Experimental Investigation on the Behavior of Supercritical CO2

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Article

Experimental Investigation on the Behavior of Supercritical CO during Reservoir Depressurization 2

Rong Li, Pei-Xue Jiang, Di He, Xue Chen, and Ruina Xu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02493 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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Environmental Science & Technology

Experimental Investigation on the Behavior of

1

2

Supercritical CO2 during Reservoir Depressurization Rong Li1,2 Peixue Jiang1,2 Di He1,2 Xue Chen1,2 Ruina Xu1,2*

3 4 1

5

Beijing Key Laboratory of CO2 Utilization and Reduction Technology, Department of Thermal Engineering, Tsinghua University

6 2

7

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University

8 9 10

*All correspondence should be addressed to Prof. Rui-Na Xu.

11

Address: Department of Thermal Engineering, Tsinghua University, Beijing 100084,

12

China

13

Telephone: (8610) 62792294;

14

Fax: (8610) 62792294;

15

E-mail: [email protected]

16

1

ACS Paragon Plus Environment


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Abstract CO2 sequestration in saline aquifers is a promising way to address

18

climate change. However, the pressure of the sequestration reservoir may decrease in

19

practice, which induces CO2 exsolution and expansion in the reservoir. In this study,

20

we conducted a core-scale experimental investigation on the depressurization of

21

CO2-containing sandstone using NMR equipment. Three different series of

22

experiments were designed to investigate the influence of the depressurization rate

23

and the initial CO2 states on the dynamics of different trapping mechanisms. The

24

pressure range of the depressurization was from 10.5 to 4.0 MPa, which covered the

25

supercritical and gaseous states of the CO2 (named as CO2(sc) and CO2(g),

26

respectively). It was found that when the aqueous phase saturated initially, the

27

exsolution behavior strongly depended on the depressurization rate. When the CO2

28

and aqueous phase co-existing initially, the expansion of the CO2(sc/g) contributed to

29

the incremental CO2 saturation in the core only when the CO2 occurred as residually

30

trapped. It indicates that the reservoir depressurization has the possibility to convert

31

the solubility trapping to the residual trapping phase, and/or convert the residual

32

trapping to mobile CO2.

33

TOC

34 2


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35 36

Introduction Geological CO2 utilization and sequestration is considered to be an effective way 1

37

to meet the challenge of climate change.

Saline aquifers, methane reservoirs, coal

38

bed methane, and shale gas reservoirs are the potential sequestration target formations.

39

The depth of these target formations mentioned in previous studies, is in the range

40

from 400 m to 2500 m, and correspondingly, the pressure is in the range from 2 MPa

41

to 28 MPa and reservoir temperature is in the range from 20 oC to 100 oC. 2-7

42

Among the aforementioned target reservoirs, saline aquifers are considered to

43

have the largest potential sequestration capacity. In saline aquifer CO2 sequestration

44

cases, reservoir depressurization can occur for several reasons e.g.a change in the

45

injection rate, withdrawal of the brine for reservoir pressure balance, and leakage

46

events.8-11 Because of the dependence of CO2 solubility on pressure in the aqueous

47

phase,12 depressurization can result in the exsolution of initially dissolved CO2 from

48

the aqueous phase.13 The exsolution and flow behavior of the CO2 during the

49

depressurization process have significant impacts on the estimation of CO2

50

sequestration safety. In most CO2 swept zones of the reservoir, CO2 not only dissolves

51

in the aqueous phase (mentioned as CO2(aq)) but also co-exists with the aqueous

52

phase as the CO2-rich phase (mentioned as supercritical CO2,

53

CO2, CO2(g) depending on whether the pressure is higher than 7.3 MPa). As presented

54

in Fig. S1 (see Supplementary information), both solubility of CO2(aq)and the

55

CO2(sc/g) density change significantly with pressure changes. Therefore, the

56

expansion of the pre-existing CO2(sc) and exsolution of CO2(aq) from the aqueous 3


CO2(sc) or gaseous


57

phase occur simultaneously during depressurization and have a coupled interaction

58

with each other.

59

Due to the complexity and significant hysteresis of two-phase flow through

60

porous media, characterization of the pore scale fluid distribution is insufficient using

61

only volumetric saturation of CO2(sc/g), SCO (sc/g) . When CO2(sc) is injected into an

62

aqueous phase saturated reservoir, the CO2(sc) is more likely to be highly

63

interconnected and mobile CO2, named as CO2,mobile in this study. However, as the

64

CO2(sc) swept zone possibly re-imbibes the aqueous phase after injection, snap-off

65

occurs and results in isolated immobile CO2(sc) ganglia, often called residually

66

trapped CO2 and abbreviated as CO2,residual in this work. This process is considered as

67

one of the important trapping mechanisms for CO2 sequestration in saline aquifer

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reservoirs.14-18 The depressurization behavior of CO2 and water-containing reservoirs

69

with a CO2,mobile and CO2,residual can differ significantly. In summary, understanding

70

the coupled process of exsolution and expansion under various initial CO2 states,

71

namely CO2(aq), CO2,mobile, and CO2,residual in the presence of reservoir

72

depressurization, has significant importance in estimating the CO2 sequestration

73

security.

2

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Experimental investigation on the exsolution behavior of CO2 from the aqueous

75

phase due to depressurization have been previously conducted to determine the

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solution gas drive19-23 process in the petroleum industry. However, most of these

77

studies were conducted under ambient pressure or slightly above ambient pressure24-26.

78

Experimental studies specifically focusing on the exsolution phenomena of the 4


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79

CO2-H2O fluid pair, under high pressure conditions, caused by pressure changes or

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thermal perturbation, were given more attention in recent years. Enouy et al.

