Visualization and Simulation of Foam-Assisted Gas Drive Mechanism in Surface Karst Slit-Hole Type Reservoirs

Abstract

Nitrogen injection technology has become an important production technology after water injection development in the karst fracture-vuggy reservoir in Tahe Oilfield. However, due to the influence of reservoir heterogeneity and the high mobility of gas fluid, nitrogen easily forms a dominant channel and gas channeling occurs, and the recovery effect time is short. Based on this, a visual surface karst model is designed and created to study nitrogen foam-assisted gas drive. The results show that after gas channeling occurs in the dominant channel of nitrogen flooding, foam injection-assisted gas flooding can improve oil recovery. In the longitudinal direction, foam-assisted gas drive mainly displaces the remaining oil because of gravity differentiation and the reduction of oil–water interfacial tension. In the horizontal direction, foam-assisted gas drive is mainly used to block the large pore cracks and dominant channels, promote the gas to turn into large tortuous and small cracks, and expand the swept efficiency of the gas. After forming the dominant channel, injecting 0.3 pv salt-sensitive foam with a gas–liquid ratio of 2:1 in the middle of the gas channel can improve the recovery rate of the model from 4% to about 25%, and the recovery rate can be increased by about 20%, which improves the effect of gas flushing and improves the development efficiency of the oil field at the same time.

1. Introduction

The domestic exploitation of conventional oil reservoirs has entered the middle and late stages, and the oil equivalent extracted cannot meet the national demand for oil and gas. With the development of science and technology, the exploitation technology of unconventional reservoirs has been improved, and more and more attention has been paid to the exploitation of unconventional reservoirs, especially fracture-vuggy carbonate reservoirs [1]. There are 68 [2,3] fracture-cave carbonate reservoirs in the world, 10 of which are in China, mainly located in Renqiu [4,5], Sichuan [6], Ordos [7,8], and Tarim [9,10]. Among them, the Ordovician reservoirs in Tahe Oilfield [11,12] are the largest fracture-cavity carbonate reservoirs that have been proved. These reservoirs [13] are mainly composed of fractures formed by tectonic deformation and pores and fractures and caverns formed by karst processes, among which large caverns are the most important reservoir spaces. The structure of the fractured cave unit is shown in Figure 1. This reservoir has a width of 200 m, a reservoir length of 400 m, and a reservoir thickness of 50 m.

Since the breakthrough of nitrogen injection extraction technology in Tahe Oilfield in 2012, nitrogen injection production has become an important technology for increasing and stabilizing production after water injection development in fracture-cavity reservoirs. However, due to the heterogeneity of the reservoir and the high mobility of the gas fluid, gas channeling easily occurs after nitrogen injection into the reservoir, and the gas extraction response time is short, so the oil and gas produced by the oil well is relatively high [14]. The gas injection production curve is shown in Figure 2. The foam fluid has the characteristics of high viscosity, mobility control, and strong migration ability, which can realize the selective plugging of “high plugging but not low plugging” and can be used to assist gas drive to improve the recovery efficiency of surface karst fracture-cavity reservoir [15,16,17,18]. However, the technology of foam-assisted gas drive control in fracture-cavity reservoir has not been reported, so it is necessary to carry out experiments to clarify the injection technology and mechanism. Therefore, a visualization model was designed and prepared for the study of foam-assisted gas drive injection parameter optimization.

2. Materials and Methods

2.1. The 2D Visualization Model of Seam-Hole Assemblies

The reservoir depth is 5300~7000 m; the formation temperature is 120~150 °C; the total mineralization of formation water is 22 × 10⁴ mg/L; the reservoir space is dominated by dissolution holes and cracks; and the oil–water relationship is extremely complex, with no uniform interface. The average porosity of the reservoir space is 0.76 percent, and the average permeability is 2.59 × 10⁻³ μm².

With reference to the typical suture-hole combination pattern of the tabular karst zone in the Tahe Oilfield, a two-dimensional visualized tabular karst model was designed and produced, made in the laboratory; as shown in Figure 3. The crack opening in the model is 0.5~2 mm; the overall size of the vertical model is 720 × 440 mm, with a total volume of 1300 mL; and the overall size of the horizontal model is 550 × 570 mm, with a total volume of 880 mL. The model is made of alexandrite plate, which is divided into two parts of the etching plate as well as the sealing plate, as shown in Figure 4.

