10th Asian Regional Conference of IAEG (2015)

CCS (Carbon dioxide Capture and Storage) demonstration project in Taiwan:
Evaluation of hydro-geological structure by laboratory tests
Masaki ONO(1), Chi-wen YU(2), Chung-Hui CHIAO(3), Shinichi HIRAMATSU(1)and Junya TAKESHIMA(1)(1) OYO Corporation, Japan
E-mail:ono-masaki@oyonet.oyo.co.jp(2) Sinotech Engineering Consultant Inc., Taiwan, (3) Taiwan Power Company, Taiwan.
Abstract
This paper outlines a CCS demonstration project in Taiwan, and introduces thegeological features of the test site and test results. A geological survey was conducted from2012 to 2013 in western Taiwan for a CO2 pilot injection test. A 3000m-deep well was
drilled to characterize the hydro-geological features at the test site. With the use of rockcores that were obtained from this deep drilling, many laboratory tests were performed,
including tests to evaluate the two-phase flow characteristics in a CO2-water system. The
permeabilities and basic physical properties of the rock cores recovered from the targeted
layers were carefully examined. A decrease in the elastic wave velocity and an increase inresistivity due to CO2 injection were observed in two-phase flow tests. The test results
indicated that CO2 migration would likely be detected by seismic or electrical survey
techniques at the site. A maximum threshold pressure of 13.4 MPa was measured inthreshold pressure tests and an inverse relationship was observed between the thresholdpressure and permeability. Based on the tests results, we can expect that both the sealcapacity and injectivity are at the same level as those in CO2 injection sites in Japan(Nagaoka and Tomakomai), which are also located within an active seismic area, like thepilot injection site in Taiwan.
Keywords: CCS, laboratory test, demonstration project1.

hydro-geological model to simulate CO2 storage.
This paper introduces a CCS demonstration
project in Taiwan, and presents the geologicalfeatures at the test site and the result of laboratory
tests that were conducted by OYO Corporation(OYO). The feasibility of CO2 sequestration is
discussed based on the obtained geological andgeophysical information.

Introduction

Many studies have sounded the alarm regarding
global climate change. One promising technique forslowing the progression of global warming is tocapture carbon dioxide and then store it deep
underground (CCS). From the perspective of thevolume of CO2 that can be expected to be stored, CO2
sequestration in a saline aquifer has been considered
to be an effective option. Before CO2 is injectedunderground, the physical properties of rocks in thecandidate injection site have to be investigated toassess the injectivity, seal capacity and/or suitabilityof geophysical exploration. To predict the behavior of
CO2 that has been injected underground, we mustalso carefully evaluate the two-phase flow
characteristics in a CO2-water system.
Table 1 shows two-phase flow characteristics.
The right-hand column shows how the respective test
result can be used. An understanding of thegeological structure is also needed to establish a

Table 1 Two-phase flow characteristics
Two-phase flow characteristics
Relative permeability
Threshold pressure

PurposeTo predict the behavior of
injected CO2
To assess the seal capacity

Elastic wave velocity versus
CO2 saturation

To interpret geophysicalResistivity versus CO2

exploration data

saturation1

10th Asian Regional Conference of IAEG (2015)

2.

Background and outline of the
project in Taiwan

better understanding of both the geological structureand geophysical information.

The Taiwanese government has set a goal of
reducing carbon dioxide emissions to the 2000 levelby 2025. CCS is considered to play an important rolein achieving this goal. CO2 emissions from the energy
sector account for 66% of the total CO2 emissions in
Taiwan (Tso, 2012). Measures to reduce CO2
emissions from thermal power plants, which accountfor approximately 80% of all generated electric power(Bureau of Energy, 2013), have become an important
issue in greenhouse gas reduction policy in Taiwan.
Taiwan Power Company started a littoral andgeological survey in 2008 to screen sites for CCS,
and the saline aquifer in the southern part of theTai-hsi basin was selected as a candidate site for theCO2 pilot injection test (Fig. 1).

3.

