GEOL. CROAT. 49/2 243 - 246 4 Figs. ZAGREB 1996 

Scientific note 

Dry Gas Injection for Miscible Displacement on 
Zutica Oil Field 

Tvanka JUTTNER 

Key words: Gas injection, Mi scibility condit ion, 
Enhanced recovery, Di sp lacement, Vaporizing gas 
drive, Multiple conlacL 

Abstract 
By the process of gas injec tion under mi scible conditions, th e 

(alai oil recovery also includes vaporized hydrocarbons from the 
residual immobile oil, in addition (0 (he oil produced by direct dis ­
placement. The process is complex and involves Ihe innucnce of the 
interaction of extracted hydrocarbons and in-situ oil at the displace­
ment fronl. Therefore, the final oil recovery under miscible cond it ions 
is higher than th e "conventionally" displaced oil. Methods of calcu­
lating multiple contacts miscibility with an Equation of State (EOS) 
determine the miscibility condit ions by simula tion processes as a 
vaporizing gas drivc or cond ensi ng gas drive. The aim of this study 
was 10 dctcmlinc a degree of miscibility. 

I. INTRODUCTION 

When a fluid is injected into a reservoir to displace 
oil towards the production well, oil recovery is not 
completc. One method to improve recovery is to reduce 
or suppress the interfacial tension between the oil and 
injeetcd nuid. This happens when injected fluid, hydro­
carbon gases for example (dry natural gas, mainly 
methanc, carbon -dioxide) are miscible with oil (BLA­
CKWELL cl al.. 1959; BENHAM cl al.. 1960; STAL­
KUP. 1984). 

2. METHODS AND RESULTS 

The aim of this research was to simulate the process 
or oil production in the Zutica field by maintaining 
reservoir pressure, and to define process characteristics 
(miscibility or immiscibility condit ions). To simulate 
the process, a unidimcntional rcservoir sim ulator 
COMP3 was uscd (Scienti fic Software Intercomp). A 
9-component system was required in the formulat ion of 
fluid composition (adjusted to PR £OS). It was also 
assumed that the pore space contains only sat urated oil 

Faculty of Mining, Geology and Petroleum Engineering , Un iversity 
of Zagrcb, Picroltijeva 6, HR- IOOOO Zagreb, Croatia. 

PROCEEDINGS 

Kljucne nJeCl: utiskivanje pUna, uvjeti mtJcSanJa, 
povecanje iscrpka, istiskivanj e, otparavanje u plin­
sku fazu, visekontaktni proces. 

Sazetak 
Pri proccsima uti skivanja plina ukupni iscrpak nafte ukljllcllje 

osim !laftc proizvedene izravnim istiskivanjem i ugljikovodikc 
otparene iz zaostale, nepokrelnc nafte. Mehanizam procesa je slo~.cn i 
obuhvaea cfckte interakcijc olparcnih ugljikovodika i nafte na frontll 
ist iskivanja. Zato je konacni iscrpak nane LIZ proccs mijcsanja veei od 
"konvcncionalno" isti sllulc i olparene naftc. 

Metodc racunanja visekontaktllog olparavanja s nckom od jed­
lIadzbi stanja odrcduju uvjcte mijdanja simulacijom proccsa; meha­
nizmom olparavanja iIi mchanizmom kondenzllcijc. Cilj si mulacijc 
procesa proizvodnje nanc polja Zutica rcZimom podrzavanja slojnog 
tlaka bio jc odrcdivanje karaktcra proccsa, Ij. stupnja priblil,avanja 
uvjctima mijcsanja izral.enog velicinom iscrpka naftc. 

(the cr iterion for iluid mobility calculated by mUltiple 
contact vaporization), and does not consider either the 
influcnce of petrophysical heterogeneities of the reser­
voir rock, or the viscous fingering of Iluid. In fact, only 
the thermodynamic aspect of the process is investigated. 

