EFFEcTS oF co2 GAs IN GEoTHERMAL WATER ON WELL CHARACTERISTICS
x.ri,qs,qNt', Eri MIYAZAKIT and Ryuichi ITolr
I
Depaltrnent ol Earth Resources Errgineering.
Faculty of Engineeling, Ki,ushu University. FukLroka 812-8581. Japan
ABSTRACT
A wellbore moclel
is
usefl
pressure ancl temperature
to examine the efects of CO2 conlenl in the vlater on well characteristics
in a v,ellbore. The equations ttsed .for CO2
irt the vvater
ctnd
profiles of
are the same as introduced by
Stttton (t976). The process in the wellbore is assunted to be isenthalpic. The well is assumed to be of trnifornt
cliameter and vertical with a single feed :one at the botlom. The vlater containing higher CO2 gas concentrcttion
starts Jlashing
at a lotver depth compared to that containing lower CO2
pressure profile shows higher wellheacl pressure
for
gas concentt'ation. Cons;equently' the
higher CO2 content. Howeve4 when the presence of scale
cleposition in thewellborc is taken into account, the calculatedwellhead pressm'e is lower compared to the wellbore
withotil
ale depos ition.
sc
INTRODUCTION
High concentration of dissolved CO2 gas in geothermal fluids is one of the main causes of scale deposition of
calcium carbonate (CaCOr) in production wells (FLriii, 1988). To avoid this problem, a scale inhibitor is injected
through a tube extending into the ploduction well. The scale deposition of CaCOi is normally found on wellbore
surt'ace near and above the flashing point. Therefore,
it is important to estimate the depth of the flashing point below
which the scale inhibitor should be injected. The flashing depth where the water starts to generate the steam depends
on the saturation pressure corresponding to the fluid temperature at depth. However, the boiling point of the water
containing CO2gas is differentfrom that of pure water. The calcite scale deposition is commonly found in production
wells that produce fluids with high concentration of CO2. As scale deposition proceeds in a well, it causes a decrease
in
rnass
flow rate of wells. Consequently, genelated electric capacity also
decreases due
to a lack of
steam
production of wells. When the scale has already been deposited in the wellbole, it must be lemoved by methods such
as
mechalical cleaning or acidizing. Howeveq there is a method to plevent the fbrmation of scale deposition, that is,
by injecting a scale inhibitor'. From the technical and economical point of view, the iniection of inhibitor seems to be
the most reliable method of preventing scale lbrmation in wellbore. Because the calcite scale is normally found
from the flashing point in the wellbore toward the rvellhead, the scale inhibitor is usually in.iected below that point'
In order to design a proper inhibitor system, it is important to locate the depth whele flashing starts. Chemically, the
calcite scale deposition occurs when the geothermal watel oliginally satulated with calcite at the reservoir
temperature. Once it becomes supersaturated with respect to calcite solubility as the water flashes and consequently
the temperature of the water is decleased.
Takahashi (1988), and Nishi and Tanaka (1988) conducted wellbore simulations in the plesence of CO2. They tried
to simulate measuled pressure and temperature profiles in production wells. Good matches were attained between
measured and simulated plofiles both in pressure and temperature. However, eff-ects of CO2 concentration on flash
commencement depth were not studied in detail. The|efore, eilbcts
olCO: concentration in the
geothermal water on
wellbore flow and depths where flashing starts were examined by numerical simulation'
GOVERNING EQUATIONS FOR WATER AND CO2
equations
The fluid in the reservoir. is a single-phase liquid corrtaining co2 gas. The nrass and energy conservation
used for water, steam and
coz
are taken fi'onr those introcluced by Sutton (1976).
V,
lv,,+Vrlvr=F,
(e)
n,V, I v, + n rV, I v, = fF,,,
H
where /1and
Z*
are the volume
[t
lvt
+ H svt]
flux of liquid and
and gaseous phases (m3/kg), naand
n,
are lhe mass
(10)
lvr = F,
(11)
gaseous phases (m/s), v/ and vsare the specific volume
of liquid
of COz per unit mass of liquid and gaseous phases, H1 and H, are
fluid'
the specific enthalpy of liquid and gaseous phases (Jikg), y is the mass of COz per unit mass of
F.
is the mass
flux (kg/m2s) and Fu is the enthalpy flux (J/m2s).
