2D eac i e anspo modeling; Dá ila e al. (2016)
1
2D eac i e anspo modeling o he in e ac ion be ween a ma l and a 1
CO2- ich sul a e solu ion unde supe c i ical CO2 condi ions 2
Gab iela Dá ila a,b,c*, Linda Luquo a,b, Josep M. Sole a,b and Jo di Cama a,b 3
a Ins i u e o En i onmen al Assessmen and Wa e Resea ch (IDAEA), CSIC. 4
Jo di Gi ona 18-26, 08034 Ba celona, Ca alonia, Spain. 5
b Associa ed Uni : Hyd ogeology G oup (UPC-CSIC). 6
c Depa men o Ci il and En i onmen al Enginee ing, Uni e si a Poli ècnica de Ca alunya (UPC). 7
Jo di Gi ona 1-3, 08034 Ba celona, Ca alonia, Spain. 8
*Co esponding au ho : Gab iela Dá ila ([email protected]) 9
Jo di Gi ona 18-26, Ba celona 08034. Ca alonia, Spain 10
Tel #:+34 934006100 11
Fax #: +34932045904 12
Abs ac 13
The ci cula ion o CO2- ich solu ions h ough ac u ed ma l co es (cap ock) unde 14
di e en low a es and supe c i ical CO2 condi ions (PTo al = 150 ba , pCO2 = 61 ba 15
and T = 60 °C) led o mine al changes caused mainly by calci e dissolu ion and o a 16
lesse ex en by aluminosilica e dissolu ion, and by gypsum p ecipi a ion adjacen o he 17
ac u e walls. Ano he signi ican esul was he o ma ion o he al e ed and highly 18
po ous zone (Dá ila e al., 2016a). Dissolu ion s uc u es anged om ace o uni o m 19
dissolu ion and wo mhole o ma ion depending mainly on he low a e. 20
2D eac i e anspo models we e used o in e p e he esul s o he pe cola ion 21
expe imen s (excep a 60 mL h-1). They ep oduced he a ia ion in he ou low 22
composi ion wi h ime and he obse ed wid h o he al e ed zone along he ac u es. A 23
good ma ch was achie ed by using ini ial De alues in he ock ma ix ha anged om 24
1 × 10-13 m2 s-1 o 3 × 10-13 m2 s-1 unde slow low a es. The De alue was highe by a 25
ac o o 20 (6 × 10-12 m2 s-1) unde as low. Mo eo e , a sligh a ia ion in he calci e 26
eac i e su ace a eas con ibu ed o he i o he model o he expe imen al da a. 27
The modeling esul s ep oduced majo dissolu ion o calci e and gypsum p ecipi a ion, 28
and mino dissolu ion o clinochlo e. Calci e dissolu ion was boos ed by inc easing he 29
low a e and gypsum p ecipi a ion inc eased a in e media e low a e (1 mL h-1). Mino 30



2D eac i e anspo modeling; Dá ila e al. (2016)
2
p ecipi a ion o dolomi e, kaolini e, boehmi e and h ee zeoli es (mesoli e, s ilbi e and 31
smec i e) along he al e ed zone occu ed. The magni ude o hese eac ions was 32
consis en wi h he measu ed inc ease in po osi y o e he al e ed zone. 33
Keywo ds: CO2 seques a ion, nume ical modeling, leakage, ma l cap ock, calci e 34
dissolu ion, gypsum p ecipi a ion. 35
1.In oduc ion 36
Leakage o injec ed CO2 in deep ese oi s may occu h ough p e e en ial pa hways 37
such as aul s and ac u es (Ru q is and Tsang, 2002) and h ough he ock cemen 38
in e ace (Dá ila e al., 2016b). Dá ila e al. (2016a) showed ha he pe meabili y o he 39
Hon omín ma l w .% calci e, < 25 w .% clay and 10 w .% 40
qua z was a leas 6 o de s o magni ude smalle han ac u e pe meabili y (k ) in 41
ac u ed co es. Fluids will he e o e low p e e en ially h ough ac u es, whe e he 42
in e ac ion be ween CO2- ich luids and he ock ma ix can b ing abou changes in 43
physical and chemical p ope ies caused by dissolu ion and p ecipi a ion (Singu indy 44
and Be kowi z, 2005; Edlmann e al., 2013; Kampman e al., 2014; Chen a al., 2014; 45
Dá ila e al., 2016a). The in e p e a ion o he p ocesses is no s aigh o wa d gi en 46
ha cap ocks a e usually composed o a la ge numbe o mine als ha eac di e en ly. 47
Reac i e anspo modeling has been pe o med o assess he impac o he 48
in e ac ion be ween CO2- ich b ines and cap ocks a he labo a o y (Ghe a di e al., 49
2007; C edoz e al., 2009; Hao e al., 2013; Tian e al., 2014) and ield scales (Gaus e 50
al., 2005). 51
Ghe a di e al. (2007) modeled luid- ock in e ac ions in in ac and ac u ed 52
ca bona e- ich shale cap ocks using 1D and 2D models, espec i ely. The dominan ole 53
o calci e ou weighed he e ec s o he o he mine alogical changes in ol ing 54
p ecipi a ion/dissolu ion o Al-silica e mine als. Mino mine al ans o ma ions induced 55
