ARTICLE pubs.acs.org/IECR
Visualization and Measurement of CO2 Flooding in Porous Media Using MRI Yuechao Zhao,† Yongchen Song,*,† Yu Liu,† Haifeng Liang,‡ and Binlin Dou† †
Key Laboratory of Ocean Energy Utilization and Energy Conservation of the Ministry of Education, Dalian University of Technology, Dalian, Liaoning, 116024, P.R. China ‡ Taiyuan University of Technology, College of Chemistry and Chemical Engineering, Taiyuan, Shanxi, 030024, P.R. China ABSTRACT: CO2 flooding is used extensively as a commercial process for enhanced oil recovery. In this study, the visualization of CO2 flooding in immiscible and miscible displacements in a high-pressure condition was studied using a 400 MHz MRI system. For CO2 immiscible displacement, the phenomenon of CO2 channelling or fingering was obviously due to the difference in fluid viscosities and densities. Thus, the sweep efficiency was small, and the final residual oil saturation was 37.2%. For CO2 miscible displacement, the results showed that pistonlike displacement occurred, and the phenomenon of the miscible regions and CO2 front was obvious. The viscous fingering and gravity override caused by the low viscosity and density of the gas were restrained effectively, and the velocity of the CO2 front was uniform. The sweep efficiency was high, and the final residual oil saturation was 13.5%, indicating that CO2 miscible displacement could recover more oil compared with CO2 immiscible displacement. Finally, the average velocity of the CO2 front was evaluated by analyzing the oil saturation profile. A special core analysis method was applied to in situ oil saturation data to directly evaluate the effect of viscosity, buoyancy, and capillary pressure on CO2 miscible displacement.
1. INTRODUCTION Enhanced oil recovery (EOR) using CO2 is an important alternative for geological CO2 storage. In the past five decades, various studies on different CO2 EOR processes have been carried out. In general, these tertiary processes can enhance oil recovery by 8%-16% of the original oil in place.1 CO2 displacement processes can be classified into two, namely, immiscible and miscible, processes.2 It is difficult to visualize the fluid movement and distribution in porous media using the traditional experimental methods. It is often assumed that the porous media has a homogeneous structure, and that the volume and composition of fluids injected and recovered can be measured. However, the fluid distribution and movement inside the porous media can only be inferred.3 X-ray computed tomography (CT) has enabled the determination of rock structure, fluid saturation, and solute concentration in the sample. Carretero-Carralero et al.4 and Du et al.5,6 have investigated CO2 foam flow in a consolidated Bentheimer sandstone core saturated with surfactant solution using a CT technique. However, CT is not well suited to observe the fluids of water and oil with low atomic number in the rock matrix; it also cannot differentiate these fluids except through the addition of high concentrations of contrast agents that tend to change the fluid properties.7 This is because X-ray attenuation is principally determined by the atomic number of the sample nuclei and is dominated by the rock matrix. MRI technique is a powerful analytical tool for noninvasive multidimensional visualization of flow and transport in porous media.8 It can provide unprecedented quantitative information about fluid-phase distributions in porous media during displacement processes, as well as information about rock structure corresponding to local regions within the porous media. Such information can improve our understanding of the storage and transport of multiphase fluids in porous media.9,10 In the past two decades, there have r 2011 American Chemical Society
been many studies on the visualization of flow and transport in porous media using MRI techniques.11-16 Recently, with the design and construction of a high-pressure core holder for MRI measurements, Suekane et al.17-19 have carried out some studies on the behavior of CO2 and water two-phase flow in porous media. Brautaset et al.20 have investigated the fluid saturation distributions and monitored the fluid flow characteristics in situ during waterflood and subsequent injection of either liquid or supercritical CO2 in four Portland Chalk core samples at different wettabilities. The recovery mechanism during CO2 injection is complicated, and obtaining in situ data is of great importance in understanding the displacement process. In this study, tests of both supercritical CO2 miscible displacement and gaseous CO2 immiscible displacement in high-permeable bead-pack core have been conducted. MRI has also been used to calculate the fluid saturations and to qualitatively monitor the displacement processes.
