Abstract
In the existing research, there are few studies on the factors affecting the effect of the CO2 huff and puff based on the law of crude oil utilization in microscopic pores. We will carry out this research on tight sandstone based on NMR. In this work, based on the law of crude oil production in microscopic pores, the relationship between various factors and huff and puff efficiency was studied, and the influence law of each factor was clarified. We characterize the dynamic changes in crude oil mobility, microporous crude oil mobilization characteristics, and huff and puff efficiency during CO2 huff and puff under different conditions based on nuclear magnetic resonance technology (NMR). The experimental results show that the injection pressure is the main factor affecting the effect of the CO2 huff and puff. The original pore structure characteristics of rock and soaking time also affect the huff and puff efficiency, but they are not the main influencing factors. In the process of CO2 huff and puff, the reservoir with better macropore development will have a better huff and puff effect. For the core with medium pore and smaller pore development, the injection pressure can be increased and the soaking time can be prolonged appropriately to improve the huff and puff effect. When the injection pressure reaches the minimum miscible pressure (MMP), the utilization degree of crude oil with larger pores reaches the maximum, and the effect of increasing the injection pressure on the huff and puff efficiency is limited. With the increase in soaking time, the huff and puff efficiency increases first and then decreases, but the overall change range is small. From the perspective of the utilization degree of crude oil in different pores, increasing the soaking time has little effect on the utilization degree of crude oil in each pore. When the injection pressure is constant, the huff and puff efficiency increases with the increase of huff and puff cycles. However, after the huff and puff cycle is increased to the third cycle, it is not helpful to improve the huff and puff efficiency and the degree of crude oil utilization. Therefore, the best condition for CO2 huff and puff is that the injection pressure reaches the minimum miscible pressure, the soaking time is 6h, and the huff and puff cycles are 3 cycles. When these conditions are reached, the utilization degree of pore crude oil in the core can reach the maximum, and the huff and puff efficiency will also reach the best. Through the experimental study of CO2 huff and puff in tight sandstone reservoirs, the influence of various factors on the production characteristics of porous crude oil was analyzed, and the mechanism of CO2 huff and puff to improve oil recovery was clarified. The research results provide a theoretical basis for optimizing the CO2 huff and puff mining system and the CO2 huff and puff mining tight sandstone reservoirs.
1. Introduction
Tight oil resources, as an important part of unconventional energy, are one of the hotspots for oil and gas resources exploration at present, with huge resource potentials.1 Compared with conventional reservoirs, tight sandstone reservoirs are characterized by strong heterogeneity, complex physical properties, diverse mineral components, complex formation conditions, and pore structure, which can lead to a lack of energy in the formation during the exploitation process, and only a very small amount of crude oil can be extracted, resulting in a low crude oil recovery rate.2 Conventional water injection development, CO2–water drive, and other extraction methods have relatively low recovery rates. There is a large amount of residual oil stored in the rock pores and not extracted. Moreover, in the process of water injection development in tight reservoirs, there will be difficulties in water injection, water flushing, insufficient energy supplementation, and so on. Not only does CO2-SAG technology have the problem of CO2 and crude oil action time being too long, and the time cost increases, but also CO2-WAG technology has the problem of high wave coefficient, easy to early breakthrough. Thereforem both technologies have not brought breakthroughs for the development of tight sandstone reservoirs.3,4 Although CO2 oil drive technology has been applied in conventional reservoirs in many oilfields, there are still many problems with this technology due to the disadvantages of huge oil consumption and small wave volume.5 In the numerically simulated mean-value model, CO2 mixed-phase drive can achieve close to 100% crude oil recovery, and after water injection development, continued CO2 huff and puff can improve recovery by about 10%.6−8 The use of CO2 also enables stratospheric sequestration, which also plays a key role in reducing the greenhouse effect and corresponds to the country’s call for green oil.9,10 CO2 huff and puff has great potential to enhance recovery in tight reservoirs compared to other recovery methods.11−14
Researchers began to study the CO2-enhanced recovery cycle in 1970. After several years of research, CO2 huff and puff technology has been gradually studied and utilized by many scholars and oil production.15 Wilmington oil field started the CO2 huff and puff extraction of thick oil test. Through the CO2 huff and puff technology, crude oil production was increased and the water content of the recovered oil was reduced, and greater success was achieved in this technology.16 Yuan Zhou's study found that the overall dissolution of the reservoir occurred after the injection of carbon dioxide.31 Hill Bruce et al. worked on an experimental study of CO2 and showed that CO2 injection not only improves crude oil recovery but also consumes a large amount of CO2, which is protective of the environment.17 Abedini Ali et al. determined the MMP in CO2 huff and puff to be 9.18 MPa to some extent and found that the improvement in recovery was not significant when the injection pressure was higher than a certain level.18 In 2017, Kim et al. investigated the effects of CO2 huff and puff technology on oil repulsion and swelling processes in tight reservoir reservoirs, and it was found that CO2 huff and puff technology exhibited stable oil repulsion effects in denser and more complex fracture systems.19 In 2019, Li et al. conducted an indoor experimental study and numerical simulation analysis of CO2 huff and puff. It was found that CO2 has a significant effect on the reduction of crude oil viscosity, and the molecular diffusion phenomenon has a great influence on the huff and puff process. In the early stage, CO2 will penetrate the core