by he ad ance o he CO2-induced acidic on in he cap ock included clay dissolu ion 56
(illi e, chlo i e and musco i e) and p ecipi a ion eac ions (Na-smec i e). O he e ec s 57
en ailed he o ma ion o new mine als such as dawsoni e, side i e, and anke i e. 58
C edoz e al. (2009) pe o med and simula ed ba ch expe imen s wi h clayey and 59
clay limes one c ushed cap ocks a pCO2 = 150 ba and T = 80 °C o 90 days. The 60
eac i i y o he wo ock samples was simila unde hese expe imen al condi ions. 61
2D eac i e anspo modeling; Dá ila e al. (2016)
3
Dissolu ion o calci e and dolomi e p edomina ed oge he wi h mino o negligible 62
p ecipi a ion o kaolini e and mon mo illoni e due o he des abiliza ion o Ca-63
mon mo illoni e and illi e. A a la ge scale, Tian e al. (2014) pe o med 1D nume ical 64
simula ions o a 30 m long e ical column wi h 1 m2 o c oss-sec ional a ea o s udy 65
he e ec o he mine al composi ion o clay- ich shale and muds one cap ocks a 101 66
ba and 47 °C. The mine al composi ion o he cap ock con olled he end o po osi y 67
change. The bu e ing capaci y o he clay- ich shale was highe han ha o he 68
muds one. The po osi y was in luenced mo e by he mine al assemblage o he 69
muds one han by he clay- ich shale. This s udy sugges ed ha he muds one was mo e 70
sui able o use as a cap ock. 71
Hao e al. (2013) used a 3D con inuum-scale eac i e anspo model o 72
simula e co e lood expe imen s o ma ly dolos one and uggy limes one eac ing wi h 73
b ines equilib a ed wi h pCO2 = 5, 10, 20, 30 ba and T = 60 °C ha we e pe o med by 74
Smi h e al. (2013). The au ho s showed he ole o physical he e ogenei ies on he 75
ca bona e ocks in he de elopmen o he dissolu ion on s. S able dissolu ion on s 76
de eloped in he ma ly dolos one we e i ed using an empi ical exponen (n = 3) in he 77
Kozeny Ca man equa ion o po ous media (K = Ko·( / o)n, whe e K and a e 78
pe meabili y and po osi y). Fo mo e impe meable and he e ogeneous co es ( uggy 79
limes one) n = 6 8. 80
A ield scale, Gaus e al. (2005) e alua ed he long e m geochemical beha io 81
o sil y clay cap ock a he Sleipne injec ion si e by pe o ming 1D eac i e anspo 82
modeling combining eac ion kine ics and di usi e anspo unde CO2 supe c i ical 83
condi ions (PTo al = 101.3 ba ; pCO2 = 100 ba and T = 37 °C). I he cap ock is assumed 84
o be a homogeneous medium, eldspa al e a ion would be he dominan long e m 85
eac ion despi e ini ial ca bona e dissolu ion. These eac ions could cause a sligh 86
dec ease in po osi y and a subsequen dec ease in pe meabili y. 87
In ou s udy, 2D eac i e anspo modeling was pe o med o quan i y he 88
dissolu ion and p ecipi a ion p ocesses ha occu ed in he labo a o y pe cola ion 89
expe imen s in which CO2- ich solu ions ci cula ed h ough ac u ed ma l co es (Dá ila 90
e al., 2016a). The changes in he ac u es and in he po osi y o he ock ma ix 91
induced by dissolu ion o calci e, clinochlo e, albi e and gypsum (in S- ee solu ions) 92
2D eac i e anspo modeling; Dá ila e al. (2016)
4
and p ecipi a ion o gypsum (in S- ich gypsum-equilib a ed solu ions) and silica e 93
mine als led o a ia ions in pe meabili y. 94
2.Summa y o he expe imen al esul s 95
Dá ila e al. (2016a) showed ha he ci cula ion o CO2- ich solu ions wi h di e en 96
sul a e con en s h ough ac u ed co es unde supe c i ical condi ions (PTo al = 150 ba , 97
pCO2 = 61 ba and T = 60 °C) and di e en low a es (0.2, 1 and 60 mL h-1) esul ed in 98
a a ie y o dissolu ion pa e ns. In all cases, dissolu ion o calci e os e ed he 99
o ma ion o a highly po ous eac ed zone made up o slowly dissol ing g ains o 100
qua z, illi e and py i e and possible seconda y mine als (e.g. gypsum). The dissolu ion 101
and p ecipi a ion eac ions con olled he a ia ions in he inal po e olume associa ed 102
wi h he eac ed co es. In all expe imen s, an inc ease in he low a e led o an inc ease 103
in he calci e dissolu ion a e and in he inal po e olume associa ed wi h he eac ed 104
co e. In he S- ich solu ion, he olume o dissol ed calci e was always la ge han he 105
olume o p ecipi a ed gypsum. The dissolu ion o calci e g ains and, o a lesse ex en , 106
