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Abstract

Over the past decades, CO₂ flooding in low-permeability porous media has continued to grow. Furthermore, propelled by the current movement toward CO₂ sequestration, the flooding of CO₂ is expected to further increase. Thus, it is of scientific significance to study the mechanism of displacement of CO₂ flooding as well as the CO₂ storage characteristics. In this study, CO₂ miscible/immiscible flooding experiments were conducted on two low permeability samples to study the difference between them. Specifically, the effect of oil displacement and CO₂ storage coefficient for different displacement models were measured and compared. In addition, nuclear magnetic resonance (NMR) tests and frozen pelleting experiments were conducted. The NMR tests clarified the lower limit of pore throat where oil can be displaced by CO₂ miscible/immiscible flooding. Furthermore, the frozen pelleting experiments analyzed the distribution characteristics of the remaining oil before and after CO₂ miscible/immiscible flooding. The investigation showed that the development efficiency of CO₂ miscible flooding is higher than CO₂ immiscible flooding in low permeability reservoirs. The results also indicated that CO₂ miscible flooding increases the recovery of hard-to-produce semi-bound oil phase and CO₂ storage coefficient with CO₂ miscible displacement is more significant than that with CO₂ immiscible flooding. The data from this study is a critical input for reservoir simulation, which sheds light on the CO₂ miscible/immiscible flooding in low permeability reservoirs.

Keywords

Low permeability porous media oil displacement efficiency

Introduction

As a fluid injected into oil reservoirs, especially unconventional reservoirs (tight sandstone, tight carbonate, and shale), carbon dioxide (CO₂) has been widely applied around the world, and CO₂ miscible/immiscible flooding has been proven to be an effective enhanced oil recovery (EOR) method (Abdassah and Kristanto, 2000; Dyer and Farouq, 1989; Khatib et al., 1981; Zhou et al., 2020). The main mechanisms of CO₂-EOR include that CO₂ could dissolve easily into the crude oil, swell the oil, reduce oil viscosity and density, and extract the light components of crude oil at moderate pressure (Al-Otaibi et al., 2015, 2019; Zhou et al., 2020). As stated by Alotaibi et al. (2019), there were more than 50 CO₂-EOR projects in the United States with oil production about 300,000 bbl/d. Therefore, it is of scientific, theoretical, and environmental significance to study the displacement mechanism of CO₂ flooding as well as the characteristics of CO₂ storage.

CO₂-EOR can be classified as CO₂ miscible flooding (Koottungal, 2014; Zhou et al., 2020) and immiscible flooding (Dyer and Farouq, 1989; Mohammed and Singhal, 2005; Yellig and Metcalfe, 1980). Koottungal (2014) suggested that, compared to other EOR methods in the United States, CO₂ miscible flooding has provided the highest daily production since 2012. Zhang et al. (2018) concluded that under reservoir conditions, CO₂ miscibility flooding could not be always achievable and CO₂ immiscible flooding would be implemented due to technical difficulties or commercial considerations. Furthermore, they also pointed out that CO₂ immiscible flooding has been successfully implemented throughout the world although the CO₂ immiscible displacement was less sufficient than CO₂ miscible displacement. For example, as stated by Dyer and Farouq (1989), CO₂ immiscible flooding was first conducted in Bartlesville, Oklahoma, in 1958. Therefore, to better understand the mechanism of CO₂ flooding, relevant researches are needed.

Nuclear magnetic resonance (NMR) technique has been successfully used to characterize rock pore structure by measuring the signal intensity of hydrogen atoms in pore spaces. Compared with conventional methods, nuclear magnetic resonance (NMR) techniques can determine the pore size distribution more accurately without damaging the sample (Hosseini et al., 2018; Liu et al., 2020). For instance, NMR was used to study the pore size distribution in tight sandstones (Wang et al., 2001). Nuclear magnetic resonance techniques can also be used to assess the porosity of mobile fluids in a reservoir (Wang et al., 1998, 2001). NMR is widely used for studying CO₂ flooding recently. Sun et al. (2016) carried out investigation of CO₂ and water exchange in coal. Cai et al. (2021) used NMR method to investigate CO₂ displacement for low permeability core in both miscible and immiscible flooding, Zhang et al. (2022) used online low-field NMR to study CO₂ flooding and water flooding mechanism in ultralow-permeability sample, and the in situ stress effect on oil recovery.