interior rapidly and then slowly into other pores. Increasing the number of huff and puff cycles can improve the wave area of CO2.20 In the same year, Ma et al. studied the influencing factors of the CO2 huff and puff in tight oil reservoirs under ultrahigh pressure. As the huff and puff cycles continue to increase, the oil production of small aperture and microporous gradually increases, and the oil production of large pores and medium pores gradually decreases.21 In 2021, Pu et al. conducted an experimental study of CO2 huff and puff in dense conglomerate reservoirs. The results showed that carbonated formation water can dissolve feldspar, montmorillonite, and chlorite in the conglomerate. It leads to a decrease in rock strength and an increase in rock porosity and pore connectivity. Supercritical CO2 increases the roughness of the core surface and weakens the adsorption of crude oil on the core surface, thus increasing the recovery rate.22 In 2020, Jia studied the availability of different pore throats in tight reservoirs through CO2 huff and puff and found that the highest degree of crude oil utilization was micron-scale pore throat and submicron-scale pore throat, and the soaking time was not sensitive to CO2 huff and puff.23 In 2021, Yan quantitatively evaluated the crude oil utilization degree of each pore in the process of CO2 huff and puff experiments of cores with three types of pore structures by nuclear magnetic resonance analysis technology. It is concluded that in the actual huff and puff mining, the appropriate injection pressure, soaking time, and huff and puff cycles should be selected according to different pore structures to achieve the best huff and puff effect.24 Since 2020, Huang has studied CO2 huff and puff technology through scanning electron microscopy and nuclear magnetic resonance. Through three different types of reservoir core experiments, it is found that the degree of crude oil utilization has a great relationship with displacement pressure and reservoir pore structure. Asphaltene is mainly deposited at the entrance of the core. When the pressure is lower than the miscible pressure, the degree of utilization of large-pore crude oil increases rapidly with the increase of pressure. When the pressure is greater than the miscible pressure, the degree of utilization of crude oil slows down.25−27
At present, the research on the influence factors of the CO2 huff and puff effect mainly focuses on shale oil reservoirs, and most of them only stay in the research on the influence of a single factor. In the past five years, injection pressure and soaking time have been the focus of CO2 huff and puff research. The influence factors of tight sandstone reservoirs have not been comprehensively studied. Therefore, this study is based on the NMR technique to characterize the dynamic changes of crude oil mobility, microporous crude oil mobilization characteristics, and huff and puff efficiency during the CO2 huff and puff under different conditions. This study elucidates the huff and puff effect under single-factor action and multifactor action and investigates the relationship between each factor and the huff and puff efficiency as well as the influence law of each factor. This study reveals the main controlling factors affecting the effect of the CO2 huff and puff. The research results improve certain theoretical bases for optimizing the CO2 huff and puff extraction system and CO2 huff and puff extraction of tight sandstone reservoirs.
2. Experimental Part
2.1. Experimental Theory
2.1.1. Principle of Nuclear Magnetic Resonance Experiment
NMR was used to collect hydrogen nuclear signals in the pores of rock samples, and the T2 relaxation time spectrum of the sample was obtained by Fourier transform fitting. The size of the relaxation time T2 represents the strength of the fluid by the surface force and the pore size. The longer the relaxation time, the smaller the surface force and the larger the pore. Conversely, the smaller the relaxation time, the greater the surface force and the smaller the pores. The closer the peak of the T2 relaxation time spectrum is to the right, the higher the amplitude, the better the physical properties of the rock sample, and vice versa.
The transverse relaxation time can be expressed as
 |
1 |
T2S is surface relaxation, T2B is Bulk relaxation, and T2D is diffusion relaxation.29
In the nuclear magnetic resonance test, the magnetic field is generally uniform; therefore, there is no effect of diffusion relaxation. The relaxation time on the surface of rock particles can be approximately characterized as the transverse relaxation time of porous media.
 |
2 |
ρ2 is the transverse surface relaxation rate of rock, S is the pore surface area, V is the void content, r is the pore width, and c is the proportionality factor.28
2.1.2. Pore Throat Size Division
The nuclear magnetic resonance was used to collect the hydrogen nuclear signal in the pores of the rock sample, and the T2 relaxation time spectrum of the sample was obtained by Bolivian transform fitting. The relaxation time T2 is proportional to the pore throat radius, which represents the strength of the fluid by the surface force and the pore size. The longer the relaxation time, the smaller the surface force and the larger the pore. Conversely, the smaller the relaxation time, the greater the surface force, and the smaller the pores. The closer the peak of the T2 relaxation time spectrum is to the right, the higher the amplitude, the better the physical properties of the rock sample, and vice versa. Therefore, in this experiment, the experimental samples were divided into four intervals according to the T2 value:29
-
1.
Micropore interval: T2 < 1 ms
-
2.
Small aperture interval: 1 ms ≤ T2 < 10 ms
-
3.
Mesopore interval: 10 ms ≤ T2 < 100 ms
-
4.
Macropore interval: T2 ≥ 100 ms
2.1.3. Utilization Degree of Pore Throat Crude Oil
The utilization degree of pore-throat crude oil is the change of oil content in a certain interval (such as micropore, small aperture, mesopore, and macropores) before and after the CO2 huff and puff experiment, which is expressed as the change of NMR curve in different intervals. The following is the calculation formula for the utilization degree of pore-throat crude oil:
 |
3 |
P is the utilization degree of pore throat crude oil in a certain interval, %; Vo is the T2 spectrum amplitude of different intervals in the state of core saturated oil; V is the T2 spectrum amplitude of remaining oil in different intervals after the CO2 huff and puff experiment.30
2.2. Experimental Scheme of Carbon Dioxide Huff and Puff
-
1.