he dissolu ion o clinochlo e and albi e, c ea ed a high po osi y zone ( anged om 48 107
o 67 %) along he ac u e walls. This po osi y zone was composed o non-dissol ed 108
qua z, py i e and illi e g ains, p ecipi a ed gypsum (in S- ich solu ions) and silica e 109
phases (e.g., kaolini e o amo phous SiO2) and Al-bea ing phases (e.g., boehmi e). 110
As ega ds he dissolu ion pa e ns, in he S- ee solu ion expe imen s ace dissolu ion 111
occu ed a slow low a es (pecle numbe (Pe) o 4 and 21). Wo mhole o ma ion was 112
obse ed a he highes low a e (Pe = 1267). Howe e , in S- ich solu ion expe imen s, 113
ace dissolu ion occu ed only a he slowes low a e. A an in e media e low a e, 114
bo h uni o m dissolu ion and wo mhole o ma ion ook place. A combina ion o 115
wo mhole o ma ion and uni o m dissolu ion occu ed a he highes low a e. Local 116
he e ogenei ies con olled mine al dissolu ion along he ac u e, which could lead o 117
unexpec ed dissolu ion pa e ns. 118
As ega ds ac u e pe meabili y, unde slow low a es (low Pe) and in S- ee solu ion 119
expe imen s, k did no signi ican ly change since ace dissolu ion did no cause ac u e 120
ape u e o a y. Mo eo e , de ached g ains along he ac u e could ha e led o he 121
obs uc ion o he luid low, p e en ing k om inc easing. In S- ich solu ions, a 122
ma ked dec ease in k was a ibu ed o gypsum p ecipi a ion. Unde as low a e (high 123
2D eac i e anspo modeling; Dá ila e al. (2016)
5
Pe), k sligh ly inc eased wi h sligh a ia ions in ape u e in he S- ee solu ion 124
expe imen . The inc ease in k in he S- ich expe imen was a ibu ed o he absence o 125
gypsum p ecipi a es. This inc ease in k sugges ed he exis ence o low h ough he 126
al e ed ock ma ix unde hese as low condi ions. 127
3.Reac i e anspo modeling 128
The simula ions we e pe o med using C unchFlow code (S ee el e al., 2015). The 129
ini ial mine al composi ion o he ock was calcula ed om he ini ial mine al 130
composi ion ob ained om he XRD-Rie eld analysis. 131
3.1 Desc ip ion o he C unchFlow eac i e anspo code 132
The eac i e anspo modeling was pe o med using he C unchFlow code (S ee el e 133
al., 2015), which sol es nume ically he mass balance o solu es exp essed as 134
j
jj
j
RCC
C
q D
(j (1) 135
whe e is po osi y, Cj is he concen a ion o componen j (mol m-3), q is he Da cy 136
eloci y (m3 m-2 s-1), Rj is he o al eac ion a e a ec ing componen j (mol m-3 ock s-1) 137
and D is he combined dispe sion-di usion coe icien (m2 s-1). 138
The o al eac ion a e Rj is gi en by 139
m
m
jmj
RR
(2) 140
whe e Rm is he a e o p ecipi a ion (Rm > 0) o dissolu ion (Rm < 0) o mine al m in mol 141
m-3 ock s-1 , and jm is he numbe o he moles o j in mine al m. 142
The eac ion a e laws used in he calcula ions a e exp essed as 143
1
2
,1
m
m
eq
i
n
i
e ms
n
HTmmm K
IAP
aakAR iH (3) 144
whe e Am is he mine al su ace a ea in m2mine al m-3bulk, km,T is he eac ion a e cons an 145
in mol m-2 s-1,
H
n
H
a
is he e m desc ibing he e ec o pH on he a e,
i
n
i
a is he e m 146
desc ibing a ca aly ic/inhibi o y e ec by ano he species on he a e, IAP is he ionic 147

2D eac i e anspo modeling; Dá ila e al. (2016)
6
ac i i y p oduc o he solu ion wi h espec o he mine al, Keq is he equilib ium 148
cons an o he dissolu ion eac ion (ionic ac i i y p oduc a equilib ium) and m2 and 149
m1 a e he pa ame e s a ec ing he dependence o he a e on solu ion sa u a ion s a e. 150
The a e cons an a empe a u e T (K) is calcula ed om 151
TTR
Ea
kk mTm 11
exp
25
25,, (4) 152
whe e km,25 is he a e cons an a 25 oC, Ea is he appa en ac i a ion ene gy o he 153
o e all eac ion (J mol-1) and R is he gas cons an (J mol-1 K-1). 154
Change in mine al su ace a ea (Am in m2mine al m-3bulk) owing o dissolu ion is 155
3
2
)(
3
2
)( imi
m
ini ial
mAA (5) 156
Change due o p ecipi a ion is 157
3
2
)(i
ini ial
mAA (6) 158
whe e (i)m is he ini ial olume ac ion o he mine al m and (i) is he ini ial po osi y o 159
he medium. This o mula ion ensu es ha as he olume ac ion o a mine al eaches 160
ze o, so does i s su ace a ea. Mo eo e , in he case o bo h dissol ing and p ecipi a ing 161