Geological feature

The island of Taiwan is located at the boundary
between the Philippine Sea Plate and the Eurasian
Plate, and is thought to have been formed by thecollision between these two tectonic plates. This longtime-scale tectonic activity has produced an
imbricated structure that mainly consists of
sedimentary rocks after the late Paleozoic and runs
north-northeast/south-southwest. The structure isdivided into six or seven geological groups and ayounger geological layer is located toward the west
(Fig. 2).
The Tai-hsi basin, the candidate site for CO2
pilot injection, is a Cenozoic sedimentary basin inwhich settlement and sedimentation have continued
to date. The geological characteristics of the basin
regarding CO2 sequestration are as follows:(1) Neogene or Quaternary geological layers thatare more than 3000 m thick constitute a gently
sloping homocline structure. Each of theselayers stretches continuously to the northnortheast. We expect that large amounts of CO2
can be stored here.
(2) Mud-stone layers that are expected to act as
cap-rock and sandstone layers that are potential
CO2 reservoirs are found at a depth of 800 m or
more. Thus, several CO2 storage scenarios canbe constructed.
(3) The candidate site is far from active faults andmicro-earthquakes rarely occur in this area.
Therefore,
withregardtogeologicalcharacteristics, the Tai-hsi basin is a feasible site not
only for CO2 pilot injection but also forcommercial-scale CO2 sequestration in Taiwan.

Fig. 1 Location of the pilot injection test site and an
isopach map of the Tai-hsi basin (based on Yu et al.,2013)

Test site

A seismic survey was conducted in 2010 toobtain further geological information (Ko et al.,2013). Based on an analysis of seismic survey data in2010 and a pre-existing 2-D seismic image, a thicksandstone layer (Kueichulin formation) at a depth ofaround 2400 m was considered to have high CO2
storage potential (up to 4.5 Gt-CO2) to be the mostpromising reservoir (Yu et al., 2013). The pilot
injection test program was launched in 2011. In thefirst stage of the program, a 3000 m vertical well was
drilled for site characterization. Non-core boring was
conducted until 1500 m, and an effort was made tosample all rock cores from 1500 m to 3000 m.
Sampled cores were used in various tests to obtain a

Fig.2 Geological profile along the southern flank of
the Tai-hsi Foreland Basin and location of the test site(based on Yu et al., 2014)
4.

Laboratory tests

4.1. Test items
The laboratory tests that were conducted by
OYO are shown in Table 2. Additional information2

10th Asian Regional Conference of IAEG (2015)

ranged from tens to thousands of millidarcy. On theother hand, the absolute permeabilities of silt stones,
which could be a candidate cap-rock, were lower than
10-2 millidarcy.

about the entire program has been reported previously(Yu et al., 2013; 2014). In addition to standard
laboratory tests to determine the basic physicalproperties, two-phase flow experiments using CO2 ina supercritical state (Photo 1) were extensively
conducted on rock cores obtained at various depths.
The test results were used not only to evaluate theinjectivity of preferred reservoirs and the sealcapacity of candidate cap-rock, but also as reference
data for determining the controlling parameters in anumerical flow simulation.
Table 2 List of tests conducted by OYO
Test items

Amount of test

Porosity measurement
Air permeability test
Water permeability test
Capillary pressure measurement by
mercury intrusion porosimetry
Thermal conductivity measurement
Threshold pressure measurement withCO2
Threshold pressure measurement with N2
Relative permeability test
Resistivity measurement during CO2
injection
P-wave velocity measurement during
CO2 injection
In-situ stress measurement by core
deformation method

20
20
20
27
20
7
12
11
10
6

Fig.3 Absolute permeability data

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Relative permeability is needed for use in anumerical flow simulation to predict the behavior of
injected CO2. Relative permeability is the ratio of theeffective permeability, the permeability of fluids in a
multiphase flow condition, to the absolutepermeability, which can be measured by apermeability test in a single-phase flow condition.
The relative permeability ranges from zero to one if
the physical properties of a porous medium are thesame in multi- and single-phase flow. The shapes ofrelative permeability curves have to be evaluatedprecisely to ensure the accuracy of the numerical flow
simulation.
Typical relative permeability curves in adrainage process that were obtained in this work areshown in Fig. 4 and Fig. 5. A core flood test with CO2
in unsteady state was used. The relative permeability
curves for both CO2 and water were calculated byusing a history matching technique and a Corey
exponent model was applied to the test data for
curve-fitting. Capillary pressure that should act at theinterface of CO2 and water and the electrochemical
force that acts around the clay mineral are different insandstone and siltstone due to the difference in the
diameter of the pore throat and the size of the rockminerals. Differences in capillary pressure andelectrochemical forces affect the shape of the relative
permeability curves. The irreducible water saturation,

Photo 1 Test apparatus for two-phase flow
experiments using supercritical CO2
4.2. Permeability
The absolute and relative permeabilities areimportant information for characterization of thereservoir. The absolute permeabilities are shown in
Fig. 3. Depending on the rock type, the absolutepermeability of the rocks ranged from 104 to 10-5
millidarcy. The absolute permeabilities of sand stones,
which are expected to be a potential CO2 reservoir,

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10th Asian Regional Conference of IAEG (2015)

which is the water saturation ratio at the endpoint of
the curves, in sandstone (approximately 0.45) issmaller than that in siltstone (0.65).

