SPE-31087-MS Selection of Screen slot with to prevent plugging and sand production

7/25/2019 SPE-31087-MS Selection of Screen slot with to prevent plugging and sand production

1/11

SPE 31 87

Society of Petroleum Engineers

Selection of Screen Slot Width to Prevent Plugging and Sand Production

P.

Markestad, SPE, and 0 Christie, RF Rogaland Research, and Aa. Espedal, SPE, Statoil,

and

0.

R0rvik, SPE, Saga Petroleum as;

Copyright, 1996, Society of Petroleum Engineers, Inc.

This paper was prepared for presentation at the SPE Formation D amage Control Symposium

held

in

Lafayette, U.S.A., 14-15 February, 1996.

This paper was selected for presentation be an SPE Program committee following review of

information contained in an abstract submitted by the author(s).

C_ontents

of the

pap.er

as

presented have not been reviewed by the society of Petroleum Eng1neers and.are subject to

correction by the author(s). The materi.al, as

~ r e s ~ n t e d

does not necessanly reflect and

position of the society of Petroleum Engineers, off1cers or members. Papers

p r e s e ~ t e

at

SPE meetings are subject to publication review by Editorial Committees of the

soc1ety

of

Petroleum Engineers. Permission to copy is restricted to an abstract of no.t more

t h ~ n

300

words. Illustrations

may

not be copied. The abstract should con a1n. con.splcuous

acknowledgement of where and by whom the paper was presented. Wnte L1branan SPE,

P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., f ax (+1) 214-952-9435.

bstract

A numerical model has been developed that addresses both

plugging of, and sand production through single wrapped

screens. The model was developed on the basis of a fractal

model for the particle size distribution of reservoir sands. A

database of sand types from the North Sea and Haltenbanken

areas was established. Principal component analysis was

used to reduce the number of significant variables in the

database, and to provide a basis for a prediction model for

critical slot widths. A series of laboratory experiments were

performed, and four critical slot widths were identified for

each sand type, defining a safe design interval for screen slot

width. A mathematical model was developed that can be

used to predict the critical slot widths for other sand types

from the area.

Introduction

Single, wire wrapped screens with keystone shaped wire

have been used to control sand production in oil and gas

wells since the 1930 s. They have the advantage over

prepacked screens in that they do not become p l ~ g g e as

easily by drilling mud. Furthermore they functiOn as a

surface filter, where the plugging material is easily removed,

whereas prepacked screens are depth filters where plugging

material tends to get trapped inside the prepack.

Single wrapped screens do, however, have a reputation

for being susceptible to plugging and/or sand production

when designed according to the various traditional criteria

(Refs. 1 and 2). This indicates that the design criteria for

single wrapped screen completions should be revised.

Sand control with screens is basically a function of the

relationship between particle size and screen slot width. The

pioneering work was published by Coberly (Ref.3) in 1937.

Coberly concluded that spherical particles could generally be

retained when the slot width was 2.5 times the particle

diameter or smaller. He also stated that in a mixture

of

particles of different size, the sand control properties of a

155

screen depends on the largest particles in the mixture. He

suggested that screen completions should be designed with

screen slots that were 2 times wider than the

1

of the

formation sand. He did not address the problem of screens

becoming plugged by fines from the formation sand. This

criterion has been used in California, while slot widths equal

to d

1

has been used on the U.S. Gulf Coast area (Ref. 4).

In this paper it is shown that the design criterion

suggested by Coberly, or any other criteria based on a single

point on the particle size distribution curve, can not

adequately describe either sand production or plugging of

single wrapped screen. Instead a method is developed where a

more complete description

of

the particle size distribution is

used

to

predict the plugging and sand control properties

of

single wrapped screens.

The study described in this paper has been limited to one

screen type, single wrapped screens, and erosion of the

screens have not been considered. An extension

of

the study

is

currently being planned that will include alternative screen

designs, and also compare the susceptibility of the various

screen types to erosion.

Description of the particle size distribution

In a traditional presentation of the results from a sieve

analysis, the accumulated mass percentage of particles larger

than a certain diameter is plotted on a semi-logarithmic

scale.