2. EXPERIMENTAL APPARATUS AND MEASUREMENT TECHNIQUES A schematic diagram of the experimental setup is shown in Figure 1. The experimental setup consisted of two circuits, namely, the displacement process line and the temperature control circulation line. In the displacement process line, the liquid CO2 was injected into a transfer vessel in the thermostatic chamber using a CO2 pump. The oil and supercritical CO2 were injected into the bead-pack holder by a syringe pump. The flow rate and back pressure were controlled by the pump and a back pressure regulator (model BP-2080-M, JASCO). The pressure Received: June 17, 2010 Accepted: February 17, 2011 Revised: January 29, 2011 Published: March 17, 2011 4707
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Figure 1. Simplified schematic of the experimental setup.
drop through the bead-pack core was measured using a low differential pressure transmitter. In the temperature control circulation line, fluorinert FC-40 was used to control the temperature of the bead-pack holder, because fluorinert contains no hydrogen atoms. Thus, the images cannot be taken, although its low dielectric properties minimize RF losses.17,21,22 The fluid was maintained at a constant flow rate and was allowed to circulate through the thermostatic bath system and holder using a recirculation pump. In this study, a new high-pressure bead-pack holder was designed and constructed for the Varian NMR Systems, which features an RF probe with an inner diameter of 40 mm. The new high-pressure beadpack holder and its cross-sectional diagram are shown in Figure 2. The holder, which was inserted vertically into the MRI system (Figure 1), was designed and constructed for MRI measurement. The maximum working pressure was 15 MPa, and the maximum working temperature was 70 °C. The holder consisted of water fittings (1), titanium end-cap (2), sealing O-ring (3, 4, 6, 10, 11), end pieces (5), filter screen (7), high-pressure polyimide tube (8), and normal-pressure polyimide sleeve (9). The high-pressure polyimide tube had an inner diameter of 15 mm and a length of 200 mm. The tube material was also selected because it is nonmagnetic and did not interfere with the RF signals used in the experiments. Temperatures at the inlet and outlet of the bead-pack holder were measured using thermocouples. The soda glass beads (BZ02) used in the experiments were made in Japan. They had a grain size distribution ranging from 0.177-0.250 mm and were employed to pack the cylindrical bead-pack holder. The contact angles of the soda glass beads with deionized water and n-decane were 78.5° and 45.3°, respectively, which were measured using a dynamic contact angle tensiometer (DCAT21, Dataphisca) at 20 °C and atmospheric pressure. The soda glass beads were oil-wet. CO2 with 99% purity was used as the gaseous phase, and n-decane was used as the oleic phase in the experiments. Fluid properties, including densities and viscosities at relevant temperature and pressure levels are listed in Table 1. The porosity of the bead-pack core was 35%, which was calculated from the traditional gravimetric measurement method. The absolute permeability to water was 13.8 D. The critical point of CO2 is typically reached with temperature and pressure levels of 31.1 °C and 7398 kPa, respectively. In previous works, minimum miscible temperature and pressure levels for the n-decane and CO2 systems have been determined at 35 °C and 7329 kPa23 and at 37.8 °C and 7894 kPa,24 respectively. In this study, a temperature of 40 °C and pressure of 8.5 MPa were selected
to ensure the supercritical properties for the supercritical CO2 miscible displacement test. Meanwhile, a temperature of 40 °C and pressure of 7.0 MPa were selected to ensure gaseous properties of CO2 for the gaseous CO2 immiscible displacement test. All MRI measurements were performed on a Varian NMR systems with 9.4-T, wide-bore (89 mm in diameter), vertical superconducting magnet. An 1H 40 mm Millipede vertical microimaging probe was used, and the gradient coils provided a maximum gradient strength of 50 G/cm. The MRI was conducted by the spin-echo multislice pulse sequence during flooding using the following experimental parameters: echo time (TE), 1.31 ms; repetition time (TR), 2 s; image data matrix, 96 96; field of view (FOV), 40 mm 40 mm with a thickness of 2.2 mm; number of slices, 7 (the position of slices is shown in Figure 3); spatial resolution, 0.42 0.42 2.2 mm3; number of images for averaging, 1; and acquisition time, 3 min 12 s. Oil contained in the bead-pack core was visualized in longitudinal planes along the flow direction.