The crude oil in the sandstone core was cleaned with toluene, and then, the core in dried at a constant temperature oven at 80 °C for 24 h. After that, the core weight was recorded every 2 h. When the core weight did not change, the core was dried.
-
2.
After the drying of the core, the core was weighed, and the experimental data were recorded. This is called core dry weight.
-
3.
The gas permeability experiment was carried out on the core, and the experimental data were recorded.
-
4.
According to the actual stratum water data, the corresponding experimental stratum water is configured. Following the vacuum ejector of the core for a period of 12 h, the stratum water is added and vacuum ejector for a further 12 h.
-
5.
After connecting the displacement device, a constant flow of 0.02 mL/min is injected into the core until the core is fully saturated with stratum water. The core is weighed, and the data are recorded. It is called core wet weight.
-
6.
Calculation of core porosity from the mass difference of the core before and after saturation.
-
7.
The core of saturated stratum water was sampled by the T2 spectrum using NMR equipment. The original water content distribution characteristics of the core were characterized by the NMR T2 spectrum.
-
8.
MnCl2 was added to the experimental stratum water in a certain proportion to form a new experimental stratum water. The stratum water is injected into the core at a constant flow rate of 0.05 mL/min. When the liquid production reaches 3–5PV, it is considered to shield the hydrogen ion signal in the stratum water. The nuclear magnetic sampling of the core after the experiment was carried out to confirm that the hydrogen ion signal in the core was completely shielded and did not affect the subsequent experiments.
-
9.
The natural core of a tight sandstone reservoir is obtained in the oilfield. According to the actual reservoir data, the experimental crude oil is con Figured, and the viscosity of the experimental crude oil is 2 mPa s. The experimental crude oil was injected into the core at a constant flow rate of 0.05 mL/min, and the amount of water and oil was recorded. When no more water came out of the outlet end, the core was considered to be fully saturated with crude oil, and the core was sampled for NMR T2 spectrum. The experimental process is shown in Figure 1.
-
10.
The CO2 huff and puff experiment was carried out in a thermostat at 65 °C. First, the CO2 cylinder and the intermediate container are connected to fill the entire intermediate container with CO2. Then the intermediate container is connected to the constant flow pump to make the CO2 pressure in the intermediate container reach the predetermined pressure requirement (6, 12, 18, 24, 30 MPa). Set the pressure to 6 MPa, put the saturated core into the core holder, and close the valves 1, 2, and 3. At the beginning of the “huff” stage, the CO2 in the intermediate container is injected into the core from the inlet 1 of the core holder at a constant flow rate of 1 mL/min. When the pressure at the inlet end shows 6 MPa and stabilizes, close the inlet 1. It begins to last for 4h of the soaking stage. After the end of the soaking stage, the “puff” stage begins. Open inlet 2 for depletion mining and collect liquid and gas production. When the pressure drops to 0 MPa and the outlet is not producing liquid gas, the first cycle of huff and puff is over. The core after the experiment was weighed and the core was sampled by NMR T2 spectrum to calculate the recovery rate of CO2 huff and puff. Repeat the experiment for 4 cycles according to the set experimental parameters, compare the experimental results of the 4 cycles, and select the best huff and puff cycle.
-
11.
For other cores, repeat steps (1–10) change the predetermined pressure to carry out experiments, and the soaking time remains unchanged. According to the experimental results, the optimal injection pressure of CO2 huff and puff is determined.
-
12.
Change the soaking time to 2, 4, 6, 8, and 10 h and repeat the experimental steps (1–10) for the remaining cores to determine the best soaking time.
Figure 1.
Flowchart of the experiment.
3. Results and Discussion
3.1. Influence of Different Pore Size Distribution on the Effect of Huff and Puff
In order to study the influence of different pore size distributions on the effect of huff and puff. Therefore, two tight sandstone cores with different pore size distributions were used in this experiment. Two cores are sampled by the T2 spectrum after saturated stratum water and saturated oil. The original oil-bearing T2 spectrum distribution of the two cores is shown in Figure 2, and the physical parameters of the cores are shown in Table 1.
Figure 2.
T2 spectrum distribution of original saturated oil in core.
Table 1. Core Physical Parameters.
| core number | length (cm) | diameter (cm) | K (10–3 μm2) | porosity (%) |
|---|---|---|---|---|
| 1# | 5.325 | 2.525 | 0.0759 | 11.32 |
| 2# | 5.411 | 2.523 | 0.0543 | 6.58 |
| 3# | 5.600 | 2.555 | 0.0610 | 11.34 |
| 4# | 5.619 | 2.555 | 0.0707 | 11.52 |
| 5# | 5.435 | 2.555 | 0.0748 | 12.04 |
| 6# | 5.478 | 2.555 | 0.0765 | 11.32 |
| 7# | 5.333 | 2.555 | 0.0872 | 12.99 |
| 8# | 5.596 | 2.555 | 0.1234 | 10.19 |
| 9# | 5.897 | 2.555 | 0.1656 | 12.35 |
| 10# | 5.562 | 2.555 | 0.2002 | 10.33 |
| 11# | 5.618 | 2.555 | 0.2014 | 12.54 |
| 12# | 5.781 | 2.555 | 0.2044 | 13.16 |
From Table 1, it can be seen that the permeability of the two cores is similar and very low, which belongs to tight sandstone. As shown in Figure 2. According to the principle of NMR, it can be obtained that core 1# mainly develops micropores and mesopores; the original oil content of micropores, small apertures, and mesopores in core 2# is relatively small, and the pore radius is relatively large. The main oil content is in macropores, indicating that the core mainly develops macropores.