Since the particle distribution is plotted as a function

of

particle mass, the distribution function will emphasise the

largest particles. When the purpose is to describe plugging

of screen slots, it is more relevant to concentrate on the

smaller particles. It is obvious that a particle matrix with

zero porosity will be able to plug a screen slot completely

as long as it contains particles large enough to be retained

by the slot.

Such a particle mixture must consist of large particles

with smaller particles that fit into the pores between the

large particles, still smaller particles that fit into the pores

between the small particles, and so on down to the

molecular level (Fig. 1). Finally, mathematical speaking,

there will be an infinite number of infinitely small particles

with an infinitely small total volume. This type of particle

size distribution is described by Kaye in Ref. 5. The

function is based on the number

of

particles instead of

particle mass. The accumulated number of particles larger

than a certain diameter is described by the p ower function


2/11

2

SELECTION OF SCREEN SLOT WIDTH TO PREVENT

PLUGGING AND SAND PRODUCTION

SPE 31 87

N d;;,di)=K

r I)

In a logarithmic plot this distribution will be represented

as a straight line with the slope

f.

The constant K is a

proportionality constant that depends on the size of the

sample. It is not important for the sand properties.

This function is also called a fractal particle size

ditribution, and the exponent

f

of eq. 1) is equal to the

fractal dimension of the sand matrix.

Theoretical arguments, that are beyond the scope of this

paper, indicate that a particle distribution with

2db the

smaller grains more than fill up the pore space between the

larger grains, and one can expect the sand to be relatively

stable. Smaller grains can not move through the matrix of

larger grains, e.g. during fluid flow through the matrix.

Also,

if

dk is not too small, this part

of

the sand taken

separately will have a finite porosity and permeability since

it is filled with connected pores of approximate size

dk.

The situation is the opposite in the part with grains

smaller than dk and f2 would make a stable,

permeable surrounding for the screen. The part with f


3/11

SPE 31087

P

MARKESTAD 0 CHRISTIE AA. ESPEDAL 0 R0RVIK

3

on PCJ

and

PC

are fine and badly sorted, while sands with

high scores on

PCJ

and

PC

are coarse and well sorted.

It was found that these two principal components were

the most important ones for describing plugging and sand

control

of

single wrapped screens. The first component,

PCJ

explained 48% of the variation in the data and

PC2

explained 24%, in total 72% of the total variance for the

first 2 principal components.

The principal component analysis effectively reduces the

number of variables from 9

to 2

A total

of

5 sand types were chosen for laboratory

testing. The sand types and their database entries are shown

in Table 1. They were chosen on the basis of their scores

on PC1 and PC2 as illustrated in Fig. 2. Partic le size

distribution curves are shown in Figs. 3,4 and 5

The principal component analysis showed that there were

no

typical variation in the particle size distribution between

the various North Sea fields. Neither were there any

significant regional difference between the North Sea and the

Haltenbanken.

Experimental

procedures

Identification

of main experimental parameters.

In a typical North Sea sandstone reservoir, the variation in

both particle size and distribution is large. The permeability

often varies by a factor

of

100 within the reservoir. Thus,

design criteria that specifies one single optimum slot width

for each sand type are not very useful because it will be very

difficult to chose which sand to use

as

a basis for the design.

It would be more relevant

to

define a range of acceptable slot

widths for each sand type, and then attempt

to

select a screen

that will fit into this range for all the sand types in the

completed interval.

This approach was adopted in the present study. Four

slot widths were determined for each sand type:

d the largest slot size where severe plugging was

frequently observed.

d the smallest slot size where no plugging was

observed.

d the largest slot size where sand production did not

occur.

d the smallest slot size where continuous sand

production did occur.

The d and d slot widths should be considered as

extreme lower and upper limits that should not normally

be

exceeded, while d and d are lower and upper limits for

an

ideal screen design.

The other parameters that were recorded during the

experiments were:

Amount

of

produced sand and sand production mode

(initial, intermittent, continuous)

Permeability ratio and skin factor for each sand type, slot

width and flow rate

Nature of plugging (permanent or removable)

Particle size distribution of produced sand.

