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Article

Mechanism and Formation Conditions of Foamy Oil During Gas Huff-n-Puff in Edge and Bottom Water Heavy Oil Reservoirs

by
Shoujun Wang
1,
Zhimin Zhang
2,
Zhuangzhuang Wang
3,*,
Fei Wang
4,
Zhaolong Yi
4 and
Yan Liu
4
1
Science and Technology Management Department, Shengli Oilfield Company, SINOPEC, Dongying 257001, China
2
Exploration Department, PetroChina Tarim Oilfiels Branch Company, Korla 841000, China
3
School of Science, Qingdao University of Technology, Qingdao 266520, China
4
Petroleum Engineering Technology Research Institute, Shengli Oilfield Company, SINOPEC, Dongying 257001, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(4), 1127; https://doi.org/10.3390/pr13041127
Submission received: 13 March 2025 / Revised: 6 April 2025 / Accepted: 8 April 2025 / Published: 9 April 2025
(This article belongs to the Section Energy Systems)
Browse Figures
Versions Notes

Abstract

:
The thermal development in heavy oil reservoirs with edge and bottom water is poor, while gas huff-n-puff development shows a high recovery and strong adaptability. The formation of foamy oil during gas huff-n-puff is one of the reasons for the high recovery. In order to determine the factors affecting the foamy oil flow during gas huff-n-puff, experiments using a one-dimensional sandpack were conducted. The influences of drawdown pressure and cycle number were analyzed. The formation conditions of foamy oil were preliminarily clarified, and the enhanced oil recovery (EOR) mechanism of foamy oil was revealed. The experimental results show that the drawdown pressure and cycle number are two important factors affecting the formation of foamy oil. Foamy oil flow is prone to forming under a moderate drawdown pressure of 0.5–0.75 MPa, and being too small or too large is unfavorable. Foamy oil is more likely to form in the first two cycles, and it becomes increasingly challenging with the increase in the cycle number. These two factors reflect two necessary conditions for the formation of foamy oil during gas huff-n-puff: one is allowing the oil and gas to flow adequately to provide the shear and mixing for the generation of micro-bubbles, and the other is that the oil content should not be too small to avoid the inability to disperse and stabilize bubbles. The formation of foamy oil, on the one hand, increases the volume of the oil phase, and on the other hand, it reduces the mobility of the gas phase and slows down the pressure decline rate in the core, thereby enhancing the driving force for oil displacement. So, under the influence of the foamy oil, the gas production volume in a cycle declined by about 26%, and the average oil recovery increased by 4.5–6.9%.