2.2. Experimental Setup and Methods

Experimental setup: Visualization experimental device—The experimental setup consists of three parts, namely the seam-hole visualization physical model system, the foaming and injection system, and the video monitoring and data acquisition system. The physical modeling system consists of a model holder and an experimental model. The model holder is composed of welded stainless steel with a retractable base to accommodate different sizes, and fixed steel clips are installed on both sides of the holder to secure the model. The foaming and injection system consists of a foam generator, a constant speed and pressure advection pump, an intermediate container, and a number of valves and pipelines. The minimum injection volume of the advection pump is 0.01 mL/min, and the maximum injection volume is 9.99 mL/min. The intermediate container is a high-pressure-resistant container with a maximum pressure of 50 MPa. The experimental flow chart is shown in Figure 5.

Experimental methods: The water used for the experimental visualization of the slit holes was homemade formation water, with a total mineralization of about 22 × 10⁴ mg/L and calcium and magnesium particles of 1 × 10⁴ mg/L, which was a highly mineralized CaCl₂-type formation water and was stained with methyl blue stain, Shanghai, China MACKlin BIOCHEMICAL Technology Co., LTD; the oil used for the experiments was stained with Sudan III reagent, Sigma Aldrich (Shanghai, China) TRADING Co., LTD. The injection method was 1 injection and 1 extraction. The experimental temperature was 25 °C. The experimental data measurement was mainly focused on the measurement of oil repellent volume. The experimental foam was generated by a foam generator with N₂, Wuhan, China Huaxin Gas Company; foaming agent, made in the laboratory; foam stabilizer, made in the laboratory; and formation water, made in the laboratory.

2.3. Experimental Steps and Program

Experimental steps: ① assemble the model as required and check the sealing of the model, confirm the sealing, and then saturate the oil and water with the stained experimental water as well as the experimental oil and record the volume of saturated oil and water, respectively; ② the saturated model is placed on the mold gripper for fixation, and the pipe connections are constructed according to the scheme; ③ open the gas cylinder, measure the gas flow rate. After reaching the predetermined injection flow rate to carry out a gas drive experiment, record the volume of oil extracted from the extraction outlet until the extraction outlet out of gas and no oil that is the end of a gas drive; ④ the configured foaming solution was placed in an intermediate container and sent to the foam generator via an advection pump for foaming, the corresponding amount of foam was injected into the model according to the experimental program for assisted gas drive, and the volume of oil recovered at the extraction point was recorded; ⑤ open the gas cylinder and measure the gas flow rate. After reaching the predetermined injection flow rate to carry out the second gas drive, record the volume of oil extracted from the extraction outlet until the gas tampering, i.e., end of the experiment.

3. Foam-Assisted Gas Drive Mechanism

(1) Gravity differentiation, vertical distribution—In the longitudinal model, nitrogen is injected to drive out the oil, and the gas, due to the density difference, is transported to the high part of the structure to form the “gas roof” under the effect of gravity differentiation, replacing the “attic oil” in the high part of the structure; at the same time, under the influence of the large difference in gas–liquid flow, the gas–oil–water distribution of the fluid medium from the high part to the low part of the model is “gas-oil-water”. At the same time, under the influence of large differences in gas–liquid flow, part of the “bypass oil” is driven out, and the model fluid medium is distributed as “gas-oil-water” from the high part to the low part, as shown in Figure 6a. After gas flurry, foam is injected to assist gas drive, and under the action of density difference, the foam is transported to the middle part of the structure along the crack, and under the action of pressure cone, the auxiliary nitrogen gas drives out the unused residual oil in the middle part of the structure; the modeled fluid medium is distributed as “gas-foam-oil-water” from the high part to the low part, as shown in Figure 6b.

(2) Gas extrusion, fracture selection—In the horizontal model, nitrogen is injected to drive out the oil, and the gas is transported deep into the model under the action of pressure, playing the role of extrusion and driving out the remaining oil in the surface layer of the model. When the gas is transported, following the principle of minimum resistance, it is prioritized to enter the cracks with large openness to drive out the remaining oil in the surface layer (Figure 7a); with the increase in the injection volume, it gradually enters the cracks with small openness and small tortuosity to drive out the remaining oil in the surface layer; then, the gas forms the dominant channel, and the gas flurry occurs (Figure 7b).

(3) Jamin effect, gas steering—In the horizontal model, after the gas scavenging, foam is injected to assist the gas drive. After foam injection, the gas is transported to the deeper part of the model, expanding the reach of the gas and displacing the remaining oil in the model (yellow line in Figure 8a). At the same time, due to the Jamin effect of the foam, the pore throats of the large cracks are blocked, and the subsequently injected gas is steered to enter the large tortuosity and small cracks to displace the remaining oil in them (blue line in Figure 8b).