In this experiment, the above two cores with different pore size distributions but similar permeabilities are used for experiments. The injection pressure is set to 12 MPa, the temperature is 65 °C, and the soaking time is 2, 4, 6, 8, and 10 h, respectively. The experimental parameters are shown in Table 2.
Table 2. Formation Water Data.
| chemical analysis | mg/L | mmol/L | molar percentage | |
|---|---|---|---|---|
| cation | K+ + Na+ | 5175 | 225 | 49.18 |
| Ca2+ | 150 | 3.75 | 0.82 | |
| Mg2+ | 61 | 2.5 | 0.55 | |
| total value | 5386 | 231.5 | 50.55 | |
| anion | Cl– | 5672 | 160 | 34.97 |
| SO42– | 1081 | 11.25 | 2.46 | |
| CO32– | 0 | 0 | 0 | |
| HCO3– | 3356 | 55 | 12.02 | |
| CH– | 0 | 0 | 0 | |
| total value | 10109 | 226.25 | 49.45 | |
| total mineralization | 457.5 mmol/L | |||
From Figures 3 and 4, it can be seen that the total crude oil consumption of core 1# is greater than that of 2#, but the overall huff and puff efficiency of 2# are greater than those of 1#. The experimental results are listed in Table 3. Since CO2 is a nonwetting phase, it will preferentially enter the large pores with relatively small capillary pressure after entering the core. During the continuous huff and puff, CO2 expands and displaces crude oil in large pores and then slowly diffuses into small apertures and mesopores. In the 2# core, the content of crude oil in the macropore is higher, and the oil content in the small apertures and mesopores is less. Therefore, the crude oil in the macropore will be preferentially utilized during the whole huff and puff process, so its huff and puff efficiency is higher. In the 1# core, because the oil content of the mesopore and micropores is large and the macropores are not developed, CO2 preferentially enters the middle pores with relatively large pores and preferentially displaces the crude oil in the mesopore. Because the original oil content of 1# is much larger than that of 2#, the contact with CO2 will be more sufficient during the huff and puff process, so that the viscosity of crude oil will decrease and the injected CO2 will be fully utilized. Therefore, the ultimate recovery of 1# is higher than that of 2#.
Figure 3.
Distribution map of crude oil production in different pores during the huff and puff process.
Figure 4.
Crude oil distribution in different states during the CO2 huff and puff.
Table 3. Experimental Parameters.
| core number | injection pressure (MPa) | soaking time (h) | overall recovery efficiency (%) |
|---|---|---|---|
| 1# | 12 | 2 | 26.42 |
| 1# | 12 | 4 | 41.61 |
| 1# | 12 | 6 | 46.48 |
| 1# | 12 | 8 | 49.12 |
| 1# | 12 | 10 | 51.48 |
| 2# | 12 | 2 | 30.59 |
| 2# | 12 | 4 | 47.36 |
| 2# | 12 | 6 | 52.29 |
| 2# | 12 | 8 | 56.46 |
| 2# | 12 | 10 | 61.29 |
It can be seen from Figure 5 that in the process of CO2 huff and puff, CO2 enters the core and dissolution with formation water and clay minerals in the rock, resulting in an increase in the number of small apertures in the core.31 Therefore, the NMR curve at the small aperture and mesopore of the 1# core moves to the left with the increase of the time of the well, resulting in a negative NMR signal of the small apertures, and the signal of the small apertures of 2# also shows a negative value. From Figure 6, it can be seen that with the increase in soaking time, the original oil production of the core is increasing, but the increase is limited. It shows that prolonging the soaking time during the CO2 huff and puff is of limited help in improving the utilization of the original oil.
Figure 5.
Crude oil production of two cores under different soaking times.
Figure 6.
Proportion of crude oil in different pores before and after two core experiments.
The XRD test results of the samples are shown in Tables 4 and 5. Although the original oil distributions of the two cores are different, they all show the same law. The crude oil in the macropores will be discharged first, followed by mesopores, small apertures, and micropores. Because CO2 enters the core and reacts with the formation water and rock clay minerals, it later leads to the dissolution of some minerals in the core. The dissolution reaction occurs, thereby increasing the number of smaller pores. The increase in micropores may also be due to the expansion of crude oil caused by CO2 entering the core, resulting in crude oil being squeezed into smaller pores. Therefore, it can be observed from Figure 7 that the proportion of crude oil in micropores and small apertures is increasing.32 Part of the crude oil in mesopores and macropores is displaced, and part is moved to micropores and small apertures. Although a large amount of crude oil remains, it is relatively difficult to extract. During the huff and puff process, light hydrocarbons in crude oil will be extracted by CO2, resulting in an increase in the relative content of heavy components. During the flow process, the heavy components will be blocked in the narrow throat, resulting in the pore throat of the micropores, and small apertures will be narrowed. Thereby affecting the flow of crude oil in the mining process, resulting in a gradual decrease in oil recovery.5
Table 4. Sandstone XRD Whole Rock Results.
