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Influence of Sedimentation Heterogeneity on CO2 Flooding
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 114 (2017) 2933 – 2941
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Influence of sedimentation heterogeneity on CO2 flooding Hyuck Parka*, Lanlan Jianga, Tamotsu Kiyamaa, Yi Zhanga, Ryo Uedab, Masanori Nakanob, Ziqiu Xuea a
Research Institute of Innovative Technology for the Earth (RITE), Japan b Japan Petroleum Exploration Co., Ltd. (JAPEX), Japan
Abstract
To study CO2 flow characteristics in heterogeneous rock, we designed a laboratory experimental system that visualizes the CO2 movements in rock specimens by using X-ray computed tomography (CT). We carried out laboratory experiments of CO2 flooding in porous sandstones, together with porosity calculation, fluid saturation monitoring based on the CT images and mass flow measurements for ejected fluids. We used Berea sandstone and Sarukawa sandstone as homogeneous and heterogeneous rock in this study. The CO2 flooding tests were carried out until the CO2 injection reaches about 2 PV (pore volume) for the Berea sandstone and about 3 PV for the Sarukawa sandstone. When injected CO2 reached 1.0 PV in both specimens, The CO2 saturations were 34.15% and 15.21% in Berea sandstone and Sarukawa sandstone, respectively. We increased the differential pressure only for the Sarukawa sandstone to confirm the influence of differential pressure on CO2 saturation and oil recovery in heterogeneous media. The oil recoveries were 74.80% and 71.93% for Berea sandstone and Sarukawa sandstone, respectively when injected CO2 reached about 2.0 PV. CO2 was spreading evenly from the injection part through the Berea sandstone. In the case of Sarukawa sandstone, almost all of the CO2 preferentially moved through the upper part of specimen. This represents that the sedimentation heterogeneity is one of the main factors that affects the CO2 flooding pattern. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: CO2 injection; CO2 flooding; heterogeneity; X-ray CT; Berea sandstone; Sarukawa sandstone;
1. Introduction Understanding the flow mechanisms of CO2 flooding in reservoir rock is important for CO2 geological sequestration and utilization, for example enhanced oil recovery. It is also essential to understand the influence of rock heterogeneity on CO2 flow. Due to the practical difficulties, few laboratory experiments have been performed on CO2-brine systems
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1422
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[1,2,3]. Perrin and Benson [3] carried out CO2/brine two-phase flow experiments to investigate the influence of subcore scale heterogeneities on the distribution of CO2 at steady state. Recently, it began to increase the laboratory experiments combined with X-ray CT scanning to measure the CO2 saturation distribution during a set of steady-state relative permeability measurements [4,5]. The heterogeneity of reservoir rock is a factor greatly affects the storage location selection. In this study, we try to understand the flooding characteristics of CO2 in heterogeneous rocks having complex sedimentary structures, which will contribute to CO2 geological sequestration and oil recovery. This paper reports the CO2 flooding test results for two different sandstones, which previously saturated by water and oil together. We first evaluated the porosity of core specimens based on the X-ray CT scanning and then visualized the CO2 flooding patterns in both specimens. Finally, we compared the CO2 saturation, total fluid recovery and oil recovery in Berea sandstone and Sarukawa sandstone. 2. Materials and methods 2.1. Rock sample Berea sandstone (diameter: 34.95 mm, length: 80.00 mm) and Sarukawa sandstone (diameter: 34.80 mm, length: 79.85 mm, from Japan) were used in this study. Physical properties of specimen are listed in Table 1. Porosities of Berea sandstone and Sarukawa sandstone determined by X-ray CT imaging are 20.21% and 31.22%, respectively. As shown in Figures 1a and 2a, Berea sandstone has bedding planes perpendicular to the core axis. The bedding planes repeatedly appear at almost same intervals and their directions are the same. Thus, we consider this specimen as homogeneous in terms of REV (Representative Elementary Volume). On the other hand, Sarukawa sandstone has a heterogeneous structure. Especially, upper part of specimen is more complex than the lower part (Figures1b and 2b). In this study, we focus on the heterogeneity of specimen for the CO2 flooding. The up-and-down directions of samples are not consistent with the directions of the fields. To prevent the infiltration of confining pressure medium or CO2 leakage, we sealed the specimen with waterproof material and epoxy. Table 1. Physical properties of specimen.
Berea sandstone Sarukawa sandstone
diameter (mm)
length (mm)
porosity (%)
specimen pore volume (cm3)
absolute permeability (KI aqueous solution) (mD)
34.95 34.80
80.00 79.85
20.21 31.22
15.51 23.71
65.52 0.57
Fig. 1. Rock specimens (a)Berea sandstone, (b)Sarukawa sandstone.