3. RESULTS AND DISCUSSION 3.1. Supercritical CO2 Miscible Displacement. 3.1.1. NMR Image Analysis. In the CO2 miscible displacement test, supercritical
CO2 was injected vertically upward into the bead-pack core saturated with oil at a pressure of 8.5 MPa and a temperature of 40 °C. This CO2 displacement experiment was conducted at constant flow rates of 0.2 and 0.15 mL/min, respectively. The NMR images of oil distribution in slices at longitudinal direction were examined. Figure 4 shows a series of NMR images (the sixth slice in Figure 3) at a constant CO2 injection rate of 0.2 mL/min; it illustrates oil saturation at different CO2 injection times of 0, 9.6, 16, 22.4, 35.2, 48, 60.8, 67.2, 83.2, 211.2, 444.8, and 812.8 min, respectively. The bright regions (red) indicate the high NMR signal intensities corresponding to high oil saturation, while the dark regions (blue) indicate the lower oil saturation. For instance, the first image shows the bead-pack core, which is 100% saturated with oil. The porosity distribution (Figure 6) can be obtained by calibrating a standard reference with known porosity, which is imaged with the sample. Then, the pistonlike displacement occurred with injection of supercritical CO2. For example, the obvious miscible regions (part of the green and yellow regions on the bottom half region) and CO2 front (the boundary of the green and blue regions on the bottom half region) can be seen from the image of 35.2 min in Figure 4. The phenomenon of viscous fingering and gravity override 4708
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Figure 3. Sketch map for the position of slices at the axial direction.
Figure 2. The high-pressure MRI bead-pack holder. The holder consisted of water fittings (1), titanium end-cap (2), sealing O-ring (3, 4, 6, 10, 11), end pieces (5), filter screen (7), high-pressure polyimide tube (8), and normal-pressure polyimide sleeve (9).
Table 1. Fluid Properties fluid CO2 n-decane
pressure (MPa) temperature (°C) density (g/cm3) viscosity (cP) 7
40
0.198
0.0193
8.5
40
0.354
0.0261
7
40
0.721
0.749
8.5
40
0.722
0.762
caused by the low viscosity and density of the gas was restrained effectively, thereby diverting injected CO2 to zones of lower permeability and improving the overall process efficiency. 3.1.2. Saturation Profiles. To analyze the evolution of oil saturation quantitatively along the bead-pack core, MRI data were converted into saturation profiles in accordance with the procedure described by Suekane et al.19 The NMR signal intensity on the spin density images from any local position is proportional to the oil
Figure 4. Distribution of NMR signal intensity in the bead-pack core at 8.5 MPa, 40 °C, and a CO2 injection rate of 0.2 mL/min.
content in the porous media. This means that the measured NMR signal intensity reflects the local oil saturation in the porous media. In the experiments with CO2 injection, the initial NMR signal intensity distributions in the porous media saturated with oil were obtained. Afterward, CO2 was injected into the porous media with time-series acquisition of NMR images. The injected CO2 displaced some oil in the porous media, thus decreasing the NMR signal intensity. First, the NMR signals of all the seven slices were added together to obtain the 2D projective distribution of 3D oil saturation distribution in the bead-pack core. Then, the oil saturation in each pixel was alculated as the ratio of the NMR signal with and without the CO2 (t = 0). Next, the oil saturation was added in a lateral direction within the bead-pack core, that is, for a given position z 4709
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Figure 6. One-dimensional distribution profile of porosity along the bead-pack core.