1. Introduction

Heavy oil has attracted much attention because of its abundant reserves and huge development potential. Against the backdrop of the increasingly tight supply of conventional fossil energy sources globally, the efficient exploitation of heavy oil has become one of the most significant approaches to safeguarding national energy security. Cycle steam stimulation (CSS, also referred to as steam huff-n-puff), steam flooding (SF), and steam-assisted gravity drainage (SAGD) are the primary thermal recovery methods for heavy oil development at present [1,2]. However, for heavy oil reservoirs with edge and bottom water, conventional thermal recovery methods such as CSS have poor effects and are not applicable. This is because, during the development of such reservoirs, a large amount of edge and bottom water will invade, causing the heat of the injected steam to be largely consumed by the formation water surrounding the wellbore, thereby preventing the further transfer of heat to the reservoir and severely impacting the utilization efficiency of steam, leading to poor economic benefits in thermal recovery [3,4,5]. Considering that the viscosity of heavy oil in reservoirs with edge and bottom water is usually not very high and has certain mobility under reservoir conditions, gas huff-n-puff is highly applicable; this has also been implemented in heavy oil reservoirs, and good development results have been achieved [6,7,8]. The main types of gas used in gas huff-n-puff include CO2, N2, natural gas, flue gas, etc. During gas huff-n-puff, chemical agents such as viscosity reducers, foaming agents, and solvents are usually injected as well so as to take advantage of the synergistic effect of gas and chemical agents and achieve an effect greater than 1 + 1. The principal mechanisms of gas huff-n-puff are as follows: on the one hand, it takes advantage of the expansibility of gas to increase the elastic energy of the reservoir and inhibit the invasion of edge and bottom water; on the other hand, the viscosity of heavy oil declined through the dissolution of gas, thus enhancing the oil mobility [9,10,11,12,13,14]. In general, gas huff-n-puff not only effectively increases the utilization and recovery of heavy oil with edge and bottom water but also reduces the consumption of steam and sequesters large amounts of greenhouse gases, giving it both economic and social significance.
As early as the 1950s, research and field trials on gas huff-n-puff for heavy oil had already been carried out. Due to the high solubility of CO2 in heavy oil and the excellent effect of reducing oil viscosity, it has become the most studied and widely used gas in gas huff-n-puff development [15,16]. Currently, the dissolution laws of CO2 in heavy oil have been fully investigated, and the quantitative relationships between its solubility and temperature, pressure, and oil composition have been well characterized. Generally, it is believed that the solubility of CO2 in heavy oil increases significantly with increasing pressure and decreasing temperature. Under reservoir conditions, the dissolution percentage of CO2 in heavy oil can reach 40% to 70%, and the viscosity of heavy oil can be decreased by more than 90% [17,18]. The dissolution of CO2 in heavy oil can also cause the volume of oil to expand, with an expansion rate of up to 10–20% [17,18]. Furthermore, CO2 is capable of becoming miscible with oil, extracting the light components from heavy oil, and reducing the oil–water interfacial tension, thereby increasing the recovery. The influence of reservoir parameters on CO2 huff-n-puff has also been widely studied, and the conditions of heavy oil reservoirs suitable for CO2 huff-n-puff have been summarized. This includes the finding that the permeability should be greater than 200 mD, the thickness of the oil layer is preferably over 40 m, the oil saturation should be above 55–60%, and the reservoir pressure should optimally be at a medium level [19]. Simultaneously, the injection parameters of CO2 have also been optimized. Research indicates that the larger the CO2 injection volume per cycle, the better the development effect. It was also found that the oil production is relatively high in the first three cycles, while it significantly declines in subsequent cycles [20,21,22,23,24,25]. In recent years, the application of chemical agents such as foaming agents and viscosity reducers to assist in CO2 huff-n-puff has gradually become a research hotspot in this field. Foam can be generated when CO2 and foaming agents are injected, which can effectively control gas mobility and, thereby, expand the sweep range. When a viscosity reducer is added during CO2 huff-n-puff, the viscosity of heavy oil can be reduced synergistically, thereby further expanding the range of viscosity reduction [26,27]. N2 huff-n-puff is also widely applied in the development of heavy oil reservoirs. Unlike CO2, N2 does not have good solubility in oil. Its main mechanism is to increase the elastic energy of due to the strong expansibility of N2. Therefore, for heavy oil reservoirs with edge and bottom water, N2 huff-n-puff may be more effective to a certain extent, as it helps to increase reservoir pressure and inhibit the invasion of edge and bottom water [28]. In addition, natural gas and flue gas are also commonly used in gas huff-n-puff for heavy oil development, and their mechanisms for enhancing recovery are essentially consistent with those of CO2 and N2 [29,30].
In the relevant research on gas huff-n-puff, the phenomenon of foamy oil has drawn particular attention [31,32]. Foamy oil is a dispersion system in which gas is dispersed in oil by the state of micro-bubbles. Due to the high viscosity of heavy oil, micro-bubbles in the oil phase are not prone to coalescence. As a result, this oil–gas dispersion system can stably exist in a foam-like state [33,34,35,36]. Compared with conventional heavy oil without micro-bubbles, the apparent viscosity of foamy oil is significantly reduced, and its expansibility is greatly enhanced due to the dispersion of a large number of micro-bubbles in the oil phase [37,38,39,40]. Foamy oil typically occurs in the depletion development of heavy oil, whose occurrence leads to depletion production exhibiting abnormal characteristics, such as high recovery, a low gas–oil ratio, and a slow pressure decline [41,42,43,44,45]. In simulation experiments on gas huff-n-puff, researchers observed that foamy oil would form during the production stage, which was also verified with the oil produced in an oilfield. The experiments showed that there was a positive correlation between the oil production of gas huff-n-puff and foamy oil. When foamy oil forms during the production stage, the oil production will significantly increase [46,47,48,49]. This experimental conclusion holds true for both N2 and CO2 [28,30]. Currently, it has been generally acknowledged that the formation of foamy oil during the production stage of gas huff-n-puff can enhance recovery. However, there is a lack of quantitative research on the formation conditions of foamy oil during gas huff-n-puff, and the factors influencing the formation of foamy oil are still unclear. The way of effectively controlling and utilizing foamy oil during gas huff-n-puff to increase production is the most concerning issue at present.
Therefore, this study focused on exploring the formation mechanism of foamy oil in gas huff-n-puff and clarifying its formation conditions so as to better utilize this enhanced oil recovery (EOR) mechanism to produce heavy oil. A one-dimensional sandpack was used to conduct gas huff-n-puff experiments. The composite of the gas injected was 50% N2 + 50% CO2, which was mainly to better align with oilfield applications. In actual gas huff-n-puff, both N2 and CO2 are generally injected to incorporate the displacement mechanisms of both gases. A total of five sets of gas huff-n-puff experiments with different drawdown pressures (0.2 MPa, 0.5 MPa, 0.75 MPa, 1.0 MPa, and 1.5 MPa) were carried out, and each experiment was conducted with three cycles. By measuring the oil and gas production in real time during the production stage and observing the state of the produced oil, the influences of the drawdown pressure and production cycle on the production behaviors were analyzed. Furthermore, the formation conditions of foamy oil during the production stage of gas huff-n-puff were clarified, and the formation mechanism of foamy oil was preliminarily revealed.

