7/24/2019 PETSOC-93-01-04-P

1/7

AN EXPERIMENTAL EVALUATION OF VISCOSITY GRADING

FOR CONTROLLING FINGERING IN MISCIBLE

DISPLACEMENTS

H.K. SARMA B.B. MAINI

this article begins on the next page FF

ic -P 7,73 ENHANCED OIL RECOVERY An experimental evaluation of viscosity grading for controlling fingering in miscible displacements HEMANTA K. SARMA and BRIJ B. MAINI Petroleum Recovery Institute ABSTRACT Displacement of oil by miscibleflooding processes is adversely affected by the hydrodynamic instability at the flood front whenever the viscosity ratio is unfavourable. Instabilities cause viscousfingers to grow and make the displacement process in- efficient and uneconomical. Therefore, it is necessary to devel- op ways of reducing or eliminating the detrimental effects of viscous

instabilities to make miscible flooding processes economically viable under adverse viscosity ratio conditions. Grading the viscosity of the displacing j7uids appears to be an effective technique for controlling viscous instabilities. This paper presents an experimental evaluation of the effi- cacy of graded-viscosity solvent banks for improving the per- formance ofsolventflooding under very adverse viscosity ratios. A scaled physical model with transparent top surface was devel- opedfor visual examination of viscousfingers during solvent floods. Base case displacement experim ents carried out to quantify the effects of viscosity ratio andflood velocity on the development of viscousfingers and the resulting displacement efficiency. The displa cement experiments were then repeated with graded-viscosity solvent slugs to determine the effective- ness of such viscosity grading. Tw o different methods of design- ing graded-viscosity banks (the Claridge and the Cos- kuner-Bentsen methods) were examined. Results show that the extent of viscousfingering present and the resulting displacement efficiency, in the base case displace- ments, varied significantly with viscratio andflood veloc- ity. The use of graded-viscosity banks always reduced, and often, totally eliminated the viscous fingering. In this study, the viscous target oil was used as an additive for increasing the solvent viscosity. Forf ield application of the graded-viscosity bank process, less expensive additives for increasing the viscosity of the solvent will be needed. Until such additives become available or the procedure can be improved upon, the use of graded-viscosity banks in miscible solventflood appears to be uneconomical Introduction Miscible flooding has received

considerable attention in recent years because of its inherent potential to achieve a higher recov- ery efficiency. A 1989 survey by Thomas et al.(') suggests that miscible flooding was the most popular method with operators in the United States during 1980-1981. However, because of the high material and operational costs the process has become less attractive at the current uncertain oil prices. Paper reviewed and accepted for publication by the Editorial 36 Although the miscible displacement process is not consider- ed an obvious choice for heavy oils, it may have some poten- tthin heavy-oil reservoirs where thermal methods are not suitable because of excessive heat losses to the underlying and _ overlying strata. However, in the case of miscible floods, high viscosity ratios between the oil and solvent often result in vis- cous instabilities at the front, and lead to a lower recovery effi- ciency. Therefor e, it is important to minimize the extent and effects of viscous instabilities. Fingers in a miscible flood tend to generate a transition zone at the interface due to mass transfer. The transition zone, whose composition exhibits continu ous gradation

viscosity due to blending, arrests finger ing of displacing fluid to an extent(3). However, in the field it is unlikely that such zone will dev elop at the initial stages of the solvent injection, and as a conse - quence, severe viscous fingering may occur. In such situations, a graded-viscosity bank ca n be used to minimize viscous finger- ing. Lake(l) suggests that the concept of graded viscos ity can become economic only when the mobility ratio is very hig h and/or the formation is very heterogeneous. Liter ature Review Literature on viscous instabili ties in immiscible and miscible dis-placements is quite extensive. Homsy(4) has presented a com- prehensive state-of-the-art review of the important papers on this subject. However , the literature on the use of graded-vis- cosity bank is limited partly because its use has remained con- fined primarily in polymer floods. So far there have been only a few field-scale pilot projects using graded-viscosity banks(i-1) . The first experimental study of the concept of graded- viscosity bank is credited to Weinaug and Ling(l) in 1959 who performed Hele-Shaw type experiments using a quarter of five- spot type model. Later,Slobod and Lestz('o) reported that graded-viscosity banks could be used to minimize viscous fingering, and suggested the use of a continuous blended zone rather than step-wise viscosity gradation to completely elimi- nate viscous fingering. Perrine (11,12) proposed a stability theory to optimize the sol- vent recovery of oil by defining the limiting conditions required for a stable miscible displacement and estimated the minimum size of the solvent slug required. Later, Kyle and Perrine(13) conducted an experimental study to validate Perrine's theory and concluded that the

concentration-gradient criterion pro- posed in Perrine's theory was an order of magnitude higher than what was observed in experiments. Board of 'Fhe Iournal of Canadian Petroleum Technology. The Journal of Canadian Petroleum Technology

7/24/2019 PETSOC-93-01-04-P

2/7

-1cP793 o/

ENHANCED

OIL

RECOVERY

An

exp,erimental evaluation

of

viscosity

grading

for

controHing fingering

miscible displacements

lUll

HEMANTA K SARMA

and BRIJ

B MAINI

Petroleum Recovery

Institute

BSTR CT

Displacement

o

oil

by

miscible

flooding

processes is adversely

afJe(,ted by the hydrodynamic instability at the

flood from

whenever the viscosity ratio ;s unfavourable. Instabilities cause

~ i s c o u s fingers to grow and make the displacement process in

efficient

and

uneconomical. Therefore, it is necessary to devel

op ways

o f

reducing or eliminating the detrimental effects

of

viSCD IS instabilities

to make

miscible

flooding

pro esses

economically viable under adverse viscosifJ ratio conditions.