the cap-rock, CO2 percolates into the cap-rock and, inan extreme case, can finally reach the surface. For
safety assurance, an evaluation of the thresholdpressure is necessary, especially when CO2 is injected
into a saline aquifer, since little geological data isavailable for a CO2 sequestration project in a saline
aquifer, compared with CO2 injection into a depleted
oil reservoir or EOR (Enhanced Oil Recovery). The
results of the threshold pressure test are shown in Fig.
6. The test results from previous studies are also
shown for reference (Li et al., 2005; Hildenbrand etal., 2004). In this work, threshold pressure tests wereconducted by the staged pressurization method (Li etal., 2005) on low-permeable silt stones that could becandidates for cap-rock. CO2 in a supercritical stateand gaseous N2 were used as the injection fluids.
The yellow squares in Fig. 6 represent thethreshold pressure determined with gaseous N2. These
values were converted to those for a CO2-water
system by considering the interfacial tension ratio
between CO2 and N2. The green triangles show thetest results obtained with CO2. An inverse correlation
was observed between absolute permeability andthreshold pressure. If we excluded the results
obtained with abnormal cores that showed cracks orveins of sand, the maximum and minimum thresholdpressures were 13.4 MPa and 0.71 MPa, respectively,in the test using supercritical CO2.

Fig.4 Relative permeability curves of sandstone(Absolute permeability is 95 millidarcy)

Fig.5 Relative permeability curves of siltstone(Absolute permeability is 0.003 millidarcy)

Fig.6 Correlation between threshold pressure
versus absolute permeability

4.3. Seal capacity
Threshold pressure is at which the injection
pressure of CO2 overcomes the capillary pressure andCO2 starts to percolate into rock. If we consider CO2
sequestration at a depth of roughly 1000 m since CO2
is less dense than water so that the buoyancy force
acts upward in CO2, a CO2 plume moves toward thesurface. When this upward force (buoyancy force ×
CO2 column height) exceeds the capillary pressure in

4.4. Changes in the elastic wave velocity andresistivity due to CO2 injection
After CO2 is injected underground, CO2monitoring with a geophysical exploration techniquewill be needed not only to ensure that CO2 issequestrated in the reservoir, but to check theaccuracy of the numerical flow simulation. Seismic
methods such as seismic reflection, velocity logging

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10th Asian Regional Conference of IAEG (2015)

and elastic wave tomography are effective tools forCO2 monitoring. Electrical methods such as
resistivity logging, resistivity tomography andelectromagnetic survey are also potential techniques
for capturing CO2 migration underground. These
techniques are based on the fact that elastic wave
velocity and electrical resistivity change depending
on changes in the fluid within the pore space of rock.
To confirm that geophysical exploration
techniques are suitable for CO2 monitoring, P wave
and electrical resistivity measurements were
performed in the laboratory during CO2 injection intoa rock core. Sandy and high-permeable rocks wereused. The change in P wave velocity during CO2
injection is shown in Fig. 7. Although the percentageof the decrease in the P wave velocity differeddepending on the rock core, in all cases a distinct
decrease in P wave velocity was observed with anincrease in CO2 saturation.
The change in electrical resistivity during CO2
injection was also measured. Rock cores weresaturated with a sodium chloride solution that was
adjusted to have approximately the same electricalcharacteristics as sea water because the electricalcharacteristics of rock are strongly influenced by thepore fluid. Three electrodes were placed on the upper,
middle and bottom parts of the cores to measure thechanges in electrical resistance. The electricalresistivities that were measured during CO2 injection
are shown in Fig. 8 (normalized by initial resistivity).
The Parameters n in Fig. 8 are the saturation exponent
in Archie’s formula. In contrast to the results in the P
wave measurement, electrical resistivity increased in
all of the tests.

Fig.8 Correlation between normalized electricalresistivity versus water saturation
5.