2. Materials and Methods

2.1. Materials

The oil sample used in the experiments was collected from one oil well in Shengli Oilfield, China. The original reservoir pressure is 10.2 MPa, the permeability is between 2000 mD and 3500 mD, and the reservoir temperature is 60 °C. The oil viscosity under the reservoir conditions is approximately 150 mPa∙s. The experiments were conducted at room temperature (25 °C). The oil used in the experiments was prepared with the oil sample collected from Shengli Oilfield and the oil-soluble viscosity reducer, giving it a viscosity of 150 mPa∙s at the experimental temperature. The treatment was feasible, as in actual gas huff-n-puff development, the slug of oil-soluble viscosity reducer is also injected before the gas injection. A water sample collected from the oil well was analyzed prior to the tests. Its salinity was about 50,000 mg/L. The water used in the tests was prepared using distilled water and chemically pure NaCl (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) at concentrations of 50,000 mg/L. The gas used in the experiment was a mixture of CO2 and N2 (Qingdao Tianyuan Co., Ltd., Qingdao, China), with each accounting for 50%. The artificial cores used in the tests were packed with silica sand with a permeability of about 3000 mD. The size of silica sand was 120 mesh, and the wettability was water-wet.

2.2. Apparatus

A schematic diagram of the gas huff-n-puff experiment is shown in Figure 1. A sandpack with a diameter of 2.54 cm and a length of 30.0 cm was used. The artificial cores were packed in the sandpack. A back-pressure regulator (BPR, open error of less than 0.01 MPa) connected with a hand pump was used to control the pressure of the sandpack. The pressure of the BPR was regulated with the hand pump. Three pressure transducers were uniformly distributed in the sandpack. The pressures of the core could be monitored and collected in real time. An ISCO double-piston pump with a flow rate range of 0.001–60 mL/min and a pressure range of 0–66 MPa was used to inject fluid into the core. The electronic balance (Model PL 2002, Metler Toledo Technology (China) Co., Ltd., Shanghai, China, measurement accuracy of 0.01 g) was used to measure the oil production. A gas mass-flow meter (Model SLA5850S, Brooks Instrumen, Philadelphia, PA, USA, measurement accuracy of 0.1 sccm) was used to measure the gas production.