Fluorescence is a phenomenon of luminescence produced by a substance subjected to laser radiation. When a molecule of matter is exposed to a special light source and absorbs a photon, the electron in the ground state is excited to jump. If energy is released by emitting the corresponding photons and returning to the ground state, fluorescence is emitted (Kaoping et al., 2004). The fluorescence methods described here are straightforward and affordable to apply and can be carried out using a basic steady-state fluorescence spectrometer (Sommer et al., 2015). This method is widely used in chemistry, medicine, and biology (Gibson et al., 2016; Miller, 2019; Sommer et al., 2015). Fluorescence analysis was also used to study the remaining oil distribution after polymer flooding (Kaoping et al., 2004; Li, 2004; Xilong, 2022). Fluorescence analysis application in CO₂ flooding is rarely reported.

In this study, CO₂ miscible/immiscible flooding experiments were, respectively, conducted on two low permeability samples. In addition, nuclear magnetic resonance (NMR) tests and frozen pelleting experiments were conducted to better understand the displacement efficiency and CO₂ storage effect by CO₂ flooding. The remainder of the article is organized as follows: “Methodology” describes the details about core samples and the process of various experiments process used in this study. “Results” presents the results and discussions of CO₂ flooding, NMR tests, and Frozen pelleting experiments. Finally, “Conclusions” concludes the main finding of the study.

Methodology

Core samples and experimental fluids

In this study, two core plugs were sampled from a low permeability formation, named Chang 8, located in Block Zhuang 46 in Changqing Oilfield, Ordos Basin. Routine core analysis was carried out to determine the length, diameter, core dry weight, total core volume, pore volume, porosity, and gas permeability of each core plug. Table 1 summarizes the basic physical properties (dimension and routine data) of these two samples. It should be noted that both samples were scanned to ensure that there were no fracture and permeability barriers in these two samples before the experiments. As a result, it is reasonable, to some extent, to consider these two samples to be homogeneous. Furthermore, each core sample (FY-1 or FY-2) was divided into different parts for CO₂ flooding, frozen pelleting, and NMR experiments. Because the divided parts were from the same homogeneous core sample, it was assumed that they had the same pore structure. Furthermore, kerosene and CO₂ were used as the oil phase and the gas phase, respectively. The formation water, whose components were listed in Table 2, was used as the water phase.

Table 1.

Properties of the core plugs used in the experiments.

No.	Length (cm)	Diameter (cm)	Dry weight (g)	Total core volume (cm³)	Pore volume (cm³)	Porosity (%)	Gas permeability (mD)
FY-1	4.29	2.53	50.251	21.567	2.373	11.005	0.102
FY-2	4.93	2.54	55.999	24.981	3.089	12.366	0.210

Notes: Core FY-1 was used for CO₂ miscible flooding and Core FY-2 was used for CO₂ immiscible flooding.

Table 2.

Composition of the formation water.

Components	NaCl	CaCl₂	MgCl₂·6H₂O	Na₂SO₄	NaHCO₃	TDS
Contents (mg/L)	3977	28	46	93	2634	6778

Note: TDS means total dissolved solids.

Experimental procedures

CO₂ flooding

The CO₂ miscible/immiscible flooding experiments on the low permeability core were carried out in the high-pressure and high-temperature (HPHT) displacement experiment device to determine the efficiency of CO₂ miscible/immiscible flooding and the corresponding storage coefficient. The minimum miscibility pressure between CO₂ and oil is 22 MPa in this study. The back pressure and temperature during the miscible flooding were 25 MPa and 70°C, respectively. However, as a comparison, during the immiscible flooding, the back pressure and temperature were 15.5 MPa and 70°C, respectively. The injection of CO₂ is in the supercritical phase. In addition, the injection rate was 0.01 mL/min. As shown in Table 1, Core FY-1 was used for the CO₂ miscible flooding experiment and Core FY-2 was used for the CO₂ immiscible flooding experiment. It is well known that quantitative characterization of the micro occurrence state of fluids can be realized through laser confocal series detection. Thus, to illustrate the results of CO₂ flooding, the distribution of oil phase before and after CO₂ miscible/immiscible flooding in these two cores were characterized through the frozen pelleting experiments. The detailed procedures for CO₂ flooding experiments could be summarized as follows:

Wash the oil in the cores using the weight balance method and dry the cores, and the criterion for qualified oil is that the core sample reaches Level 3 under fluorescent light.

Place the core samples (FY-1 and FY-2) into the core holder, and maintain the confining pressure of the sample at 3 MPa by using a hand pump.