mine als, he e m ( / (i))2/3 equi es he su ace a ea o a mine al in con ac wi h luid 162
o each ze o when he po osi y o he medium eaches ze o. This o mula ion is used 163
p ima ily o p ima y mine als (i.e., mine als wi h ini ial olume ac ions > 0). In he 164
case o seconda y mine als which p ecipi a e, he alue o he ini ial bulk su ace a ea 165
speci ied is used p o ided ha p ecipi a ion occu s. I his phase subsequen ly dissol es, 166
he abo e o mula ion is used wi h an a bi a y ini ial olume ac ion o 0.01. 167
3.2 Nume ical disc e iza ion 168
The 3D cylind ical co e samples used o he labo a o y expe imen s (Fig. 1a) we e 169
con e ed o a 2D ec angula symme y. Fi s , hal o he ci cula sec ion o he co e 170
was ans o med in o a ec angle. The a ea o his ec angle was equal o he a ea o he 171
2D eac i e anspo modeling; Dá ila e al. (2016)
7
hal ci cle (a ea = x·l = ½· ·(l/2)2; whe e l is he diame e o he co e and x is he wid h 172
o he ec angle). A 2D longi udinal sec ion o he esul ing p ism was used as he 173
calcula ion domain (Fig. 1b and c). This domain was composed o a ac u e (high 174
pe meabili y zone) and ock ma ix (low pe meabili y zone; Fig. 1d). The domain was 175
di ided in o 36 and 30 nodes om he inle o he ou le (L) and om he cen e o he 176
ac u e o 3.5 mm in he model, espec i ely (Table 1). F ac u e co esponded only o 177
he i s node in he x di ec ion. The model conside ed ad ec ion and dispe sion only 178
along he ac u e. Solu e anspo in he ock ma ix was caused only by di usion. 179
3.3 Pa ame e s 180
3.3.1.Rock composi ion 181
The ock conside ed in he models was he ma l desc ibed in Dá ila e al. (2016a). I is 182
composed o calci e (71.2 w .%), qua z (9.7 w .%), illi e (7.1 w .%), albi e (6.5 w .%), 183
gypsum (2.0 w .%), clinochlo e (2.8 w .%), anhyd i e (0.5 w .%), and py i e (0.2 w .%). 184
The ini ial po osi y o he ock ma ix ( (i)) was es ima ed by image segmen a ion 185
p ocessing applied o ESEM images, which led o he sepa a ion o ock and oids. The 186
ini ial mine al olume ac ions we e calcula ed om he es ima ed po osi y (6.7 %; 187
Table 2). An analysis o he selec ion o he seconda y mine als was pe o med based 188
on equilib ium ba ch modeling (Gaus, 2010; Sole e al., 2011; Tian e al., 2014). Ini ial 189
es ima es o he eac i e su ace a eas o he p ima y mine als we e calcula ed by 190
assuming sphe es wi h adii es ima ed om he ESEM images (Table 2). The seconda y 191
mine als we e allowed o s a p ecipi a ing when he solu ion eached supe sa u a ion. 192
The ini ial eac i e su ace a eas o all he seconda y mine als we e assumed o be he 193
same and su icien ly high (1.0 × 104 m2mine al m-3 ock) o allow as p ecipi a ion (local 194
equilib ium). Tu olo e al. (2015) pe o med simula ions and obse ed ha when 195
solu ions we e supe sa u a ed wi h espec o Al-bea ing mine als (boehmi e, gibbsi e 196
and diaspo e), only boehmi e was aken in o accoun since i s p ecipi a ion was 197
obse ed unde high empe a u e and p essu e condi ions. 198
3.3.2.Solu ion composi ion 199
The wo solu ions used in he models we e he ones used by Dá ila e al., 2016a (Table 200
3). In bo h solu ions, he ini ial concen a ion o CO2 was calcula ed o be 6.52 × 10-1 201
mol kgw-1 acco ding o he Duan and Sun (2003) model wi h an imposed pCO2 o 61 202
2D eac i e anspo modeling; Dá ila e al. (2016)
8
ba and bea ing in mind ha PTo al = 150 ba , T = 60 °C and I = 0.6 M. Also, he ini ial 203
concen a ion o O2 was es ima ed o be 3.06 × 10-4 mol L-1 (a mosphe ic and CO2-204
bo le concen a ions). The S- ich solu ion was unde sa u a ed wi h espec o calci e 205
(SICal = -3.20) and nea equilib ium wi h espec o gypsum (SIGp = -0.02) a 60 °C. The 206
S- ee injec ed solu ion was unde sa u a ed wi h espec o bo h calci e (SICal = -3.50) 207
and gypsum (SIGp = -7.54) a 60 °C. The calcula ed ini ial pH alues we e 3.26 and 3.29 208
o he S- ich and S- ee solu ions, espec i ely. 209
Since he ini ial po e wa e composi ion o hese samples was unknown and dis u bed 210