2.3. Experimental Procedures

Five sets of one-dimensional gas huff-n-puff experiments were carried out under different drawdown pressures, with the core parameters and other experimental conditions kept the same. The five drawdown pressures were 0.2 MPa, 0.5 MPa, 0.75 MPa, 1.0 MPa, and 1.5 MPa. The parameters of each test are shown in Table 1. Three cycles of huff-n-puff were simulated in each experiment. The specific procedures were as follows.
(1) The sandpack was prepared with pre-set permeability and porosity. Then, the sandpack was saturated with the prepared brine after vacuum for 4 h. The pore volume and permeability were measured and calculated.
(2) The back pressure was set to 10 MPa. The sandpack was saturated with the prepared oil at a rate of 0.1 mL/min or a smaller rate for more than 2 PV. After that, the irreducible water saturation and initial oil saturation were calculated.
(3) Before gas huff-n-puff, depletion production was simulated first according to the pre-set drawdown pressure. Here, the depressurization operation is illustrated with a drawdown pressure of 0.2 MPa as an example. Before depressurization, both the pressures of the sandpack and the BPR were 10 MPa. The back pressure was set to 9.8 MPa first, thereby creating a pressure difference of 0.2 MPa for production. As the fluid in the core was produced, the sandpack pressure gradually decreased until it equaled the back pressure. At this point, the back pressure was reset to 9.6 MPa to allow the core fluid to continue to be produced under a pressure difference of 0.2 MPa. This process was repeated, with each pressure decline being 0.2 MPa, until the model pressure dropped to atmospheric pressure. Finally, the produced oil mass and recovery of the depletion production were calculated.
(4) After the depletion, three cycles of gas huff-n-puff were simulated. First, the experimental gas (50%CO2 + 50%N2) was injected into the sandpack until the initial pressure of 10 MPa was restored. Then, the inlet valve was closed, and it was allowed to soak. In the soaking process, gas was continuously injected to maintain the system pressure at 10 MPa. When the system pressure did not decrease for 2 h, it was considered to have reached equilibrium. Then, the outlet valve was opened to conduct the puffing process according to the pre-set drawdown pressure. The depressurization operation was the same as that in step (3). The oil and gas production rates were measured in the puffing process. When the sandpack pressure dropped to atmospheric pressure and no more oil was produced, this cycle was finished, and we proceeded to the next cycle. The gas injection, soaking, and puffing processes described above were repeated.