Vacuum the core samples for 24 h by using a vacuum pump.

Inject the simulated formation water into the cores and record the pressure. When the pressure gauge reading was kept at 0.5 MPa, terminate the water injection process.

Kerosene was injected into the core samples at a rate of 0.005 mL/min until no water was produced at the outlet of the device. Measure the volume of water and determine the irreducible water saturation.

Place the core holder in a thermostat for 24 h, take out the cores (FY-1 and FY-2) and cut the cores end of about 1.5 cm to prepare the frozen thin slices, then carry out the CO₂ miscible/immiscible flooding experiments on the remaining cores at a constant rate of 0.01 mL/min. After the CO₂ miscible/immiscible flooding experiments, cut the core end by about 1.5 cm to prepare the frozen slices.

Use the laser confocal cryopreservation technology to analyze the microscopic occurrence of fluids and the remaining oil distribution after CO₂ miscible/immiscible flooding in the two cores.

Record the oil and gas output at the outlet at an interval (about 1 h), and terminate the experiment when no oil is observed at the outlet. Finally, determine the CO₂ storage coefficient by using the following equation:

R_{s} = \frac{M_{e}}{N} = \frac{(V_{z g} - V_{c g}) ρ_{g}}{V_{o} ρ_{o}}

(1)

where M_e is the CO₂ effective storage volume in the core, g; N is the oil in place in the core sample, g; R_s is the CO₂ storage coefficient, g/g; V_zg is the cumulative volume of CO₂ injected, ml; V_cg is the cumulative volume of CO₂ produced, ml; ρ_g is the CO₂ density, g/cm³; ρ_o is the oil density, g/cm³; and V_o is the saturated oil volume, ml.

NMR experiments

To further explain the results of CO₂ flooding, the NMR relaxation time T₂ tests were carried out with the high frequency (23 MHz) NMR instrument in the study. The specific procedures were as follows:

Weigh the dry core samples and measure the length and diameter. Then, pressurize and saturate the dry core samples with the simulated formation water and weigh the core wet weight. For core samples saturated with water, perform an NMR T₂ spectrum measurement.

Conduct centrifugal experiments on rock samples under 500 psi for 3 h, then measure the T₂ spectrum of the centrifuged cores with NMR measurement.

Dry the centrifuged core, inject kerosene into the cores, and weigh the core saturated with kerosene. Then, measure T₂ spectrum of the cores saturated with kerosene.

Carry out CO₂ flooding experiments on cores saturated with kerosene, and measure the T₂ spectrum of the cores after CO₂ displacement.

For a given core sample, since the T₂ spectrum measured by NMR reflects the number of hydrogen atoms in the pore space, it is proportional to the pore radius (Fang et al., 2018; Li et al., 2020; Xiao et al., 2016). T₂ spectrum is a function of surface relaxation and the ratio of pore volume V_p to surface area S_p (Gao et al., 2019; Gao and Li, 2015; Saidian and Prasad, 2015; Xiao et al., 2016). Thus, one has the following equation:

T_{2} = \frac{1}{β} \frac{V_{p}}{S_{p}}

(2)

where β is the relaxation of the surface; V_p is pore volume, cm³; S_p is surface area, cm². Mathematically, for the spherical pore with pore radius r, the ratio of V_p to S_p equals to r/3. However, for the cylindrical pore with pore radius r, the ratio of V_p to S_p equals to r/2. Thus, to simplify the model, Kenyon (1992) suggested that equation (2) could be rewritten as:

T_{2} = c r

(3)

where c is the conversion coefficient. Equation (3) reveals that the T₂ spectrum is proportional to the pore radius. In addition, after the c is determined, the pore radius of porous media will be determined according to the measured T₂ spectrum. In general, parameter c is not a constant but varies with r and T₂ spectrum. By combining the longitudinal interpolation method, mercury injection data, measured T₂ spectrum data, and inverse modeling, Liu et al. (2020) obtained the optimum c of three low permeability samples. In this study, we also used this method to optimize the parameter c.

Figure 1 is the flow chart of the experimental equipment.

Figure 1.

Schematic of CO₂ injection with NMR.