du ing sample p epa a ion, i was assumed o be in equilib ium wi h calci e and gypsum 211
o he S- ich solu ion expe imen s a oom T and a mosphe ic pCO2 (concen a ion o 212
CO2 = 4.14 × 10-4 mol kgw-1), yielding a pH 7.7 (Table 3). A e y small concen a ion 213
(10-6 mol kgw-1) was assumed o Na+, K+, Al3+, Cl-, B -, Fe2+ and SiO2(aq). 214
3.3.3.Flow and anspo p ope ies 215
Da cy eloci y (q), ini ial e ec i e di usion coe icien (De (i)), and longi udinal 216
dispe si i y used in he simula ions a e shown in Table 2. The low ield used in he 217
eac i e anspo assumed cons an Da cy eloci y in he ac u e and no low in he 218
ock ma ix. The e ec i e di usion coe icien (De ) a 60 °C in he longi udinal and 219
ans e sal di ec ions was calcula ed as
o
m
e
DD
, whe e m is he cemen a ion 220
exponen (m = 2.5; Re il and Ca hles, 1999) and Do is he di usion coe icien in wa e . 221
3.3.4.The modynamic and kine ic da a 222
One hund ed and se en aqueous species we e conside ed in he simula ions. The 223
equilib ium cons an s we e aken om he EQ3/6 he modynamic da abase (Wole y e 224
al., 1990) included in C unchFlow and a e lis ed in Table A1 (Appendix 1). The ac i i y 225
coe icien s we e calcula ed using he ex ended Debye-Hückel o mula ion (b-do 226
model; Helgeson, 1969) wi h pa ame e s om he same da abase. Twen y solid phases 227
(eigh p ima y mine als (calci e, qua z, illi e, albi e, gypsum, clinochlo e, anhyd i e and 228
py i e) and wel e seconda y mine als (kaolini e, SiO2(am), dolomi e, mesoli e, s ilbi e, 229
smec i e, mo deni e, scoleci e, analcime, wai aki e, laumon i e, gismondine and 230
boehmi e) we e conside ed in he calcula ions. Equilib ium cons an s o he mine al 231
dissolu ion eac ions (C unchFlow, EQ3/6) a e gi en in Table A2 (Appendix 1). The 232
gypsum equilib ium cons an (log KGp) alue a 60 °C used in his s udy we e lowe 233
2D eac i e anspo modeling; Dá ila e al. (2016)
9
han ha ini ially implemen ed in he da abase and we e log KGp(60°C) = 4.7383 as 234
epo ed by No ds om (2013) and Ga cia e al. (2014). 235
Kine ic a e laws o he p ima y and seconda y mine als, a e pa ame e s and 236
ac i a ion ene gies a e lis ed in Table 4. The mine al dissolu ion and p ecipi a ion a es 237
we e aken om he li e a u e o each mine al (Paland i and Kha aka, 2004; Xu e al., 238
2012; Bands a e al., 2008; Bibi e al., 2011; Chou and Wollas , 1985; Hellmann e al., 239
2010; Hame e al., 2003; Zhang e al., 2015; Domènech e al., 2002 ; Cama e al., 240
2000). The pa allel a e laws o mine als desc ibe he explici dependence o he a es 241
on pH. Ra e cons an s a a empe a u e di e en om 25 °C we e calcula ed acco ding 242
o Eq. (4). 243
4.Resul s and discussion 244
4.1 Dissolu ion and p ecipi a ion eac ions 245
4.1.1S- ich injec ed solu ion 246
Adjus men o he alues o he mine al eac i e su ace a eas (Am) and he ini ial 247
e ec i e di usion coe icien (De ) we e used o ma ch he a ia ion in aqueous Ca, S, 248
Mg, K, Si and Fe concen a ions wi h ime (Table 2). 249
Fig. 2 shows he ma ch be ween he expe imen al and modeled a ia ions in he ou pu 250
concen a ions o e ime in he S- ich injec ed solu ion expe imen a 1 mL h-1 (exp. 4). 251
The adjus ed su ace a ea o calci e was dec eased by one o de o magni ude om he 252
calcula ed geome ic su ace a ea alue o ma ch he ou pu Ca concen a ion (Table 2), 253
which was highe han he inpu one owing o calci e dissolu ion (Fig. 2). The inc ease 254
in Ca concen a ion o he i s 8 h was associa ed wi h he ini ially la ge eac i e 255
su ace a ea o calci e, esul ing in signi ican calci e dissolu ion and in a dec ease in 256
calci e con en in he ock ma ix close o he ac u e. Wi h ime, he eac ion was 257
con olled by di usion h ough he ock ma ix be o e slowing down. 258
The expe imen al and simula ed ou pu S concen a ions we e always smalle han he 259
inpu ones (Fig. 2). The de ici o S was a ibu ed o gypsum p ecipi a ion gi en he 260
excessi e Ca concen a ion and he posi i e alues o he gypsum sa u a ion index (0 < 261
SI he co e du ing he expe imen (Fig. S1; supplemen a y da a). 262
2D eac i e anspo modeling; Dá ila e al. (2016)
16