Discussion

The CCS demonstration project in Nagaoka,
Japan in 2003 is one of the few CCS projects that has
been conducted in a seismically active area. Anotherdemonstration project is now on-going in Tomakomai,
Japan. These two projects can be used as a reference.
In the Nagaoka project, the lower part of the Haizume
layer (early Pleistocene), which had an absolutepermeability of several millidarcy, was selected as thereservoir. The upper part of the Haizume layer, which
had an absolute permeability on the order of 10-2
millidarcy, was expected to act as a cap-rock(Research Institute of Innovative Technology for the
Earth, 2002). A total of one M-ton CO2 was injectedsuccessfully, and the geophysical exploration data
have indicated that the CO2 has remained
sequestrated in the reservoir to date. In the
Tomakomai project, 10 M-ton/year CO2 has beenscheduled for injection into two different reservoirs(Ministry of Economy, Trade and Industry, 2011).
Based on an investigation that included laboratory
tests, physical properties have been determined toestablish the hydro-geological structure and a seriesof numerical flow simulations have been conducted.
The permeability tests in this work revealed thatsandy stones, which could be a candidate reservoir,
have high permeability that was almost the same oreven higher than that at the Nagaoka site. Thissuggests that the sufficient injectivity could be
expected in the preferred reservoir at thedemonstration site in Taiwan.
A maximum threshold pressure of 13.4 MPa anda minimum threshold pressure of 0.71 MPa wereobtained by the threshold pressure test using
supercritical CO2. The minimum value is almost thesame as the minimum threshold pressure of
mudstones that was obtained from a characterization
well (Tomakomai CCS-2 well) in Tomakomai.

Fig.7 Correlation between P wave velocity
versus CO2 saturation

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10th Asian Regional Conference of IAEG (2015)

Test data for both P-wave and electricalresistivity during CO2 injection showed clear changes
in these physical parameters. In the CCS
demonstration project in Nagaoka, Japan, a decreasein the P wave velocity of up to 13.4 % was observedby acoustic logging analysis (Xue and Matsuoka,2008). Although there are some significant
differences between a laboratory-scale test and a fieldsurvey such as the wavelength of the P wave, thecharacteristics of the measuring device and themeasuring environment, it is highly likely that we
could determine the area of CO2 by acoustic logging
if the decrease in P wave velocity measured in thefield is comparable to that in a laboratory-scale test.
The change in electrical resistivity during CO2
injection also suggests that the electrical geophysicalexploration technique could be a useful for
supplementing a seismic survey.
6.

important consideration in the numerical flow
simulation.
Acknowledgement
All of the findings described in this paper wereobtained through the CCS demonstration project,“On-site investigation and planning for a pilot test
program in a preferred carbon sequestration reservoir(Phase-1)”, funded by Taiwan Power Company. The
project was successfully carried out by Sinotech
Engineering Consultant, Inc. The authors are gratefulto the editor and anonymous reviewers for theircomments and suggestions.
References
Tso, CT. (2012): Conceptual Strategy of Developing
CCS technology roadmap and industry cluster in
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Yu, CW., Ko, WC., Chiao, CH., Hwang, LT. andYang, MW. (2013): Planning a Pilot Injection Test
for a 3000m Deep Saline Aquifer in a Preferred
Carbon Sequestration Site, Energy Procedia, Vol.37,
pp.4960-4967.
Ko, WC., Yu, CW., Chiao, CH., Hwang, LT., andYang, MW. (2013): Seismic Reflection Survey to
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Characterizing a Candidate Deep Saline Aquifer inWestern Taiwan, Energy Procedia, Vol.63,
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Nickel, E. (2005): Gas breakthrough pressure forhydrocarbon reservoir seal rocks: implications forthe security of long-term CO2 storage in the
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Hildenbrand, A., Schlomer, S., Krooss, BM., andLittke, R. (2004): Gas breakthrough experiments onpolitic rocks: comparative study with N2, CO2 andCH4, Geofluids, 4, pp.61-80.
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Conclusion

The conclusions that can be drawn from thegeological features and the test results can besummarized as follows:(1) Based on the geological features of the test site,
such as the presence of thick and continuous
geological layers, the availability of potential
cap-rocks and potential storage layers and the
distance from an active fault, the Tai-hsi basin isa feasible site for CO2 pilot injection.
(2) The absolute permeabilities of sandy stones
obtained in this work were almost the same oreven greater than those in the reservoir at the
Nagaoka site. This result supports the feasibilityof a CO2 pilot injection test at the test site.
(3) The minimum threshold pressure obtained withthe use of supercritical CO2 was 0.71 MPa. Thisvalue is comparable to the minimum thresholdpressure of mudstones that was determined froma characterization well (Tomakomai CCS-2 well)in Tomakomai.
(4) Changes in both the P-wave velocity andelectrical resistivity were measured in a CO2
flooding test. This result suggests that CO2
migration would likely be detected by ageophysical exploration technique in situ.
This study attempted to evaluate the validity of apilot test of CO2 injection that has been planned in
Taiwan by considering only limited information
regarding the geology and laboratory test data. The
geological study and laboratory data indicate that
there is a high possibility of successful CO2 storage at
the preferred site in Taiwan. However, further studies
will be needed for a precise evaluation of thefeasibility of the project. For example, the continuityof preferred storage and seal layers has to beinvestigated. Heterogeneities in each layer are an

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