Figure 5. Evolution of oil saturation profiles in the bead-pack core at 8.5 MPa, 40 °C, and a CO2 injection rate of 0.2 mL/min: (a) one-dimensional saturation profile along the bead-pack core; (b) saturation profile of the total FOV versus volume of CO2 injection; and (c) saturation profile of the total FOV based on material balance versus MRI.
along the flow direction from the inlet of the bead-pack core (z = 0). A one-dimensional saturation profile along the bead-pack core at different volumes of injected CO2 was obtained (Figure 5a). Finally, the oil saturation was also averaged in the total FOV. The saturation profile of the total FOV versus volume of CO2 injection was also obtained. The results are shown in Figure 5b. Figure 5a shows two stages of the oil displacement by CO2. The first stage started from the beginning of the injection and ended with the CO2 breakthrough. In this stage, the pistonlike displacement occurred. It can be seen from the profiles of 35.2 and 48 min in Figure 5a. This stage consisted of three regions: (a) a region with low
oil saturation, where the CO2 front had passed; (b) a region with high oil saturation, in which the CO2 front did not arrive; and (c) a transition region, where the CO2 front was located. With CO2 injection, the oil saturation decreased gradually from the inlet of the bead-pack core, and the low oil saturation region gradually increased. The CO2 front proceeding in the bead-pack core can be measured clearly. The slope of the front tended to be at a shallow angle to the migration direction. The second stage began after CO2 breakthrough in the total bead-pack core. The total height of the bead-pack core was 200 mm, and the height of the FOV was 40 mm. The inlet of the FOV was located 10 cm close to the outlet of the bead-pack core. The second stage ended with the unchanged oil saturation distribution. This stage was characterized by the secondary oil desaturation, which started in the inlet of the bead-pack core and propagated toward the outlet. Finally, the oil saturation decreased by 10%-20%. Oil saturation in the total FOV as a function of CO2 injection time determined from MRI is shown in Figure 5b. The oil saturation decreased gradually with the injection of CO2 and the continuous displacement of oil during CO2 displacement; this occurred until the residual oil saturation of 13.5% was reached (oil flow ceases) after the injection of 892.8 min of CO2. In the oil saturation profile, seven specific times can be found during CO2 front moving through the FOV in the experiment. The specific times are shown in Figures 4 (real schematic) and 7 (simplified schematic by analysis). Table 2 summarizes oil recovery results with the eight parts of CO2 injection for the total experimental process. The final oil recovery was 86.5%. The process of the CO2 motion can be divided into the following eight regions: (1) Part OA: The oil saturation decreased gradually from 100% to 93.8% with the increase of the injected CO2. The decrease of the oil saturation was more rapid in the end of the part OA due to the irregular characteristic of the head of miscible regions and the CO2 front. (2) Part AB: The oil saturation decreased gradually to 88.3%. The decrease of the oil saturation was also more rapid in the end of part AB due to the irregular characteristic of the CO2 front. (3) Part BC: The oil saturation decreased linearly to 37.3% due to the uniform motion of the CO2 front. (4) Part CD: The oil saturation decreased exponentially to 30.4% with the process of miscible region breakthrough. 4710
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FOV can be expressed as follows: ΔVo ¼ AφΔh
ð1Þ
where A is the cross-sectional area of bead-pack core (cm2), φ is porosity (fraction), and Δh is the displacement of the CO2 front (cm). The oil saturation (So) can be expressed as follows: So ¼
Vo Vp
ð2Þ
where Vo is the oil volume of bead-pack core in FOV (cm3), and Vp is pore volume of bead-pack core in FOV (cm3) and may be expressed as: Vp ¼ ALφ
ð3Þ
where L is the height of bead-pack core in FOV (cm). From eqs 1 to 3, we can evaluate the average velocity of the CO2 front (v): v¼
Figure 7. Simplified schematic of the specific times when the CO2 front moved through the FOV. (O) The head of miscible regions move into the FOV corresponding to time 0 min; (A) the head of the CO2 front moves into the FOV; (B) the tail of CO2 front moves into the FOV; (C) the head of miscible regions broke through the FOV; (D) the head of CO2 front broke through the FOV; (E) the tail of CO2 front broke through the FOV; (F) the tail of CO2 front broke through the total bead-pack core; and (G) the process of secondary oil desaturation was over.