4. Optimization of Foam Injection Parameters

4.1. Vertical Model

(1) Foam Type Optimization

The comparison of the effect of injecting different types of foams for assisted oil drive was studied when the injection volume was 0.4 pv, the gas–liquid ratio was 3:1, and the foam was injected for assisted gas drive at the late stage of gas flushing, and the experimental results are shown in

Table 1. Experimental results of foam type optimization.

Foam Type	Oil Recovery Rate %
	Inject Gas	Inject Foam	Inject Gas
Salt-sensitive foam	13.46	26.62	2.77
Gel foam	13.46	25.77	1.93
Dispersion foam	13.46	23.62	1.19

and Figure 9.

The experimental results show that the recovery rate of the salt-sensitive foam-assisted gas flooding model is the best among dispersed foam, gelatin foam, and salt-sensitive foam. The main reason is that compared with gelatin foam and dispersion foam, salt-sensitive foam has a longer half-life, defoaming is more difficult, and after injection into the model, the foam is more stable and more parts of the remaining oil can be started. Therefore, the injection of salt-sensitive foam-assisted gas flooding provides the best economic benefits after gas cannot be enhanced.

(2) Foam injection volume optimization

For the salt-sensitive foam, the comparison of the oil repelling effect under different foam injection volumes was studied when the gas–liquid ratio was 3:1 and foam-assisted gas drive was injected at the late gas flurry stage, and the experimental results are shown in

Table 2. Experimental results of foam injection volume optimization.

Foam Injection Volume/pv	Oil Recovery Rate/%
	Inject Gas	Inject Foam	Inject Gas
0.2	13.46	11.46	0.46
0.3	13.46	21.16	1.84
0.4	13.46	26.62	2.77

and Figure 10.

When 0.2 pv foam is injected, the recovery range of each 0.1 pv foam-assisted gas is 5.73%. When 0.3 pv foam is injected, the recovery range of each 0.1 pv foam-assisted gas is 7.05%. When 0.4 pv foam was injected, the recovery range of each 0.1 pv foam-assisted gas displacement was 6.65%. When the foam injection rate increases from 0.2 pv to 0.3 pv, the recovery rate increases by 11.08%, and when the foam injection rate increases from 0.3 pv to 0.4 pv, the recovery rate increases by 6.39%. The increase in recovery rate decreased.

According to the mean and range analysis, the optimal foam injection amount is between 0.3 pv and 0.4 pv. The optimized injection volume was 0.3 pv.

(3) Foam gas–liquid ratio optimization

For the salt-sensitive foam, the comparison of oil repelling effect under different gas–liquid ratios was investigated when the injection volume was 0.4 pv and the foam-assisted gas drive was injected at the late stage of the gas flurry, and the experimental results are shown in

Table 3. Experimental results of foam gas–liquid ratio optimization.

Foam Gas–Liquid Ratio	Oil Recovery Rate/%
	Inject Gas	Inject Foam	Inject Gas
1:1	13.46	25.39	0.15
2:1	13.46	29.36	0.5
3:1	13.46	28.16	1.23

and Figure 11.

When the gas–liquid ratio of foam injection is 1:1, the foam extraction range is 6.35% for every 0.1 pv. When the gas–liquid ratio of foam injection is 2:1, the foam extraction range is 7.34% for every 0.1 pv. When the gas–liquid ratio of foam injection is 3:1, the foam extraction amplitude is 7.04% for every 0.1 pv. When the gas–liquid ratio of foam is increased from 1:1 to 2:1, the recovery efficiency is increased by 4.32%, and when the gas–liquid ratio is increased to 3:1, the recovery efficiency is decreased by 0.47%.

According to the mean and range analysis, the foam gas–liquid ratio of 2:1 is the best gas–liquid ratio, and the foam-assisted gas drive effect is the best among the three kinds of gas–liquid ratio in the experiment.

(4) Foam injection timing optimization

Salt-sensitive foam with 0.4 pv and a gas–liquid ratio of 3:1 was injected at the early stage, middle stage, and late stage of gas flushing to assist gas flushing, and the oil repelling performance of salt-sensitive foam at different injection times was compared. The experimental results are shown in

Table 4. Experimental results of foam injection timing optimization.

Injection Timing	Oil Recovery Rate/%
	Inject Gas	Inject Foam	Inject Gas
Gas channeling early	12.92	25.7	0.52
Gas channeling middle	13.31	27.15	1.47
Gas channeling late	13.46	28.14	1.25

and Figure 12.

