m. stavropoulou coupled wellbore erosion and stability analysis

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS

Int. J. Numer. Anal. Meth. Geomech., 22, 749769 (1998)

COUPLED WELLBORE EROSION AND STABILITYANALYSIS

M. STAVROPOULOU1, P. PAPANASTASIOU2,* AND I. VARDOULAKIS1

1Department of Engineering Sciences, National Technical University of Athens, GR-157 73, Athens, Greece2Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge CB3 OEL, U.K.

SUMMARY

This paper extends earlier work1 on sand erosion and presents an attempt to couple sand erosionto mechanical damage of rock around a wellbore. Porosity which evolves in time and space as sur-face erosion progresses, is chosen as the coupling parameter. Both rock elasticity and strength (cohesion)are assumed to depend on porosity in such a way that the material becomes weaker with increasingporosity. The mathematical model, consists of erosion equations, mixture ow equations and stressequilibrium equations, is solved numerically by Galerkin nite element method. Numerical results suggestthat erosion, resulting in sand production, is high close to the free surface. Erosion is accompained bychanges in porosity and a signicant permeability increase. Erosion in the vicinity of the wellbore inducesalterations in the mechanical behaviour of the medium. Weakening of rock stiness leads to severe alterationof both eective stresses and pore pressure near the cavity. Since cohesion decreases with increasingporosity, one can also identify the time instant at which rock mechanical failure starts. ( 1998 John Wiley& Sons, Ltd.

Key words: wellbore stability; sand erosion; sand production; sand prediction; radial ow; poroelasticity

1. INTRODUCTION

Wellbore instabilities and production of solids (mainly sand) are common problems in oilproducing wells embedded in formations that are inherently weak. Sand prediction models aimat optimizing uid production rates while avoiding near-wellbore instabilities and damageof reservoir structure. The prediction of sand production is essential to the decision whethersand control techniques will require to be installed during the well completion phase. Thehigh completion cost and the reduction of well productivity which are associated with sandcontrol measures, especially in the case of gravel packing, make sand prediction models a valu-able tool.

The dierent processes involved in sand production are associated with uid and solidsmovement, uid/rock interaction and rock deformation. From the modelling point of view,sand production is usually related to the following two failure mechanisms: (1) mechanicalinstabilities and localized failure (damage) of the rock in the vicinity of the wellbore due to stress

*Correspondence to: P. Papanastasiou, Schlumberger Cambridge Research, High Cross, Madingley Road, CambridgeCB3 OEL, U.K.

CCC 03639601/98/09074921$17.50 Received 7 March 1997( 1998 John Wiley & Sons, Ltd. Revised 12 December 1997

concentration2 and (2) hydro-mechanical instabilities due to internal and surface erosion,which manifest themselves in releasing and displacing of particles, caused by the action ofseepage forces. The mechanical failure and erosion mechanisms are coupled to one another, sincestress concentrations lead to localized damage, which in turn increases the amount of looseparticles that can leave the matrix and enter the population of particles in transport. On the otherhand, washing-out of particles increases the porosity of the intact rock and may cause re-adjustment of the intergranular forces leading to further strength decrease and damage of therock.

All the existing sand prediction models treat the problem similar to the wellbore stabilityaddressing only the problem of sand initiation, see, for example, References 3 and 4. The rstwork considering sand production as an erosion phenomenon was reported recently in a paper bythe authors.1 The paper examined the hydro-mechanical aspect of the sand production problemand set the basic frame of a mathematical model. Surface erosion and piping eects were studiedon the basis of mass balance and particle transport considerations as well as Darcys law. In therst model, all deformation and strength characteristics of the rock were suppressed assuminga cohesionless material similar to loose sand, and the emphasis was put on mass transport. Thepaper considered Darcy ow normal to the free surface of an inhomogeneous half-space withpermeability which depends on the porosity. The results showed that surface erosion is accom-pained by high changes of porosity and permeability close to the free surface. The paper suggestedthat quantities which can be measured in experiment, like the amount of produced soils or uiddischarge, can be used in an inverse way to determine the constitutive parameters of the problem.We emphasize here that the experimental studies of sand production given more qualitative thanquantitative information on the erosion phenomenon [5]. The shortage of quantitative experi-mental data may partly be attributed to the lack of theoretical framework which could guide thedesign of basic sand erosion experiments.