(5) Part DE: The oil saturation decreased exponentially to 23.2% with the process of total CO2 front breakthrough. (6) Part EF: The oil saturation only slightly decreased to 22.2%. (7) Part FG: The oil saturation gradually decreased to 13.6% with the process of secondary oil desaturation, which was due to the velocity of CO2 flooding increase brought about by the decrease of the residual oil saturation and the quantity of CO2 dissolution when the CO2 front broke through the total bead-pack core. (8) Part GH: The oil saturation was kept constant, and residual oil saturation of 13.5% was reached (oil flow ceased). Through an analysis of part BC in the oil saturation profile, we can evaluate the average velocity of the CO2 front in the following manner. The variation of oil saturation (ΔVo) in the
Δh ΔSo ¼ L Δt Δt
ð4Þ
where ΔSo is the variation of oil saturation in the FOV (fraction), and Δt is the time variation (min). Meanwhile, ΔSo/Δt in eq 4 can be obtained by assuming that the oil saturation is linear in relation to the volume of CO2 injection. The average velocity of the CO2 front was obtained as 0.0539 cm/min. The velocity of the CO2 front was smaller than the CO2 injection rate because of the dissolution of CO2 into oil. Through an analysis of part BF in the oil saturation profile, the period of CO2 front displacement time was found to be 188.8 min, and the approximate CO2 front displacement (i.e., the total length of bead-pack core from the inlet of the FOV to the exit of the bead-pack holder) was evaluated as 10.18 cm. The results are consistent with the real length of CO2 front displacement. Figure 5c shows the saturation profiles of the total FOV based on the material balance method versus MRI method in part BC. On the assumption that the oil saturation of the total FOV based on the two methods was the same (88.3%) in time B, the oil saturation decreased linearly from 88.3% to 40.4% (in the MRI method) and from 88.3% to 37.3% (in the material balance method). The error rate for the MRI method was 6.5%. In this study, the bulk relaxation times were measured using the CPMG method. Prior to conducting the test, when the beadpack sample was 100% saturated with oil, the 50% line width was 341 Hz with the single pulse, and T2 was 59.7 ms. After 444.8 min (i.e., the time point K can be seen in the profile of Figure 5b), the oil saturation in the bead-pack sample became 22.9% for the MRI method, and T2 was 16.5 ms. After the test was completed, the oil saturation in the bead-pack sample was 13% for the same method, and T2 was 10.8 ms. T2 changed with oil saturation, but TE was about 8 times smaller than the bulk T2. The beadpack sample did not have a large T2 distribution when the fairly uniform diameter of the beads was used. The inhomogeneity of the magnetic field caused by the susceptibility gradients was largely refocused by the 180 pulse. The images can be treated as spin-density images, and the quantitative analysis of saturation was considered as true. 3.1.3. Core Analysis Methods. The coreflood interpretation method25 was applied to the data to determine the local Darcy phase velocities in the bead-pack core. The phase volumes per unit cross-sectional area between the core inlet (ζ= 0) and 4711