When the injection time is adjusted from the initial stage of gas channeling to the middle stage of gas channeling, the recovery rate increases by 2.79%. When adjusted to the late stage of gas channeling, the recovery rate increases by 0.92%. The recovery rate of foam-assisted gas drive in the early stage of gas channeling is 0.52%, that of foam-assisted gas drive in the middle stage of gas channeling is 1.47%, and that of foam-assisted gas drive in the late stage of gas channeling is 1.25%.

The comprehensive analysis of oil recovery at the range and secondary gas injection stage shows that foam injection-assisted gas flooding in the middle stage of gas channeling has the best economic benefit.

4.2. Horizontal Model

(1) Foam Type Optimization

The comparison of the effect of injecting different types of foams for oil repulsion was studied when the injection volume was 0.4 pv, the gas–liquid ratio was 2:1, and foam-assisted gas drive was injected at the late stage of gas flushing, and the experimental results are shown in

Table 5. Experimental results of foam type optimization.

Foam Type	Oil Recovery Rate %
	Inject Gas	Inject Foam	Inject Gas
Salt-sensitive foam	27.09	28.61	4.18
Gel foam	27.09	26.18	3.49
Dispersion foam	27.09	12.23	1.61

and Figure 13.

The experimental results show that after gas flooding reaches the maximum recovery rate, the injection of dispersed foam-assisted gas flooding can increase the recovery rate by 13.84%. The injection of gel foam-assisted gas drive can increase the recovery rate by 29.67%. The injection of salt-sensitive foam-assisted gas drive can increase the recovery rate by 32.79%. It can be seen that salt-sensitive foam has the best auxiliary benefits in auxiliary gas flooding, mainly because the flow channel plugging ability of salt-sensitive foam is stronger than that of dispersion foam and gelled foam. After injection, the effect of deep gas migration and steering is increased, and the sweep coefficient of gas is increased.

(2) Foam injection volume optimization

For the salt-sensitive foam, the comparison of the oil repelling effect under different foam injection volumes was studied when the gas–liquid ratio was 2:1 and foam-assisted gas drive was injected at the late gas flurry stage, and the experimental results are shown in

Table 6. Experimental results of foam injection volume optimization.

Foam Injection Volume/pv	Oil Recovery Rate/%
	Inject Gas	Inject Foam	Inject Gas
0.2	27.09	11.72	0.96
0.3	27.09	22.24	2.64
0.4	27.09	29.14	3.65

and Figure 14.

It was found that when 0.2 pv foam was injected, the recovery enhancement per 0.1 pv foam-assisted gas drive was 6.34%; when 0.3 pv foam was injected, the recovery enhancement per 0.1 pv foam-assisted gas drive was 8.3%; and when 0.4 pv foam was injected, the recovery enhancement per 0.1 pv foam-assisted gas drive was 8.19%. When the foam injection amount was increased from 0.2 pv to 0.3 pv, the recovery rate was improved by 12.2%, and when the injection amount was increased from 0.3 pv to 0.4 pv, the recovery rate was improved by 3.65%; the recovery rate improvement was reduced.

From the mean value as well as the extreme deviation analysis, it can be seen that there exists an economically optimal foam injection volume between 0.3 pv and 0.4 pv. The optimized result of injection volume was 0.3 pv.

(3) Foam gas–liquid ratio optimization

For the salt-sensitive foam, the comparison of oil repelling effect under different gas–liquid ratios was investigated when the injection volume was 0.4 pv and the foam-assisted gas drive was injected at the late stage of the gas flurry, and the experimental results are shown in

Table 7. Experimental results of foam gas–liquid ratio optimization.

Foam Gas–Liquid Ratio	Oil Recovery Rate/%
	Inject Gas	Inject Foam	Inject Gas
1:1	27.09	20.38	1.1
2:1	27.09	28.61	4.18
3:1	27.09	25.81	3.06

and Figure 15.

When the foam injection gas–liquid ratio was 1:1, the recovery enhancement was 5.09% per 0.1 pv foam; when the foam injection gas–liquid ratio was 2:1, the recovery enhancement was 7.15% per 0.1 pv foam; when the foam injection gas–liquid ratio was 3:1, the recovery enhancement was 6.45% per 0.1 pv foam. When the foam gas–liquid ratio was increased from 1:1 to 2:1, the recovery rate was increased by 11.31%, and when the gas–liquid ratio was increased to 3:1, the recovery rate was decreased.

From the mean value as well as the extreme deviation analysis, it can be seen that the foam gas–liquid ratio of 2:1 is the best gas–liquid ratio.