This study extends our previous work1 and presents an attempt to couple sand erosion tomechanical damage of rock around a wellbore. We considered a wellbore in a deep rockformation as illustrated in Figure 1. The uid ow is considered to be radial (towards the freesurface of the wellbore). Also radial symmetry around the well axis is assumed. Under theseconditions any deformation of the rock will take place in a plane normal to the borehole axis. Therock is treated as a porous and permeable material and erosion is mainly governed by thepossibility of grains passing through the interconnected void space and the existence of defects aserosion starters points.

The studied approach is based on a three-phase mixture theory for a continuum consisting ofskeleton solids, uidized-solids and uid.1 The formulation of the dierential equations pertain-ing to the erosion kinematics is superseded by the numerical solution based on Galerkin niteelement scheme. Accordingly, an approximate solution is obtained assuming that the velocity ofthe uid ow is much greater than the deformation rate of the solid skeleton. The solids skeletonis deformed only due to variation of the stress eld. Further on, a theory of partial stressesis developed and the stress equilibrium equations, as well as the constitutive relations ofa poroelastic medium are presented. Porosity which evolves in time and space as surface erosionprogresses is chosen as a coupling parameter. Both rock elasticity and strength (cohesion) areassumed to depend on porosity in such a way that the material becomes weaker with increasingporosity. Combining the above relationships a single non-linear dierential equation whichdescribes the radial displacement is obtained. This equation is solved numerically at each timestep using a second Galerkin nite element scheme. Finally, based on the derived stress solution

750 M. STAVROPOULOU E A.

( 1998 John Wiley & Sons, Ltd. Int. J. Numer. Anal. Meth. Geomech., 22, 749769 (1998)

Figure 1. Conguration of the problem

around the circular opening, a stability analysis is performed using a modied MohrCoulombfailure criterion expressed in terms of Terzaghi principal eective stresses. The degradationof the medium is described by a simple damage law according to which the diminution ofthe cohesion is quantiatively proportional to the porosity. Since cohesion decreases with in-creasing porosity, one can identify the time instant at which the rock starts failing and producingsand.

2. DEFINITION OF BASIC HYDRO-MECHANICAL EQUATIONS

2.1. Mass balance and particle transport

The saturated porous medium is modelled as a three-phase system consisting of skeleton solids(s), uidized-solids (fs) and uid (f ).1 Fluidized particles are particles in suspension that movewith the uid. Any other loose particles which are trapped inside the void space are seen aspart of the solid phase. Moreover, it is assumed that uid and uidized particles share at anyinstant the same velocity. The solid is assumed to be rigid. In other words, a solid particle haseither zero velocity and is assigned to the solid phase or has the velocity of the uid and isassigned to the mixture which lls the void space. The volume fraction of voids is expressed by the

COUPLED WELLBORE EROSION AND STABILITY ANALYSIS 751


bulk porosity /

/"d7d (1)

where d indicates a volume element and d7is the volume of interconnected void space of pores

which is fully occupied by the mixture of uid and uidized particles. The transport concentrationof the uidized solids c is dened by

c"d&4d

7

(2)

where d&4

indicates the volume of uidized particles. It should be noticed that / and c arefunctions of location and time. The partial densities of the solid phase (index 1) and the mixturephase (index 2) coincide with the bulk densities of the corresponding constituents

o(1)"dM4d"(1!/)o4 (3)

o(2)"dMMd"

dMdM

&4d "/oN oN "co4#(1!c)o& (4)

where dM4is the mass of solids, dM

&is the mass of uid phase, dM

&4is the mass of uidized-

particles, dMM is the mass of mixture, o4is the density of solids, o

&is the density of uid and oN is the

density of mixture. The volume discharge rate of the mixture is dened by the relation

qNi" dM

dSidt

(5)

where dM is the volume of ow through the cross-sectional area dSiin time dt.