3. Results

3.1. Production Behaviors at Different Drawdown Pressures

(1)
Drawdown pressure of 0.2 MPa
The changes in oil production rate and gas production rate with time for each cycle at the drawdown pressure of 0.2 MPa are shown in Figure 2. It can be seen that the oil production rates in the three cycles were very low. Specifically, oil almost was not produced in the third cycle, and the oil production rates in the first two cycles were merely slightly higher than that of the third cycle. The gas production rates in the three cycles were approximately the same, with the average being basically at the level of 80 mL/min. Moreover, within a complete cycle, the gas production rate remained relatively stable from the beginning to the end. Under this relatively small drawdown pressure, it was difficult for the oil phase to be displaced, resulting in the dominance of gas flow during the production stage. Consequently, it was characterized by a large gas production rate and a very small oil production rate. From the perspective of the observed oil production behavior, during the entire production stage of the three cycles, heavy oil flowed out slowly, and there were no obvious micro-bubbles in the produced oil. The oil and gas were basically in a separated state, as shown in Figure 3, which indicates that foamy oil did not form under this drawdown pressure.
(2)
Drawdown pressure of 0.5 MPa
The changes in the oil production rate and gas production rate with time for each cycle at the drawdown pressure of 0.5 MPa are shown in Figure 4. It can be seen that there were significant differences in the oil production rates between the three cycles. With the increase in the cycle number, the oil production rate declined. Specifically, the oil production rate in the first cycle was basically maintained above 0.6 g/min, that in the second cycle was approximately sustained at around 0.3 g/min, and that in the third cycle was very small, with almost no oil production. The gas production rates in the three cycles were close and significantly higher than that at the drawdown pressure of 0.2 MPa. The gas production rate in the initial stage of production gradually increased with the increase in the cycle number, while the gas production rates in the middle and later stages of production did not vary much with the cycle. From the perspective of the observed oil production behavior, oil puffed out rapidly with gas in the first two cycles, accompanied by crackling and popping sounds of bubbles bursting. A number of micro-bubbles existed in the produced oil, as shown in Figure 5. This indicated that foamy oil was prone to forming at the drawdown pressure of 0.5 MPa.
(3)
Drawdown pressure of 0.75 MPa
The changes in the oil production rate and gas production rate with time for each cycle at the drawdown pressure of 0.75 MPa are shown in Figure 6. It can be seen that the oil production rates in the first two cycles were similar in their variation patterns, with both increasing initially and then rapidly declining. However, the oil production rate in the first cycle was higher than that in the second cycle. In the third cycle, the oil production rate was significantly lower, with an average of less than 0.2 g/min, and oil was produced only for a brief period in the early stage. In terms of the gas production rate, the variation patterns in the three cycles were similar, with all showing a relatively low gas production rate in the early stage, a sudden increase in the middle stage, and then a slow decline. However, the gas production rate in the early stage of the first two cycles was significantly lower than that of the third cycle. From the perspective of the observed oil production behavior, oil was produced in a mixture with gas in the first two cycles. The produced oil exhibited distinct foamy oil characteristics, with a large number of tiny bubbles densely and stably dispersed in the heavy oil, as shown in Figure 7. It was also observed that the oil production rate was relatively high while the gas production rate was relatively low in the early period of the production stage, and the bubbles were small and uniform. However, the bubble size in the foamy oil significantly increased in the later period of the production stage, and the duration of the bubbles’ existence in the foamy oil was significantly shorter. In the third cycle, oil was produced mainly through the carrying of gas, and the foamy oil characteristics were not prominent, with only a small number of bubbles present in the produced oil.
(4)
Drawdown pressure of 1.0 MPa
The changes in the oil production rate and gas production rate with time for each cycle at the drawdown pressure of 1.0 MPa are shown in Figure 8. It can be seen that the oil production rate in the first cycle was much higher than that in the second cycle. The oil production rate in the early period of the second cycle was still relatively large, but it dropped to a very low level in the later period. The oil production rate throughout the entire third cycle was very small. The gas production rate at this drawdown pressure increased significantly compared with the previous tests with smaller drawdown pressures. Even though the gas production rate in the first cycle was the lowest, it still reached 400 mL/min. The gas production rate in the third cycle was the highest and even reached 600 mL/min. The high gas production level was maintained throughout almost the entire first cycle, while in the second and third cycles, the gas production rate started out high but soon dropped rapidly. From the perspective of the observed oil production behavior, there were lots of bubbles of varying sizes dispersed in the produced oil in the first cycle, as shown in Figure 9. Obvious gas channeling was observed in the second cycle, and oil was mostly carried out by the rapid-flowing gas, with fewer bubbles being dispersed in the produced oil than in the first cycle. In the third cycle, almost no oil was produced, and gas was produced in large quantities, with no foamy oil phenomenon. Taking these results together, we believe that foamy oil formed in the first cycle but not in the second cycle. Due to the effect of foamy oil, the gas production rate was relatively low, and the gas production duration was relatively long in the first cycle. After the first cycle, some flowing channels formed in the sandpack, and gas channeling could easily occur in subsequent cycles. Once gas channeling occurs, it becomes extremely difficult to produce oil again, and foamy oil cannot form either. Consequently, the characteristics of a low oil production rate and high gas production rate were exhibited in the second and third cycles.
(5)
Drawdown pressure of 1.5 MPa
The changes in the oil production rate and gas production rate with time for each cycle at the drawdown pressure of 1.5 MPa are shown in Figure 10. It can be seen that the oil production rate in the first cycle was extremely high, reaching approximately 2.7 g/min in the early period. However, it declined sharply soon after due to gas channeling. Although the oil production rates in the second and third cycles were much lower than that in the first cycle, the initial oil production rates were still higher than those of the previous four tests. This indicates that heavy oil could be displaced easily under such a high drawdown pressure. The gas production rates in the three cycles were all very high, basically remaining at 600 mL/min, which was much higher than those of the previous four tests. This suggests that gas channeling was severe during gas huff-n-puff under this drawdown pressure. From the perspective of the observed oil production behavior, due to the large drawdown pressure, gas channeling was quite severe, and the pressure in the sandpack dropped rapidly. As a result, the duration of each cycle was short, less than 10 min. A large amount of oil was produced in the early period of the first cycle, and there were many bubbles. However, the bubble size was relatively large, and the bubbles coalesced and collapsed soon after production, resulting in a short-lived foamy oil state in the first cycle. In the second and third cycles, gas production was mainly observed, and a small amount of oil was carried out rapidly by the gas without forming foamy oil.