Results

CO₂ miscible/immiscible flooding

Figure 2 shows the results (e.g. recovery, storage coefficient, and gas–oil ratio) between CO₂ miscible flooding of Core FY-1 and CO₂ immiscible flooding of Core FY-2. It suggests that CO₂ miscible flooding leads to better oil displacement effects in ultralow permeability reservoirs than CO₂ immiscible flooding. For example, although irreducible water saturation of Core FY-1 (46.66%) is similar with that of Core FY-2 (45.56%), the oil displacement efficiency of CO₂ miscible flooding in Core FY-1 is 76.19%, which is much higher than that of CO₂ immiscible flooding in Core FY-2 (about 40.98%). In addition, Figure 1 also reveals that the gas breakthrough for CO₂ miscible flooding is later than that for the immiscible flooding (the delay time is about 0.11 PV). Moreover, based on equation (1), the CO₂ storage coefficient for CO₂ miscible flooding was 0.834, which is much higher than that for CO₂ immiscible flooding (0.135). It illustrates that CO₂ miscible flooding leads to a better CO₂ storage effect in ultralow permeability reservoirs.

Figure 2.

Comparison of the CO₂ displacement results between Core FY-1 and Core FY-2. (a) CO₂ miscible flooding, (b) CO₂ immiscible flooding.

NMR tests

Figure 3 presents the results of the NMR of CO₂ miscible/immiscible flooding. More specifically, Figure 2(a) and (b) shows the results of the NMR test for CO₂ miscible flooding and Figure 2(c) and (d) shows the results of the NMR test for CO₂ immiscible flooding. As depicted in Figure 2, the measured T2 distribution curve can reflect the pore radius distribution and porosity components of the sample. Furthermore, based on the results in Figure 2, the lower limit of rock pore throat for CO₂ flooding could be determined (Tables 3 and 4). Table 3 suggests that the lower limit of Core FY-1 pore throat for CO₂ miscible flooding is 0.04 μm, and the 0.04–0.5 μm pore throats contribute most to the recovery factor. For example, the 0.04–0.5, 0.5–1, 1–3, 3–5, and 5–10 μm pore throats lead to the contribution of 54.17%, 23.31%, 15.33%, 6.59%, and 0.35% to the recovery factor, respectively. Table 4 shows that the lower limit of the Core FY-2 pore throat for CO₂ immiscible flooding is 0.08 μm and the 0.08–0.5 μm pore throats contribute the most to the recovery factor (71.46%). As a consequence, the lower limit of the pore throat for CO₂ miscible flooding is 0.04 μm, which is less than the value for CO₂ immiscible flooding (0.08 μm). The range of the pore throats contributing the most to the oil recovery is 0.04–0.5 μm in CO₂ miscible flooding and is 0.08–0.5 μm in CO₂ immiscible flooding. That is, the pore throats that contribute the most to the recovery factor in CO₂ miscible flooding are less than that in CO₂ immiscible flooding.

Figure 3.

Comparison of the NMR test results between Core FY-1 and Core FY-2. (a) Core FY-1 saturated with water and after centrifugation, (b) Core FY-1 saturated with kerosene and after CO₂ flooding, (c) Core FY-2 saturated with water and after centrifugation, (d) Core FY-2 saturated with kerosene and after CO₂ flooding.

Table 3.

NMR test results of the ultralow permeability Core FY-1.

Fluid distribution in different stages		Ratio of fluids in the pores corresponding to the pore-throat of different sizes (μm)						Total
Fluid distribution in different stages		<0.04	0.04–0.5	0.5–1	1–3	3–5	5–10	Total
Saturated with water	Water volume/core volume (%)	1.25	5.14	1.60	1.29	0.39	0.13	9.81
Saturated with water	Water volume/pore volume (%)	12.75	52.41	16.33	13.20	4.02	1.29	100
After centrifugation	Irreducible water volume/core volume (%)	0.91	2.39	0.45	0.40	0.18	0.08	4.41
After centrifugation	Irreducible water volume/pore volume (%)	9.32	24.41	4.56	4.12	1.81	0.77	45.00
Saturated with kerosene	(Irreducible water + oil volume)/core volume (%)	1.81	5.19	1.53	1.25	0.40	0.02	10.20
Saturated with kerosene	(Irreducible water + oil volume)/pore volume (%)	17.77	50.84	15.02	12.30	3.92	0.15	100
After flooding	(Bound water + residual oil volume)/core volume (%)	1.80	2.82	0.51	0.59	0.11	0	5.83
After flooding	(Bound water + residual oil volume)/pore volume (%)	30.88	48.35	8.81	10.03	1.93	0	100
Ratio of recovery factor	Produced oil volume/core volume (%)	0.01	2.37	1.08	0.67	0.29	0.02	4.37
	Produced oil volume/pore volume (%)	0.11	23.19	9.98	6.56	2.82	0.15	42.80
	Recovery factor (produced oil volume/oil volume) (%)	2.29	82.44	83.63	75.11	88.65	100	75.46
	Contribution rate (produced oil volume/total produced oil volume) (%)	0.25	54.17	23.31	15.33	6.59	0.35	100

Table 4.