Experimental

set

up. The screen filtration experiments

were performed in a radial flow cell as illustrated in

Figs. 6 and 7. The experimental set-up consisted of an

adjustable pump, a radial flow cell representing a 22.5

section of a well with a 7 5 screen, a sand trap and a fluid

reservoir. The radial cell was fitted with 2 differential

157

pressures sensors. One measures the differential pressures

created by flow through 150 mm

of

sand pack well away

from the screen. The other measures the differential pressure

across the screen and 5 em of sand adjacent to the screen.

The positions

of

the differential pressure measuring points

is illustrated in Fig. 6.

The concept

of

using a flow cell filled with loose,

unstressed sand was chosen because it was felt that this

would represent the worst case situation for sand production.

Differential pressures and flow rates as indicated in

Fig.

7 were logged on a computer running a data

acquisition program. Sand production was measured in a

graduated cylinder placed below the sand trap.

The actual particle size distribution of the formation sand

was approximated by mixing a range

of

sands with known

particle size distribution. The cell was flooded with single

phase, synthetic seawater during all the tests since this

represents a worst case situation with no capillary forces.

Test procedures.

Each sand was tested against screens

with slot widths ranging from

100

to

800 microns.

Each test .consisted of two parts. Initially the cell was

completely filled with sand and oriented with the screen at

the bottom. In this situation the screen was always in direct

contact with the sand. This corresponds

to

a well where the

annulus outside the screen is completely filled with sand.

In the second part of the test the cell was oriented with

the screen on top, and

3 4

em of sand was removed. This

was done

to

simulate an annulus that is not completely

filled with sand.

In

this situation liquid flowing towards the

screen will fluidize the sand and lift it towards the screen.

Fluid flow through the sand typically caused some

separation

of

fine material. Both sand production and

plugging did occur more easily in this situation.

Experimental results

The critical slot widths, determined from the experiments are

presented in Table

2

Discussion

General

flow

properties.

A sand control screen will

necessarily restrict the fluid flow into the well to some

degree, even when it

is

functioning

as

intended. Intuitively,

one should expect that the degree of flow restriction would

be a function of the screen slot width, the particle size

distribution of the sand, and maybe the rate

of

flow through

the screen. This turned out not

to

be the case, however, as

the skin factor varied unsystematically between 0.0 and 0.5

for all the sands and screen slot widths. Even

if

the

permeability of the sand varied from 0.2 Darcy for All to

20 Darcy forB

19

This can not be considered

to

be a serious

flow restriction, and it can be concluded that single screen

completions will not significantly restrict well production

as

long

as

they function

as

intended.

The slot area

is

typically only 5-10% of the total screen

area. Fluid flow will converge

on

the slots, and the fluid

velocity will increase

by

a factor

of

10 20 through the

slots, depending on the slot width and the width

of

the

wrapped wire. The converging flow results in a differential

pressure that is higher than expected from the Darcy

equation, where it appears as the observed skin factor. In

this way the flow properties

of

the screen is very dependent


4/11

4

SELECTION OF SCREEN SLOT WIDTH TO PREVENT

PLUGGING AND SAND PRODUCTION

SP 31087

on the permeability

of

a thin layer

of

sand immediately next

to the screen.

If

the permeability

of

this layer is decreased by

contamination by fine particles, the skin factor will be high.

If the finer particles are produced away from this sand layer,

the skin factor will be low

or

negative. This conclusion is

supported by Runar Mfl)ller in Ref. 6. He showed that the

fines content

of

the layer of sand next to a screen tends to

become reduced with time, resulting in a small and often

negative skin factor.

Plugging

of single wrapped

screens. Plugging was

defined as a situation where the differential pressure across

the screen is more than two times as high as expected from

the Darcy equation.

Plugging

was

never

observed in the screen

down

position. In the screen up position, representing an annulus

which is not completely filled with loose sand, plugging

occurred to some degree for all the tested sand types except

for

All. The

severity

of

the plugging, and the width

of

the

slots that can be

plugged

depends on the particle size

distribution of the sand. Plugging was more likely to occur

when the flow was started suddenly at a relatively high rate.