3.2. Formation Conditions of Foamy Oil During Gas Huff-n-Puff

Whether foamy oil formed or not in each cycle of the gas huff-n-puff experiments is summarized in Table 2. Through comparisons, the following results were noted:
(1)
Stable foamy oil formed under a moderate drawdown pressure. It was unfavorable for foamy oil formation when the drawdown pressure was too small or too large.
(2)
Foamy oil was mainly formed in the first and second cycles. No foamy oil was formed in the third cycle, suggesting that the formation of foamy oil became increasingly challenging with the increase in the cycle number.
At the drawdown pressure of 0.2 MPa, the flow of oil and gas was relatively weak, and it was especially difficult for the oil phase to be driven. This made it hard for the two phases to fully collide and mix during the puffing process, but instead, they gradually separated. Since the gas could not be effectively dispersed in the oil phase, foam oil could not form. At the drawdown pressure of 1.5 MPa, the flow rate of the gas phase was overly high, which caused continuous channels to be easily generated in the sandpack. The gas flowed rapidly in these channels—i.e., gas channeling—which also resulted in poor mixing of the gas with the oil phase. As a result, a stable foamy oil flow could not form either. When the drawdown pressure was between 0.5 MPa and 1.0 MPa, the oil and gas phases could stably flow in the sandpack during the puffing process. The continuous gas phase could be continuously sheared into tiny bubbles during the flow and fully mixed with the oil. As the bubbles were surrounded by the oil phase with high viscosity, it was not easy for them to coalesce, and thus, foamy oil gradually formed and flowed stably. Therefore, it can be inferred that full shearing, collision, and mixing during the flow of the oil and gas phases are necessary conditions for the formation of stable foamy oil flow, and thus a moderate drawdown pressure is required in the puffing process for improved oil production.
The influence of the gas huff-n-puff cycle number on the formation of foamy oil is intrinsically determined by the ratio of the oil and gas phases. In the first two cycles, the oil saturation was relatively high, and the gas phase could fully contact and mix with the oil phase during the puffing process. The oil phase with high viscosity acted to prevent the coalescence of tiny bubbles, thus enabling the gas bubbles to stably disperse in the oil phase. After two cycles of gas huff-n-puff, the oil content in the sandpack was very low, and seepage channels were formed. At this time, when the gas was injected, it occupied these large and continuous channels. Once the outlet valve was opened, the gas rapidly flowed out along these channels, with the system pressure declining quickly. In the puffing process, it was difficult for the gas phase to effectively mix with the oil phase. Consequently, gas channeling predominated in the third cycle without foamy oil flow. That is to say, an adequate oil phase is necessary for the formation of foamy oil, which plays a role in dispersing and stabilizing bubbles after fully mixing with gas.
For the formation conditions of foamy oil obtained from experiment, there is a certain range of application. For example, the drawdown pressure of 0.5–0.75 MPa is only suitable for heavy oil with a viscosity of 150 mPa∙s. With the change in reservoir conditions, the formation conditions of foamy oil should be changed accordingly.