| core number | remark | total amount of clay minerals (%) | quartz (%) | calcite (%) | ankerite (%) | dolomite (%) |
|---|---|---|---|---|---|---|
| 1# | pre-experiment | 0.4 | 97.4 | 1.1 | 1.1 | |
| after the experiment | 0.3 | 98.5 | 0.6 | 0.6 | ||
| rate of change (%) | –25 | 1.1 | –45.5 | –45.5 | ||
| 2# | pre-experiment | 16.8 | 80.1 | 2.6 | 0.5 | |
| after the experiment | 14.2 | 83.7 | 2.1 | |||
| rate of change (%) | –15.5 | 4.5 | –19.2 | –100 |
Table 5. Sandstone XRD Clay Mineral Results.
| core number | remark | K (%) | C (%) | I (%) | I/S (%) |
|---|---|---|---|---|---|
| 1# | pre-experiment | 44.3 | 4.9 | 34.1 | 16.7 |
| after the experiment | 70.1 | 5 | 11.9 | 13 | |
| rate of change (%) | 58.2 | 2 | –65.1 | –22.2 | |
| 2# | pre-experiment | 11 | 12.7 | 41 | 35.3 |
| after the experiment | 12.6 | 14.6 | 45 | 27.8 | |
| rate of change (%) | 14.5 | 15 | 9.8 | –21.2 |
Figure 7.
Crude oil distribution under different injection pressures during the CO2 huff and puff process.
In general, in the process of CO2 huff and puff, CO2 will preferentially enter the larger pores after entering the core. Macropores are the main accumulation and circulation areas of CO2, and then they enter smaller pores by diffusion. Cores with macropores and mesopores have the best huff and puff effect, while crude oil in micropores is difficult to exploit.
3.2. Influence of Different Injection Pressure on the Huff and Puff Effect
During CO2 huff and puff, the higher the injection pressure the faster CO2 enters the interior of the core. The easier it is to form a finger in the core, the more easily CO2 can enter the interior of the core. The higher the injection pressure, the easier it is for the CO2 to dissolve in the crude oil. This reduces the viscosity of crude oil and improves its flowability. As a result, the effect of the CO2 huffing and puffing becomes better, and more crude oil is extracted.
In order to avoid the influence of other factors on the experiment, we used 3–7 samples with basically the same pore size distribution to carry out the experiment. In this experiment, the injection pressure is set to 6, 12, 18, 24, 30 MPa. It is known that the minimum miscibility pressure (MMP) of crude oil and carbon dioxide in this block is 22.18 MPa. The experiment set a pressure point below the supercritical state and two points before and after the MMP. The effect of injection pressure on CO2 huff and puff was studied. In this experiment, five cores of the same block were selected and the physical properties were similar. The physical parameters are shown in Table 1, and the experimental parameters are shown in Table 6.
Table 6. Experimental Parameters.
| core number | injection pressure (MPa) | soaking time (h) | CO2 huff and puff efficiency/% | ||||
|---|---|---|---|---|---|---|---|
| the first cycle | the second cycle | the third cycle | the fourth cycle | footing | |||
| 3# | 6 | 4 | 15.49 | 14.87 | 6.11 | 3.12 | 39.59 |
| 4# | 12 | 4 | 16.15 | 13.78 | 14.54 | 9.01 | 53.48 |
| 5# | 18 | 4 | 22.34 | 17.24 | 10.98 | 6.01 | 56.57 |
| 6# | 24 | 4 | 37.88 | 12.63 | 9.95 | 6.08 | 66.55 |
| 7# | 30 | 4 | 40.34 | 14.24 | 14.65 | 3.57 | 72.79 |
The following Figure 8 is the NMR T2 spectrum distribution of crude oil in five cores under different injection pressures during CO2 huff and puff.
Figure 8.
Cumulative huff and puff efficiency of each huff and puff cycle under different injection pressures.
It can be seen from Figure 7 that there is a consistent change trend in the five cores with similar properties under different injection pressures. With the increase of huff and puff cycles, the NMR signals in the mesopores and macropores are obviously decreasing. It shows that the utilization degree of crude oil is increasing, but the crude oil in micropores basically does not change.
It can be seen from Figure 8 that the cumulative recovery rate and injection pressure show a positive correlation trend. It can be demonstrated that the higher the injection pressure, the greater the cumulative recovery rate. It can be seen from Figures 9 and 10 that with the increase of injection pressure, the huff and puff efficiency also increases. With the increase in the injection pressure, the effect of the first two cycles of huff and puff is more obvious. However, when the pressure reaches the minimum miscibility pressure, the huff and puff effect will no longer increase significantly.
Figure 9.
Relationship between huff and puff efficiency and injection pressure of a single huff and puff cycle.
Figure 10.
Producing degree of crude oil in different pores under different injection pressures.
The results of the huff and puff efficiency of each pore under different injection pressures are shown in Table 7. It can be seen from Figure 10 that when the injection pressure is 6 MPa, the crude oil utilization degree of micropores and small apertures is negative. The reason is that the total amount of CO2 in the micropores and small apertures is limited. The pressure conduction in micropores and small apertures is relatively slow. Therefore, the effect of the dissolved gas drive will be relatively weak. At the same time, CO2 reacts with water to form a weak acid, which then reacts with clay minerals in the rock. As a result, the number of micropores and small apertures increases, so crude oil in other pores will enter micropores and small apertures. Therefore, in the process of CO2 injection, the utilization degree of crude oil in micropores and small apertures is negative.