Plugging in the screen up position was initiated by the

following mechanism: When fluid was flowing towards the

screen, the finest fraction of particles from the sand was

separated from the bulk of the sand and transported towards

the screen. These particles formed a filter cake along the

screen slots with a

much

lower permeability than the bulk

sand, restricting flow through the screen slots. This process

was typical for situations where the flow was initiated

suddenly, corresponding to a well that is brought on stream

suddenly at a high rate.

When the rate of flow was increased gradually, the first

particles that were separated from the bulk of the sand tended

to be small enough to pass through the screens. As the fluid

velocity increased, particles large enough to be retained by

the screen was lifted. But since most of the fines had already

been

produced

the resulting filtercake

did

not

have a

sufficiently low permeability to significantly restrict the

flow through the slots.

This mechanism can also explain why plugging was not

observed for the

All.

This sand has a very high fines

content, dominating the permeability of the bulk sand which

is very low. A filtercake consisting of the finest fraction of

the sand will not have a permeability that is significantly

different from the permeability

of

the bulk sand, and thus do

not reduce productivity through the screens.

The

filter

cake

that formed along the slots when they

became plugged, was generally thin and could be easily

removed.

I t

would often fall

off

by gravity alone if left

without fluid flow for some time. But some particles were

able to invade the slots and got trapped there. There are

however, no indications from the differential pressure

measurements that the trapped particles reduce the overall

flow efficiency of the screens.

Sand control properties

of

single screens. In

Table 3 the results from the present study are compared

with Coberly s criterion of

d

1

(Ref. 3) and with the

Gulf

Coast criterion of ld O (Ref. 4 .

It

is clear that sand is

generally produced through

much

narrower slots than 2

times the

d

1

of the sand.

The

d

1

diameters are very similar

for the A 11, B 10 and C31 sands,

but

the largest sand free

158

slot width, d , varied from 100

micron

for

All

to

400

micron for B 10. In all the tests the risk of sand production

is underestimated by the Coberly criterion, while the

Gulf

Coast criterion both over and underestimates it. The results

presented in

Table

3

clearly show that other parameters

than

d

1

must be important when choosing the slot width

of

sand control screens.

Prediction of critical slot widths. A mathematical

model for prediction of the critical slot widths was fitted to

the experimental data by the least squares method. Several

models, both using the principal components and various

combinations

of

the

9

original variables in the database,

were tested against the experimental data.

The

best results

were achieved with the following model

dail =

f o

fJ/1 J t J12t 2

(2)

Here dcrit is the predicted critical slot width,

{3

0

, ..

2

is a

set

of

constants, and t

and t

2

are the score values,

or

co

ordinates, on the first

two

principal components.

The

predicted values are compared with the observed values from

the laboratory experiments in

Table 4. The

difference

between the observed and predicted values are less than 50

microns which is approximately ha lf the typical step of 100

micron between two consecutive screen sizes. This indicates

that the accuracy of the prediction model is equal

or

better

than the accuracy

of

the experiments.

The

accuracy

of

the

prediction model cannot be evaluated statistically because of

the limited number of experiments. Two more sand types

have been tested to verify the model, however,

and

the

observed results are very similar to the predicted critical slot

widths.

The

predicted values for the critical

slot

widths are

plotted as a function of PC

1

and

PC2

in Figs. 8 to 11.

From

Figs. 8 and 9

one

can

see that

the risk of

screens being plugged is high for fine sands and for coarse

sands with a large fraction

of

fine material. As expected, the

risk

of

plugging the screens is low for coarse, well sorted

sands. But the risk of plugging is also reduced for fine sands

with a high fines content. This is

maybe

surprising, but it

means that the original permeability of these sands are so

low that it is in the same range as the permeabi lity of the

filter cake.

In

Fig. 10 one can

observe

that sand control is a

function both of the particle size and the degree of sorting

and content of small particles. For fine sands, a low score

on PC2, indicating a badly sorted sand with a lot of fines,

will increase the risk

of

sand production. But for coarse sand

with high scores on

PCI

a low score on

PC2

will reduce

the risk of sand production.