(4) Foam injection timing optimization

Foam-assisted gas drive was carried out by injecting 0.4 pv salt-sensitive foam with a gas–liquid ratio of 2:1 at the early, middle, and late stages of gas flushing to compare the oil repelling performance of salt-sensitive foam at different injection times. The experimental results are shown in

Table 8. Experimental results of foam injection timing optimization.

Injection Timing	Oil Recovery Rate/%
	Inject Gas	Inject Foam	Inject Gas
Gas channeling early	22.56	31.51	2.25
Gas channeling middle	25.47	30.53	2.72
Gas channeling late	27.09	30.4	2.39

and Figure 16.

When the foam injection timing was adjusted from the early stage of gas flushing to the middle stage of gas flushing, the recovery rate was increased by 2.4%, and when it was adjusted to the late stage of gas flushing, the recovery rate was increased by 1.16%. The recovery rate of foam-assisted gas drive injected at the beginning of gas scram is 2.25% in the secondary gas injection and oil drive stage, the recovery rate of foam-assisted gas drive injected at the middle of gas scram is 2.72% in the secondary gas injection and oil drive stage, and the recovery rate of foam-assisted gas drive injected at the late stage of gas scram is 2.39% in the secondary gas injection and oil drive stage.

Comprehensively analyzing the extreme difference and the recovery rate in the second gas injection and oil driving stage, the gas scrambling middle injection foam-assisted gas drive has the best economic benefit.

4.3. Vertical and Horizontal Integrated Effects Evaluation

Integrated recovery was introduced to evaluate the combined role of foam in the vertical and horizontal models. It is calculated by multiplying the vertical model recovery rate by the vertical model recovery rate. Considering the effect of foam in both vertical and horizontal models, the results are shown in

Table 9. Summary of modelled overall recoveries.

Experimental Factors		Oil Recovery Rate/%
		Vertical Model	Horizontal Model	Combined Recovery Rate
Type of foam	Salt-sensitive foam	42.85	59.88	25.66
	Gel foam	41.16	56.76	23.36
	Dispersion foam	38.27	40.93	15.66
Foam injection volume	0.2	25.38	39.77	10.09
	0.3	36.46	51.97	18.95
	0.4	42.85	59.88	25.66
Foam gas–liquid ratio	1:1	39.00	48.57	18.94
	2:1	43.32	59.88	25.94
	3:1	42.85	55.96	23.98
Timing of foam injection	Gas channeling early	39.14	56.32	22.04
	Gas channeling middle	41.93	58.72	24.62
	Gas channeling late	42.85	59.88	25.66

and Figure 17.

Comparing and analyzing the combined recovery data, it is clear from the mean and extreme deviation that the salt-sensitive foam has the best extraction effect, with an injection volume of 0.3 and a gas–liquid ratio of 2:1, and that injecting salt-sensitive foam in the middle of the gas scavenging has the best economic benefits.

4.4. Oilfield Field Applications

Salt-sensitive foam was applied in the field for gas tampering control, and the application results showed that the injection of salt-sensitive foam of 0.3 pv with a gas–liquid ratio of 2:1 after gas tampering can improve the recovery rate by about 10%, and the recovery rate increases with the increase of the foam injection volume.

5. Conclusions and Discussion

5.1. Conclusions

(1) The foam system can be used to assist gas drive to improve the recovery rate and can meet the requirements of residual oil after gas drive in the startup reservoir.

(2) The foam-assisted gas drive visualization experimental results show that foam-assisted gas drive mechanism is divided into ① gravity differentiation, vertical distribution; ② gas extrusion, fracture selection; and ③ Jamin effect, gas steering.

(3) The experimental results of foam-assisted gas drive injection parameter optimization show that: ① different types of foam-assisted gas drive have different effects, and among the three types of foams studied, the salt-sensitive foam has the best economic benefits; ② as foam injection volume increases, the extraction capacity shows a trend of increasing and then decreasing, and there exists the best economic efficiency of the injection volume (between 0.3 and 0.4 pv); ③ as the foam gas–liquid ratio increases, the foam-assisted gas drive effect first increases and then decreases, and when the gas–liquid ratio is 2:1, the foam-assisted gas drive effect is the most significant; and ④ injecting foam-assisted gas drive in the middle of gas scavenging has the best economic benefit.

5.2. Discussion

The long-term effect of foam on the reservoir and the environmental benefits were not carried out in this study, and subsequent experimental studies can be carried out on this issue to further improve the mechanism of foam tampering.