Hrcanbethe sum of the enthalpy oldissolved CO2 and the water:
H
, = noH ,,,, + (l -
n,)(U
t,
+ Pvt')
(12)
(J/kg) and P
where 116 is the specitic enthalpy of CO2 in water (Jlkg), Ul is the specific internal energy of the liquid
is the plessure (Pa). SimilaLly for the gaseous phase:
H
r
= nrH r, +
(-
(13)
nr)(U, + Pv r)
(J/kg).
where Hg. is specific enthalpy of CO2 in steam (J/kg), U, is the specific intelnal energy of the steam
The solution ofEqs. (9) to (11) should satisfy:
n,H, - nrH n -
T(H r
- H,) + (F, I F,,,)(n, -
From this equation, the relation between the pressure P (Pa) and the temperature
/71,
(
ne) = 0
I(K)
can be introduced. When
l4)
n*l
wo cao deflne a function:
G(y, P,T) =ln,H
r
-, rH, - l(H r - H )ll@ r - n,)
(15)
In a system with y and F"/F,,frxed:
G(y, P,T)
=
for initial values P, and I". The thermodynamic quantities, such &s
onPandL
(17)
constant = G(Y, P",To)
ttg,
tt(tand Hg" in the definition of G depend only
DETERMINATION OF THE THERMODYNAMIC VARIABLES
as
The nrass of coz pel. unit mass of water and steam phases, n1 ?nd nr. are expressed
nt = d(T)P'
(18)
ns = P, I P
(1e)
results
where a (f with the unit of pa-r in Eq. ( l8) is an approximate quadratic tlt to the experimental
d(T) =ls.+ - t .s1r tl
The partial pressure, pc, at temperature
I
00) + | .2(T
/l 00)' l'
I
and given as
04
(20)
is defrned to be the dillerence between the total pressure P and
saturated vapour pressure of steam at telnperatufe
I
the
and can be written as
(2t)
P,=P-P,(T)
volume of the liquid phase, v1, is assumed to be
Since the amount of co2 in the liquid phase is small' the specific
a fixed temperature' the molar ratio of CO:
equal to that of water, v,,,, at the same temperature. Considering that fbr
gaseous phase, hence
in the liquid phase is proportional to the partial pressul'e of CO2 in the
n, = d(T)'
(22)
P,
pressLlre'
When the iluid entering the wellbore is in a single-phase liquid, the flash starting
P,',b =
nt= nclhen'
P,(T)+
P,,,16 ce;tr
be expressed as
(23)
P,
(24)
Pu,,b=P,(T)+n"la(T)
on the initial values of P,,,
The calculation of the parameters for two-phase condition is carried out based
Toandy'ln
the pressut'es, P, for given
this study, subscript o refers to the condition at the well bottom. In order to determine
successive temperatures,
TlEq. (17) can be modified
as:
(25)
G(y,P,T,) = G(Y,P,,To)
and Physics (Weast, 1964) as
The specific enthalpy of COz gas, Hg,(Jikg), is given in the Handbook of Chemistly
H
r, =-2.18x
105
+732(T +273)+0'252(T +273)'z
(26)
-2.63x10{(Z +2n)3
The specific enthalpy of CO2 in water, Ht"Qlkg), is given by
(27)
Hor=Hrotr*Hg"
where FI,,,r,, is the heat of solution of CO2 in water (J/kg) is calculated by
H,ot, =(-1.351 + 0.0 16927 -7.5524'
l0-tf2
+ 1.3 18'
On the basis of all thermodynamic valiables above, the pressure loss
deter.mined
to obtain the
pr.essure and temperatule
l0-tf3)'
tbl the liquid
l0u
(28)
phase and the gaseous phase are
profiles in the wellbore. The first step is the input data for
diameter, D, well length' l'
calculation given as initial pressure, P,,, initial temperature fl,, mass flow rate, lul,,well
Wellbore model was used
pipe
e, and CO2 gas content, n". Then the flash starting pressul'e is determined.
roughness,
(2002) and thermodynanric properlies above
to simulate steam-water two-phase flow developed by Khasani et al.
are incorporatecl into the wellbore model to consider the presence
ol coz.