5.F om he labo a o y o he ield scale 449
The expe imen al and modeling esul s o he pe cola ion expe imen s shown in he 450
cu en pape and in he p e ious one by Dá ila e al. (2016a) indica e ha bo h he 451
o ma ion o dissolu ion pa e ns and he a ia ion in ac u e pe meabili y a e highly 452
dependen on he low a e which, being he ac u e olume p ac ically he same in all 453
he expe imen s, de e mines he esidence ime o he luid ci cula ing h ough he 454
ac u es. An inc ease in he esidence ime (slow low a e) p o okes signi ican 455
dissolu ion o calci e only a he ac u e inle , o ming ace dissolu ion pa e ns, and 456
allows o gypsum p ecipi a ion, yielding ei he li le a ia ion o a educ ion in ac u e 457
pe meabili y. Opposi ely, sho e esidence imes ( as low a e) do no allo o la ge 458
changes in solu ion composi ion, esul ing in he o ma ion o wo mholes o uni o m 459
dissolu ion and an inc ease in ac u e pe meabili y. 460
The esidence ime in he expe imen s anged om 0.1 o 12 s (Table 2), which is 461
ex emely sho compa ed o likely esidence imes in cap ock ac u es du ing CO2 462
injec ion (e.g., om yea s, Bachu e al., 1994). Hence, acco ding o ou sho - e m 463
labo a o y-scale esul s, i can be concluded ha he slowe low a es in he eposi o y 464
sys em will lead o limi ed calci e dissolu ion and e ec i e gypsum p ecipi a ion, which 465
would in u n p omo e li le change o e en a dec ease in ac u e pe meabili y. This 466
impo an ole o low eloci y has al eady been poin ed ou by B une e al. (2016) o 467
Po land cemen - CO2 - b ine in e ac ions. 468
A his poin , howe e , we would like o emphasize ha he 2D eac i e anspo 469
modeling applied o quan i y he ongoing p ocesses in he sho ac u ed pe cola ion 470
expe imen s unde pCO2 and empe a u e simila o hose a he ield si e canno be 471
di ec ly used o p edic he long- e m beha io in cap ock ac u es a he ield scale. 472
Fo such modeling, pa ame e s like (1) he dimensions (ape u e and leng h) and 473
mo phology o he ield-scale ac u es, (2) he eac i e mine al su ace a eas, and (3) 474
he luid low egime should be known. Wi hou his in o ma ion, a sensi i i y analysis 475
conside ing he possible a iabili y o hese pa ame e s should be pe o med. Such a 476
modeling exe cise is clea ly beyond he objec i es o his s udy. 477

2D eac i e anspo modeling; Dá ila e al. (2016)
17
6.Summa y and conclusions 478
Two dimensional eac i e anspo models we e employed o in e p e he esul s om 479
pe cola ion expe imen s wi h single ac u e ma l co es du ing he injec ion o CO2- ich 480
solu ions unde supe c i ical condi ions (PTo al = 150 ba , pCO2 = 61 ba and T = 60 °C). 481
In he model, low ci cula ed h ough he ac u e, and solu e anspo in he ock 482
ma ix was caused only by di usion. Calcula ed solu ion concen a ion o he ou le 483
compa es ai ly well wi h he a ia ion in he measu ed concen a ions unde slow low 484
a es whe eas unde as low a poo ma ch was ob ained. The simula ions ep oduced 485
he dimensions o he dissolu ion pa e ns obse ed in ESEM and XMT images, excep 486
o he S- ee expe imen un unde high low a e (60 mL h-1) in which a wo mhole 487
o med (Dá ila e al., 2016a) in a po ous al e ed zone. These esul s conside ably 488
imp o e ou unde s anding o he di e en dissolu ion and p ecipi a ion p ocesses 489
obse ed du ing he pe cola ion expe imen s. 490
A success ul ma ch (excep o he as low a e) be ween he expe imen al and 491
calcula ed ou pu concen a ions was achie ed by using De alues be ween 1 and 3 × 492
10-13 m2 s-1 and a single alue o calci e i espec i e o he ype o solu ion wi h he 493
same eac i e su ace a ea (1.9 ± 1.5 × 104 m2mine al m-3 ock). In he as low a e 494
expe imen s (60 mL h-1) o p o ide mo e calci e eac i i y, he De alue showed a 495
sligh inc ease (6 × 10-12 m2 s-1) along wi h he calci e eac i e su ace a ea (2.5 ± 0.1 × 496
105 m2mine al m-3 ock). The su ace a ea alues o he o he mine als did no change. 497
Di e en composi ions o he injec ed solu ions p oduced di e se e ec s on he mine al 498
dissolu ion and p ecipi a ion eac ions in he Hon omín cap ock. The main eac ion ha 499