Once the data from the principal component analysis and

the set

of

constants (from eq. 2) for the critical slot widths

are known, the prediction model is easily implemented in a

standard spreadsheet. A simple, user-friendly

computer

program for the design of screen slot widths is currently

being developed.

Screen

design. The screens

for

a completion will

typically have to be ordered long before the well is actually

drilled through the reservoir, and the screen slot width will

have to be based

on

samples from other wells in the area.

For

this reason, the ideal solution

would

be to identify a


5/11

SPE

31087

P MARKESTAD

0.

CHRISTIE AA. ESPEDAL

0.

R0RVIK

5

typical screen slot width in advance that could be used for

the whole reservoir with small variations.

A possible method to achieve this is presented in

Fig.

12.

Here the critical slot widths are plotted for all the

samples from Field A. Screen slot width design is then a

matter

of

drawing a straight, horizontal line through the

graph that intersects the critical slot width curves

as

seldom

as possible. A possible solution is shown in Fig.

12.

By this method one can:

Find the optimal screen slot width for a reservoir or part

of

a reservoir.

Identify

sand

types that are well suited to screen

completions.

Identify sand types that may cause problems for the

chosen screen size.

From Fig.

12

one can see that the All and A12 may

cause sand production through the suggested slot width of

250 micron. In this case it is known from the laboratory

testing that the All sand will require a 100 micron screen,

and it may be necessary to reduce the screen size across the

A11 sand. In general, when such potential problem sands are

identified, it will be necessary to go back and study the cotes

and find out whether these sands are typical for the reservoir,

whether they strong or weak, and whether they are likely to

produce at all.

Conclusions

1. No sand types have been identified during the reported

work that are not suited to screen completions. For all the

sand types tested it has been possible to identify an interval

of

screen slot widths that will neither be plugged nor

produce sand. The width

of the design interval varies as a

function of the particle size distribution of the sand.

2. A well functioning screen represents a skin factor

of

less than 0.5.

3. The risk

of

sand production is increased in a situation

corresponding to an open annulus, partially filled with sand.

Plugging

of

screens by formation sand has only been

observed in this situation.

4. The risk

of

plugging the screen is decreased when the

fluid flow velocity through the screen is increased gradually.

This corresponds to bringing a well on stream slowly.

5. Design crite ria for screen slot width based on one

single point on the particle distribution curve can not

accurately predict neither plugging

of

the screens nor sand

production through the screens.

6. By introducing a fractal description for the particle

size

distribution of

the formation sand,

and

using

multivariate analysis, it has been possible to develop a

quantitative method for design of screen slot widths. The

method identifies a safe interval

of

slot widths where

plugging and sand production are not likely to occur.

7. The prediction model is applicable to sands from the

North Sea area and Haltenbanken, and can easily be extended

to other areas.

8. A method is proposed, where the prediction model

can be used to design screen completions for specific

reservoirs or parts

of reservoirs.

Nomenclature

159

dx = x -percentile diameter is here defined

as

the

theoretical sieve size that will retain

percent of the particles by weight.

N d d . = number of particles

l

K

= proportionality constant

f

=

exponent

of particle size

distribution

function (and fractal dimension

of

sand

matrix)

el l

d4ofd90

dcrir=

critical slot widths

d , d_, d ord++)

0

,

12

=

constants in the prediction model

ti

= score value,

or

co-ordinate, on principal

component i

cknowledgements

We would like to thank Saga Petroleum as and Statoil for

the permission to publish the material; Bjarne Aas

of

RF

Rogaland Research for helpful review comments; and Jorunn

0vsthus

of

RF for her accurate laboratory work.

References

1. Penberthy, W. L. and Shaugnessy, C. M.: Sand

Control,

SPE, 1992

2. Sparlin, D. D. and Hagen, R. W.,

Selection and

design of sand control methods,

Course Manual,

ICCI, 1991

3. Coberly, C. J.: Selec tion

of

screen openings for

unconsolidated sands ,

Drill.

and

Prod. Prac.,

API,

1937.