Fig. 2. CT images of dry state specimens (a)Berea sandstone, (b)Sarukawa sandstone.
Hyuck Park et al. / Energy Procedia 114 (2017) 2933 – 2941
2.2. Test system Schematic diagram of test system is shown in Fig. 3. The test system is composed of a pressure vessel for specimen and syringe pumps to control the pressure and flow rate of fluids. We used a high-pressure vessel having high transparency for X-ray in this study. As shown in the Fig. 3, syringe pump number-1(SP1), SP2 and SP3 control the CO2, water and oil, respectively. SP4 controls the confining oil pressure. Fluids, which have passed through the specimen flow into the SP5. SP5 is also used for the control of backpressure. Time-series data of flow rate, pressure and volume of fluid pumps are stored in the computer. In order to measure the differential pressure, we placed pressure transducers at upstream and downstream of specimens (PT1, PT2). Confining pressure and pore pressure were set to 12 MPa and 10 MPa in this study. The temperature of test system was maintained at 40 degrees Celsius. For this, all of the syringe pumps were set in a box made from a heat insulating material. Tube-lines from syringe pumps to the specimen were also kept at a constant temperature by using warm water circulators. Moreover, we prepared a sheet shape heater that specially designed for this study to keep temperature of pressure vessel. It is also highly transparent for the X-ray.
Fig. 3. Schematic diagram of test system (SP㸸syringe pump (SP1: CO2, SP2: KI aqueous solution, SP3: oil, SP4: confining pressure oil, SP5: back pressure & recovery), PT: pressure transducer, d㸸distributer㸪s㸸specimen).
2.3. CO2 flooding experiment Water used in this study is 12.5% KI (potassium iodide) aqueous solution. We used this aqueous solution for clarity of X-ray CT imaging. To prevent CO2 dissolving in the KI aqueous solution, we saturated the KI aqueous solution with CO2 prior to the CO2 flooding experiments. I-Decane (12.6%) was used as a pseudo-crude oil. We kept upstream pressure as 10 MPa by using CO2 pump (SP1 in Fig. 3), and decreased downstream pressure to generate differential pressure in the specimen using backpressure pump (SP5). Details of the test methods and CO2 injection volume are listed in Tables 2 and 3. For the Berea sandstone, we used only one kind of flow rate for backpressure pump as -0.1 ml/min. In the case of Sarukawa sandstone, three kinds of flow rates (-0.1, -1.0, -1.5 ml/min) for downstream pump were selected. Note that there is different test method to generate the differential pressure between the two samples. There were five and ten steps of CO2 flooding for Berea sandstone and Sarukawa sandstone, respectively. For each step, KI aqueous solution and oil were carefully recovered from the backpressure pump (SP5). We measured the weight of KI aqueous solution and oil. Each recovered fluid weight was converted to the volume using the density
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Table 2. Back pressure pump flow rate and CO2 injection volume (Berea sandstone). test method back pressure pump flow rate (ml/min)
method
method1
-0.10
step
mean differential Pressure (MPa)
CO2 pump mean flow rate (ml/min)
injection volume (PV)
step 1 step 2 step 3 step 4 step 5
0.007 0.005 0.005 0.005 0.006
0.119 0.111 0.067 0.075 0.076
0.26 0.53 1.05 1.58 2.11
Table 3. Back pressure pump flow rate and CO2 injection volume (Sarukawa sandstone). test method method
back pressure pump flow rate (ml/min)
method1
-0.10
method2
-1.00
method3
-1.50
step
mean differential Pressure (MPa)
CO2 pump mean flow rate (ml/min)
injection volume (PV)
step 1 step 2 step 3 step 4 step 5 step 6 step 7 step 8 step 9 step 10
0.107 0.020 0.015 0.015 0.182 0.205 0.311 0.269 0.267 0.244
0.115 0.050 0.047 0.048 0.150 0.099 0.100 0.100 0.100 0.100
0.25 0.50 0.75 1.00 1.43 1.75 2.05 2.38 2.70 2.98
value, which had been previously confirmed. Maximum differential pressures applied to the Berea sandstone and Sarukawa sandstone were about 0.007 MPa and 0.311 MPa in this study. Porosity and fluid saturation were calculated for each pixel of the digital image. In this study, the average of CT values for each axial images were estimated for identifying the distribution of porosity and fluid saturation. We used the formula of Akin and Kovscek [6] for the calculation of specimen porosity (I) and CO2 saturation (SCO2).