occu ed was dissolu ion o calci e in bo h ypes o injec ed solu ion. Fo he same 500
expe imen al ime and unde he same low a e, he olume o dissol ed calci e was 501
always la ge in he S- ee solu ion expe imen s han in he S- ich ones. In S- ich 502
solu ion expe imen s, gypsum p ecipi a ed mo e a 1 mL h-1. Dissolu ion o clinochlo e, 503
albi e, illi e and py i e, and p ecipi a ion o dolomi e, illi e, kaolini e, mesoli e, smec i e, 504
s ilbi e and boehmi e also occu ed. P ecipi a ion o seconda y Al and Si ich mine als 505
ook place o a lesse ex en as was expec ed om he mass balance calcula ions, bu 506
his was no obse ed in he ESEM and XMT images. 507
The calci e dissolu ion a e a he con ac be ween he ac u e and he ock ma ix, 508
whe e he acid solu ion eac ed wi h calci e unde an ad ec i e low egime, was as e 509
2D eac i e anspo modeling; Dá ila e al. (2016)
18
han he a es calcula ed as he eac ion mo ed owa ds he ock ma ix. This occu ed 510
because he accessibili y o he solu ion o he calci e su ace diminished du ing he 511
o ma ion o he al e ed zone whe e solu e anspo was con olled by di usion. This 512
phenomenon did no occu unde he as - low a e expe imen . 513
In bo h ypes o solu ion, he calcula ed po osi y in he eac ed zone was highe a he 514
ac u e wall con ac , dec easing wi h dis ance no mal o ac u e. An inc ease in he 515
low a e caused a ise in po osi y. The inc ease in po osi y was highe a he inle o he 516
co e han a he ou le . This di e ence is diminished a e speeding up he low a e. The 517
ise in po osi y om 6.7 70 % was simila o he olume o dissol ed 518
60 %). 519
In con as o he po osi y inc ease, ac u e pe meabili y unde slow low a es ended 520
o dec ease in he S- ich expe imen s and emained ai ly cons an in S- ee 521
expe imen s. In bo h ypes o solu ion, he calcula ed po osi y was g ea e unde he as 522
low a e (60 mL h-1) han unde slow low a es and k was conside ably highe in he S-523
ee injec ed solu ion. A new model accoun ing o he wo mhole o ma ion is 524
wa an ed o ully in e p e he esul s om he expe imen s un unde as low. 525
Acknowledgemen s 526
This wo k was inanced by p ojec s CGL2010-20984-C02-01 and CGL2014-54831-C3-527
1-R o he Spanish Go e nmen , p ojec 2014SGR (G up de Rece ca SGR) 1456 o he 528
Ca alan Go e nmen 529
F amewo k P og amme FP7/2007-2013 unde g an ag eemen numbe 282900). 530
Suppo was p o ided o GD by a JAE- a a la 531
LL by a Juan de la Cie a pos doc o al g an 532
(MINECO, Spain). We would like o hank Na àlia Mo eno and Jo di Bellés (IDAEA), 533
and E a Peleg í and Mai e Rome o (SCT-Ba celona Uni e si y) o analy ical 534
assis ance. We also wish o hank he anonymous e iewe s o hei cons uc i e 535
commen s ha ha e imp o ed he quali y o he pape . 536
Appendix 1 537
Table A1 Equilib ium cons an s (log Keq) o he homogeneous eac ions conside ed in 538
he eac i e anspo model. Reac ions a e w i en as he dissolu ion o 1 mol o he 539
2D eac i e anspo modeling; Dá ila e al. (2016)
19
species in he able and in e ms o Ca2+, Mg 2+, HCO3-, H+, SO42-, Na+, K+, Al3+, Cl-, 540
B -, Fe2+, SiO2(aq) and O2(aq). 541
Table A2 Mine al and gas equilib ium cons an s (log Keq) conside ed in he eac i e 542
anspo model. Reac ions a e w i en as he dissolu ion o 1 mol o he species Ca2+, 543
Mg 2+, HCO3-, H+, SO42-, Na+, K+, Al3+, Cl-, B -, Fe2+, SiO2(aq) and O2(aq). 544
Appendix 2 545
Figu e B1 Simula ed pH a ia ion wi h espec o ime in he S- ich (le ) and in he S-546
ee ( igh ) injec ed solu ions a he di e en low a es (0.2, 1 and 60 mL h-1). 547
548
2D eac i e anspo modeling; Dá ila e al. (2016)
20
Re e ences 549
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657
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24
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e ec s on chlo i e dissolu ion kine ics unde geological CO2 seques a ion ela ed 661
condi ions. Chem. Geol. 396, 208 217. 662
663
2D eac i e anspo modeling; Dá ila e al. (2016)
25
Figu e cap ions 664
Figu e 1 Schemes showing: a) simpli ied expe imen al se up (mo e de ails in Dá ila e 665