Grading the

~ I i s c o s i t y

o he displacing fluids appears fO be an

effective technique jor controlling viscous instabilities.

This paper presents an experimental evaluation

o

Ihe effi

cacy o graded-viscosity solvent banks for improving the per

ormance ojsolvent flooding under very adverse viscosity ratios.

A scaled physical model with transparent top surface was devel

oped Jor

visual examination o viscous fingers during solvent

loods. Base case displacement experiments were carried out to

quantify the effects

o j

\ iscosity ratio and flood velocity on the

development o viscous fingers

and

the resulting displacement

efficiency. The displacement experiments were then repeated

with graded-viscosity solvent slugs to determine the effective

ness 0/ uch viscosity grading. Two different melhods

o

design

ing graded-l- ;scosity banks (the Claridge

and

the Cos

kuner-Bentsen methods) were examined.

Results show that the extem

o

viscousjingering present and

the resulting displacement efficiency, in the base case displace

ments, varied significantly with \ iscosity ratio andf/ood veloc

ily. The use

o

graded-viscosity banks always reduced. and

often, totally eliminated the viscous fingering.

In this study, the viscous target oil was used as an additive

or

increasing thesolvellt viscosity. For field application oj he

graded-viscosity bank process, less expensive additives

for

increasing the viscostty o he solvent wilJ be needed. Until such

additives become available or the procedure can be improved

upon, the

lise of

graded-viscosity banks in miscible solvent

flood

appears to be uneconomical.

Introduction

Miscible flooding has received considerable atten tion in recem

years because of its inherent pOlential to achieve a higher recov

Although the miscible displacement process

is

not co

ed an obvious choice for heavy oils, it may have some

tial in thin heavy-oil reservoirs where thermal methods

suitable because

of

excessive heat losses to the underly

overlying strata. However, in the case

of

miscible nood

viscosity ratios between the oil and soh em orten re .ul

cous instabilities

at

the front, and lead to a lower recove

ciency. Therefore, it is

important

to minimize the ext

effects of viscous instabilities.

Fingers in a miscible flood tend to generate a transiti

at the interface due

to

mass transfer. The transition zone

composition exhibits continuous

gradation

in viscosity

blending, arrest'S" fingering

of

displacing fluid to an e

However. in the field it is unlikely that such zone will

at

the initial stages

of

the solvent injection, and as a

quence, severe viscous Fingering may occur. In such situ

a graded-viscosity bank can be used to miOimize viscous

ing. Lake(J) suggests that the concept

of

graded viscos

become economic only when the mobility ratio i ;;

Vc

and/or the formation is very heterogeneous.

Literature lReview

Literature on viscous instabilities in immiscible and misc

placements is quite extensive. HomsyH) has prescnted

prehensive state-of-the-art review of the important pa

this subject. However, the literature

on

the lise of grad

cosity

bank

is

limited

panly

because its use has remain

fined primarily

in

polymer floods. So far there have be

a fel,.\ field-scale pilot projects using graded-viscosity ban

The first experimental study of the concept of

viscosity

bank

is

credited to Weinaug and Ling(9) in

19

performed Hele-Shaw type experiments using a quarter

spot

type model. Later, Slobod

and Les[z(lO)

reporte

graded-viscosity banks could be used

[

minimize

fingering, and suggested the use of a continuous blende

rather than

step-wise viscosity gradation to completely

nate viscous fingering.

Perrine(l),12) proposed a smbility theory [Q optimize

vent recovery

of

oil by defining the limiting conditions r

for a stable miscible displacement

and

estimated the mi

7/24/2019 PETSOC-93-01-04-P

3/7

In 1963, Koval(l4) proposed the K-factor method for pre

dicting the performance

of

unstable immiscible displacements

in heterogeneous media by combining the effects

of

a

number

of

factors, such as: viscosity ratio, channelling and longitudi

nal dispersion_ The K-faclor

is

essentially the

product

of two

other factors: the heterogeneity factor (H) and effective viscos

ity ratio (E)-

A heterogeneous system as defined by Koval is the one in

whrch the recovery

at

one pore volume solvent injected in a

matched viscosity flood is less

than

991lJo _According to Koval,

the heterogeneity factor is a function

of

dispersion, channel

ling and the length

of

the system_ Empirically. he observed that

the shorter the length

of the

core, the higher is the heteroge

neity factor_In a recent study , Menzie(l5) a bserved

that

the H

factor was higher for rocks with higher dispersivity_

When the transition or mixing zone develops at the inter

face, the viscosity of the oil changes significantly, and in most

cases, the viscosity decreases. Thus, in effect, the oil is displaced

by a mixture

of

the solvent

and

oil with a viscosity intermediate

between

that of

the pure solvent and

pure

oiL Koval's effective

viscosity ratio takes this into account when calculating the vis

cosity ratio between the displaced oil and the displacing oil

solvent mixture_

For

systems with minimum effects

of

disper

sion and channelling, the empirrcal relationship between E and

V is given by:

o

-I-

E (0_78

+ 0.22

V )

. . . . . ._. __

._ .. _.