81

conducted a column experiment and relevant numerical modeling of the CO2

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supersaturated water injection process. The pressure applied in their study was

83

slightly higher than ambient pressure.27 Luhmann et al. and Tutolo et al. conducted

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high pressure experiments of CO2 exsolution caused by increasing temperature.28,29

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Zuo et al. conducted a series of experimental studies on the exsolution behavior due to

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depressurization of the CO2-saturated aqueous solution at a relatively high pressure,

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which corresponds to typical reservoir conditions of deep saline aquifer CO2

88

sequestration sites.13,18,

89

behaviors using sandstone core samples and a micromodel, respectively. Xu et al.32

90

conducted

91

re-compression behavior of high pressure CO2 during the cyclic pressure oscillation

92

process. Xu et al.

93

exsolution process and the pore-scale morphology of exsolved CO2 showed strong

94

dependency on the depressurization rate. However, the coupled process of exsolution

95

and expansion under various initial CO2 states has not been investigated clearly. The

96

environmental effect of the status changing of dissolved CO2, mobile CO2, and

97

residually trapped CO2 in the presence of reservoir depressurization need to be studied

98

deeply.

a

30,31

core-scale

33

The studies focused on the core-scale and pore-scale

experimental

observation

on

the

exsolution

and

also obtained a pore-scale micromodel study on the CO2

99

In this study, we conducted a core-scale investigation on the depressurization of

100

CO2-containing sandstone using an experimental system combined with a high 5



101

pressure core-flooding set-up and Nuclear Magnetic Resonance (NMR) equipment.

102

To investigate the influence of the depressurization rate and the initial condition, three

103

different series of experiments were designed with various depressurization rate

104

controls and pre-treatment processes before the depressurization. The pressure range

105

of the depressurization ranged from 10.5 MPa to 4.0 MPa, and this pressure range

106

covered the states of the CO2(sc) and CO2(g).

107 108

Experimental Set-up

109

The diagram of the experimental setup is presented in Fig. S2 (see

110

supplementary information), which is a modification based on our previous

111

studies.32,34 The core sample was placed horizontally in a core holder. The core holder

112

was specially designed for the NMR scanner using PEEK material. The core sample

113

was wrapped and flexed in a thermal shrinking plastic. The blank NMR test result

114

showed that the signal responses of the core holder, the thermal shrink plastic and the

115

dry core were negligible.

116

FC-40 (3M Fluorinert) was selected as the confining fluid to avoid any additional

117

signal influencing the NMR scanning. The confining fluid was continuously supplied

118

by a high-pressure syringe pump and flooding through the confined space with a

119

continuous 50-ml/min flow rate. The confining pressure was controlled by a

120

backpressure regulator. The value of the effective confining pressure, namely the

121

difference between the confining and pore fluid pressure, was adjusted from 1.5MPA

122

to 2.5MPa over the course of the depressurization stage of each experiment. A heater 6


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123

located upstream of the confining fluid flow path controlled the temperature of the

124

confining fluid measured upstream of the core holder adjacent to the inlet of the

125

holder (PT100 thermo-resistance sensor). The confining fluid cycles with 50-ml/min

126

flow rates ensured the uniformity and consistency of the core temperature. The

127

temperature difference between the upstream and downstream regions of the core

128

holder (confining flow path) was less than 1°C throughout the experimental process.

129

The fluid was equilibrated at 10.5 MPa and 40 °C in a par-reactor equilibrium

130

cell. A high-pressure syringe pump (SSI/Laballiance Series 1500) supplied

131

pre-equilibrated CO2-saturated water or DI water to the core sample at the designated

132

flow rate.

133

The core sample was a water-wet Berea sandstone with a 25.2-mm diameter and

134

50.4-mm length. The porosity and absolute permeability of the core sample were 21%

135

and 685 mD, respectively, which were measured by weighing (measured at ambient

136

pressure) and steady state water-flooding experiments (measured at 10 MPa with a

137

2.5-MPa effective confining pressure).

138

A nuclear magnetic resonance imaging scanner (Niumag, MesoMR23-060H-I,

139

21.3 MHz, 0.5 T) was set-up for online visualization of images and quantitative

140

measurements. A CPMG (Carr-Purcell-Meiboom-Gill) impulse sequence was applied

141

to characterize the average CO2 saturation, SCO over the entire core sample. In

142

addition, the T2 intensity curve was obtained from the signal response, and the

143

pore-fluid distribution information was investigated based on this T2 intensity

144

curve.35,36

2

7



145

The uncertainty of the pressure and fluid saturation measurement based on our

146

experimental set-up was 0.02 MPa and 0.5%, respectively.

147

Experimental Procedure

148

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Three series of experiments, namely Series A, B, and C, were designed to

149

investigate the influence of the depressurizing rate and initial condition on the SCO

150

change during depressurization process.

2

151

Before the first experimental run, the core was dried and loaded onto the core

152

holder. Then, the core and tubing were vacuumed and flooded with 30 pore volumes

153

of low pressure CO2 (about 0.5 MPa) to remove the air. Subsequently, more than 30

154

PV of DI water was injected to displace and dissolve the CO2. Then, the system was

155

pressurized to 10.5 MPa through continuous injection of DI water. Before each

156

individual experimental case, at least 30 pore volumes of DI water were injected and

157

allowed to flow through the sample to displace and dissolve the residual CO2 and

158

maintain the initial conditions of the sample fully saturated with DI water. Then, at

159

least 30 pore volumes of CO2 saturated aqueous phase was flooded through the core,

160

which was initially saturated with DI water, at a 1.0-ml/min flow rate to ensure the DI

161

water was completely displaced by the CO2 saturated aqueous phase. Three types of

162

pretreatment processes, namely injecting the CO2 saturated aqueous phase, drainage

163

(refers to the non-wetting CO2(sc/g) injecting process) and imbibition (refers to the

164

wetting aqueous phase injection process) after drainage were applied to establish the

165

initial pore-fluid configuration states in three series of experiments.