Table 7. Huff and Puff Efficiency of Each Pore under Different Injection Pressures.
| core number | injection pressure (MPa) | the degree of crude oil utilization (%) | ||||
|---|---|---|---|---|---|---|
| micropore | small aperture | mesopore | macropore | entirety | ||
| 3# | 6 | –3.18 | –28.82 | 71.91 | 81.96 | 39.59 |
| 4# | 12 | 4.39 | 16.97 | 89.03 | 91.60 | 53.48 |
| 5# | 18 | 7.52 | 16.93 | 89.58 | 92.69 | 56.57 |
| 6# | 24 | 8.91 | 46.48 | 93.08 | 94.98 | 66.55 |
| 7# | 30 | 9.82 | 59.06 | 93.91 | 96.02 | 72.79 |
In the process of huff and puff, with the increase of injection pressure, the utilization degree of all pore crude oil increases. However, after the pressure reaches 12 MPa, the crude oil in the middle and large pores remains basically unchanged. When the injection pressure reaches 18 MPa, the degree of utilization of the small hole has a greater increase. Before that, it increased slowly. This is because the increase of injection pressure leads to the enhancement of CO2 extraction capability. When the injection pressure reaches the minimum miscibility pressure, the extraction capability of CO2 reaches the maximum. The effect on the viscosity of crude oil no longer increases.
In general, injection pressure is the main influencing factor of carbon dioxide huff and puff. The injection pressure can reach the minimum miscible pressure, and the increase of the injection pressure is limited to the improvement of huff and puff efficiency. With the increase of injection pressure, huff and puff efficiency has two turning points. When the injection pressure reaches these two turning points, the CO2 huff and puff efficiency is greatly improved. One is that CO2 reaches a supercritical state, and the other is that CO2 and crude oil reach a miscible state. With the increase of the injection pressure, the crude oil in the large hole is basically used passively. When the injection pressure exceeds the minimum miscibility pressure, the interfacial tension between CO2 and crude oil will gradually decrease. At this time, the degree of crude oil production in small apertures will increase.
3.3. Influence of Different Soaking Time on the Huff and Puff Effect
It takes time for CO2 to enter the core and dissolve in contact with crude oil, so the length of the soaking time will affect the effect of the huff and puff. A too-short soaking time will result in a too-short contact time between CO2 and crude oil, which will in turn lead to insufficient dissolution of CO2 and thus affect the extraction and expansion of crude oil by CO2. If the soaking time is too long, the solubility of CO2 in crude oil reaches the maximum. If the soaking time continues to increase, the expansion energy of crude oil will be lost, which is not conducive to the recovery of crude oil.
In this experiment, the injection pressure is set to 12 MPa, the temperature is 65 °C, and the soaking time is 2, 4, 6, 8, and 10 h. The physical parameters of the five cores are shown in Table 1, and the experimental parameters are shown in Table 8.
Table 8. Experimental Parameters.
| core number | injection pressure (MPa) | soaking time (h) | CO2 huff and puff efficiency (%) | ||||
|---|---|---|---|---|---|---|---|
| the first cycle | the second cycle | the third cycle | the fourth cycle | footing | |||
| 8# | 12 | 2 | 28.82 | 10.64 | 7.15 | 2.34 | 48.95 |
| 9# | 12 | 4 | 21.09 | 22.33 | 4.56 | 5.05 | 53.03 |
| 10# | 12 | 6 | 28.02 | 18.23 | 5.83 | 3.64 | 55.72 |
| 11# | 12 | 8 | 27.65 | 11.36 | 4.96 | 5.20 | 49.17 |
| 12# | 12 | 10 | 21.89 | 13.87 | 7.92 | 2.35 | 46.04 |
The following Figure 12 shows the nuclear magnetic resonance T2 spectrum distribution curve of crude oil in five cores under different soaking times during CO2 huff and puff.
Figure 12.
Cumulative huff and puff efficiency of each huff and puff cycle under different soaking time.
From Figure 11, it can be seen that under different injection pressures, the five cores with similar properties all have basically the same rules. With the increase in soaking time, the NMR signal in the mesopore and macropore is obviously reduced, and the NMR curve moves to the left. It shows that the utilization degree of crude oil in mesopores and macropores is increasing, while the crude oil in micropores is basically unchanged.
Figure 11.
Distribution of crude oil in core under different soaking times during the CO2 huff and puff process.
From Figure 12, when the soaking time is small, the huff and puff efficiency and soaking time show a positive correlation trend. The results of the crude oil-producing degree of each pore under different soaking times are shown in Table 9. After the soaking time reaches 6h, the huff and puff efficiency no longer increases, and there is a downward trend. The reason is that the contact between CO2 and crude oil is not sufficient when the soaking time is short. Increasing the soaking time can increase the contact time between CO2 and crude oil and improve the effect of the dissolved gas drive. When the soaking time is too long, it will lead to an increase in the CO2 diffusion distance and the occurrence of an escape phenomenon. The entire huff and puff process will result in a decrease in efficiency due to insufficient energy.