I
CTW .r CTair .r CTW CTair
S CO2
CTexp.r CTW .r CTCO2 .r CTW .r
(1)
,
Sw
1 S CO2
(1)
where CTW.r and CTair.r are CT values of water-saturated rock and dry rock, CTW and CTair are CT values of water and air, CTexp.r is CT value of rock during the experiment, CTCO2.r is CT value of rock saturated with CO2. Based on the above contents, the CO2-flooding experiment was performed in the following procedure: 1) Acquisition of CTW and CTair 2) Installation of the core specimen to the pressure vessel 3) Acquisition of CTair.r and CTCO2.r 4) Acquisition of CTW.r 5) Measuring the absolute permeability with KI aqueous solution 6) Replacement with CO2-saturated KI aqueous solution 7) Oil injecting to make the specimen water-oil mixed condition (CTW+O.r) 8) CO2 flooding start 9) Measurement of differential pressure from PT1 and PT2, monitoring the CO2 saturation by CT image analysis 10) Fluid recovery from SP5 (step1 completed)
Hyuck Park et al. / Energy Procedia 114 (2017) 2933 – 2941
11) Repeat 8) - 10), increasing the CO2 saturation 3. Results 3.1. Porosity We could get the porosity after injection of KI aqueous solution into the dry specimen. CT images of dry specimens are shown in Figures 4a and b. There are vertical and horizontal cross-section images and five axial cross-section images. The positions of each axial cross-section (Ax1-5) are marked in the vertical cross-section image. As shown in the Fig. 4a, Berea sandstone has bedding planes perpendicular to the core axis. The bedding planes repeatedly appear at almost same intervals and their directions are the same. On the other hand, Sarukawa sandstone has a heterogeneous structure. Especially, upper part of specimen is more complex than lower part of it (Fig. 4b). After injection of KI aqueous solution into the dry specimen, we could obtain the porosity of specimen by using the Formula (1) mentioned above. Mean porosities of Berea sandstone and Sarukawa sandstone are 20.21% and 31.22% in this study. Then we injected oil into the water-saturated specimens to make water-oil mixed condition.
Fig. 4. X-ray CT images of dry and water saturated specimens (a, c) Berea sandstone, (b, d) Sarukawa sandstone.
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Fig. 5. Differential CT images of CO2 flooding experiments a) Berea sandstone, 0.24PV CO2 injected (breakthrough point), b) Sarukawa sandstone, 0.06PV CO2 injected (breakthrough point), c) Berea sandstone, 1.05PV CO2 injected, d) Sarukawa sandstone, 1.00PV CO2 injected, e) Berea sandstone, 2.11PV CO2 injected, f) Sarukawa sandstone, 2.05PV CO2 injected.
Fig. 6. CO2 saturation profile change during CO2 flooding; a) Berea sandstone, b) Sarukawa sandstone.
Hyuck Park et al. / Energy Procedia 114 (2017) 2933 – 2941
3.2. CO2 saturation distribution The CO2 flooding tests were carried out until the CO2 injection reaches 2.11 PV (pore volume) and 2.98 PV for the Berea sandstone and Sarukawa sandstone. Figures 5a and b show the differential CT images when CO2 break through the specimen. For the brake through, it took 0.24PV CO2 injection for Berea sandstone and 0.06 PV for Sarukawa sandstones. As shown in the vertical section images, there is big difference of CO2 injection amount between Berea sandstone and Sarukawa sandstone for the brake through. After 1 PV CO2 flooding for both specimens, there is also different flow patterns. In the case of Sarukawa sandstone, CO2 saturation increased in only upper half part of specimen (Fig. 5d). It implies that the heterogeneity of specimen significantly affects the flow patterns of CO2. The correlations of CO2 injection and CO2 saturation are shown in Fig. 6. The CO2 saturation profiles in the graphs are corresponding to each step of experiments mentioned in Tables 2 and 3. In the case of Berea sandstone, the CO2 saturation initially increases from upstream side and then, with the step increases, CO2 saturation of the downstream side is gradually increased (Figures 5a and 6a). After 5 steps of CO2 injection, the CO2 saturation of Berea sandstone was 39.29% in this study. In the case of Sarukawa sandstone, the CO2 saturation slightly increases in the full length of specimen with 4-steps of CO2 injection (Fig. 6b). Then, the CO2 saturation of Sarukawa sandstone increased with two times increments of differential pressure. After totally 10 steps of CO2 injection, the CO2 saturation of Sarukawa sandstone reached 34.41%. 4. Comparison of homogeneous and heterogeneous sedimentary structure 4.1 CO2 saturation Comparison of CO2 saturation change during the CO2-flooding tests for both specimen is shown in Fig. 7. In the case of Berea sandstone, the saturation reaches 34.15% with 1.05 PV of CO2 flooding then it finishes at 39.29% with 2.11 PV of CO2 flooding. In the case of Sarukawa sandstone, the CO2 saturation reaches 15.21% after 1.00 PV of CO2 flooding. After the first increasing of differential pressure, it reaches 21.35% with 1.75 PV of CO2 flooding. Finally, it finishes at 34.41% with 2.98 PV of CO2 flooding after another increasing of differential pressure. Comparing the results, when the CO2 flooding reached 1.00 PV, the Berea sandstone has more than twice of CO2 saturation than Sarukawa sandstone. However, the two times increments of differential pressure for Sarukawa sandstone made it possible to increase the CO2 saturation, even though the Sarukawa sandstone was expected lower CO2 saturation if we kept the test method as same as Berea sandstone..