Mass balances for the solid phase, uidized particles and uid phase are given by the followingexpression:1

L/Lt

"mRo4

(6)

Lc/Lt

#div(cqNi)"mR

o4

(7)

div qNi"0 (8)

where mR is a mass generation term, which corresponds to the rate of net mass eroded and uidizedat any time and point. Notice that in this study the hydro-mechanical dispersion is disregarded.Equations (6) (8) constitute the set of mass balance equations for the considered problem. Theseequations do not suce for solving the problem since a constitutive relation for the massgeneration term is needed. A constitutive law for the rate of eroded mass, suggesting that erosionis driven by the discharge of the uidized particles cq

i, is given by the following relation:6,7,1

mRo4

"j (1!/) Ac!c2

c#3B DqN i D (9)



where c#3

is a critical value of c for which the two competing phenomena, erosion and deposition(plugging) balance each other. The coecient j has dimension of inverse length, [j]"~1, and isprobably related to the spatial frequency of the potential erosion starters points in the solidskeleton of the porous medium and must be determined experimentally. In case of couplingbetween erosion process and rock weakening, j is expected to be an increasing function of rockdamage.

2.2. Stresses, partial stresses, damage

For the formulation of stressstrain relationships the concept of partial stress in a mixturetheory is utilized. The medium is treated as a two-phase mixture equipped by a total (Cauchy)stress tensor p

ijand by partial stresses p(a)

ijwhich are assigned to the solid phase (a"1) and uid

phase (a"2).8 The total stress is the summation of the partial stresses

pij"p(i)

ij#p(2)

ij(10)

The partial stress for the uid phase is directly related to the pore uid pressure p, via

p(2)ij"!/pd

ij(11)

and the partial stress for the solid phase is given by

p(1)ij"(1!/)pN

ij(12)

where pNij

is the constitutive part that is related to the strains. Here we adopt the simplest possiblemodel; i.e. an isotropic linear elastic model

pNij"2Ge

ij#"e

kkdij

(13)

where " and G are the Lames parameters, given below in terms of the Poissons ratio, l, and theYoungs modulus, EM ,

"" EM l(1!l) (1!2l) , G"

EM2(1!l) (14)

We notice that combination of equations (12) and (13) leads to a poroelastic model withinternal damage and with porosity, 0(/(1, being the damage parameter9

E"EM (1!/) (15)

It should be noticed that in this analysis the total porosity and the porosity of the interconnec-ted void space are assumed to be the same, having in mind a material like a porous sandstone.

According to Terzaghis eective stress principle the total stress is decomposed into an eectivestress, assigned to the soil skeleton, and into a pore uid pressure

pij"p@

ij!pd

ij(16)

We notice that based on experimental evidence, strength criteria are formulated in terms ofTerzaghis eective stress. Combining equations (10)(12) and (16) the relation between the



Figure 2. Coordination number

Terzaghis eective stress p@ij, the constitutive stress, pN

ij, and the pore water pressure, p, can be

derivedp@ij"(1!/)[pN

ij#pd

ij] (17)

Erosion phenomena in turn, cause the degradation of the granular medium through porosityincrease and diminution of the cohesion between the grains. The parameter that describes thenumber of grains in contact is the coordination number, k, Rumpf.8 The coordination number isinversely proportional to porosity of the medium (Figure 2). Considering Rumpf s observation, inthe present analysis the degradation of the medium is described by a simple damage lawaccording to which the diminution of the cohesion C is quantitatively proportional to theporosity10

C"CM (1!/) (18)Taking into account relation (18), the MohrCoulomb failure criterion, expressed in terms of

principal eective stresses, is modied as follows:11

12(p@

1#p@

2) sin'!1

2(p@

1!p@

2)"C cos '"CM (1!/) cos ' (19)

ortm"!q

msin '#C cos ' (20)

where ' is the rock friction angle, tm

is the shear stress and qm

is the mean pressure.