NMR test results of the ultralow permeability Core FY-2.

Fluid distribution in different stages		Ratio of fluids in the pores corresponding to the pore-throat of different sizes (μm)					Total
Fluid distribution in different stages		<0.08	0.08–0.5	0.5–1	1–3	3–5	Total
Saturated with water	Water volume/core volume (%)	3.50	3.08	1.61	2.32	0.59	11.09
Saturated with water	Water volume/pore volume (%)	31.52	27.74	14.53	20.93	5.28	100.00
After centrifugation	Irreducible water volume/core volume (%)	3.06	1.03	0.29	0.33	0.06	4.77
After centrifugation	Irreducible water volume/pore volume (%)	27.58	9.28	2.66	2.99	0.52	43.03
Saturated with kerosene	(Irreducible water + oil volume)/core volume (%)	3.33	6.22	1.28	1.27	0.17	12.25
Saturated with kerosene	(Irreducible water + oil volume)/pore volume (%)	27.15	50.72	10.46	10.33	1.35	100.00
After flooding	(Bound water + residual oil volume)/core volume (%)	3.31	3.63	1.05	0.65	0	12.25
After flooding	(Bound water + residual oil volume)/pore volume (%)	27.03	29.66	8.53	5.31	0	100.00
Ratio of recovery factor	Produced oil volume/core volume (%)	0.01	2.58	0.24	0.61	0.17	3.61
	Produced oil volume/pore volume (%)	0.12	21.06	1.93	5.02	1.35	29.47
	Recovery factor (produced oil volume/oil volume) (%)	2.44	58.04	22.22	50.91	100	48.28
	Contribution rate (produced oil volume/total produced oil volume) (%)	0.40	71.46	6.53	17.02	4.58	100.00

Frozen pelleting experiments

State of fluid occurrence before and after CO₂ miscible flooding

Based on the core fluorescence software (Image J), frozen slice images of Core FY-1 before CO₂ miscible flooding could be analyzed. The initial state of the mobile oil in throat, cluster, and corner shapes before CO₂ miscible flooding for Core FY-1 are depicted in Figure 4(a). The immobile oil mainly is bound in intergranular adsorption and slit shapes (Figure 4(b)). Water is mainly irreducible water. After CO₂ miscible flooding, the remaining oil is bound in throat-shaped, cluster-shaped, corner-shaped, intergranular adsorption-shaped, and slit-shaped.

Figure 4.

Frozen slice images of Core FY-1. (a) Before CO₂ miscible flooding, (b) after CO₂ miscible flooding.

The comparisons in Figure 5 suggest that throat-shaped oil, cluster-shaped oil, corner-shaped oil, intergranular adsorption-shaped oil, slit-shaped oil, and irreducible water account for 7.91%, 24.72%, 8.54%, 12.45%, 1.27%, and 44.16%, respectively, before CO₂ miscible flooding. After CO₂ miscible flooding, the throat-shaped oil, cluster-shaped oil, corner-shaped oil, intergranular adsorption-shaped oil, slit-shaped oil, and irreducible water account for 3.63%, 5.27%, 4.73%, 2.27%, 0.36%, and 40.53%, respectively. It is obvious that both oil saturation and water saturation decrease after CO₂ miscible flooding. Before and after CO₂ miscible flooding, oil saturation drops from 54.89% to 16.26%, with a change of 38.63%. The order of significant change from largest to smallest is the cluster-shaped oil, the intergranular adsorption-shaped oil, the throat-shaped oil, the corner-shaped oil, and the slit-shaped oil. In addition, water saturation declines from 44.16% to 40.53%, with a change of 3.63%.

Figure 5.

Analysis of the fluid microscopic occurrence of Core FY-1. (a) Before CO₂ miscible flooding, (b) after CO₂ miscible flooding.

State of fluid occurrence before and after CO₂ immiscible flooding

With the same method, frozen slice images of Core FY-2 before and after CO₂ immiscible flooding were analyzed. As depicted in Figure 6, the initial state of the oil phase (mobile oil and immobile oil) can also be classified as throat shape, cluster shape, corner shape, intergranular adsorption shape, and the slit shape for core FY-2 before CO₂ immiscible flooding. After CO₂ immiscible flooding, the remaining oil distributes in throat, cluster, corner, intergranular adsorption, and slit shapes.