166 8


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[Table 1 about here]

167

168

In Series A experiments, the core was directly depressurized with a constant

169

pressure variation rate. For the three experimental cases, the depressurization rates

170

were 8 KPa/min, 37 KPa/min, and 160 KPa/min. The depressurization process was

171

controlled using a programmable Isco pump. The depressurization process was

172

paused, and the pressure was maintained at a constant value to apply the CPMG NMR

173

sequence and to measure SCO at a given pressure. The duration of the measurement

174

was approximately 1 min, and the depressurization was restarted immediately after

175

the measurement. When the depressuization process was finalized at 4.0 MPa, the

176

final CO2 saturation SCO ,final was measured.

2

2

177

In experiment Series B, the depressurization process occurred after a drainage

178

process. Before drainage, the core was initially saturated with the aqueous phase as in

179

the aforementioned treatment process in Series A. To establish different CO2

180

saturation values, 3 different flow rates (0.1 ml/min, 0.5ml/min and 1.5 ml/min) for

181

the drainage were applied. Correspondingly, Capillary number for the displacement,

182

Ca=uµ/σ (u represents to the superficial velocity of displacement, µ represents to the

183

viscosity of displacing phase, and σ represents to the interfacial tension between

184

CO2(sc/g) and aqueous phase) ranged from 6.1×10-9 to 9.2×10-8. The steady states

185

of the drainage process were identified based on the criterion that the variation of the

186

NMR-measured SCO was less than 1%. The CO2(sc) saturation after pre-treatment

187

and before depressurization,

188

depressurized from 10.5 MPa to 4.0 MPa at a depletion rate of 37 KPa/min.

2

SCO2 ,P0 was

measured. Then, the system was

9



189

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In Series C experiments, the depressurization occurred after an imbibition

190

process following the drainage process, similar to Series B

191

from 6.1×10-9 to 7.4×10-7). The imbibition process was lasted for at least 20 pore

192

volumes of aqueous phase injection and stopped when a steady state was reached.

193

Therefore, the depressurization in Series C was initiated from the conditions

194

corresponding to the residually trapped cases. Then the CO2 saturation before

195

depressurization, SCO ,P was measured. For experimental Series C, the pressure also

196

decreased from 10.5 MPa to 4.0 MPa, and the depletion rate was 37 KPa/min. The

197

detailed information of the experimental cases is listed in Table 1.

2

(Ca numbers ranged

0

198

In experimental case A-1 and A-2, after the depressurization process, the

199

imbibition process with a 0.5-ml/min flow rate (Ca=3.9×10-7) was applied. The

200

imbibed aqueous phase was equilibrated at the final depleted pressure, 4.0 MPa. After

201

10 pore volumes of continuous injection, SCO was measured based on the CPMG

202

NMR sequence in each pore volume. When the relative SCO

203

successive measurements was less than 1%, the imbibition process was finalized since

204

the remaining mobile CO2(sc/g) was considered to be negligible. In these cases, the

205

CO2 saturation after exsolution and before imbibition, was named as initial CO2

206

saturation, SCO ,i , and the CO2 saturation after imbibition process was the residual

207

CO2 saturation, SCO ,R .

2

2

variation of two

2

2

208 209

Results and discussion

210

Exsolution from aqueous phase In Series A, the depressurization process was 10


Page 11 of 35


211

followed by the CO2 equilibrated aqueous phase injection. The change in SCO with

212

the pressure during the depressurization processes is shown in Fig. 1. The theoretical

213

maximum saturation of exsolved CO2 at given pressure P, SCO ,exsolution − max ( P) can be

214

calculated from the density of the CO2(sc/g) ρCO , and the solubility of the CO2(aq)

215

(defined in the form of mass fraction), xCO (aq)

2

2

2

2

217 218

(1)

SCO2 ,exsolution- max ( P)=( xCO2 (aq), P0 − xCO2 (aq), P ) ρH2O / ρCO2 , P

216

The theoretical maximum saturation SCO ,exsolution- max ( P) was calculated by Eq.1 and 2

plotted in Fig.1.

[Fig. 1 about here]

219 220

Generally, SCO increased with the decrease in the pore pressure results from the

221

exsolution and expansion. SCO changed from 0 to 21.1%, 13.1% and 2.3% in the

222

A-1, A-2 and A-3 experimental cases, respectively as the pore pressure decreased

223

from 10.5 MPa to 4.0 MPa. In case A-1 and case A-2, the depressurization could be

224

divided into two stages. In the initial stage, a very small change in SCO was observed

225

as the pore pressure decreased from 10.5 MPa to 7.5 MPa. After the initial 3 MPa of

226

the depressurization, and SCO exhibited a significant increase as the pore pressure

227

continued to decrease. During the initial stage, although the CO2(sc) starts to nucleate

228

and exsolve from the supersaturated aqueous phase, but due to the small accumulative

229

exsolved mass and the large CO2 density, the increment of SCO was relatively small.

230

In our recent pore-scale experiment, the nucleation of exsolved CO2 was observable

231

under the pressure around 7.3 MPa while the depressurization of saturated CO2(aq)

232

was initiated from 9.85 MPa under 40 oC.

2

2

2

2

2

33

The occurrence of the SCO trend

11


2


Page 12 of 35

233

transition under a pressure of approximately 7.3 to 7.5 MPa also resulted from the

234

specific density-pressure dependency of the CO2 (sc/g). The CO2 (sc/g) density

235

showed a sharp decrease when the depressurization ranged from about 9.5 MPa to 7.5

236

MPa, as shown in Fig. S2. The saturation contribution of the newly nucleated

237

CO2(sc)become observable after a significant expansion process. In this study, 7.5

238

MPa was the “critical” pressure at which the CO2 nuclei expanded sufficiently such

239

that the saturation started to show significant increment.