Table 9. Crude Oil Producing Degree of Each Pore under Different Soaking Time.
| core number | soaking time (h) | the degree of crude oil utilization (%) | ||||
|---|---|---|---|---|---|---|
| micropore | small aperture | mesopore | macropore | entirety | ||
| 3# | 2 | 14.16 | –30.48 | 73.04 | 87.14 | 48.95 |
| 4# | 4 | 16.73 | –20.68 | 79.23 | 93.12 | 53.03 |
| 5# | 6 | 0.21 | –28.42 | 84.86 | 94.89 | 55.72 |
| 6# | 8 | 9.43 | –72.13 | 82.00 | 93.83 | 49.17 |
| 7# | 10 | 6.42 | –17.07 | 76.32 | 91.00 | 46.04 |
With the increase of soaking time, the utilization degree of crude oil in macro- and mesopores increases first and then decreases. From Figure 13, when the soaking time reaches 6h, the degree of crude oil utilization reaches the maximum, but the increase and decrease are not large. This is because the energy is not enough to carry crude oil outflow, so the degree of utilization of microporous crude oil is gradually reduced. The relative content of heavy components will also increase, resulting in an increase in crude oil viscosity and the deterioration of fluidity. From the perspective of the overall crude oil utilization degree, there is a trend of increasing first and then decreasing, which is consistent with the trend of the huff and puff efficiency.
Figure 13.
Crude oil producing degree of each pore under different soaking time.
In general, with the increase of soaking time, the huff and puff efficiency increases and then decreases. However, the overall change in the huff and puff efficiency is relatively small, and the difference between the highest and the lowest is only 6.77%. From the perspective of the utilization degree of crude oil in different pores, increasing the soaking time has little effect on the utilization degree of crude oil in each pore. It can be seen that the soaking time is not the main factor affecting the puff efficiency.
3.4. Influence of Different Huff and Puff Cycle on the Huff and Puff Effect
In the process of CO2 huff and puff, the appropriate huff and puff cycles can make CO2 fully in contact with the crude oil in the core and make full use of the injected CO2 energy. In this experiment, CO2 huff and puff under different injection pressures and different soaking times were carried out, and other experimental parameters remained unchanged. Five pressure points and five soaking time points were set up in this experiment. In order to study the effect of huff and puff cycles on the efficiency of the CO2 huff and puff, four cycles of huff and puff were performed at each experimental point.
3.4.1. Different Injection Pressure
The results of the proportion of huff and puff efficiency of each cycle under different injection pressures are shown in Table 10. From Figure 14, when the soaking time is constant, no matter how the pressure changes, the CO2 huff and puff efficiency gradually decreases with the increase of huff and puff cycles. However, the overall huff and puff efficiency still shows an increasing trend. When the number of puffs and huffs increases, CO2 will first displace crude oil from larger pores and then enter smaller pores. However, crude oil from smaller pores is difficult to displace, so the huff and puff efficiency will gradually decrease. Comprehensively looking at the huff and puff efficiency under different injection pressures, the huff and puff efficiency before the third cycle is basically above 90%. Therefore, the actual production huff and puff cycles should not be too much.
Table 10. Proportion of the Huff and Puff Efficiency of Each Cycle under Different Injection Pressures.
| core number | injection pressure (MPa) | soaking time (h) | CO2 huff and puff efficiency (%) | |||
|---|---|---|---|---|---|---|
| the first cycle | the second cycle | the third cycle | the fourth cycle | |||
| 8# | 6 | 4 | 39.13 | 37.56 | 15.43 | 7.88 |
| 9# | 12 | 4 | 30.20 | 25.77 | 27.19 | 16.85 |
| 10# | 18 | 4 | 39.49 | 30.48 | 19.41 | 10.62 |
| 11# | 24 | 4 | 56.92 | 18.98 | 14.95 | 9.14 |
| 12# | 30 | 4 | 55.42 | 19.56 | 20.13 | 4.90 |
Figure 14.
Huff and puff efficiency of each cycle under different injection pressures.
From Figure 15, when the injection pressure is less than the MMR, the effect of increasing the number of huff and puff cycles to improve the degree of utilization of microporous and small aperture crude oil is not obvious. Only when the injection pressure is greater than the MMR, can the degree of crude oil utilization in macro- and mesopores be increased by increasing the number of huff and puff cycles. Because the crude oil in the macro- and mesopores is basically displaced in the early stage of huff and puff. However, the degree of production of crude oil in smaller pores increased with the increase of injection pressure. However, the slow increase in rate accounts for a small proportion and has little effect on the total recovery rate. Therefore, the effect of increasing huff and puff cycles in the later period is gradually weakened.
Figure 15.
Degree of production of pore crude oil in each cycle under different injection pressures.
In general, when the injection pressure is low, it is better to increase the utilization degree of crude oil by increasing the number of huff and puff cycles. When the injection pressure is greater than the minimum miscible pressure, increasing the miscible pressure can increase the degree of production of crude oil with smaller pores. However, considering the utilization rate of CO2 and the actual economic factors, the huff and puff cycles should not be too many. No matter what the injection pressure, huff and puff cycles should not exceed 3 times.
3.4.2. Different Soaking Times
The results of the proportion of huff and puff efficiency of each cycle under different soaking times are shown in Table 11. When the injection pressure is constant, with the increase in soaking time, there is no significant change in the overall huff and puff efficiency before the third cycle. From Figure 16, the cumulative huff and puff efficiency before the third cycle is 89.42–95.22%. Improving the huff and puff efficiency of the next few cycles of huff and puff can only be used to increase the injection pressure. This can increase the solubility of CO2 in crude oil, thereby increasing the expansion capacity of crude oil and reducing the viscosity. And the extraction capacity of CO2 is greatly increased. So that more light hydrocarbons in crude oil are extracted, so that the huff and puff efficiency is improved.
