6. (Color online) The permeability of porous media versus e®ective stress with di®erent

Poisson's ratio.

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the stress e®ect and basic °uid dynamics, an entire analytical expression of perme-

ability of porous media can be constructed.

Acknowledgments

This work was jointly supported by Open Fund (PLN201721) of State Key Labo-

ratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum

University), National Natural Science Foundation of China (51704246), the National

Science and Technology Major Project (2016ZX05052 -002-04, 2016ZX05024-005-

008), and PetroChina Innovation Foundation (2016D-5007-0209).

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