240

In the A-3 experiment (8 KPa/min depressurization rate), when the pore pressure

241

decreased from 10.5 MPa to 4.0 MPa, the exsolved SCO only increased from 0 to

242

2.3%. It is somehow unreasonable to expect the CO2(sc/g) to be interconnected and

243

mobilized. The CO2(aq) possibly remained in a supersaturated form in the aqueous

244

phase without significant exsolution. The diffusive mass transfer of CO2(aq) to the out

245

of the core can also possibly cause the supersaturation reduction without exsolution.

246

When the depressurization rate decreases, there are more sufficient time for diffusive

247

mass transfer, thus possibly reduce the SCO increase caused by exsolution. However,

248

due to the lack of measured CO2 concentration data in the aqueous phase based on the

249

current experimental setup, it is difficult to determine whether the CO2 component

250

remained in a supersaturated form or migrated out of the core through a diffusive

251

mass transfer.

2

2

252

In general, a faster depressurization rate results in higher exsolved SCO . Similar

253

to this observation, in previous studies on the solution gas drive process, this

254

depressurization rate dependency19 was also observed. Bauget and Lenormand20

2

12


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255

suggested that an increase in gas phase saturation with an increase in the

256

depressurization rate was the result of enhanced nucleation. Indeed, the more

257

nucleation events that occur in the pore space, the more dispersed the CO2(sc/g)

258

morphology. The dispersed CO2 (sc/g) is more difficult to interconnect with the flow

259

boundary and be mobilized. Thus, the exsolved CO2 is more likely to be trapped in

260

the sample and results in a higher SCO ,i as observed in this study. 2

261

[Fig. 2 about here]

262

The T2 intensity curves were obtained from the CPMG NMR signal responses

263

measured during the experiment A-2 as shown in Fig. 2. Similar to our previous

264

study32, during depressurization, the signal with a T2 relaxation time longer than 50

265

ms preferentially decreased, and the signal with a shorter relaxation time remained

266

almost unchanged. This result indicates that the exsolved CO2 preferentially occupies

267

the larger pores instead of finer pores, since the signal response with a longer T2

268

represents the occupancy of the aqueous phase in the larger pores.

269

The residually trapped CO2 refers to the CO2 remaining after the imbibition

270

process.

In

this

study,

the

imbibition

process

271

depressurization-induced CO2 exsolution for experimental case A-2 and A-3. As the

272

baseline for comparison, the drainage-imbibition processes were applied at 10.5 MPa

273

during the pretreatment process of the Series C experiments. For all imbibition

274

processes, the flow rate was controlled at 0.5 ml/min. The corresponding to the Ca

275

number were 3.9×10-7 under 4.0 MPa and 2.3×10-7 under 10.5 MPa calculated using

276

the aqueous phase viscosity and the interfacial tension of CO2 and the aqueous phase 13


was

applied

after

a


277

Page 14 of 35

under experimental conditions.

278

[Fig. 3 about here]

279 280 281

As shown in Fig. 3, the residual CO2 saturation values after exsolution, SCO ,R ,

282

were 12.7% and 20.4%, corresponding to exsolved CO2 saturation, SCO ,i equal to

283

21.1% and 14.0%, respectively. The residual trapping ratios (defined as SCO ,R / SCO ,i )

284

were approximately 90% in these cases. In the case of the drainage/imbibition process

285

(measured during the pre-treating stage of series C experiments), the residual trapping

286

ratio of the CO2 was approximately 40% to 60% in our experiments. It can be

287

observed that the exsolution-imbibition case showed a much higher residual CO2

288

trapping ratio compared with the drainage-imbibition case. Similarly, Zuo et al.18 and

289

Xu et al.32 also measured extraordinarily high residual trapping ratios in the

290

exsolution-imbibition cases. The pore-scale experimental investigation indicated that

291

the exsolution-generated CO2 (sc/g) had highly dispersed morphology and a small

292

ganglia size compared with the continuous displacement cases. The dispersed small

293

ganglia of CO2, therefore, are more difficult to be mobilized during the following

294

aqueous phase forced imbibition, as mentioned in previous studies. Based on this

295

perspective,

296

imbibition-induced residually trapped CO2. The exsolution process can be considered

297

as the conversion of CO2 from the solubility trapping phase to the residual trapping

298

phase.

2

2

2

the

exsolved

CO2

phase

showed

some

14


similarity

with

2

the

Page 15 of 35


299 300

Depressurization in the presence of CO2 with high mobility Reservoir

301

depressurization possibly occurs not only under the aqueous phase saturated condition

302

but also in the presence of mobile or trapped CO2. The influence of an initial

303

pre-existing CO2 (sc) was investigated in experimental cases in Series B and Series C.

304

In all of the experimental cases, the depressurization rate was 37 KPa/min.

305

[Fig. 4 about here]

306 307

In Series B, as presented in Table 1, three different initial conditions were

308

obtained by draining the core with different CO2 flow rates. In the three cases, the

309

CO2 saturation values before the depressurization, namely SCO ,P , were 22.9%, 28.1%,

310

and 40.4%. The change in SCO as the pressure decreased is shown in Fig. 4. SCO

311

increases with the pressure decrease. The finial CO2 saturation, SCO2-final, was

312

measured under the condition that the pore pressure was depleted to 4.0 MPa. The

313

values of SCO2-final were 32.1%, 35.6%, and 40.4%.

2

0

2

2

314

Similar to the experiments A-1 and A-2, the SCO remained almost unchanged as

315

the pressure decreased from 10.5 MPa to 7.5 MPa. As the pressure continued to

316

decrease, the SCO exhibited an increase in experiments B-1 and B-2. As the SCO , P

317

increased, the incremental saturation, ∆SCO =SCO ,final − SCO , P decreased.