CALCULATION RESULTS
The wellbore geometry used in this study has a lengtlr
of
1500 m, a diametet'
of 0.2 m and a pipe
roughness
of
values of COz contellts (nc) of nc:0 wt%'
4.6x10'5 m. The mass flow rate including CO2 gas is given as 50 kg/s. The
liquid water are evaluated. The well bottom pressure and tempe|atu|e as the boundary
0.5 wt% and
I
wt7o in the
to the possibility
conditions are 75 bar.and 260oc, respectively. These values ale selected due
deposition with this condition. The presence
of
scale deposition
to lorm
calcite
in the wellbore is introduced by giving the
shrinkage of well diameter tiom the flashing point to specific depth'
Effects
Figures
of CO2Gas Content on the Pressure and Temperature Profiles
I (a) and (b) show the pressure and temperatures protiles in a wellbore fbr different
CO2 content' Tanaka
is 2'8 wt%' In this
andNishi (1988) reported the field data of sixteen wells in which the maximum CO2 content
are given. It can be observed that when
study, three differ.ent values ol gas contents of 0 wtolo, 0.5 wt% and I wt%
gives similar pressure profiles on the
the COz gas present is in the single-phase liquid, the different amount of COz
after the pressure in the
lower.most part of the curves as shown in Fig. I (a). In this study the coz degassing occurs
pa|tial pressure ol COz. As soon
wellbore reaches the flash starting pressure that consists of saturation pressure and
the pressure gradient deffease
as the liquid water starts to generate the steam and COz gas when flowing upwat'ds,
This is probably due to the
starts from that point. The decrease rate fbr 0 wt% CO2 shows largest among othels.
partial pressure ofCO2 gas that is proportional to
11 and
r,
values and contlibutes to total pressure.
TEMPERATURE (C)
PRESSURE 6ar)
20 40 60
80
100
160
200
,/'
E
E
E 1000
F
rIl
F
o-
1500
1
000
COz
- 0 wt%
td
0 wt%
-
280
500
500
o-
240
ctr
0.5 wt%
l wt%
0.5 wt%
I 500
1
v'tlo/o
1 flashing points
2000
2000
(a)
(b)
Figure 1. Pressure and temperature profiles tbr diff'erent co2 gas content.
lowel temperature at
From Fig. I (b), it can be seen that the higher CO2 content gives the lower flashing depth. The
the flashing
of the temper.atur.e profile for 0 wt% CO2 is due to lowel boiling pl'essure. In this study,
the uppermost
m and 1430 m for the cases olcoz
depth is calculated to be 1130 m for pure water. The flashing depths are 1270
by the increase in the
of 0.5 wt% and I wtyo, respectively. The increase in the COz content is also followed
content
wellhead pressure. The corresponding P',,t,for Coz
of 0, 0.5 and 1 wtTo are 1,l.5 bar, 22'7 bar and 23.9 bar'
respectively.
(in wt%) in liquid and gas phase, respectively'
Figures 2 (a) and (b) show the distribution of COz gas concentration
remains constant.
It can be observed that below the flaslring point, the CO2 content in the liquid
MASS FRACTnN 0F C0z fuf/')
MASS FRACTDN 0F C02 fuf/o)
0.5
0
1
010203040
1.5
500
500
flashing points
E
F
o-
E
:tr 1000
F
o-
1000
fr.l
-
rl]
-
1500
2000
nc = 0.5 wflo
1
nc = 1 w{/o
500
nc = 0.5 w9o nc = 1w9o
2000
(b) in gas phase
(a) in liquid phase
Figure 2. Distribution of CO2 concentration in liquid and gas phases.
than that in liquid phase'
once flashing starts, the concentrations of co2 in the gas phase show much lalger values
gas and liquid phases' The COz
The higher initial CO2 content in the fluid results in higher CO2 content both in
fluid temperature decreases from
content decreases as the fluid flows upwards in the wellbore. This is because the
to Eq. (20) and affects
the f'lashing point towards the wellhead. Consequently, the constant a(T) changes accolding
the COz concentratior.rs both in liquid and steam phases. For nc
=I
wt7o,
it gives about 8 wt% at wellhead for nr'
This value is relatively high compared with that found in Momotombo field that has a range between
I
and 4 wt%
(Porras, pers. comm.).