Fig. 7. CO2 saturation change during CO2 flooding.
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Fig. 8. Total recovery change during CO2 flooding.
Fig. 9. Oil recovery change during CO2 flooding.
4.2 Total fluid recovery and oil recovery Fig. 8 shows the comparison of total fluid recovery between Berea sandstone and Sarukawa sandstone. Here, the total fluid represents the sum of KI aqueous solution and oil volume after recovery from the backpressure pump. The results of total recovery are in agreement with the CO2 saturation results. Paying attention to the recovery of oil, in the case of Sarukawa sandstone, we could obtain almost the same oil recovery with the Berea sandstone after about 2.0 PV of CO2 flooding. As shown in Fig. 9, the oil recoveries are 74.80% and 71.93% for Berea sandstone and Sarukawa sandstone, respectively. It reveals that the capillary pressure difference due to the heterogeneity is the main cause of the oil recovery difference between the Berea sandstone and Sarukawa sandstone. Finally, about 77.68% of oil recovery for the Sarukawa sandstone was confirmed after 2.98 PV of CO2 flooding. 5. Conclusion To study the influence of sedimentation heterogeneity on CO2 flooding, we carried out CO2 flooding laboratory
Hyuck Park et al. / Energy Procedia 114 (2017) 2933 – 2941
tests of porous sandstones. Berea sandstone and Sarukawa sandstone were used as a homogeneous and heterogeneous rock, respectively. X-ray CT visualization and recovery of fluids were performed for every test steps. This study can be summarized as follows: x
x x
x
According to the X-ray CT visualization results, the CO2 flows in both specimens showed different patterns. In the case of Berea sandstone, CO2 was spreading evenly from the injection part through the specimen after the breakthrough. However, for Sarukawa sandstone, almost all of the CO2 preferentially moved through the upper part of specimen. The results of CO2 saturation obtained from the CT image analysis were in agreement with the results of fluid recovery amount measured form the sum total of KI aqueous solution and oil. For the Sarukawa sandstone, it was possible to increase the CO2 saturation and oil recovery almost the same as Berea sandstone with two times increments of differential pressure. After about 2.0 PV of CO2 flooding, the oil recoveries were 74.80% and 71.93% for Berea sandstone and Sarukawa sandstone, respectively. The experimental study reveals that the capillary pressure difference due to the heterogeneity of rock is one of the main cause of CO2 flooding characteristics.
Acknowledgements 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. References [1] Perrin, JC, Krause M, Kuo CW, Miljkovic L, Charoba E, Benson SM. Core-scale experimental study of relative permeability properties of CO2 and brine in reservoir rocks. Energy Procedia 2009;1:3515–3522. [2] Shi JQ, Xue Z, Durucan S. History matching of CO2 core flooding CT scan saturation profiles with porosity dependant capillary pressure. Energy Procedia 2009;1:3205-3211. [3] Perrin JC, Benson S. An Experimental Study on the Influence of Sub-Core Scale Heterogeneities on CO2 Distribution in Reservoir Rocks. Transp Porous Med 2010;82:93-109. [4] Shi JQ, Xue Z, Durucan S. Supercritical CO2 core flooding and imbibition in Tako sandstone—Influence of sub-core scale heterogeneity. International Journal of Greenhouse Gas Control 2011;5:75–87. [5] Kogure T, Nishizawa O, Chiyonobu S, Yazaki Y, Shibatani S, Xue Z. Effect of sub-core scale heterogeneity on relative permeability curves of porous sandstone in a water-supercritical CO2 system. Energy Procedia 2013;37:4491–4498. [6] Akin S, Kovscek AR. Computed tomography in petroleum engineering research. Geological Society Special Publication 215:23-38; 2003.