2

2

2

2

2

2

0

0

318

After several pore volumes of continuous drainage, the pore-scale morphology of

319

the CO2 tended to be highly interconnected from the inside of the sample to the flow

320

boundary37. The remaining aqueous phase was more likely to be distributed in the 15



Page 16 of 35

321

finer pores and throats. Thus, the aqueous phase was largely immobile when the

322

drainage process continued. For the initial period following pressure depressurization,

323

the behavior of the volumetric expansion of the pre-existing CO2 (sc) remained

324

similar to the usual pressure gradient-driven displacement, in the case where the

325

expanding CO2 phase was highly interconnected as shown in Fig. 4(a). Therefore, it

326

was difficult for the residual aqueous phase after drainage to be mobilized by the

327

expansion mechanism. In this case, the expansion of CO2 preferentially resulted in the

328

continuous single phase migration to the outside of the sample.

329

When the system continued to depressurize, the contribution of newly nucleated

330

CO2 from the aqueous phase could be observed through the SCO change similar to

331

the results of the Series A experiments, as shown in Fig. 4(b). In experiment B-1 and

332

B-2, a significant SCO

333

approximately 7.5 MPa. However, in experiment B-3, which was the experimental

334

case with the highest SCO , P , the SCO

335

saturation, ∆SCO

336

saturation resulted in a lower potential exsolution capability because the

337

supersaturated mass of CO2 is proportional to the mass of aqueous phase when the

338

pressure changing range is constant. The incremental theoretical maximum CO2

339

saturation induced by exsolution under a given pressure P, can be estimated via the

340

solubility of the CO2(aq), xCO (aq), P and the density of the CO2(sc/g) ρCO , P .

2

2

increase occurred when the pressure decreased to

2

2

0

did not increase. The incremental CO2

2

decreased as the SCO , P increased. The lower aqueous phase 2

0

2

2

(2)

341

∆SCO2 ,exsolution- max ( P)=(1 − SCO2 , P0 )( xCO2 (aq), P0 − xCO2 (aq), P ) ρH2O / ρCO2 , P

342

Calculation results using Eq.2 under 4 MPa showed that ∆SCO ,exsolution- max (4.0MPa) 2

16


Page 17 of 35


343

decreased from 19.0% to 14.7% when SCO , P increased from 22.9% to 40.3%.

344

However, the results showed that even the aqueous phase saturation reduced, there are

345

still CO2(aq) mass can provide considerable amount of ∆SCO ,exsolution − max .

2

0

2

346

Moreover, the CO2(aq) less-likely exsolve as independent ganglia in the pores

347

that have pre-existing CO2(sc/g) as this exolved CO2 would immediately be

348

incorporated into the existing CO2(sc/g) ganglia. The higher SCO , P resulted in the

349

more pores with pre-exist CO2, and therefore, the lower ∆SCO contributed by the

350

exsolution. The higher SCO , P also resulted in the shorter average distance of aqueous

351

phase blobs from the interface of large CO2 ganglia connected to the flow boundary.

352

Therefore, in the high SCO , P case, the exsolution of aqueous phase occurred

353

preferentially at the pre-existing CO2 interface rather than in the bulk of the aqueous

354

phase or at the aqueous phase/pore wall interface through the nucleation mechanism.

355

Similar to the aforementioned volumetric expansion, the mass addition on the large

356

interconnected CO2 ganglia due to the interfacial mass transfer from the

357

supersaturated aqueous phase was not able to mobilize the aqueous phase either.

358

Depressurization in the presence of residually trapped CO2 In experiment Series C,

359

the depressurization process followed CO2 residual trapping. Four different cases

360

were obtained with various residual CO2 saturation values. The core was drained by

361

CO2 using different flow rates until the aqueous phase saturation stopped decreasing.

362

The CO2 saturation, SCO , P (identical to the SCO2-r in the drainage-imbibition case of

363

Fig. 3), ranged from 12.4% to 26.8% as the drainage flow rate varied. The change in

364

CO2 saturation with the pressure depletion is presented in Fig. 5.

2

0

2

2

0

2

2

0

0

17



Page 18 of 35

365

[Fig. 5 about here]

366 367 368

The SCO change during depressurization behaved significantly different from

369

that in the Series A and B experiments. As the pressure decreased from 10.5 MPa to

370

7.5 MPa, SCO increased significantly. When the pressure decreased from 10.5 MPa

371

to 7.5 MPa, the change in SCO increased as the saturation of residually trapped CO2

372

before depressurization, SCO , P increased. When the pressure continued to decrease

373

from 7.5 MPa to 4.0 MPa, the increase in SCO decreased as the CO2 saturation

374

measured at 7.5 MPa, SCO2,7.5, increased. In experiment case C-1, the SCO increased

375

from 20.3% to 27.3%, but in case C-4, the SCO remained almost constant.

376

2

2

2

2

0

2

2

2

The calculation based on Eq.2 showed that that

∆SCO2 ,exsolution- max (7.5MPa)

ranged

377

from 1.4% to 1.6%. This result showed that the contribution of exsolution on SCO

378

change was very limited. The theoretical maximum saturation change induced by the

379

CO2 expansion,

380

under given pressure, P.

∆SCO2 ,exp ansion − max (7.5MPa)

can be estimated by the density of CO2

(3)

∆S CO2 ,exp ansion − max ( P )=SCO2 , P0 × ( ρ CO2 , P0 / ρCO2 , P − 1)

381

2

382

The ∆SCO ,exp ansion- max (7.5MPa) ranged from 22.9% to 48.7% as SCO , P increased

383

from 12.4% to 26.8% in series C experiments. This calculation indicated that the

384

expansion of pre-existing CO2(sc) is more likely the major driving force for the SCO

385

change in pressure range of 10.5 MPa to 7.5 MPa. Thus, the significant saturation

386

increase resulted from the expansion of the CO2 (sc). As presented in Fig. 5(a), after

2

2

18


0

2

Page 19 of 35


387

re-imbibition treatment, the pre-existing CO2 (sc) in Series C occupied the pore space