4. Suman, G.

0.

Ellis, R. C. and Snyder, R. E.,

Sand

Control

Handbook,

Gulf Publishing Company,

Houston, 1985

5. Kaye, Brian H., Fractal dimensions in data space;

New descriptors for fineparticle systems,

Particle and

particle system characterization,

Vol4 No 10, 1993

6. M ISller, R.:

The influence

of

formation grain size

distribution on production through a

sand

screen,

Thesis, Stavanger College, 1994


6/11

6

Table

Sand

A11

810

C31

815

819

SELECTION OF SCREEN SLOT WIDTH TO PREVENT PLUGGING

AND SAND PRODUCTION

1 - Characterisation of the particle

size distribution

of the

d 10

[micron]

213

219

249

475

491

d 40

d 50 d 90

ell

f1

f2

[micron] [micron] [micron]

109 89

38

2.91

-3.22 -8.86

136 126 68 1.99

-1.12 -6.49

210 197 131

1.61

-0.45

-9.23

340 316 169

2.01

-2.20 -7.99

353 329 197 1.79

-0.78 -8.41

Table 2- Experimentally determined

critical

slot widths micron)

Sand

d

d

d

d

A11

0*

100 100 200

810

100 250 250 300

C31

0*

200 400 600

815 200 300

600 800

819

0*

100 500 800

were set equal to 0 when severe plugging o the 100

micron slot was not observed.

Table

3-The experimental

data

compared

with

the Coberly

and Gulf Coast criteria

Sand

d1o

2d o

d

d

Gulf

Coast)

Coi:JOOy)

A11 213

427 100 200

810

219 439 250 300

C31

249 498

400 600

815 475

949 600 800

819 491

982 500 800

160

tested

lnt1

[micron]

173

104

152

379

306

SPE

31087

sands

ln t2

[ ]

18.47

60.87

84.55

29.25

59.59


7/11

SPE 31087

P.

MARKESTAD,

0.

CHRISTIE,

AA.

ESPEDAL, 0. R0RVIK

Table 4 Predicted critical

slot widths

compared with experimental

results

d

d

d

d

Sand

Measured Predicted Measured Predicted Measured Predicted Measured

Predicted

A11

810

C31

815

819

values values

values values values

values values values

0

6 100

100

100

93 200

185

100 79

250 249

250

273

300

351

0

21

200 201

400

377

600 550

200

205 300

300

600

594 800 787

0

-11 **

100 100 500

512 800 827

d

were set equal to 0 when severe plugging

of

the 100 micron slot was not observed.

Negative slot widths are artefacts

of

the prediction model.

C31

++

q.

t '+

++

++

+

:\:

/ 8 1 9

::t

N

u

ll

t

++

t

-1

++

815

2

3

A11

4

4

2 4

PC1

7

-1

2

3

4

Fig. 1 lllustration

in

2 dimensions of fractal

particle

structure with porosity

equal to zero.

Fig.2 AII the sand types in the database plotted

by

their scores on PC1

and

PC2

161


8/11

8

SELECTION OF SCREEN SLOT WIDTH TO PREVENT PLUGGING

AND SAND PRODUCTION

SPE 31087

A 11

100

100

10

10

0.1

0.1

z

0.01

L--- ' ------ ' --L-. .L. . . . . J .. . . . .L. . .L. . J . . . .L_---- --- ' --- ' - . . . . . .. .__. . . . . . ._L.J .. . J 0.001

10

100

1000

Particle

size

micron)

Fig. 3-The fractal particle

size

distribution

curve for the

A sand.

819

100

n

10

10

i

0

0.1

0.1

e

0.01

I

z

0.001

L _ _ _l____j__L_..J........I.....L...J....Ll...-_ ___..L._L-...JL....L.....J.....J.....LU 0.0001

100

1000

Particle size mi cron)

Fig. 4-The fractal particle size distribution

curve for

the 815

sand.

100

100

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ -

80


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Documents

SPE-31087-MS Selection of Screen slot with to prevent plugging and sand production