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Table 2. Oil Recovery with a CO2 Injection Rate of 0.2 mL/min. specific
time of CO2 injection
volume of CO2 injection
pore volume of CO2
regions
(min)
(ml)
injection
OA
16
oil recovery based on MRI oil recovery based on material balance (%)
3.2
1.29
6.2
1.28
0.52
5.5
AB
6.4
BC
38.4
7.68
3.11
CD
6.4
1.28
0.52
6.9 7.2
51
47.9
DE
16
3.2
1.29
EF
128
25.6
10.35
1
FG GH
601.6 80
120.32 16
48.66 6.47
8.6 0.1
total
892.8
178.56
72.21
86.5
current position (ζ= z) are given by Z z φðζÞ Sg ðζ, tÞ dζ Vg ðz, tÞ ¼
(%)
ð5Þ
0
where φ(z) is the porosity at position z (Figure >/>), and Sg(ζ,t) is the CO2 saturation at position z and time t (Figure 5a). Using a material balance approach, the local CO2 and oil Darcy phase velocities can then be expressed in the following form: Ug ðz, tÞ ¼ UðtÞFginj ðtÞ -
DVg ðz, tÞ Dt
ð6Þ
and Uo ðz, tÞ ¼ -
DVo ðz, tÞ Dt
ð7Þ
where Ug(z,t) and Uo(z,t) are the local Darcy phase velocity of CO2 and oil, respectively; U(t) is the total Darcy velocity; and inj Finj g (t) is the fractional flow of CO2 injection at time t. Fg (t) should be corrected by the average velocity of the CO2 front because of the dissolution of CO2 into oil. Afterward, the local Darcy phase velocities of CO2 and oil were obtained with this method and shown in Figure 8. In Figure 8a, the local Darcy phase velocities of CO2 increased with the CO2 injection time, from the inlet up to the outlet, along the flow direction in the bead-pack core in FOV. For example, the profile of 48 min consists of three regions: (a) a region with high velocity where the CO2 front has passed; (b) a region with low velocity in which the CO2 front does not arrive; and (c) a transition region, where the CO2 front is located. Once the CO2 front broke through (83.2 min), the velocity from the inlet to the outlet along the flow direction in the bead-pack core in the FOV reached the peak values. The Darcy law in the new interpretation methods for the phase velocities is expressed according to DpR þ FR g ð8Þ UR ¼ - kλR Dz where R denotes each phase of CO2 (g) and oil (o), U is the local Darcy velocity; λR = krR/μR is the mobility, k is the absolute permeability, p is the pressure, F is the density, g is the gravity constant, kr is the relative permeability, and μ is the viscosity. The capillary pressure pco(So) is defined in the standard convention by pco ðSo Þ ¼ pg ðSo Þ - po ðSo Þ
ð9Þ
Figure 8. Local phase velocity of (a) CO2 velocity and (b) oil velocity.
From eqs 8 and 9, the local Darcy velocity of CO2 can be expressed as follows: ! kgðFo - Fg Þ λg λo 1þ Ug ¼ UðtÞ λg þ λo UðtÞ -k
λg λo dpco DSo λg þ λo dSo Dz
ð10Þ
where U is the total flow rate. The viscous-dominant fractional flow function, gravity countercurrent flow function, and capillary 4712
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Figure 10. Viscous-dominant fractional flow function at 8.5 MPa and 40 °C.
Figure 9. Capillary dispersion rate at 8.5 MPa and 40 °C.