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ScienceDirect Energy Procedia 114 (2017) 2933 – 2941
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Influence of sedimentation heterogeneity on CO2 flooding Hyuck Parka*, Lanlan Jianga, Tamotsu Kiyamaa, Yi Zhanga, Ryo Uedab, Masanori Nakanob, Ziqiu Xuea a
Research Institute of Innovative Technology for the Earth (RITE), Japan b Japan Petroleum Exploration Co., Ltd. (JAPEX), Japan
Abstract
To study CO2 flow characteristics in heterogeneous rock, we designed a laboratory experimental system that visualizes the CO2 movements in rock specimens by using X-ray computed tomography (CT). We carried out laboratory experiments of CO2 flooding in porous sandstones, together with porosity calculation, fluid saturation monitoring based on the CT images and mass flow measurements for ejected fluids. We used Berea sandstone and Sarukawa sandstone as homogeneous and heterogeneous rock in this study. The CO2 flooding tests were carried out until the CO2 injection reaches about 2 PV (pore volume) for the Berea sandstone and about 3 PV for the Sarukawa sandstone. When injected CO2 reached 1.0 PV in both specimens, The CO2 saturations were 34.15% and 15.21% in Berea sandstone and Sarukawa sandstone, respectively. We increased the differential pressure only for the Sarukawa sandstone to confirm the influence of differential pressure on CO2 saturation and oil recovery in heterogeneous media. The oil recoveries were 74.80% and 71.93% for Berea sandstone and Sarukawa sandstone, respectively when injected CO2 reached about 2.0 PV. CO2 was spreading evenly from the injection part through the Berea sandstone. In the case of Sarukawa sandstone, almost all of the CO2 preferentially moved through the upper part of specimen. This represents that the sedimentation heterogeneity is one of the main factors that affects the CO2 flooding pattern. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: CO2 injection; CO2 flooding; heterogeneity; X-ray CT; Berea sandstone; Sarukawa sandstone;
1. Introduction Understanding the flow mechanisms of CO2 flooding in reservoir rock is important for CO2 geological sequestration and utilization, for example enhanced oil recovery. It is also essential to understand the influence of rock heterogeneity on CO2 flow. Due to the practical difficulties, few laboratory experiments have been performed on CO2-brine systems
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1422
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[1,2,3]. Perrin and Benson [3] carried out CO2/brine two-phase flow experiments to investigate the influence of subcore scale heterogeneities on the distribution of CO2 at steady state. Recently, it began to increase the laboratory experiments combined with X-ray CT scanning to measure the CO2 saturation distribution during a set of steady-state relative permeability measurements [4,5]. The heterogeneity of reservoir rock is a factor greatly affects the storage location selection. In this study, we try to understand the flooding characteristics of CO2 in heterogeneous rocks having complex sedimentary structures, which will contribute to CO2 geological sequestration and oil recovery. This paper reports the CO2 flooding test results for two different sandstones, which previously saturated by water and oil together. We first evaluated the porosity of core specimens based on the X-ray CT scanning and then visualized the CO2 flooding patterns in both specimens. Finally, we compared the CO2 saturation, total fluid recovery and oil recovery in Berea sandstone and Sarukawa sandstone. 2. Materials and methods 2.1. Rock sample Berea sandstone (diameter: 34.95 mm, length: 80.00 mm) and Sarukawa sandstone (diameter: 34.80 mm, length: 79.85 mm, from Japan) were used in this study. Physical properties of specimen are listed in Table 1. Porosities of Berea sandstone and Sarukawa sandstone determined by X-ray CT imaging are 20.21% and 31.22%, respectively. As shown in Figures 1a and 2a, Berea sandstone has bedding planes perpendicular to the core axis. The bedding planes repeatedly appear at almost same intervals and their directions are the same. Thus, we consider this specimen as homogeneous in terms of REV (Representative Elementary Volume). On the other hand, Sarukawa sandstone has a heterogeneous structure. Especially, upper part of specimen is more complex than the lower part (Figures1b and 2b). In this study, we focus on the heterogeneity of specimen for the CO2 flooding. The up-and-down directions of samples are not consistent with the directions of the fields. To prevent the infiltration of confining pressure medium or CO2 leakage, we sealed the specimen with waterproof material and epoxy. Table 1. Physical properties of specimen.