al., 2016a), b) ac u ed co e sample and low di ec ion, and he spa ial disc e iza ion 666
co esponding o he ac u ed ma l co es: c) cylind ical coo dina es, d) ec angula 667
coo dina es and e) mesh dis ibu ion along he co e sample. 668
Figu e 2 Va ia ion o he ou pu concen a ions wi h ime unde pCO2 o 61 ba and 60 669
°C in S- ich injec ed solu ion a 1 mL h-1 (exp. 4) o Ca, S, Mg, K, Fe and Si. Solid 670
symbols and solid line ep esen he expe imen al and calcula ed a ia ions, 671
espec i ely. The do ed line ep esen s he inpu solu ion concen a ion. 672
Figu e 3 Simula ed pH a ia ions o he ou le solu ion wi h espec o ime (le ) and 673
wi h dis ance no mal o ac u e and a a ious posi ions along he ac u e ( igh ) Exp. 4 674
a Q = 1 mL h-1 and S- ich injec ed solu ion. 675
Figu e 4 Va ia ions in he ou pu species concen a ions wi h ime unde pCO2 o 61 676
ba and 60 °C du ing S- ich injec ed solu ion expe imen s a Q = 0.2 mL h-1 (exp. 2; 677
le ) and a Q = 60 mL h-1 (exp. 7; igh ). Solid symbols and lines ep esen he 678
expe imen al and calcula ed (model A, B and C) a ia ions, espec i ely. The do ed 679
line ep esen s he inpu solu ion concen a ion. 680
Figu e 5 Va ia ions in he ou pu concen a ions wi h ime unde pCO2 o 61 ba and 60 681
°C in S- ee injec ed solu ion expe imen s a 0.2, 1 and 60 mL h-1 (exps. 1, 3 and 6): Ca, 682
S, Fe and Si. 683
Figu e 6 Va ia ions o he simula ed dissolu ion and p ecipi a ion a es o he p ima y 684
mine als (mol L-1 s-1) wi h espec o dis ance no mal o ac u e a di e en imes a he 685
ou le o he co e sample, o he 1 mL h-1 expe imen (S- ich): calci e (Cal), gypsum 686
(Gp), clinochlo e (Cln), albi e (Ab), qua z (Q z), py i e (Py), anhyd i e (Anh) and illi e 687
(Il ). 688
Figu e 7 Va ia ions o he simula ed p ecipi a ion a es o he seconda y mine als (mol 689
L-1 s-1) wi h espec o dis ance no mal o ac u e a di e en imes in a 1 mL h-1 690
expe imen (S- ich): dolomi e (Dol), mesoli e (Ms), smec i e (Smc), kaolini e (Kln), 691
s ilbi e (S l) and boehmi e (Bhm). 692
Table 5
p ima y phases seconda y phases
mine als
Cal
Il
Gp
Cln
Ab Kln
Dol
Ms
Slb
Bhm
VTo al-diss
[mm3]
VTo al-
pp
[mm3]
S- ich
solu ion
exp 2; Q = 0.2 mL h
-
1
Vmodel [mm
3
]
-
2.90
-
0.03
0.41
-
1.49
-0.01
0.01
0.78
0.19
1.35
0.13
-4.43 2.86
Vbalance [mm
3
]
-
2.80
nc
0.39
-
3.29
-(0-
1.50)
2.13-
4.34
nc
nc
nc
nc
-(6.09-
7.59)
2.51-
4.72
Vmodel/V
balance
1.04
n
c
1.05
0.45
exp 4; Q = 1 mL h
-
1
Vmodel [mm
3
]
-
18.84
-
0.16
7.05
-
3.70
-0.18
0.002
2.01
0.38
2.37
0.31
-22.88 12.13
Vbalance [mm
3
]
-
20.34
nc
7.17
-
4.51
-(0-
2.06)
2.69-
5.73
nc
nc
nc
nc
-(24.85-
26.91)
9.87-
12.90
Vmodel/V
balance
0.93
nc
0.98
0.82
exp 7; Q = 60 mL h
-
1
Vmodel [mm
3
]
-
7.83
-
0.17
4.73
-1.2
1
-0.15
0.001
0.46
-
0.04
0.11
-9.36 6.13
Vbalance [mm
3
]
-
9.09
nc
13.98
-
1.17
-(0-
0.53)
0.59-
1.38
nc
nc
nc
nc
-(10.25-
10.79)
14.57-
15.36
Vmodel/V
balance
0.86
nc
0.34
1.0
3
S- ee solu ion
exp 1; Q = 0.2 mL h
-
1
Vmodel [mm
3
]
-3.
07
-0.
03
-0.1
5
-1.
64
-0.01
0.002 0.
83
0.12
1.21
0.15
-4.90 2.31
Vbalance [mm
3
]
-
3.07
nc
-
0.14
-
2.29
-(0-
1.05)
1.49-
3.03
nc
nc
nc
nc
-(5.36-
6.41)
1.63-
3.17
Vmodel/V
balance
1.00
nc
1.07
0.72
exp 3; Q = 1 mL h
-
1
Vmodel [mm
3
]
-7.3
0
-
0.03
-0.8
3
-1.9
3
-0.02
0.002
0.8
7
0
.13
1.33
0.17
-10.11 2.51
Vbalance [mm
3
]
-
7.30
nc
-
0.62
-
1.53
-(0-
0.70)
0.96-
1.99
nc
nc
nc
nc
-(8.83-
9.53)
1.58-
2.61
Vmodel/V
balance
1.00
nc
1.34
1.26
exp 6; Q = 60 mL h
-
1
Vmodel [mm
3
]
-44.3
1
-
0.1
5
-2.
52
-1.8
9
-0.15
0.001
0.5
5
0.
03
0.
90
0.16
-49.02 1.63
Vbalance [mm
3
]
-
45.82
nc
nc
-
1.98
-(0-
0.90)
(0.98-
2.31)
nc
nc
nc
nc
-(47.80-
48.70)
19.00-
20.33
Vmodel/V
balance
0.97
nc
nc
0.95
VTo al-diss is he o al olume o dissol ed mine al and VTo al-pp is he o al olume o p ecipi a ed mine al.
V = V inal - Vini ial, whe e V > 0 and V < 0 indica e mine al p ecipi a ion and mine al dissolu ion,
espec i ely.
Values in pa en heses indica e he V ange conside ing he wo hypo he ical calcula ions.
nc indica es ha he olume could no be calcula ed om he mass balance (Il , Dol, Ms and S l).

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