3. COUPLED WELLBORE STABILITY ANALYSIS

3.1. Wellbore erosion

Combining equation (6) with equations (7) and (9) and taking into account the continuityequation (8) the formulation of the axisymmetric erosion kinetics is obtained

L(c/)Lt

#qN3

LcLr

"L/Lt

(21)

L/Lt

"j(1!/) Ac!c2

c#3B DqN 3 D (22)



On the other hand, ow in a porous medium is governed by Darcys law which says that theow rate or mixture discharge q

3is driven by the gradient of pore uid pressure

qN3"! k

g,oN

LpLr

, qN3"!q0 (23)

where g,

is the kinematic viscosity of the uid, p is the pore uid pressure and oN is the partialdensity of the mixture dened in equation (4).

We recall that the physical permeability k of a porous medium in general depends on porosity/. For example, the CarmanKozeny equation,12,8 gives

k"k0

/3(1!/)2 (24)

The physical permeability k has the dimensions of surface, which means that k stands asa measure of the cross-sectional area of the microscopic channels in the interconnected voidspace. The physical permeability k is usually measured in millidarcy (1 md"0)987]10~15m2).

Substituting qN3from Darcys law (23), in continuity equation (8), the following expression is

obtained:

L2pLr2

#LpLr C

1

r#a L/

Lr#b Lc

LrD"0 (25)where a and b are in general functions of c and /. Here due to the assumptions made in (23)and (24)

a" 3!// (1!/) , b"!

o4!o

&(1!c)o

co

4

(26)

The system of equations to be solved simultaneously consists of :(a) mass balance equation (21) and evolution law for porosity (22) which both govern the erosion

process,(b) Darcys law (23) with the appropriate denitions for partial density (4) and permeability (24)

and(c) continuity equation (25) which models the ow of the mixture.

The coupled non-linear governing dierential equations with unknowns /, c, q and p aresupplemented with the following boundary conditions at the remote boundary r

0and at the

wellbore surface r!,

c (r0, t)"c

0, p (r

0, t)"p

0, p(r

a, t)"p

a

For simplicity the initial porosity and transport concentration are assumed to be constant

/ (r, 0)"/0, c(r, 0)"c

0)

The initial proles for ow rate and pore pressure are determined from Darcys law assum-ing a homogeneous medium. However, it should be noticed that other initial and boundary



conditions for / and c are also possible which may lead to smooth or non-smooth solutions dueto the hyperbolic character of equation (21).

3.2. Wellbore stresses, deformation and failure

For the problem considered, a plane-strain axially symmetric deformation analysis is per-formed. Taking into account equations (10) (14), the elastic constitutive relations between totalstress and total strain can be derived from

p3" EM (1!/)

(1#l) (1!2l) [(1!l)e3#leh]!/p (27)

ph"EM (1!/)

(1#l) (1!2l) [(1!l)eh#le3]!/p (28)

For the considered axisymmetric planestrain conditions the corresponding strains are ex-pressed in terms of the radial displacement u

3"u(r, t) only

e3"Lu

Lr(29)

eh"u

r(30)

The equation of stress equilibrium for p3"p

3(r, t) and ph"p((r, t), neglecting body forces,

takes the form

Lp3

Lr#p3!ph

r"0 (31)

Substituting equations (27) (30) into equilibrium equation (31), the following dierentialequation describing the radial displacement appears:

L2uLr2

#g1

LuLr

!g2u"g

3

L(/p)Lr

(32)

where gi"g

i(r, t) are functions of l and /"/ (r, t) which are given in Appendix II.

For the open hole of Figure 1 the stress boundary conditions are

p3(ra, t)"!p

a, p

3(r0, t)"!p

0

which in terms of displacement u become

LuLr K

r/ra

"! l1!l

u(ra)

ra

!g3(ra, t)[/(r

a, t)p

a!p

a] (33)

LuLr K

r/r0

"! l1!l

u(r0)

r0

#g3(r0, t)[/

0p0!p

0] (34)



The radial displacement, u, can be obtained solving numerically the partial dierential equa-tion (32) with boundary conditions (33) and (34). Once u and Lu/Lr are determined, the stresssolution can be derived by introducing the result into equations (27) and (28).