388

as ganglia with various sizes but isolated from each other by the interconnected

389

aqueous phase. When the residually trapped CO2 expanded, the adjacent aqueous

390

phase was displaced as shown in Fig. 5(b). Therefore, the SCO increased. Once the

391

pathway connected to the boundary of the sample was established for certain ganglia,

392

the expansion effect preferentially resulted in the flow along the pathway rather than

393

draining the aqueous phase in the adjacent pores. In this case, the expansion-induced

394

SCO2 increase weakened as the SCO2 itself continued to increase. As the depleted

395

pressure ranged from 7.5 MPa to 4.0 MPa, the incremental CO2 saturation showed a

396

trend similar to that observed in the experiments in Series B. This indicates that the

397

pore-scale morphology of the CO2(sc/g) was almost converted from isolated ganglia

398

to the interconnected shape, similar to Series B. The incremental CO2 saturation

399

resulted from the newly nucleated CO2 bubbles in the aqueous phase when the

400

pressure decreased from 7.5 MPa to 4.0 MPa.

2

401

In summary, based on the experimental series B and C, the initial CO2 states

402

significantly influenced the initial depressurization stage (10.5 MPa to 7.5 MPa in our

403

case), dominated by the CO2(sc/g) expansion effect. In the following stage, the

404

CO2(sc/g) expansion more likely resulted in mobilization without draining the

405

aqueous phase, and the SCO

406

newly nucleated CO2ganglia.

407

2

increase was dominated by the exsolution-induced

[Fig. 6 about here]

408 19



409

Page 20 of 35

The final CO2 saturation, SCO ,final and incremental CO2 saturation, ∆SCO 2

2

410

measured at 4.0 MPa versus the CO2 saturation before depressurization, SCO ,P

411

presented in Fig. 6. The results indicate that the ∆SCO in Series B (after drainage)

412

was lower than the ∆SCO in Series C (after imbibition). More CO2 was easily

413

mobilized in Series B since the CO2(sc/g) had a continuous flow pathway to the

414

sample boundary. By contrast, the isolated CO2 ganglia after imbibition needed to

415

drain more surrounding aqueous phase before becoming interconnected and mobile.

416

Therefore, the well-connected CO2 phase was more problematic due to the concern of

417

unexpected mobilization compared with the trapped CO2 in the residual phase in the

418

presence of reservoir depressurization.

2

0

are

2

2

419 420

Environmental Implications

421

The behavior of supercritical CO2 during reservoir depressurization processes

422

with various initial conditions was investigated in this study. Three different types of

423

initial condition corresponded to three types of common states of CO2 in saline

424

aquifers, namely, mobile, residually trapped, and solubility trapped CO2.

425

[Fig. 7 about here]

426

As presented in Fig.7, in absence of the event induces the depressurization, the

427

mobile CO2 is converted into residually trapped CO2 through imbibition and the

428

snap-off effect, and both mobile and residually trapped CO2 forms are converted to

429

the solubility trapping phase through dissolution mass transfer as the time passes after

430

CO2 injection.1 Consequently, the safety performance of the sequestration increases. 20


Page 21 of 35


431

In the presence of reservoir depressurization, the solubility trapping phase tends

432

to be converted into residually trapped or mobile CO2 due to the exsolution effect.

433

The reservoir depressurization rate influences the proportion of the conversion. The

434

result of the imbibition test following by depressurization indicated that the exsolved

435

CO

436

residual trapping phase when the exsolved CO2 saturation was relatively low. This

437

trend implies less security concern with respect to uncontrollable CO2 mobilization

438

following reservoir depressurization. However during depressurization, the initially

439

isolated CO2 ganglia can expand. Isolated ganglia can expand and connect with

440

ganglia in adjacent pores creating a mobile pathway. In this case, the residually

441

trapped CO2 is gradually converted into mobile CO2, which results in potential

442

concern with respect to sequestration security, especially when the amplitude of the

443

depressurization is relatively large. In the presence of an initial interconnected CO2

444

phase, the expansion effect can possibly result in unfavorable mobilization even from

445

the early stage of the depressurization process. Compared with the residual trapping

446

case, the interconnected initial condition is more problematic in terms of sequestration

447

security due to the high mobility in the early period.

2

converted from the solubility trapping phase behaved most similarly to the

448 449 450 451

Supplementary Information Fig. S1, CO2 phase density and CO2 solubility in the aqueous phase below 40°C; Fig. S2, Experimental set-up of core-scale CO2 depressurization.

452 453

Acknowledgement 21



454 455

This project was supported by the National Natural Science Foundation of China (No. 51536004 and 51621062).

456 457

References

458

(1) IPCC. Underground Geological Storage. In IPCC Special Report on Carbon

459

Dioxide Capture and Storage, Prepared by Working Group III of the

460

Intergovernmental Panel on Climate Change; Metz, B.; Davidson, O.; de Coninck,

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H. C.; Loos, M.; Meyer, L. A.; Eds. Cambridge University Press: Cambridge,

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United Kingdom and New York, 2005; pp 195−276.

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(2) Jiang, P.X.; Li, X.L.; Xu, R.N.; Wang, Y.S.; Chen, M.S.;

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Wang H.M.; Ruan, B.L. Thermal modeling of CO2 in the injection well and rese

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rvoir at the Ordos CCS demonstration project, China. Int. J. Greenh. Gas Control.,

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2014, 23, 135-146.

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(3) Ahmed, T.K.; Nasrabadi, H. Case study on combined CO2 sequestration and

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low-salinity water production potential in a shallow saline aquifer in Qatar.

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Journal of Environmental Management, 2012, 109, 27-32

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(4) Jun, Y.S.; Giammar, D.E.; Werth, C.J. Impacts of geochemical reactions on

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geologic carbon sequestration. Environ. Sci. Technol. 2012, 47(1), 3−8.