Berea sandstone Sarukawa sandstone
diameter (mm)
length (mm)
porosity (%)
specimen pore volume (cm3)
absolute permeability (KI aqueous solution) (mD)
34.95 34.80
80.00 79.85
20.21 31.22
15.51 23.71
65.52 0.57
Fig. 1. Rock specimens (a)Berea sandstone, (b)Sarukawa sandstone.
Fig. 2. CT images of dry state specimens (a)Berea sandstone, (b)Sarukawa sandstone.
Hyuck Park et al. / Energy Procedia 114 (2017) 2933 – 2941
2.2. Test system Schematic diagram of test system is shown in Fig. 3. The test system is composed of a pressure vessel for specimen and syringe pumps to control the pressure and flow rate of fluids. We used a high-pressure vessel having high transparency for X-ray in this study. As shown in the Fig. 3, syringe pump number-1(SP1), SP2 and SP3 control the CO2, water and oil, respectively. SP4 controls the confining oil pressure. Fluids, which have passed through the specimen flow into the SP5. SP5 is also used for the control of backpressure. Time-series data of flow rate, pressure and volume of fluid pumps are stored in the computer. In order to measure the differential pressure, we placed pressure transducers at upstream and downstream of specimens (PT1, PT2). Confining pressure and pore pressure were set to 12 MPa and 10 MPa in this study. The temperature of test system was maintained at 40 degrees Celsius. For this, all of the syringe pumps were set in a box made from a heat insulating material. Tube-lines from syringe pumps to the specimen were also kept at a constant temperature by using warm water circulators. Moreover, we prepared a sheet shape heater that specially designed for this study to keep temperature of pressure vessel. It is also highly transparent for the X-ray.
Fig. 3. Schematic diagram of test system (SP㸸syringe pump (SP1: CO2, SP2: KI aqueous solution, SP3: oil, SP4: confining pressure oil, SP5: back pressure & recovery), PT: pressure transducer, d㸸distributer㸪s㸸specimen).
2.3. CO2 flooding experiment Water used in this study is 12.5% KI (potassium iodide) aqueous solution. We used this aqueous solution for clarity of X-ray CT imaging. To prevent CO2 dissolving in the KI aqueous solution, we saturated the KI aqueous solution with CO2 prior to the CO2 flooding experiments. I-Decane (12.6%) was used as a pseudo-crude oil. We kept upstream pressure as 10 MPa by using CO2 pump (SP1 in Fig. 3), and decreased downstream pressure to generate differential pressure in the specimen using backpressure pump (SP5). Details of the test methods and CO2 injection volume are listed in Tables 2 and 3. For the Berea sandstone, we used only one kind of flow rate for backpressure pump as -0.1 ml/min. In the case of Sarukawa sandstone, three kinds of flow rates (-0.1, -1.0, -1.5 ml/min) for downstream pump were selected. Note that there is different test method to generate the differential pressure between the two samples. There were five and ten steps of CO2 flooding for Berea sandstone and Sarukawa sandstone, respectively. For each step, KI aqueous solution and oil were carefully recovered from the backpressure pump (SP5). We measured the weight of KI aqueous solution and oil. Each recovered fluid weight was converted to the volume using the density
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Table 2. Back pressure pump flow rate and CO2 injection volume (Berea sandstone). test method back pressure pump flow rate (ml/min)
method
method1
-0.10
step
mean differential Pressure (MPa)
CO2 pump mean flow rate (ml/min)
injection volume (PV)
step 1 step 2 step 3 step 4 step 5
0.007 0.005 0.005 0.005 0.006
0.119 0.111 0.067 0.075 0.076
0.26 0.53 1.05 1.58 2.11
Table 3. Back pressure pump flow rate and CO2 injection volume (Sarukawa sandstone). test method method
back pressure pump flow rate (ml/min)
method1
-0.10
method2
-1.00
method3
-1.50
step
mean differential Pressure (MPa)
CO2 pump mean flow rate (ml/min)
injection volume (PV)
step 1 step 2 step 3 step 4 step 5 step 6 step 7 step 8 step 9 step 10
0.107 0.020 0.015 0.015 0.182 0.205 0.311 0.269 0.267 0.244
0.115 0.050 0.047 0.048 0.150 0.099 0.100 0.100 0.100 0.100
0.25 0.50 0.75 1.00 1.43 1.75 2.05 2.38 2.70 2.98
value, which had been previously confirmed. Maximum differential pressures applied to the Berea sandstone and Sarukawa sandstone were about 0.007 MPa and 0.311 MPa in this study. Porosity and fluid saturation were calculated for each pixel of the digital image. In this study, the average of CT values for each axial images were estimated for identifying the distribution of porosity and fluid saturation. We used the formula of Akin and Kovscek [6] for the calculation of specimen porosity (I) and CO2 saturation (SCO2).