Table 11. Proportion of Huff and Puff Efficiency of Each Cycle under Different Soaking Times.
| core number | soaking time (h) | injection pressure (MPa) | CO2 huff and puff efficiency (%) | |||
|---|---|---|---|---|---|---|
| the first cycle | the second cycle | the third cycle | the fourth cycle | |||
| 8# | 2 | 12 | 28.88 | 21.74 | 14.61 | 4.78 |
| 9# | 4 | 12 | 39.77 | 42.11 | 8.60 | 9.52 |
| 10# | 6 | 12 | 50.29 | 32.72 | 10.46 | 6.53 |
| 11# | 8 | 12 | 56.23 | 23.10 | 10.09 | 10.58 |
| 12# | 10 | 12 | 47.55 | 30.13 | 17.20 | 5.10 |
Figure 16.
Huff and puff efficiency of each cycle under different soaking time.
From Figure 17, through the NMR T2 spectrum curve of 4 huff and puff cycles under different soaking times, the cumulative crude oil producing degree of each pore under different soaking times was obtained. Comparing the cumulative crude oil production degree of each cycle of pores under different pressures, the law is basically the same. With the increase of huff and puff cycles, the cumulative crude oil utilization degree of large and small apertures gradually increases, but the increase is small. Because the injection pressure is constant, the CO2 diffusion path gradually enters from the larger pores into the smaller pores and some crude oil enters from the larger pores into the smaller pores. Due to the large capillary force of smaller pores, the resistance of crude oil flow needs to be overcome. Therefore, it is difficult for crude oil to flow after flowing into smaller pores. Under the condition of constant injection pressure, huff and puff cycles can be continuously increased, so that CO2 can fully contact the crude oil in small apertures, and the utilization degree of crude oil in small apertures and micropores can be improved.
Figure 17.
Producing degree of crude oil in each pore under different soaking time.
In general, when the soaking time is constant, regardless of the pressure, the CO2 huff and puff efficiency gradually decrease with the increase in the huff and puff cycles. When the number of huff and puff cycles is increased, CO2 will first extract crude oil from larger pores and then enter smaller pores, but crude oil from smaller pores is difficult to extract, so the huff and puff efficiency will gradually decrease. When the injection pressure is constant, no matter how much the soaking time is, with the increase of the huff and puff cycles, the cumulative huff and puff efficiency basically does not increase much after the third cycle. At this time, if you want to continue to increase the huff and puff efficiency, you can increase the injection pressure to meet
4. Conclusions
-
1.
In the process of CO2 huff and puff, injection pressure is the most important factor. The core with micropore throat development needs higher injection pressure, and the maximum economic benefit can be achieved when the huff and puff cycles reach 3 cycles.
-
2.
CO2 huff and puff experiments were carried out on cores with different pore size distributions under the same conditions. After CO2 enters the core, it will first enter the larger pores. The macropores are the main accumulation and circulation areas of CO2 and then enter the smaller pores by diffusion. The core with macropores and mesopores has the best huff-and-puff effect. For the core with mesopores and smaller pores, the injection pressure can be increased and the soaking time can be appropriately extended to improve the huff and puff effect.
-
3.
Through CO2 huff and puff experiments with different injection pressures, it can be seen that with the increase of injection pressure, there are two turning points in huff and puff efficiency. One is that the efficiency of CO2 huff and puff is greatly improved when CO2 is in the supercritical state, and the other is that the efficiency of CO2 huff and puff is greatly improved when CO2 and crude oil are in a miscible state. In this experiment, the huff and puff efficiency reached the highest at 30 MPa, up to 72.79%. As the pressure increases, the huff and puff efficiency eventually increases by 33.20%. It can be seen that the injection pressure is the main factor affecting the huff and puff effect. However, the injection pressure does not need to be too high to reach the minimum miscible pressure. Continue to increase the injection pressure to improve the huff and puff efficiency is limited.
-
4.
The experimental results of CO2 huff and puff with different soaking times show that with the increase of soaking time, the huff and puff efficiency increases first and then decreases. When the well is soaked for 6h, the huff and puff efficiency reaches a maximum of 55.72%. However, with the increase in soaking time, the overall change in huff and puff efficiency is not large, and the difference between the highest huff and puff efficiency and the lowest huff and puff efficiency is only 6.77%. From the perspective of the utilization degree of crude oil in different pores, increasing the soaking time has little effect on the utilization degree of crude oil in each pore. Therefore, soaking time is not the main factor affecting the effect of CO2 huff and puff.
-
5.
When the number of huff and puff cycles increases, CO2 will first extract crude oil from larger pores and then enter smaller pores. However, crude oil from smaller pores is difficult to extract, so the huff and puff efficiency will gradually decrease. When the injection pressure is constant, no matter how much the soaking time is, with the increase of the huff and puff cycles, the cumulative huff and puff efficiency basically does not increase much after the third cycle. At this time, if you want to continue to increase the huff and puff efficiency, you can increase the injection pressure to meet.
-
6.
When the injection pressure is small, increasing the injection pressure is not helpful to improve the utilization degree of pore crude oil. When the injection pressure is greater than MMP, increasing the injection pressure can greatly improve the utilization degree of pore crude oil. Under the condition of constant injection pressure, increasing the number of huff and puff cycles can improve the utilization degree of crude oil in small apertures and micropores.
Acknowledgments
This research is supported by the National Natural Science Foundation of China (No. 52174031) and the Youth Innovation Team of Shaanxi Universities.
The authors declare no competing financial interest.
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