Figure 6.

Frozen slice images of Core FY-2. (a) Before CO₂ miscible flooding, (b) after CO₂ miscible flooding.

Results from Figure 7 suggest that oil saturation drops from 57.33% to 35.76%, with a change of 21.57% after CO₂ immiscible flooding. Before and after CO₂ immiscible flooding, the oil saturation difference in the throat-shaped, cluster-shaped, corner-shaped, intergranular adsorption, and slit is 2.51%, 10.19%, 2.10%, 6.23%, and 0.54%, respectively. Water saturation reduces from 40.11% to 38.65%, with a change of 1.46%. Thus, compared with CO₂ immiscible flooding, CO₂ miscible flooding can produce more oil and water in various types of pore.

Figure 7.

Analysis of the fluid microscopic occurrence state of FY-2 core before CO₂ immiscible flooding. (a) Before CO₂ miscible flooding, (b) after CO₂ miscible flooding.

Based on the experimental measurements, the fluid occurrence state before and after CO₂ flooding, the recovery enhanced by CO₂ miscible/immiscible flooding, and the production characteristics of various types of remaining oil were compared and analyzed. The comparison results are shown in Figures 8 and 9. Before CO₂ miscible and immiscible flooding, the irreducible water saturation and the occurrence state of various oil types are basically the same (Figure 7(a)). However, the difference between various types of bound oil becomes obvious after CO₂ flooding (Figure 7(b)). The recovery of CO₂ miscible flooding is much higher than that of immiscible CO₂ flooding. Figure 8 suggests that CO₂ miscible flooding increases the recovery of hard-to-produce oil (throat-shaped oil and corner-shaped oil) by 3.48%.

Figure 8.

Statistics of the fluid occurrence state. (a) Before CO₂ miscible flooding, (b) after CO₂ miscible flooding.

Figure 9.

Proportion of different types of remaining oil after CO₂ flooding.

Conclusions

In this article, we carried out CO₂ miscible/immiscible flooding and the nuclear magnetic resonance (NMR) experiments on two low permeability samples to study the mechanism of CO₂ displacement in ultralow permeability sandstone reservoirs. The difference in oil displacement efficiency, CO₂ storage coefficient, and the lower limit of pore throat for CO₂ miscible and immiscible flooding were analyzed. Furthermore, by combining the HPHT displacement equipment and the frozen pelleting technology, the distribution of the oil before and after CO₂ miscible/immiscible flooding was analyzed. Based on the results and observations of the study, the following conclusions can be drawn.

For low permeability reservoirs, the gas breakthrough in CO₂ miscible flooding is generally later than that in CO₂ immiscible flooding, and the recovery of CO₂ miscible flooding is 32.74%, which is higher than that of immiscible CO₂ flooding.

For both CO₂ miscible and immiscible flooding, the pore with higher permeability is easier to produce, meanwhile, compared with immiscible flooding, CO₂ miscible could produce more oil in low permeability pore, such as throat shape and corner shape.The lower limit of pore throat, where oil can be produced with CO₂ miscible flooding, is 0.04 μm, which is smaller than that of Core FY-2 with CO₂ immiscible flooding (0.08 μm).

For the low permeability reservoirs, CO₂ storage coefficient (0.834) of Core FY-1 with CO₂ miscible displacement is greater than that (0.135) of Core FY-2 with CO₂ immiscible flooding, which reveals that CO₂ miscible flooding provides a better CO₂ storage alternative.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This project was supported by the National Natural Science Foundation of China (No. 52074250, No. 41802195).

ORCID iD

Weirong Li

Nomenclature

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Displacement efficiency and storage characteristics of CO 2 in low permeability reservoirs: An experimental work

Abstract

Keywords

Introduction

Methodology

Core samples and experimental fluids

Experimental procedures

CO2 flooding

NMR experiments

Results

CO2 miscible/immiscible flooding

NMR tests

Frozen pelleting experiments

State of fluid occurrence before and after CO2 miscible flooding

State of fluid occurrence before and after CO2 immiscible flooding

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

Nomenclature

References

CO₂ flooding

CO₂ miscible/immiscible flooding

State of fluid occurrence before and after CO₂ miscible flooding

State of fluid occurrence before and after CO₂ immiscible flooding