dispersion rate can be respectively defined as follows: fg ðSo Þ ¼
λg λ g þ λo
Gg ðSo Þ ¼ gðFo - Fg Þ
λg λo λg þ λo
ð11Þ
ð12Þ
and λg λo dpco ð13Þ λg þ λo dSo The parameters can be evaluated independently by imaging the distribution of each phase in the core at various injection flow rates. The relative permeability and the capillary dispersion rate can be evaluated without further measurements, such as pressure drop across the core. Figure 9 shows that the capillary dispersion rates vary with the oil saturations under the condition of 8.5 MPa and 40 °C. The capillary dispersion rate was calculated independently from two data sets of different flow rates. As the oil saturation increased, the capillary dispersion rate increased first and then decreased eventually. When the oil saturation was between 0.7 and 0.8, the capillary dispersion rates reached the peak values. Although the profiles are different, their actual profiles are identical. The difference may be caused by the experimental results; for example, it was difficult to control the CO2 displacement speed in the test because of the dissolution of CO2 into oil. Figure 10 shows that the viscous-dominant fractional flow function varies with the oil saturation at 8.5 MPa and 40 °C. The viscous-dominant fractional function began to decrease below 1 at oil saturation of approximately 0.2. This saturation corresponded to the irreducible oil saturation. At low oil saturations, the migration of CO2 phase is said to be governed by viscosity. The profile of the gravity countercurrent flow function is shown in Figure 11. The gravity countercurrent flow function corresponded to λgλo/(λg þ λo) because the buoyancy g(Fo - Fg) was constant against oil saturation. The mobility of oil λo was zero at So = 0 and increased with increasing oil saturation. On the other hand, the mobility of CO2 λg was zero at So = 1 and increased with a decrease in oil saturation. Therefore, the gravity countercurrent flow function took a maximum at the saturation of 0.65 (Figure 11). 3.2. Gaseous CO2 Immiscible Displacement Test. 3.2.1. NMR Image Analysis. In the CO2 immiscible displacement test, gaseous CO2 was injected vertically upward into the bead-pack core dcpo ðSw Þ ¼ -
Figure 11. Gravity countercurrent flow function at 8.5 MPa and 40 °C.
saturated with oil at a pressure of 7.0 MPa and a temperature of 40 °C. This CO2 displacement experiment was conducted at a constant flow rate of 0.15 mL/min. Figure 12 shows a series of NMR images that illustrate oil saturation at different CO2 injection times of 0, 19.2, 41.6, 64, 105.6, 144, 326.4, and 915.2 min, respectively. The CO2 moved upward rapidly because of fluid viscosity and buoyancy, after which the CO2 broke through the bead-pack core in FOV. Given that the bead-pack core was not perfectly homogeneous, the injected CO2 tended to channel through the highpermeable zones. Therefore, some thin channels were established, and the CO2 ran through the channels vertically in a short period. These are shown in the images of 19.2, 41.6, and 64 min in Figure 12. Once these channels were established, the secondary oil desaturation started, and CO2 ran through these channels continuously. These are shown in the images of 105.6, 144, and 326.4 min in Figure 12, bypassing most of the residual oil in the matrix. The oil saturation decreased gradually and the residual oil became immobilized as shown in the image of 915.2 min. 3.2.2. Saturation Profiles. The evolution of oil saturation profiles in the bead-pack core at 7.0 MPa and 40 °C with CO2 injection was obtained. The results are shown in Figure 13, which also shows the two stages of the oil displacement by CO2. In the first stage, starting from the beginning of the injection up to the CO2 breakthrough, the CO2 penetrated through the more permeable regions. The second stage started after CO2 breakthrough and ended with the unchanged oil saturation distribution. 4713
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Figure 12. Distribution of NMR signal intensity in the bead-pack core at 7.0 MPa, 40 °C, and a CO2 injection rate of 0.15 mL/min.
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The profile of the CO2 motion can also be divided into four regions. In part OA, starting from the beginning of the injection up to the total of CO2 displacement front move into the FOV, the oil saturation decreased from 100% to 89.7% (as shown in the point A) after the CO2 injection of 41.6 min. In part AB, CO2 breakthrough oil saturation decreased linearly from 89.7% to 76.3% (in the point B) with the CO2 injection of 22.4 min. In part BC, the oil saturation decreased exponentially to 37.8% with the CO2 injection of 262.4 min; meanwhile, in part CD, continuous displacement of oil occurred during CO2 flooding until it reached the point of residual oil saturation (37.2%) and oil flow ceased. The CO2 additional injection of 588.8 min recovered only little oil because, at this point, the relative permeability to oil was close to zero since oil saturation was equal to residual oil saturation and oil did not flow. In addition, the final oil recovery was 62.8%. The sweep efficiency in this test was also lower compared with the supercritical CO2 miscible displacement test. The oil saturation in the bead-pack sample after the test was 37.2% based on the MRI method, and T2 was 22.9 ms.