I
CTW .r CTair .r CTW CTair
S CO2
CTexp.r CTW .r CTCO2 .r CTW .r
(1)
,
Sw
1 S CO2
(1)
where CTW.r and CTair.r are CT values of water-saturated rock and dry rock, CTW and CTair are CT values of water and air, CTexp.r is CT value of rock during the experiment, CTCO2.r is CT value of rock saturated with CO2. Based on the above contents, the CO2-flooding experiment was performed in the following procedure: 1) Acquisition of CTW and CTair 2) Installation of the core specimen to the pressure vessel 3) Acquisition of CTair.r and CTCO2.r 4) Acquisition of CTW.r 5) Measuring the absolute permeability with KI aqueous solution 6) Replacement with CO2-saturated KI aqueous solution 7) Oil injecting to make the specimen water-oil mixed condition (CTW+O.r) 8) CO2 flooding start 9) Measurement of differential pressure from PT1 and PT2, monitoring the CO2 saturation by CT image analysis 10) Fluid recovery from SP5 (step1 completed)
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11) Repeat 8) - 10), increasing the CO2 saturation 3. Results 3.1. Porosity We could get the porosity after injection of KI aqueous solution into the dry specimen. CT images of dry specimens are shown in Figures 4a and b. There are vertical and horizontal cross-section images and five axial cross-section images. The positions of each axial cross-section (Ax1-5) are marked in the vertical cross-section image. As shown in the Fig. 4a, Berea sandstone has bedding planes perpendicular to the core axis. The bedding planes repeatedly appear at almost same intervals and their directions are the same. On the other hand, Sarukawa sandstone has a heterogeneous structure. Especially, upper part of specimen is more complex than lower part of it (Fig. 4b). After injection of KI aqueous solution into the dry specimen, we could obtain the porosity of specimen by using the Formula (1) mentioned above. Mean porosities of Berea sandstone and Sarukawa sandstone are 20.21% and 31.22% in this study. Then we injected oil into the water-saturated specimens to make water-oil mixed condition.
Fig. 4. X-ray CT images of dry and water saturated specimens (a, c) Berea sandstone, (b, d) Sarukawa sandstone.
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Fig. 5. Differential CT images of CO2 flooding experiments a) Berea sandstone, 0.24PV CO2 injected (breakthrough point), b) Sarukawa sandstone, 0.06PV CO2 injected (breakthrough point), c) Berea sandstone, 1.05PV CO2 injected, d) Sarukawa sandstone, 1.00PV CO2 injected, e) Berea sandstone, 2.11PV CO2 injected, f) Sarukawa sandstone, 2.05PV CO2 injected.
Fig. 6. CO2 saturation profile change during CO2 flooding; a) Berea sandstone, b) Sarukawa sandstone.
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3.2. CO2 saturation distribution The CO2 flooding tests were carried out until the CO2 injection reaches 2.11 PV (pore volume) and 2.98 PV for the Berea sandstone and Sarukawa sandstone. Figures 5a and b show the differential CT images when CO2 break through the specimen. For the brake through, it took 0.24PV CO2 injection for Berea sandstone and 0.06 PV for Sarukawa sandstones. As shown in the vertical section images, there is big difference of CO2 injection amount between Berea sandstone and Sarukawa sandstone for the brake through. After 1 PV CO2 flooding for both specimens, there is also different flow patterns. In the case of Sarukawa sandstone, CO2 saturation increased in only upper half part of specimen (Fig. 5d). It implies that the heterogeneity of specimen significantly affects the flow patterns of CO2. The correlations of CO2 injection and CO2 saturation are shown in Fig. 6. The CO2 saturation profiles in the graphs are corresponding to each step of experiments mentioned in Tables 2 and 3. In the case of Berea sandstone, the CO2 saturation initially increases from upstream side and then, with the step increases, CO2 saturation of the downstream side is gradually increased (Figures 5a and 6a). After 5 steps of CO2 injection, the CO2 saturation of Berea sandstone was 39.29% in this study. In the case of Sarukawa sandstone, the CO2 saturation slightly increases in the full length of specimen with 4-steps of CO2 injection (Fig. 6b). Then, the CO2 saturation of Sarukawa sandstone increased with two times increments of differential pressure. After totally 10 steps of CO2 injection, the CO2 saturation of Sarukawa sandstone reached 34.41%. 4. Comparison of homogeneous and heterogeneous sedimentary structure 4.1 CO2 saturation Comparison of CO2 saturation change during the CO2-flooding tests for both specimen is shown in Fig. 7. In the case of Berea sandstone, the saturation reaches 34.15% with 1.05 PV of CO2 flooding then it finishes at 39.29% with 2.11 PV of CO2 flooding. In the case of Sarukawa sandstone, the CO2 saturation reaches 15.21% after 1.00 PV of CO2 flooding. After the first increasing of differential pressure, it reaches 21.35% with 1.75 PV of CO2 flooding. Finally, it finishes at 34.41% with 2.98 PV of CO2 flooding after another increasing of differential pressure. Comparing the results, when the CO2 flooding reached 1.00 PV, the Berea sandstone has more than twice of CO2 saturation than Sarukawa sandstone. However, the two times increments of differential pressure for Sarukawa sandstone made it possible to increase the CO2 saturation, even though the Sarukawa sandstone was expected lower CO2 saturation if we kept the test method as same as Berea sandstone..