4. NUMERICAL SOLUTION

The system of governing equations of erosion kinetics (21) (23) and (25) is solved numerically bya discretization procedure using the Galerkin nite element method in space and nite dierencesin time. The rst step of this scheme is to discretize the space domain replacing the original set ofPDE with a set of ODE and then to apply a second discretization in time alone. Considering thenon-linearity of equation system, the solution is obtained with Newtons method. The system canbe put in a compact form:

R (/, c, q, p)"0 (35)

where R is the vector of residuals. Newtons method is an iterative method where, given an initialapproximation y (0) to the solution y, the (m#1)th approximation is obtained from the mth bysolving

J(y(m) )[y(m`1)

!y(m)

]"!R(y(m)

) (36)

where J is the Jacobian matrix which contains the partial derivatives of the residuals R withrespect to the unknowns, i.e. J

ij"(LRi/Ly

j). The derivatives are evaluated at y"y

(m). The integer

m is the iteration counter. Time discretization is accomplished through a fully explicit nitedierence scheme using the following two approximations. They are derived from Taylor seriesexpansions and denote evaluation of a quantity y and its time derivative dy/dt at timet"t

n#h*t which falls in the time interval *t"t

n`1!t

n

yn`h+(1!a)yn#ayn`1 dydt K

n`a+yn`1!yn

*t(37)

The unknown variables are expressed in terms of their nodal values as

/" N+j/1

/j'j c" N+

j/1

cj'j q" N+

j/1

qj'j p" N+

j/1

pj'j (38)

where ' j are the approximation functions (basis functions) over the element. For these approxi-mations the Lagrange quadratic interpolation functions were used.

The Galerkin-weighted residuals of the system equations are formed by multiplying them byeach basis function 'i in turn and then integrating over the entire space-domain [r

!, r

0]. At each

step of Newtons iteration equation (36) can be written in an expanded form

KLR1

i/L/ LR1

i/Lc LR1

i/Lq 0

LR2i/L/ LR2

i/Lc LR2

i/Lq 0

LR3i/L/ LR3

i/Lc LR3

i/Lq LR3

i/Lp

LR4i/L/ LR4

i/Lc 0 LR4

i/Lp K

m

Kd/

dc

dq

dp K(m`1)

"!KR1

iR2

iR3

iR4

iKm

(39)



The derivation of Galerkin-weighted residuals Rji

and their partial derivatives Jij"(LRi/Ly

j)

which form the elements of the Jacobian matrix is given in the Appendix I. The set of linearequations (39) can be solved repeatedly until the iteration converges to the solution (y"/, c, q, p)of the equation set (35). Convergence is achieved when the Eucledian norm of the solution update,dy, approximately vanishes.

In parallel, since the values of variables /, c, q and p at every node and at each time step werecalculated, the partial dierential equation (32) can be solved at each time step by employinga second Galerkin nite element scheme. The unknowns of the second numerical scheme are thevalue of the function u and its rst derivative, Lu/Lr, at the nodal points. The solution isapproximated by the expression

u" N+j/1

uj'j (40)

where the basis functions ' j here are Hermite cubic polynomials. This leads to two doublenodes/element with two basis functions for interpolating the function u and other two forinterpolating the slope Lu/Lr.

Equation (32) with boundary conditions (33) and (34) can be assembled in matrix form asfollows:

[a]MuN"MbN (41)

The elements of matrix [a] and vector MbN are derived from the Galerkin form of equation (32)and they are given in Appendix II.

Finally, a non-uniform mesh with element length increasing according to a simple geometriclaw is used for the spatial discretization.

5. NUMERICAL RESULTS AND DISCUSSION

Computational results for the coupled wellbore erosion and stability analysis problem will bepresented for the set of parameters shown in Table I. With these data the process starts at timet"0 from a Darcy ow. The time factor is described by the ow rate which governs the timeevolution of porosity and consequently the lifetime of the wellbore.