3.3. EOR Mechanisms of Foamy Oil in Gas Huff-n-Puff

The gas production volume and oil recovery in each cycle under five different drawdown pressures are shown in Figure 11. It can be seen that the gas production volume decreased significantly while oil recovery increased remarkably for the puffing stage with stable foamy oil flow. The gas production volume in a cycle without foamy oil was approximately 4500–5000 mL under standard conditions. However, the gas production volume in a cycle with foamy oil was only about 3500 mL, representing a decline of about 26%.
For the first cycle of the tests, a stable foamy oil flow formed at the drawdown pressures of 0.5 MPa, 0.75 MPa, and 1.0 MPa, with corresponding oil recovery as high as 16.9%, 18.34%, and 14.34%, respectively. In comparison, foamy oil flow did not form at the drawdown pressures of 0.2 MPa and 1.5 MPa, with corresponding oil recovery of only 9.41% and 9.83%. For the second cycle of the tests, a stable foamy oil flow formed at the drawdown pressures of 0.5 MPa and 0.75 MPa, with corresponding oil recovery as high as 11.72% and 9.09%. In comparison, foamy oil flow was not observed at the drawdown pressures of 0.2 MPa, 1.0 MPa, and 1.5 MPa, with corresponding oil recovery of only 9.02%, 6.19%, and 4.42%, respectively. For the first two cycles, the average oil recovery influenced by foamy oil increased by 6.9% and 4.5%, respectively, compared with the tests without foam oil generation.
The reasons for the above phenomenon are as follows: If foam oil forms, some of the produced gas exists in the oil phase in the form of micro-bubbles and flows with the oil phase. The state of gas changes from the continuous phase to the dispersion phase in porous media with an evident relative decline in permeability. This results in more gas remaining in the pores for a longer time, thereby slowing down the system pressure drop rate and enhancing the driving force for oil displacement. Meanwhile, the volume of the oil phase can expand due to the micro-bubbles, which can further improve oil recovery. Consequently, the characteristics of high oil recovery and a low gas production rate are usually shown when the phenomenon of foamy oil is observed in the puffing stage of gas huff-n-puff. Thus, it can be summarized that the EOR mechanisms of foamy oil mainly consist of two aspects: one is the reduction in the mobility of the gas phase and prolongation of the gas retention time in the reservoir, thereby maintaining a higher displacement pressure; the other is the promotion of the expansion of the oil phase’s volume and increasing the elastic energy.

4. Conclusions

(1) Foamy oil can form in the puffing stage of gas huff-n-puff under certain conditions, which can greatly reduce the gas production rate and increase the oil production rate, thereby significantly improving the oil recovery.
(2) According to five experiments on gas huff-n-puff, foamy oil is more likely to form in the first two cycles under a drawdown pressure of 0.5–0.75 MPa. It can be concluded that the drawdown pressure and cycle number of gas huff-n-puff are two crucial factors influencing the foamy oil flow, thus reflecting the two necessary conditions for the formation of foamy oil. Firstly, the gas and oil in porous media should flow adequately to provide the necessary shear for the generation of micro-bubbles. Secondly, the proportion of the oil and gas phases should be moderate; in particular, the oil phase’s content should not be too low to avoid the inability to disperse and stabilize bubbles.
(3) The main EOR mechanisms of foamy oil consist of two aspects: one is the promotion of the expansion of the oil phase, and the other is the mobility reduction of the gas phase, allowing more gas to remain in the pores for a longer time, slowing down the decline rate of system pressure, and thereby enhancing the driving force for oil displacement.

Author Contributions

Conceptualization, S.W. and Z.W.; methodology, S.W.; investigation, S.W., Z.Z., Z.W. and F.W.; writing—original draft preparation, S.W., Z.Z. and F.W.; writing—review and editing, Z.W., Z.Y. and Y.L.; supervision, Z.W.; funding acquisition, S.W. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This project was financially supported by the National Natural Science Foundation of China (No. 42306236) and the Natural Science Foundation of Shandong Province (ZR2021QD077).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