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(5) Yang, F.; Bai, B.; Dunn-Norman, S.; Nygaard, R.; Eckert, A. Factors affecting

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CO2 storage capacity and efficiency with water withdrawal in shallow saline

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aquifers. Environ Earth Sci, 2014, 71, 267–275

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(6) Mathias, S.A.; Gluyas, J.G.; Oldenburg, C.M.; Tsang, C. Analytical solution for

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dioxide mixtures with phase transition during geological storage, International

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Journal of Multiphase Flow, 2014, 63, 82-92

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(8) Viswanathan, H. S.; Pawar, R. J.; Stauffer, P. H.; Kaszuba, J. P.; Carey, J. W.;

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Olsen, S. C.; Keating, G. N.; Kavetski, D.; Guthrie, G. D. Development of a

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hybrid process and system model for the assessment of wellbore leakage at a

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geologic CO2 sequestration site. Environ. Sci. Technol. 2008, 42 (19), 7280−7286.

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(9) Shukla, R.; Ranjith, P.; Haque, A.; Choi, X. A review of studies on CO2

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sequestration and caprock integrity. Fuel. 2010, 89(10), 2651−2664.

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(10) Li, Q.; Wei, Y. N.; Liu, G.; Lin, Q. Combination of CO2 geological storage with

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deep saline water recovery in western China: insights from numerical

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analyses. Appl. Energy. 2014, 116, 101−110.

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(11) Birkholzer, J. T.; Cihan, A.; Zhou, Q. Impact-driven pressure management via

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targeted brine extraction—Conceptual studies of CO2 storage in saline formations.

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Int. J. Greenh. Gas Control. 2012, 7, 168−180.

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(12) Spycher, N.; Pruess, K.; Ennis-King, J. CO2-H2O mixtures in the geological

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sequestration of CO2. I. Assessment and calculation of mutual solubilities from 12

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to 100 oC and up to 600 bar. Geochim. Cosmochim. Acta. 2003, 67 (16),

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3015−3031.

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depressurization. Transp. Porous Media.2012, 91, 459−478. (14) Suekane, T.; Nobuso, T.; Hirai, S.; Kiyota, M. Geological storage of carbon

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dioxide by residual gas and solubility trapping. Int. J. Greenh. Gas Control. 2008,

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2(1), 58−64.

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(15) Reynolds, C. A. and S. Krevor. Characterizing ﬂow behavior for gas injection:

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Relative permeability of CO2 brine and N2-water in heterogeneous rocks. Water

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Resour. Res. 2005, 51, doi:10.1002/2015WR018046.

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Trapping for Geologic Storage. Environ. Sci. Technol. 2015, 49(17),

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10265−10284.

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(17) El-Maghraby, R. M.; Blunt, M. J. Residual CO2 trapping in Indiana limestone.

Environ. Sci. Technol. 2012, 47(1), 227−233. (18) Zuo, L.; Benson, S. M. Process-dependent residual trapping of CO2 in sandstone.

Geophys. Res. Lett.2014, 41(8), 2820−2826. (19) Handy, L. L. A Laboratory study of oil recovery by solution gas drive. Petroleum

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in porous media: a critical review. SPE Annual Technical Conference and

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drive process in heavy oil reservoirs using a glass micromodel. SPE Reserv. Eval.

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Eng.2000, 3(03), 224−229.

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(23) George, D. S.; Hayat, O.; Kovscek, A. R. A microvisual study of

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solution-gas-drive mechanisms in viscous oils. J. Pet. Sci. Eng.2005, 46(1),

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101−119.

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(24) Satik, C.; Li, X.; Yortsos, Y. C. Scaling of single-bubble growth in a porous medium. Phys. Rev. E, 1995, 51(4), 3286. (25) Li, X.; Yortsos, Y. C. Theory of multiple bubble growth in porous media by solute diffusion. Chem. Eng. Sci.; 1995, 50(8), 1247−1271. (26) Li, X.; Yortsos, Y. C. Visualization and numerical studies of bubble growth during pressure depletion. SPE, 1991, 22589, 6−9. (27) Enouy, R.; Li, M.; Ioannidis, M. A.;& Unger, A. J. A.. Gas exsolution and flow

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during supersaturated water injection in porous media: II. Column experiments

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and continuum modeling. Adv. Water Resour. 2011, 34(1), 15−25.

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(28) Luhmann, A. J.; Kong, X. Z.; Tutolo, B. M.; Ding, K.; Saar, M. O.; Seyfried, W.

536

E. Permeability reduction produced by grain reorganization and accumulation of

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exsolved CO2 during geologic carbon sequestration: A new CO2 trapping

538

mechanism. Environ. Sci. Technol.2012, 47(1), 242−251.

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(29) Tutolo, B. M.; Luhmann, A. J.; Kong, X. Z.; Saar, M. O.; Seyfried Jr, W. E.

540

Experimental observation of permeability changes in dolomite at CO2

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sequestration conditions. Environ. Sci. Technol., 2014, 48(4), 2445-2452.

542

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543

2 exsolution from carbonated water in sedimentary rocks. Adv. Water Resour.2013,

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53(1), 188−197.

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(31) Perrin, J. C.; Falta, R. W.; Krevor, S.; Lin, Z.; Ellison, K.; Benson, S. M.

546

Laboratory experiments on core-scale behavior of CO2 exolved from

547

CO2-saturated brine. Energy Procedia, 2011, 4(22), 3210−3215.

548

(32) Xu, R.N.; Li, R.; Ma, J.; Jiang, P. X.; CO2 Exsolution from CO2 Saturated Water:

549

Core-Scale Experiments and Focus on Impacts of Pressure Variations. Environ.

550

Sci. Technol., 2015, 49 (24), 14696–14703.

551

(33) Xu, R.N.; Li, R.; Huang, F.; Jiang, P.X. Pore-scale visualization on a

552

depressurization induced CO2 exsolution. Sci. Bull. 62 (11), 795–803.