__

._.

__

._._._._ .. _ . _._ . . . . _ . _

...

(I)

The

solvent mixture compositions

of 781170

oil and 221lJo solvent

were chosen arbitrarily as such compositions gave the best fit

of

experimental data_

Using Koval's K-factor metho d repeatedly for each viscos

ity zone, Claridge('6) proposed a method for designing graded

viscosity banks to minimize viscous fingering in linear systems

and suggested a criterion

for

the minimum size

of

a continuously

graded bank:

TABLE 1. Typical scaling parameters

for

Tellus320

oil

Paramelers

Prolotype Model

L,

(m)

150.0 0.50

L,

(m)

60.0 0.20

h

(m) 3.0

0.Q1

( ,I ,)

1406.3 1406.3

k ( m')

3.0 159.0

v (m/day)

0.1

3.6

K, (m /s)

1.63E6 1.96E7

K,

(m /s)

5.14E-8

6.17E9

Objectives

The objectives

of

this paper were to study

how

viscous instabil

i[ies affected cumulative oil recoveries in miscible displacements

of

viscous oils, and to detennine the efficacy of viscosity graded

slugs in suppressing the detrimental effects of viscous instabil

ities_

To meet these objectives, the following tasks were

undertaken_

1_

Derive a set

of

scaling criteria and use these criteria to design

a scaled physical model to carry out flow-visualization studies

of

the displacement process_

2_ Conduct

displacement experiments using three different

methods: (a) pure solvent flood. (b) Claridge method and

(c) Coskuner-Bentsen method_

3_

Compare

and evaluate experimental findings from the three

methods.

Scaling Criteria

In order to make this study more relevant to field situations,

scaled laboratory experiments were designed with following

scaling criteria. These criteria were derived

on

the basis

of

the

dimensional and inspectional analyses_

~ ~ P = (1

-

IIMD-44)

--.--------.,----.------------------.------------ (2)

Geometric Similarity

For a very large M, the size

of

the continuously graded

bank

is nearly equal [ 0 one pore volume_

According to Claridge, if 0,

I,

2, ... were the successive slugs

of

the displacing fluid with descending viscosities away from

the oil, the size

of

a panicular slug can be estimated by using

the following equation:

[Mo

M

o

- I]

s1ug

= -'------ ------'''----_''----

_________

.

________

. .

____ (3)

[ Mn

l\:l

n

_

1

___

M2, lvl

l

Mo] 2

Coskuner and Bentsen(17) proposed a stability theory for

miscible displacement processes. Using

Chuoke's

unpublished

work reported by

Gardner

and Ypma(lB) to account for the

boundary conditions at side walls

of

the porous medium, they

demonstrated that this theory could be used to predict the initial

length and the upper and lower bounds

of

the transition region

between the solvent and oiL

Upper bound:

.4k [ ~ d . ~ g ]

h

2

L2

JlZ = _ . . . : : . k _ . : : d ~ c _ ~ d ~ c - - - -

---:-:---- :-:-___________

. .

(4)

p rp

7r1 KI l +

Lower bound:

.k ~ ~ _ d ~ g ]

k de de

.;.z

hl L2

,

.. _ . _ .... (5)

h

2

+ L2

,

Although Coskuner and Bentsen validated their theory by using

data from the literature, they did

not

report

any

experimental

validation of their own_

January 1993, Volume 32, No_ 1

L,

[ L ] Model

,

= [ ~ ] ProLOlype

---.--. ,--.--------. . ----.----

(6)

,

Viscous Ratio

[ ]

Model =

[ ~ ]

Pr010L}'pe -----------------------.------

(7)

lis

Ratio of Viscous to Gravity Forces

Lx II 11

[ k

g

h

lip

-------':........:...::_ _ ._._

..

_

.

_ (8)

] [

Lxvpo ]

Model

k

g

h o lt.p Prololype

Longitudinal Dispersion Effects

K,

[ ]

Model

L,

V

K,

[

L

V ] ProlOlypc ------------------------

(9)

Transverse Dispersion Effects

[

K, L,

v Ll

] Model

[

K,

L,

v

Ll

] ProtOlype ----------------------

(10)

For

the sake of ilIus[ration, a set of typical scaling paramete rs

for Tellus-320 oil are presented in Table

1.

37

7/24/2019 PETSOC-93-01-04-P

4/7

7/24/2019 PETSOC-93-01-04-P

5/7

7/24/2019 PETSOC-93-01-04-P

6/7

7/24/2019 PETSOC-93-01-04-P

7/7