Development of Carbon Dioxide Microbubble Sequestration
into Saline Aquifer and CO2-EOR Reservoirs
Introduction
To study the effect of microbubble injection for CO2 dissolution, we carried out laboratory experiments of CO2 flooding in porous sandstone (Berea sandstone). Using X‐ray CT image analysis, porosity
estimation and CO2 saturation monitoring were conducted. A long core specimen was used to determine how much the microbubble effect reached in our experiment system. On the basis ofexperimental results, we try to evaluate the superiority of microbubble injection for CO2 dissolution by comparing the difference between microbubble and normal‐bubble injections. We expect that themicrobubble CO2 injection technique will contribute to geological CO2 sequestration.

Rock specimen

Test system

Berea sandstone (diameter: 34.85mm, length: 288.00mm) was used in this study. It has bedding planes parallel to the core axis.
We set the bedding planes horizontally. Microbubble filter (diameter: 34.80mm, length: 4.00mm) was located in between
distributer and core specimen in upstream side.

The experiments were carried out under the pressure and temperature conditions that simulate
underground environments; pore pressure: 10MPa, temperature: 40 degrees Celsius. The confiningpressure of 15MPa was selected in this study. The syringe pumps on the upstream side was controlled tomaintain 0.05ml/min (constant flow control). And the syringe pump on the downstream side wascontrolled to maintain 10 MPa.

SPAP

microbubble filter

288mm long core specimen

CP

Physical properties of specimen & MB filter

Schematic diagram of test system
SP：syringe pump (SP1: CO2, SP2: water, SP3: confining oil, SP4: back pressure), S： rock sample, PT：
pressure transducer, d：distributor, f： microbubble filter

microbubble

normal‐bubble

CO2 saturations

Major crossSections in CT

Field application

Micro‐bubble
filterConclusions•••

CO2‐flooding laboratory experiments of porous sandstone (Berea sandstone; long core specimen; 288mm) and X‐ray CT visualization were carried out to study the effect of microbubble injection for CO2 dissolution.
We could estimate the porosity of specimen and visualize the process of water injection and CO2‐flooding process by the X‐ray CT image analysis. CO2 saturations during the experiments were also obtained.
The CO2 saturation distributions along the specimen were different in between the cases of microbubble and normal‐bubble CO2 injections. At each breakthrough point, there was a difference of about 5% points of CO2 saturation. It reveals that the microbubble
CO2 injection has more advantage to the CO2 dissolution for geological CO2 sequestration.

Acknowledgement :This work is part of an R&D project, “Research and Development of Safety Technology for Geological CO2 Storage” by the Ministry of Economy, Trade and Industry (METI) of Japan.

海水中のCO2濃度（pCO2）と溶存酸素（DO）の関係を用いた異常値判定基準について－大阪湾の長期観測データの解析事例－
目的 観測されたpCO2から異常値を検出するための次の2つの基準値についてその有効性について検討を行う。
① pCO2一定基準値：季節ごとに設定した一定のpCO2値が閾値 ② pCO2-DO基準値：pCO2とDO（溶存酸素）の関係を用いた閾値（Uchimoto et al., 2017）
解析データ
海域：大阪湾。20測点（図1）のうち測点2-8を鉛直混合の強い西部、測点1, 9-20を成層の強い東部として解析（cf. エスチュアリ
循環: 城, 1989; 中嶋・藤原,2007; 図2）
データ：浅海定線調査データ（ 2002年～2010年の2月、5月、8月、11月に底層で観測された
水温、塩分、DO、pH;
http://kouwan.pa.kkr.mlit.go.jp/kankyo-db）
pCO2：塩分から求めた全アルカ
リ度（田口ら, 2009）とpHを使ってCO2SYSで計算【冬季】

図1: 測点位置と水深（カラー）

基準値

西部

東部

【夏季】

西部

東部

図2: 大阪湾のエスチュアリ循環概念図。(a) 冬季は西部で混合された水が海底に沿って東部に流れ込むため東部底層も高DO, 低pCO2。(b) 夏季は東部の底層より上に流れ込むため東部底層が低DO, 高pCO2になりやすい。（中嶋・藤原, 2007）①pCO2一定基準値：各月ごとに基準値を決定する。ここでは例として平均値＋2.57σを基準値とした（表1）②pCO2-DO基準値：pCO2とDO（%）の線形回帰の予測範囲上限を基準値とする。ここでは例として回帰直線の99%予測範囲上限を基準値とした（図3）
2月

5月

8月

東部

455.7

811.9 1496.2 619.7

西部

453.7

541.3

707.4

11月

東部

西部

601.7

図3（→）: 黒太点線がpCO2‐DO基準値（回帰直線（緑実線）の99%予測範囲の上限）。(a)東部、(b) 西部。黒細実線（横線）はpCO2全データの平均値、黒細点線はpCO2平均値＋2.57σ。黒縦実線はDOの最小値と最大値。
False-positive（正常値を異常値と誤判定；表2, 3）とFalse-negative（異常値を正常値と誤判定＝漏出の見落とし；図4）
False-positive：通年で考えると2つの基準値に大きな違いはないが、pCO2-DO基準値は特定の月にFalse-positiveが集中（東部：8月、西部：11月）
False-negative：海域・月によってどちらの基準がよいか（Falsenegativeが少ないか）は異なる。しかしpCO2-DO基準値はΔpCO2が350μatmであれば海域・季節によらずほぼ漏出の見落としが起きない（東部の8月でも15%以下）のに対し、pCO2一定基準値は東部の夏季に見落としが約50%以上になる。
表2: 東部の2002年～2010年のデータに対するFalse‐positiveの数。