553

(34) Manceau, J. C.; Ma, J.; Li, R.; Audigane, P.; Jiang, P. X.; Xu, R. N.; Tremosa, J.;

554

Lerouge, C.; Two‐phase flow properties of a sandstone rock for the CO2/water

555

system: Core‐flooding experiments, and focus on impacts of mineralogical

556

changes. Water Resour. Res.2015, 51(4), 2885−2900.

557

(35) Mitchell, J.; Chandrasekera, T. C.; Holland, D. J.; Gladden, L. F.; Fordham, E. J.

558

Magnetic resonance imaging in laboratory petrophysical core analysis. Phys.

559

Rep.2013, 526(3), 165−225.

560 561

(36) Coates, G. R.; Xiao, L.; Prammer, M. G. NMR Logging Principles and Applications. Halliburton Energy Services, Houston. 1999.

562

(37) Iglauer, S.; Paluszny, A.; Pentland, C. H.; Blunt, M. J. (2011). Residual CO2

563

imaged with X-ray micro-tomography. Geophysical Research Letters, 2011,

564

38(21). 26


Page 26 of 35

Page 27 of 35


565

27



Page 28 of 35

Table 1. The details of the experimental cases

566

Series

A

Case

Depressurization rate

A-1

160 KPa/min

A-2

37 KPa/min

A-3

8 KPa/min

B-2

Initial CO2 state CO2(aq)

0.5 ml/min drainage(Ca=3.1×10-8) 1.5 ml/min

B-3

0% 0%

(Ca=6.1×10-9) 37 KPa/min

SCO2 ,P0

0%

0.1 ml/min drainage

B-1 B

Pre-treatment procedure

22.9% CO2,mobile and CO2(aq)

28.1% 40.4%

drainage(Ca=9.2×10-8) 0.1 ml/min drainage (Ca=6.1×10-9)+ 0.5 ml/min imbibition

C-1

12.4%

(Ca=3.9×10-7) 0.5 ml/min drainage(Ca=3.1×10-8) + 0.5 ml/min imbibition

C-2 C

37 KPa /min C-3

(Ca=3.9×10-7) 1.5 ml/min drainage (Ca=9.2×10-8)+ 0.5 ml/min imbibition

18.0% CO2,residual and CO2(aq) 23.2%

(Ca=3.9×10-7) 12.0 ml/min drainage C-4

(Ca=7.4×10-7)+ 0.5 ml/min imbibition (Ca=3.9×10-7)

567 568

28


26.8%

Page 29 of 35


569

Figures

570 571

Fig. 1 SCO during the depressurization process in Series A experiments. The

572

discrete data point was the saturation measured under different pressures when the

573

system is depressurizing. Figures numbered as (a), (b) and (c) present the schematic

574

diagram of the possible pore-scale mechanism. The uncertainty of the SCO is 0.005.

2

2

575

29



Page 30 of 35

576

250

10.5 MPa 7.5 MPa 5.6 MPa 4.8 MPa 4.0 MPa

Signal Intensigy

200

150

100

50

0

0.1

1

10

T2 (ms)

100

1000

577 578

Fig. 2 T2 distribution of the remaining H2O NMR signal after the exsolution

579

process in the A-2 experimental case. The larger T2 relaxation time represents a

580

coarser pore size in general. The signal intensity corresponding to a T2 relaxation time

581

longer than 50 ms shows a significant reduction in the depressurization rage from 7.5

582

MPa to 4.0 MPa. The signal intensity of T2 shorter than 50 ms does not show

583

significant variation.

584 585

30


Page 31 of 35


586 587

Fig. 3 SCO2 ,R vs SCO2 ,i in Drainage-Imbibition and Exsolution-Imbibition cases.

588

The drainage-imbibition saturation data were collected during the pre-treatment

589

process of Series C experiments. The Exsolution-Imbibition data were collected

590

during the following up forced imbibition after experimental case A-1 and A-2. SCO ,R

591

and SCO ,i represent the CO2 saturation before and after imbibition process,

592

respectively. The uncertainty of the SCO is 0.005.

2

2

2

593

31



594

595 596

Fig. 4 SCO during the depressurization processes in Series B experiments. The 2

597

results of experimental case A-2 was also plotted as the baseline without initial

598

CO2,mobile. The discrete data point was the saturation measured under different

599

pressures when the system is depressurizing. Figures numbered as (a) and (b) present

600

the schematic diagram of the possible pore-scale mechanism. The uncertainty of the

601

SCO2 is 0.005.

602

32


Page 32 of 35

Page 33 of 35


603 604

Fig. 5 SCO during the depressurization processes in Series C experiments. The 2

605

results of experimental case A-2 was also plotted as the baseline without initial

606

CO2,residual. The discrete data point was the saturation measured under different

607

pressures when the system is depressurizing. Figures numbered as (a) and (b) present

608

the schematic diagram of the possible pore-scale mechanism. The uncertainty of the

609

SCO2 is 0.005.

610

33



Page 34 of 35

611 612

Fig. 6 SCO ,final and ∆SCO vs SCO ,P . SCO , P refers to the value of SCO measured

613

after the pre-treatment stage and before depressurization stage at 10.5 MPa. SCO ,final

614

refers to the SCO measured at the end of depressurization stage(under 4.0 MPa).

615

∆SCO2 =SCO2 ,final − SCO2 , P0 , refers to the incremental CO2 saturation during the

616

depressurization stage. The uncertainty of the SCO is 0.005.

2

2

2

0

2

0

2

2

2

2

617

34


Page 35 of 35


618 619

Fig. 7 Schematic diagram of CO2 state conversion after injection. Arrows in the

620

left side refers to the usual condition without reservoir depressurization. In case the

621

reservoir depressurizes, the processes mentioned in the right side possibly occurs.

622

35


Experimental Investigation on the Behavior of Supercritical CO2

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