The sequence of calculations is correlated with the 
practical procedure of oil production which consists of: 

I. Gas injeclion (SLOBOD & KOCH. 1963; CAU­
DLE & DYES, [958) into a reservoir until a certain 
pressure is attained, and 

2. Maintenance of constant reservoir pressure with a 
given valuc of oil production. Part of the injected dry 
gas is dissolved in the reservoir oil and the cons(> 
q uenccs are: 

• increasing oil volume (oil-swell); 

• changes in phase composition, density and viscosity 
of oil; 

• changes in composition and density of equi librium gas 
phase. 

Changes of these properties of Zutica oil calculated 
for various prcssures ranging from the initial saturation 
pressure Pbi :::: 128.5 bar up to 200 bars are shown in Fi g. 
L In this particular case, injected dry gas is poorly dis­
solved in already saturated oil and swelling of oil is 6%, 
while oil density decreases by 3% from 0.687 10 0.665 
g/cm3

. Simulation of the dynamics of displacement pro-



244 Geologia Croatica 49/2 

1.08 
! 

0.70 

, I v-----Y density r I • 

~ 
_________ swem,g facW / 

1.06 

0.69 

"E 

~ 

.~r 
'0 

'0 
> 1.04 " • 
~ 

0.68 

0.67 

/ -

15 
?: ." 0 
0 
u 

1.02 

/ 1.00 
125 150 175 

gas injection pressure (bar) 

cess at set constant pressure or gas injection has already 
been performed unde r var ious pressure va lues. Typical 
curves of oil recovery versus volume of injected gas are 
shown in Fig. 2. Oil recovery is not a distinct function 
of inj ection pressure , therefore the oil recovery at a 
pressure or 135 bars (pressure close to actual reservoir 
condi ti ons) is a little lower than oil recovery at 180 bars 
pressure. Gas injection of one pore volume (P.Y.) at a 
pressure of 135 bar gave a recovery of 5 1 %, while at a 
pressure of 180 bar the recovery is 55% of the original 
oil in place. 

A practical method of defining a characteristic mul­
tiple contact miscibility between injected fluid and oil 
is the "slim-tube" test from which the minimum misci ­
bi lit y press ure (MMP) was de termined . Accord ing to 

100 

~v 

200 

0.66 

0.65 Fig. 1 Changes in properties of 
reservoir oi l during gas injec­
tion (after JUlTNER, 1995) . 

the c riterion (YELLING & METCALFE, 1980) the 
minimum miscibility pressure (MMP) is that particular 
gas inject ion pressure when 1.2 P.V. of injected gas dis­
p laced ovcr 90 % of present oil. Results of the "slim­
tube" tes t are shown in Fi g. 3 whi ch showcs that the 
miscible conditions in the sys tern (saturated oil Zutica -
methane, respectively dry natural gas), can be achieved 
only after application of very high pressure (MMP of a 
sys tem is 500 bar). In other words, in oil product ion 
from the Zutica fi e ld, th e regime of press ure mainte­
nance should make modest contri butions by the mecha­
nism of multiple contact vaporization to oil displace­
ment, since in the interval of rea l applicable press ures 
of gas injection the process will proceed undcr immi sci­
bl e conditions. 

80 
I 0------0 injection pressure", 135 bar I ................. , ................. I I · ··············· ······ · ···~ 

,: 
"- 60 E 
g 
;J! 

" 0 
> 
0 

40 f-- .... u 
~ 

'0 

20 C-.. .. 

o 
o 

......................... 

0.5 

I 0----0 injection pressure", 180 bar I 

1.0 1.5 

amount of gas injection (P.V.) 

Fig. 2 Oil recovery by multiple -
2.0 conlac\ process Caner JUT­

TNER, 1995). 



Ju1tner: Dry Gas Injection for Miscible Displacement on Zut ica Oil Field 245 

100 I 

-----~ 

~ ..-------
/ 

:....----80 

V 
/ :> 

0: 60 

/ 
/ 

/' 
V 

;f. 

'" • > 
0 
u 
~ 

'0 40 

I il recovery at 

20 

o 
100 200 300 

gas injection pressure (bar) 

Quantitative dynamic changes of composition with 
dependence on quantity of injected gas is shown in Fig. 
4. The enrichment of dry gas with for example ethane is 
about 80% at the first contact and aboltt50% at the 16th 
contact (which equals the total injected volume from 
0.8 P.V.). It must be noted that these data relate to 
processes of gas injection into saturated oil. The gas 
cap in the reservoir has not been considered. In this par­
ticular process, composition of lhe existing gas cap 
affects the phase equilibria and final composition of the 
produced gas phase. 