Shoujun Wang was employed by the Science and Technology Management Department, Shengli Oilfield Company, SINOPEC; Zhimin Zhang was employed by the Exploration Department, PetroChina Tarim Oilfiels Branch Company; and Fei Wang, Zhaolong Yi and Yan Liu were employed by the Petroleum Engineering Technology Research Institute, Shengli Oilfield Company, SINOPEC. The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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Processes 13 01127 g001
Figure 1. Schematic diagram of the gas huff-n-puff experiment in the laboratory.
Figure 1. Schematic diagram of the gas huff-n-puff experiment in the laboratory.
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Figure 2. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 0.2 MPa. (a) Oil production rate. (b) Gas production rate.
Figure 2. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 0.2 MPa. (a) Oil production rate. (b) Gas production rate.
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Figure 3. Oil produced at the drawdown pressure of 0.2 MPa.
Figure 3. Oil produced at the drawdown pressure of 0.2 MPa.
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Figure 4. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 0.5 MPa. (a) Oil production rate. (b) Gas production rate.
Figure 4. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 0.5 MPa. (a) Oil production rate. (b) Gas production rate.
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Figure 5. Oil produced at the drawdown pressure of 0.5 MPa (the left picture shows the oil produced in Cycle 1; the right picture shows the oil produced in Cycle 2).
Figure 5. Oil produced at the drawdown pressure of 0.5 MPa (the left picture shows the oil produced in Cycle 1; the right picture shows the oil produced in Cycle 2).
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Figure 6. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 0.75 MPa. (a) Oil production rate. (b) Gas production rate.
Figure 6. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 0.75 MPa. (a) Oil production rate. (b) Gas production rate.
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Figure 7. Oil produced at the drawdown pressure of 0.7 MPa (the left picture shows the oil produced in Cycle 1; the right picture shows the oil produced in Cycle 2).
Figure 7. Oil produced at the drawdown pressure of 0.7 MPa (the left picture shows the oil produced in Cycle 1; the right picture shows the oil produced in Cycle 2).
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Figure 8. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 1.0 MPa. (a) Oil production rate. (b) Gas production rate.
Figure 8. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 1.0 MPa. (a) Oil production rate. (b) Gas production rate.
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Figure 9. Oil produced at the drawdown pressure of 1.0 MPa.
Figure 9. Oil produced at the drawdown pressure of 1.0 MPa.
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Figure 10. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 1.5 MPa. (a) Oil production rate. (b) Gas production rate.
Figure 10. Changes in the oil production rate and gas production rate with time at the drawdown pressure of 1.5 MPa. (a) Oil production rate. (b) Gas production rate.
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Figure 11. Comparison of gas production and oil recovery in each cycle with different drawdown pressures. (a) Gas production. (b) Oil recovery.
Figure 11. Comparison of gas production and oil recovery in each cycle with different drawdown pressures. (a) Gas production. (b) Oil recovery.
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Table 1. Characteristic parameters of the artificial cores in the tests at different drawdown pressures.
Table 1. Characteristic parameters of the artificial cores in the tests at different drawdown pressures.
No.Porosity
(%)		Permeability
(mD)		Initial Oil Saturation
(%)		Drawdown Pressure
(MPa)
131.23306587.630.2
230.65295086.710.5
330.45305086.380.75
431.72312087.521.0
531.06296087.331.5
Table 2. The situation of foamy oil in each cycle of different tests.
Table 2. The situation of foamy oil in each cycle of different tests.
Drawdown Pressure/MPaCycle 1Cycle 2Cycle 3
0.2No foamy oilNo foamy oilNo foamy oil
0.5Foamy oil flowFoamy oil flowNo foamy oil
0.75Foamy oil flowFoamy oil flowNo foamy oil
1.0Foamy oil flowNo foamy oilNo foamy oil
1.5No foamy oilNo foamy oilNo foamy oil
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Wang, S.; Zhang, Z.; Wang, Z.; Wang, F.; Yi, Z.; Liu, Y. Mechanism and Formation Conditions of Foamy Oil During Gas Huff-n-Puff in Edge and Bottom Water Heavy Oil Reservoirs. Processes 2025, 13, 1127. https://doi.org/10.3390/pr13041127

AMA Style

Wang S, Zhang Z, Wang Z, Wang F, Yi Z, Liu Y. Mechanism and Formation Conditions of Foamy Oil During Gas Huff-n-Puff in Edge and Bottom Water Heavy Oil Reservoirs. Processes. 2025; 13(4):1127. https://doi.org/10.3390/pr13041127

Chicago/Turabian Style

Wang, Shoujun, Zhimin Zhang, Zhuangzhuang Wang, Fei Wang, Zhaolong Yi, and Yan Liu. 2025. "Mechanism and Formation Conditions of Foamy Oil During Gas Huff-n-Puff in Edge and Bottom Water Heavy Oil Reservoirs" Processes 13, no. 4: 1127. https://doi.org/10.3390/pr13041127

APA Style

Wang, S., Zhang, Z., Wang, Z., Wang, F., Yi, Z., & Liu, Y. (2025). Mechanism and Formation Conditions of Foamy Oil During Gas Huff-n-Puff in Edge and Bottom Water Heavy Oil Reservoirs. Processes, 13(4), 1127. https://doi.org/10.3390/pr13041127

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