Effects of Well Bottom Temperature on Flashing Point
at the well bottom for two
Figures 3 shows the pressure distributions in the wellbore for different fluid temperature
cases
of CO2 concentr.ations, (a) nc = 0.5 wt% and (b)
nc: I wtolo. The decrease
fiom 260oC to 250oC causes the deqease in wellhead pressure
=
|
(P',,r,)
in well bottom temperature by l0"C
of 3.56 bar for nc = 0-5
wto/o and 3.60 bar
for nc
1280 m for nc =
wto/o.The pr.essure profiles are identical from the well bottom (1500 m) to the depth of about
0.5 % and 1420 mlor nc
=
1ol0.
This is becar-rse the liquid phases are found fiom well bottom up to those depths that
higher CO2 content (nc : 1
clearly identified in temperatule plofiles shown in Fig. 4. It can be noted that for the
upwards by 77 m from 1426
wt%), the temperature decrease of l0"C fl'om 260"C to 250'C shifts the flashing level
(nc = 0'5 wt%)' This is because the lower
m to 1349 m. It gives larger flashing shift of 85 m fbr lower COz content
ar.e
temperatut'e corresponds
to the lower saturation
pressllre' Fof the same
flow rate, the f'lashing point occurs
at
shallower Ievel fbr Iowet' satltration pl'essure.
PRESSURE bad
PRESSURE 6ar)
0
20 40 60 B0
0
100
26ooc
500
20 40 60 B0
100
z.'-2600C
500
2500C
E
:tr 1000
F
o-
F
o-
1000
GI
-
G.l
o
1
1500
500
2000
2000
(a)
COz:0.5 wt%
(b)
CO:: I wt%
Figure 3. Pressure profiles in the wellbore for different well bottom temperature.
Effects of CO2 Gas Content and Scale Deposition on Well Deliverability
Care should be taken during the analysis of pressure profiles in the wellbore in the presence of CO2. This is because
that the presence of COz forms calcite scale deposition in the wellbole. Thelefore, when the data of CO2 gas and
flowing pressure in the wellbore are available, the wellbole analysis should consider the presence of CO2 in the fluid
and ofthe deposition ofscale.
CC)
20A 240 280
TEM PERATURE
160
TEM PERATURE fC )
160 200
240
280
500
500
2500C
2500C
E
F
ord
260"C
1
000
2600C
E
F
1000
o.
l!]
U
o
I 500
1
500
o flashing point
o flashing point
2000
2000
(a) COz = 0.5 wt%
(b)CO2:7wto/o
Figure 4. Temperature profiles in the wellbore fbr diff'elent well bottom tetnperatut'e.
Most of the numerical studies about wellbole analyses for the pressure profiles in the presence of CO2 did not
include the scale deposition ef'fects. and consequently a leduction of wellbore diameter.
Hokkaido, Japan' He cart'ied out
Fujii (19s8) reported calcite scale deposition in plocluction'"vells at Nigorikawa,
scale was mainly fbrrned along wellbore fiom
caliper. sur.vey at the production wells and fbund out calcite
just above
part of the lvells were negligible. TheIefole, in ofder to
the flashing commencernent point. Scale deposits at shallow
olscale deposited section is given as 200 m fiom
evaluate the effects ofscale formation on pressure profiles, length
3 cm. In othe| words, well diameter is reduced
the flash starting point and above with hornogeneous thickness ol
f'r.om 0.2 m to 0.194
m lorthe depth from 1270mto 1170 m forthe caseof
TEN,IPERATURE
PRESSURE 6ar)
20 40 60
nc:0'5 wt%'
80
200
160
100
fC)
240
280
CO2=657'
uniform diameter
2,.