4. CONCLUSION This study has demonstrated the experimental results on supercritical CO2 miscible displacement test and gaseous CO2 immiscible displacement test in high-permeable bead-pack core. A new highpressure bead-pack holder has been designed and constructed for the MRI system using an RF probe with an inner diameter of 40 mm. Its maximum working pressure is 15 MPa, and its maximum working temperature is 70 °C. The process of CO2 being injected into the bead-pack core at a high pressure and a high temperature by the highpressure holder can be visualized using MRI. Using this method, the fundamental characteristics of the flooding process, such as the piston-like miscible regions and CO2 front, onset of CO2 channelling or fingering, and the distribution of oil in porous media, can be accurately detected. In addition, the phenomenon of CO2 channelling or fingering occurred in CO2 immiscible displacement due to the difference of fluid viscosities and densities. The sweep efficiency was small, and the final residual oil saturation was 37.2%. For CO2 miscible displacement, the piston-like displacement also occurred. The phenomenon of CO2 channelling or fingering was restrained effectively, and the velocity of the CO2 front was uniform; in addition, the sweep efficiency was high, and the final residual oil saturation was 13.5%. CO2 miscible displacement could enhance oil recovery evidently more than CO2 immiscible displacement. The oil saturation distributions have also been monitored in situ as a function of the volume of CO2 injection. Through the analysis of the oil saturation profile, the velocity of the CO2 front has been evaluated. A special core analysis method has also been applied to in situ oil saturation data to directly evaluate the effect of viscosity, buoyancy, and capillary pressure on CO2 miscible displacement. ’ AUTHOR INFORMATION Corresponding Author
Figure 13. Evolution of oil saturation profiles in the bead-pack core at 7.0 MPa, 40 °C, and a CO2 injection: (a) one-dimensional saturation profile along the bead-pack core; and (b) saturation profile of the total FOV versus volume of CO2 injection.
Figure 13b shows the oil saturation in the total FOV as a function of the volume of CO2 injection determined from MRI.
*Tel.: þ86-411-84706608. Fax: þ86-411-84708015. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors are grateful for the financial support given by the National High Technology Research and Development Program of China (863 Program, Grant Nos. 2008AA062303 and 4714
dx.doi.org/10.1021/ie1013019 |Ind. Eng. Chem. Res. 2011, 50, 4707–4715
Industrial & Engineering Chemistry Research 2009AA063402), the National Basic Research Program of China (973 Program, Grant No. 2006CB705804), and the National Natural Science Foundation of China (Key Program, Grant No. 50736001).
’ NOMENCLATURE A = the cross-sectional area of bead-pack core, cm2 dcpo(Sw) = capillary dispersion rate, s-1 fg(So) = viscous-dominant fractional flow function Finj g (t) = the fractional flow of CO2 injection at time t g = gravity constant, ms-2 Gg(So) = gravity countercurrent flow function, m-1 s-1 k = absolute permeability, m2 kr = relative permeability L = the height of FOV, cm p = pressure, Pa pco(So) = the capillary pressure, Pa Sg(ζ,t) = the CO2 saturation at position z and time t So = the oil saturation U(t) = the total Darcy velocity, m s-1 Ug(z,t) = the local Darcy phase velocity of CO2, m s-1 Uo(z,t) = the local Darcy phase velocity of oil, m s-1 v = the average velocity of CO2 front, cm min-1 Vo = the oil volume of bead-pack core in FOV, cm3 Vp = pore volume of bead-pack core in FOV, cm3 R = each phase of CO2 (g) and oil (o) φ = porosity φ(z) = the porosity at position z λR = mobility, Pa-1 s-1 μ = viscosity, Pa s F = density, kg m-3 Δh = the displacement of CO2 front, cm ΔQ = the injected variation of CO2 volume in FOV ΔSo = the variation of oil saturation in FOV Δt = the time variation, min ΔVo = the variation of oil saturation in FOV, cm3 Subscripts
a = phase label g = gas phase o = oil phase
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