The time history and the spatial proles at dierent time steps of the varying properties areshown in Figures 312. Figure 3 shows that the alteration of porosity is high close to the freesurface and it diminishes rapidly within a small distance from it. This indicates that the erosionprocess is more active close to the free surface. Such a porosity distribution results in permeabilityinhomogeneity, according to the CarmanKozeny equation (24), which in turn results in lowerpressure gradients in the eroded region (Figure 4). For the same reason the pore pressure atr"1)0 m is relatively low (Figure 4) although the porosity at this radius has not increased farbeyond its initial value (Figure 3). As we can see, the original logarithmic pressure prole for theaxisymmetric problem considered is convex and as time progresses the curvature changes in theopposite direction yielding a concave prole. The next plots, Figures 5 and 6 depict the timehistory of porosity and transport concentration at the free surface of the wellbore. The resultsshow that porosity and transport concentration increase rapidly (after the elapsed of sometime) in the beginning and as time progresses the rate drops. The transport concentration in



Table I. Input parameters of computational example

Wellbore radius ra"0)1 m

Outer boundary radius r0"5 m

Initial porosity /(r, 0)"/0"0)25

Initial transport concentration c(r, 0)"c0"10~3

Critical transport concentration c#3"0)3

Initial permeability k (r, 0)"k0(/3

0/(1!/

0)2)"373 md

Initial Youngs modulus E"2 GPaPoissons ratio l"0)3Kinematic viscosity of the uid g

k"5]10~6m2 s~1

Density of uid o&"840 kgm~3

Density of solids o4"2650 kg m~3

Wellbore uid pressure p (ra, t)"5 MPa

Far eld pore pressure p (r0, t)"8 MPa

Uniform exterior stress p0"20 MPa

Frequency j"5 m~1

Figure 3. Spatial proles of porosity at various time steps

Figure 6 would eventually converge to the asymptotic value c#3"0)3. Figure 7 depicts the time

history of ow rate. The ow rate increases as time progresses and nally approaches innity asthe permeability of the medium becomes very large.

Furthermore, erosion in the vicinity of the wellbore induce alterations in the mechanicalbehaviour of the medium (Figures 812). Figure 8 shows the spatial proles of radial displacement



Figure 4. Spatial proles of pore pressure at various time steps

Figure 5. Time variation of porosity at free surface, r"ra

at various time steps and Figure 9 demonstrates the variation with time of radial displacementand its slope at the wellbore surface. From those two diagrams, we can observe that the absolutevalue of the radial displacement and its slope increase with time. These increases cause in turna decrease in the gradient of the eective radial stress (Figure 10), and at the same time a decreasein the value of the eective tangential (or hoop) stress (Figure 11). The curves t"6800 in Figures3 and 4 have twice as many circles as Figures 8, 10 and 11 due to the two dierent levels of mesh



Figure 6. Time variation of transport concentration at free surface, r"ra

Figure 7. Time variation of ow rate at free surface, r"ra

discretization. Figure 11 shows that the gradient of the eective tangential stress decreasesmonotonically until it reaches the value of zero and then it changes sign and it increases. Sucha stress distribution in the vicinity of the wellbore results in a decrease of the stress concentrationfactor

K" p@h!p0#p

!

(42)

at the free boundary of the hole as time evolves (Figure 12).



Figure 8. Distribution of radial displacement at various time steps

Figure 9. Time variation of radial displacement and its slope at the wellbore surface, r"ra

Considering the solution of the elastic stress boundary value problem, a failure analysis of theborehole was carried out using a modied MohrCoulomb criterion (19). In the present analysis,the cohesion is related to the damage of the medium through a conceptual cohesionsofteningmodel (Figure 13). The results are demonstrated in Figures 14 and 15. The shrinkage of the failure



Figure 10. Distribution of radial eective stress at various time steps

Figure 11. Distribution of tangential eective stress at various time steps

envelope and at the same time the decrease of shear stress is shown in Figure 14. This shrinkageeect is due to the decrease of the cohesion of the medium whereas the decrease of shear stress iscaused by the relief of the stress concentration in the eroded regions around the hole. After theelapse of 3300 s, the decrease of the shear strength (cohesion) at the borehole wall is greater thanthe decrease of the shear stress and collapse of the rock formation is anticipated. This is shown



Figure 12. Time variation of the stress concentration factor at the wellbore surface r"ra

Figure 13. Conceptual cohesion-softening model, C"CM (1!/)

more clearly in Figure 15 where the time variation of the failure envelope and the stress eld at thefree boundary of the wellbore is plotted. The present analysis can be easily extended to accountfor elastoplastic deformation. In such a case the stress state will remain on the yield surface whichshrinks with increasing porosity until a critical condition for failure is met.