500
uniform diameter
COz= 0.5%;
shrinkage diameter
:tr
F.
o-
COz=
E
E
1
000
L
500
CO2= 6o1o,
uniform
F
orll
a
fr.l
a
1000
0.5o/o,
COz= 0.5'h:
2000
(b)
(a)
gas content and well diameter
Figure 5. pressure and temperatule profiles in the wellbore fot different CO2
shrinkage.
in the wellbore for different CO2 gas
Figures 5 (a) and (b) show the pressure and temperature profiles, respectively,
diarneter. length and rotlghness and
content in the 1uid. The wellbore parameters fbr calculation consist of wellbore
pressLrre and temperature values are 50
are given as 0.2 m, 1500 m and 4.6x10-5 m. The mass flow rate, well bottom
water of 0 wt% and 0'5 wt%
kgls,T5bar and 260oC, respectively. The initial COz gas concentrations in the liquid
are evaluated.
and uniform diameter, 2) with 0'5 wt%
The wellhead pressures fbr three conditions; 1) pure water'(0 wt% of COz)
co2 with uniform diameter' and 3) with 0.5% of
co:
with scale length ol 200 m and thickness of 3 cm
of
are
without scale deposition and co2 gas
calculated as 11.5 bar, 22.7 and20.6bar,respectively. The results fol the case
(case-2) results in lower
(case-l) shows lowest p",r, (11.5 bar). The case with co2 and wellbore diameter shfinkage
pu4,coffipar€d with the case without scale deposition (case-3). Therefore, the scale deposition should be taken into
show lowest value in case-l as shown in
account for more reliable analysis. The temperatures at the wellhead also
and co2 content, reduction of well radiLrs due to scale deposition results in
Fig. 5 (b). For. the same flow r.ate
decrease the welllread pressure.
This is because lalger pressut'e dlops due to accelet'ation and friction compollents
no scaling'
occur in the scale deposited section of the well compared with the case ol
CONCLUSIONS
1.
Nuu.rerical analysis olrvellbore tlow irr the presence
The presence
of
CO2
will
olCOl
irr the oliginal geothermal water were carried out'
increase the boiling pressure conrpared to that
containing high COz discharges,
it
flashes
at a
cleeper point conrpared
of pure water' When the
water
with that containing lowel
CO2
concentration.
2.
temperature gives shallower flashing point
For a given C02 concentration in the liquid water, the lor,ver fluid
fluid temperature, higher CO2 concentration results in the deeper flashing level'
level. At the same
3.
into account in numerical analysis in the
The presence of scale deposition in the wellbore should be taken
presence
of COz. This is
because the fbrmation
of scale leads to shrinkage of wellbofe diameter, and
consequentlyitacceleratespressufedropinthescaledepositedsection.
ACKNOWLEDGEMENTS
of Education, culture. Sports, Science and
The first author. gr.atefirlly acknowledges the scholarship by the Ministry
Technology, Government of JaPan.
REFERENCES
Japan Geothern.ral Association'
Fujii, y. CaCOj Scale problents in the Nigorikawa Geothermal Area, Hokkaiclo.
25(4), pp.5 4-65 ( 1 988);
Khasani, Itoi, R., Tanaka, T., and Fukuda, M. An Analysis of Presntre Drops
in Wellbore, Under Low Flow
Rate
Workshop, Auckland Unive|sity,
Conditions, Stvo, in A Deliverability Curve. Proc.24th New Zealand Geothermal
pp. 115-120 (2002).
Water and Carbon Dioxide' New Zealand
Sutton, F M. presstre-Tentperanne Ctntes fot' a Ti,o-Phase Mixtttre of
Journal of Science, 19, pp. 297-301 (1976).
Takahashi,
M.
Wellbore Model
in the
Presence
of COt Gas. Proc. l3'h Wo|kshop on
Geothern.ral Reservoir
Engineering, Stanlbrd, CA, Jan. 19-21, pp. I 51-1 57 (1988)'
Tanaka, S. and Nishi,
K. Compyter Code of
T\uo-Phase Flou,
in Ceothermal
Ll/ells Producing Ll/ater and/or
Engineering, Stanford, CA, Jan'
Water-Carbon Dioxicte Mixtttres. Proc.23tl'Workshop on Geothermal Reservoir
26-28, pp. 159- 164 ( 1988).
Weast, R. C. Handbook of Chemistty
and Physics.46'h ed. Chemical Rubbel Co' Ltd', Cleveland' Ohio ( 1964)'