Fig. 7. CO2 saturation change during CO2 flooding.
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Fig. 8. Total recovery change during CO2 flooding.
Fig. 9. Oil recovery change during CO2 flooding.
4.2 Total fluid recovery and oil recovery Fig. 8 shows the comparison of total fluid recovery between Berea sandstone and Sarukawa sandstone. Here, the total fluid represents the sum of KI aqueous solution and oil volume after recovery from the backpressure pump. The results of total recovery are in agreement with the CO2 saturation results. Paying attention to the recovery of oil, in the case of Sarukawa sandstone, we could obtain almost the same oil recovery with the Berea sandstone after about 2.0 PV of CO2 flooding. As shown in Fig. 9, the oil recoveries are 74.80% and 71.93% for Berea sandstone and Sarukawa sandstone, respectively. It reveals that the capillary pressure difference due to the heterogeneity is the main cause of the oil recovery difference between the Berea sandstone and Sarukawa sandstone. Finally, about 77.68% of oil recovery for the Sarukawa sandstone was confirmed after 2.98 PV of CO2 flooding. 5. Conclusion To study the influence of sedimentation heterogeneity on CO2 flooding, we carried out CO2 flooding laboratory
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tests of porous sandstones. Berea sandstone and Sarukawa sandstone were used as a homogeneous and heterogeneous rock, respectively. X-ray CT visualization and recovery of fluids were performed for every test steps. This study can be summarized as follows: x
x x
x
According to the X-ray CT visualization results, the CO2 flows in both specimens showed different patterns. In the case of Berea sandstone, CO2 was spreading evenly from the injection part through the specimen after the breakthrough. However, for Sarukawa sandstone, almost all of the CO2 preferentially moved through the upper part of specimen. The results of CO2 saturation obtained from the CT image analysis were in agreement with the results of fluid recovery amount measured form the sum total of KI aqueous solution and oil. For the Sarukawa sandstone, it was possible to increase the CO2 saturation and oil recovery almost the same as Berea sandstone with two times increments of differential pressure. After about 2.0 PV of CO2 flooding, the oil recoveries were 74.80% and 71.93% for Berea sandstone and Sarukawa sandstone, respectively. The experimental study reveals that the capillary pressure difference due to the heterogeneity of rock is one of the main cause of CO2 flooding characteristics.
Acknowledgements 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. References [1] Perrin, JC, Krause M, Kuo CW, Miljkovic L, Charoba E, Benson SM. Core-scale experimental study of relative permeability properties of CO2 and brine in reservoir rocks. Energy Procedia 2009;1:3515–3522. [2] Shi JQ, Xue Z, Durucan S. History matching of CO2 core flooding CT scan saturation profiles with porosity dependant capillary pressure. Energy Procedia 2009;1:3205-3211. [3] Perrin JC, Benson S. An Experimental Study on the Influence of Sub-Core Scale Heterogeneities on CO2 Distribution in Reservoir Rocks. Transp Porous Med 2010;82:93-109. [4] Shi JQ, Xue Z, Durucan S. Supercritical CO2 core flooding and imbibition in Tako sandstone—Influence of sub-core scale heterogeneity. International Journal of Greenhouse Gas Control 2011;5:75–87. [5] Kogure T, Nishizawa O, Chiyonobu S, Yazaki Y, Shibatani S, Xue Z. Effect of sub-core scale heterogeneity on relative permeability curves of porous sandstone in a water-supercritical CO2 system. Energy Procedia 2013;37:4491–4498. [6] Akin S, Kovscek AR. Computed tomography in petroleum engineering research. Geological Society Special Publication 215:23-38; 2003.
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