6. CONCLUSIONS

This paper presents a way of coupling sand erosion to mechanical damage of rock neara wellbore. Porosity which evolves in time and space, as surface erosion progresses, is chosen asthe coupling parameter. Both rock elasticity and strength (cohesion) are assumed to depend on



Figure 14. Failure envelopes and corresponding critical stresses at the free boundary of the wellbore, r"ra, at various

time steps ('"37, CM "7)5 MPa)

Figure 15. Time variation of the failure envelope and the stress eld at the free boundary of the wellbore, r"ra, ('"37,

CM "7)5 MPa)

porosity in such a way that the material becomes weaker with increasing porosity. The time factoris described by the ow rate which governs the time evolution of porosity and consequently thelifetime of the wellbore. The mathematical model, consists of erosion equations, mixture owequations and stress equilibrium equations, is solved numerically by Galerkin nite elementmethod. Based on the numerical results the following conclusions are drawn:

(1) Erosion is high close to the free surface and decays rapidly with short distance from it. Theerosion near to the free surface is accompained by changes in porosity and a signicant



permeability increase. Inhomogeneity in permeability results in dierent pressure prolesas time progresses with lower pressure gradients in the eroded regions.

(2) Erosion in the vicinity of the wellbore induces alterations in the mechanical behaviour ofthe medium. The mechanical damage causes reduction of the stress concentration factor inthis region. Furthermore, the increase of porosity results in reduction of cohesion betweenthe grains and shrinkage of the failure envelope. Since cohesion decreases with increasingporosity, the time instant at which rock starts failing and producing sand can be identied.

APPENDIX I

Discretization of the governing hydromechanical equations

The Galerkin-weighted residuals of the governing dierential equations of erosion kinetics(21)(23) are given by13

R1i"P

r0

raC

(c/)n`1!(c/)n*t

!aqn`1 CLcLrD

n`1!(1!a)qn CLcLrD

n

!/n`1!/n*t D'i dr (43)

R2i"P

r0

raC

/n`1!/n*t

!aj@(1!/n`1) Acn`1!c(n`1)2

c#3B qn`1!(1!a)j@ (1!/n)

]Acn!cn2

c#3B qnD 'i dr (44)

R3i"P

r0

raCa C

LpLrD

n`1#(1!a) CLpLrD

n!a g,k0

(1!/n`1)2/n`13

o~n`1qn`1!(1!a)

]g,

k0

(1!/n )2/n3

o~nqnD'i dr (45)In order to derive the residual of equation (25) and to utilize the appropriate boundary

conditions we lower rst the order of equation by applying integration by parts on the second-order derivative term and then we apply a similar procedure

R4i"!P

r0

raCa C

LpLrD

n`1#(1!a) CLpLrD

n

DL'iLr

dr

#Pr0

raGa C

1

r# 3!/n`1

/n`1(1!/n`1) CL/Lr D

n`1! o4!o&(1!cn`1)o

cn`1o

4C

LcLrD

n`1

D]C

LpLrD

n`1#(1!a) C1

r# 3!/n

/n(1!/n ) CL/Lr D

n! o4!o&(1!cn )o

cno

4C

LcLrD

n

D]C

LpLrD

n

H 'idr (46)



The (i-th row, j-th column) element of Jacobian is

LR1i

L/"P

r0

raC

c

*t'i' j! 1

*t'i' jD dr (47)

LR1i

Lc"P

r0

raC

/

*t'i'j!aq'i L'j

Lr D dr (48)

LR1i

Lq"P

r0

ra

!a LcLr

'i'j dr (49)

LR2i

L/"P

r0

raC

1

*t'i'j#aj@ Ac!

c2

c#3B q'i'jD dr (50)

LR2i

Lc"P

r0

ra

!aj@(1!/) A1!2c

c#3B q'i' j dr (51)