基準値
pCO2一定pCO2‐DO

2月
5月
8月
11月
計
0 (0%) 3 (2.59%) 1 (0.86%) 1 (0.85%) 5 (1.08%)
0 (0%) 0 (0%) 6 (5.17%) 2 (1.71%) 8 (1.72%)

表3: 西部の2002年～2010年のデータに対するFalse‐positiveの数。

基準値
pCO2一定pCO2‐DO

2月
5月
8月
11月
計
0 (0%) 1 (1.59%) 2 (3.17%) 1 (1.59%) 4 (1.59%)
0 (0%) 0 (0%)
0 (0%) 3 (4.76%) 3 (1.19%)

図4（→）: ΔpCO2（CO2漏出によるpCO2の増加）に対するfalse‐negativeの割合（%）。赤がpCO2一定基準値、青がpCO2‐DO基準値。(a) 東部、(b) 西部。
False‐negativeが100%（0%）であれば漏出検出が0%（100%）という意味。
結論 pCO2一定基準値とpCO2-DO基準値のどちらがよいかは海域や季節に依存する。pCO2の変動が海面ガス交換の影響を強く受ける海域や季節はpCO2一定基準値の方がよく、海面ガス交換の影響を受けにくい海域や季節ではpCO2-DO基準値の方がよい。成層の強い大阪湾東部海域では、底層に海面ガス交換の影響を受けた西部の海水が供給される冬季はpCO2一定基準値の方がよく、底層に西部からの海水が供給されにくい夏季はpCO2一定基準値では漏出検出が極めて困難であり、pCO2-DO基準値が必須となる。一方、年間を通して海面ガス交換の影響の強い西部では、pCO2一定基準値の方がpCO2-DO基準値よりも優れているかまたは同等であり、pCO2の正常異常の判定のためにDOを導入する必要がないと考えられる。
参考文献 Uchimoto et al. (2017) Energy Procedia, 114, pp.3771-3777. 城 (1989) 沿岸海洋研究ノート, 26(2), pp.87-98. 田口ら (2009) 沿岸海洋研究, 47(1), pp.71-75. 中嶋・藤原 (2007) 沿岸海洋研究, 44(2), pp.157-163.

謝辞 本研究は、経済産業省の「安全なCCS実施のためのCO2貯留技術の研究開発事業」の成果の一部である。

Dynamic Stability Monitoring of GeologicalFormations using Fiber-Optic Sensing
It is important to monitor deformations of geological formations as well as temperature and pressure for a safety evaluation at the CCS site.
We are developing a technology to monitor the deformations of the formations using a distributed fiber‐optic sensing. Our goal is to deploy
fiber‐optic cables behind the casing into a deep borehole and cement in place, and the formation deformations (strain) from the surface to thebottom hole are to be monitored continuously in the depth direction.

1. Motivation•••••

2. Measuring method

At the In Salah field, surface uplift around CO2 injection wells were mapped by InSAR.
Reservoir expansion induced by pore pressure build‐up would go up to the surface.
If the expansion gets larger safety of a seal layer may be decreased.
It is important to monitor the deformations from the reservoir to the surface.
Since measure points are limited for conventional tools, research and development of
distributed (multi‐point measurable) sensors are required.3

fig. 1 Geological formations monitoring
using the optical fibers at the CCS site•

When pulsed light is entered into an optic fiber, back scattering lights are induced throughout fiber’s length.
They produce frequency shifts in proportion to each variation of physical quantity (temperature, pressureand strain).
These frequency shifts consists of temperature, pressure and strain changes, It is necessary to separate eachparameter in case of borehole monitoring (fig. 3).
Hybrid Brillouin‐Rayleigh measurement enables to distinguish among them.

••fig. 2 Brillouin gain spectrum
measurement

fig. 3 Physical quantities in the frequency shift

4. Water pumping test

3. Pilot test ‐300 m deep borehole‐•••• The fiber‐optic cables were deployed behind the casing (annulus) andcemented in place.

• A little amount of CO2 was injected into a sand layer through perforation zones
of a monitoring well during a day.
• The fibers cemented behind the casing has been successfully applied tomeasure deformations occurred by CO2 injection at the injection zone.
•••

6. Future works•••

Development and improvement of the high sensitivity and high strength fiber optic cable with theinstallation behind the casing.
Improvements of the temperature, pressure and strain separation technique with the hybrid Brillouin‐
Rayleigh processing.
Automation of data processing and analysis, realization of remote operation, and packaging of theobservation equipment.
This work is part of an R&D project “the Development of Safety Management Technology for Large‐Scale
CO2 Geological Storage, commissioned to the Geological Carbon Dioxide Storage Technology Research
Association by the Ministry of Economy, Trade and Industry (METI) of Japan.

fig. 4 Separation of temperature,pressure and strain

Pumping test was performed using two wells, 280 m and 175m away from the monitoring well.
The pumping zones are sand rich sand‐silt alternate layer (aquifer) of 150 m‐230 m depth.
The fibers detected the compression of the aquifer linked with pumping works.

5. Distributed acoustic sensing (DAS)DAS allows seismic monitoring with fiber‐optic cable and has developed recent years.
A DAS applicability test was performed using the optic fibers which were already deployed behind thecasing for the formation deformations monitoring.
Through the stacked waveform obtained by the fiber included some noises, P‐wave first break was able tobe read as well as a geophone data.