3, CONCLUSION 

By processes of gas injecl ion into a reservoir, the 
composition of fluids in the area near the critical point 

;f. 
15 

100 

80 

E 60 
o 
15 
c 
.2 
1i 
E 40 
• u 
c 
o 
u 

20 

o 

-

I 

-

I 

r--

r--
I 

-Ia 
D 

r--

1 1 I I 

II I I 
11 

c, co, 

components 

'. at injected gas I 

4DC 500 

Fig. 3 Determination of minimum 
miscibili ty pressure ( MMP) 
("s li me-tube" test) (afte r JOT­
TNER, 1995). 

distinctly diffe r from the composition of the original 
reservoir fluid. Therefore it is necessary to create a 
good model of a fluid from 9 to 15 components adjust­
ed with one Equations of State (EOS). 

To produce more oil, the pressure in the reservoir 
must be maintained by injecting another fluid. Oil dis­
placement in the Zutica oil field by maintaning reser­
voir pressure by gas injection at an actual pressure of 
130 bar occurs under immisci ble conditions in accor­
dance with the expected phase behaviour of a methane­
oil system, because the minimum miscibility pressure 
(MMP) of injected gas in reservoir oil , determined by 
the "slim-tube" test, is about 500 bar. If the process 
should be performed at higher pressure (up to maxi­
mum possible reservoir pressure of 200 bar), it cannot 
be expected to produce a greater contribution to misci-

10 

injected gas I 
after 1st contact 

afte r 15th contact r-- 8 

...... 

6 

. . . .. . 

4 

2 

III . o 

;f. 
15 
E 

J 

" " 
£ 
15 
c 
0 
~ 

~ 
E • u 
c 
0 
u 

Fig. 4 Composition of equilibrium gas 
(after JOlTNER, 1995). 



246 

bility displacement in the LOtal production. At the pre­
sent production regime, injection of the dry gas leads to 
enrichment of gas phase by multiple contact vaporiza­
tion of liglll hydrocarbons. Low volumes of methane 
dissolve in the oil in place, so changes in the propcI1ics 
of the sa turated oil arc indistinct. The contribution of 
multiple contact mechanism of hydrocarbon vaporizing 
in \olal oil displacement is negligible. 

Acknowledgment 

Thi s work was supported in part by Ministry of Sci­
ence and Technology of the Repub lic of Croatia (Pro­
ject 2-16-230: " Improvement of Hydrocarbon Recovery 
by Gas Injection Processes"), 

Gt:ologia CrOiltiCii 49/2 

4. REFERENCES 

BENHAM, A.L., DOWDEN, W.E. & KUNZMAN , 
W.J. (1960): Miscible fluid displacement prediction 
of miseibility. - Trans. AIME, 219, 219-237. 

BLACKWELL, R.l., RAYNE, 1.R. & TERRY, W.M . 
(1959): Factors influencing the e fficiency of misci­
ble displacement.- Trans. AIME, 216, 1-8. 

CAUDLE, B.H. & DYES, A.B . (1958): Improving mis­
cible displacement by gas-water injection.- Trans. 
AIME, 213, 281 -284. 

lUTTNER, l. (1995): Proces optimalization for recov­
ery improvement by gas recycl ing in oil reservoir 
with gas-cap .- Unpublished PhD Thesis, [-'acuity of 
Mining, Geology and Petroleum Engineering, Uni­
versity of Zagreb, 122p. 

SLOBOD, R.L. & KOCH, H.A. l r. (1963): High pres­
sure gas injection-mehanism of recovery increase.­
Drill. and Prod. Prac., 20, 82-96. 

STALKUP, F.l. lr. (1984): Miscible dispiacement.­
Monograph Series, L. H. Doherty Memorial Funf. 
of AiME SPE, Dallas, 8, 2-135. 

YELLING, W.F. & METCALFE, R.S. (1980): Deter­
mination and prediction of CO2 MMP. - 1. Petr. 
Techn., 32/1,160-168.