LR2i

Lq"P

r0

ra

!aj@(1!/) Ac!c2

c#3B 'i'j dr (52)

LR3i

L/"P

r0

raCa

g,

k0

/2!4/#3/4

oN qD 'i'j dr (53)

LR3i

Lc"!P

r0

raCa

g,

k0

(1!/)2/3

(o4!o

&)qD 'i'j dr (54)

LR3i

Lq"!P

r0

raCa

g,

k0

(1!/)2/3

oN D'i'j dr (55)

LR3i

Lp"P

r0

ra

aL' jLr

'i dr (56)

LR4i

L/"P

r0

ra

aLpLr C

3!// (1!/)

L' jLr

!/2!6/#3/2(1!/)2

L/Lr

' jD 'i dr (57)

LR4i

Lc"P

r0

ra

!a LpLr C

o4!o

&(1!cn`1)o

cn`1o

4

L' jLr

! (o4!o&)2[(1!cn`1)o

cn`1o

4]2

LcLr

'jD 'i dr (58)

LR4i

Lq"P

r0

raC!a

L'jLr

L'iLr

#a L' jLr

'i C1

r# 3!/

/ (1!/)L/Lr

! o4!o&(1!cn`1)o

cn`1o

4

LcLrDD dr (59)



APPENDIX II

Discretization of stress equilibrium equation

The Galerkin-weighted residuals of stress equilibrium equation (32) are formed followinga similar procedure.13 The Galerkin form is rst obtained by multiplying equation (32) with eachbasis function (Hermite cubic polynomials). Then the boundary conditions can be applied bylowering the order of equation and nally, the residuals are derived by integrating over the entirespace-domain [r

!, r

0],

Ri"C'i

LuLrD

r0

ra

#Pr0

raC!

L'jLr

LuLr

#'i LuLr

g1(r)!'ig

2(r)!'ig

3(r)

L (/p)Lr D dr (60)

with

g1(r)"1

r! 1

1!/L/Lr

g2(r)"1

r2#1

r

l1!l

1

1!/L/Lr

(61)

g3(r)" (1#l)(1!2l)

EM (1!/) (1!l)

Equation (41) can be written in the following form:

aijuj"b

i(62)

where the matrix coecients are given by

aij"P

r0

raC!

L'jLr

L'iLr

#'i L'jLr

g1(r)!'i'jg

2(r)D dr, i"1,2, N, j"1,2, N (63)

a11"a

11# l

1!l1

ra

, aN~1,N~1

"aN~1,N~1

! l1!l

1

r0

(64)

and

bi"P

r0

raC'ig3(r)

L/pLr D dr, i"1,2, N (65)

b1"b

1#(1#l)(1!2l)

EM (1!l) pa , bN~1"bN~1!(1#l) (1!2l)

EM (1!/0) (1!l) (/0p0!p0) (66)

Finally, all the involved integrals were evaluated numerically using Gaussian integration.

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3. M. G. A. Veeken, D. R. Davies, C. J. Kenter and A. P. Kooijman, Sand production review: developing an integralapproach, SPE 22792, 335346 (1991).

4. I. Bradford and J. Cook, A semi-analytic elastoplastic model for wellbore stability with applications to sanding,Eurock94, Balkema, 347353 (1994).

5. J. Tronvoll and F. Fjaer, Experimental study of sand production from perforation cavities, Int. J. Rock Mech. Min.Sci. Geomech. Abstr., 31(5), 393410 (1994).

6. R. Sakthivadivel, Theory and mechanism of ltration of non-colloidal nes through a porous medium, HEL 15-5,6,7,University of California, Berkekey, 1966.

7. R. Sakthivadivel and S. Irmay A review of ltration theories, HEL 15-4, University of California, Berkekey, 1966.8. I. Vardoulakis and J. Sulem, Bifurcation Analysis in Geomechanics, Blackie, London, 1995.9. J. Lemaitre, A Course on Damage Mechanics, Springer, Berlin.

10. R. A. Plumb, Inuence of composition and texture on the failure properties of clastic rocks Eurock94, 1320.Balkema Rotterdam, 1994.

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