Experimental Study of the Dynamic Water–Gas Ratio of Water and Gas Flooding in Low-Permeability Reservoirs

Abstract

WAG flooding is a dynamic process of continuous reservoir flow field reconstruction. The unique advantages of WAG flooding cannot be utilized, due to the fixed water–gas ratio. Therefore, we must investigate the dynamic adjustment of the water–gas ratio for WAG flooding. Using nine cases of long-core displacement experiments in low-permeability reservoirs, the development effects of three different displacement methods, namely, continuous gas flooding, WAG flooding with a fixed water–gas ratio, and WAG flooding with a dynamic water–gas ratio, were investigated after elastic development, water flooding, and gas flooding. This study shows that for early elastic development in low-permeability reservoirs, WAG flooding can significantly improve oil recovery, but WAG flooding with a dynamic water–gas ratio is not conducive to the control of the water cut rise and gas channeling. As a result, it is more suitable to adopt WAG flooding with a fixed water–gas ratio. For early water flooding in low-permeability reservoirs, WAG flooding more clearly improves oil recovery and suppresses gas channeling, but WAG flooding with a dynamic water–gas ratio exhibits a higher oil recovery and thus is recommended. For early gas flooding in low-permeability reservoirs, whether the development effect of WAG flooding can improve oil recovery and inhibit gas channeling strongly depends on whether the water–gas ratio is adjusted. The development effect of WAG flooding with a dynamic water–gas ratio is significantly better than that with a fixed water–gas ratio. Therefore, WAG flooding with a dynamic water–gas ratio is recommended to achieve the best displacement effect. This research has important practical significance for further improving the development effect of WAG flooding in low-permeability reservoirs.

1. Introduction

Studies conducted nationally and internationally have shown that WAG (alternate water and gas injection) flooding is a CO₂-enhanced oil recovery (EOR) displacement technology, which has been intensively developed in terms of research and application in recent years [1,2,3,4,5]. Compared with water flooding or gas flooding, WAG flooding can rapidly replenish the formation energy and improve the sweep efficiency of injected fluid through the mixing effect of CO₂ and crude oil [6,7,8]. On the other hand, WAG flooding can improve the fluidity of injected fluid, reduce the water cut, and slow gas channeling, which can significantly improve the final development effect of low-permeability reservoirs [9,10]. Currently, the water–gas ratio of WAG flooding is usually designed as a fixed value. However, the displacement process of WAG flooding is a dynamic process in which the equilibrium state of each fluid phase in the reservoir is constantly broken and the reservoir flow field characteristics are continuously reconstructed [6]. Therefore, it is clearly inappropriate to use a fixed water–gas ratio throughout the WAG flooding process. The initial stage takes into account whether the water–gas ratio of WAG flooding needs to be dynamically adjusted and whether there are significant differences between the final development effect of the dynamic and fixed gas–water ratios. In addition, there is little relevant research at present [11].

Currently, the optimal design of the water–gas ratio is mainly based on the reservoir numerical simulation method [12,13,14]. Depending on the influence of reservoir characteristics on the development effect of WAG flooding, such as crude oil properties, reservoir permeability, and non-homogeneity, the value of the water–gas ratio usually ranges from 0.5 to 2 (1:2, 1:1, 2:1). For low-permeability reservoirs and weak reservoir inhomogeneity, to obtain a higher oil recovery, the water–gas ratio of WAG flooding is usually taken as 1:2 or 1:1 to maximize the mixing effect of CO₂ and crude oil and improve the sweep efficiency of injected fluid [15,16,17]. When the reservoir is more inhomogeneous (permeability gradient < 50), the water–gas ratio of WAG flooding is usually taken as 1:1 or 2:1 to improve the fluidity of injected fluid and better suppress gas channeling, thus improving the utilization rate of injected CO₂ and the oil exchange rate [18,19,20]. Research results have greatly narrowed the range of values for the design of the water–gas ratio, and there is already a theoretical basis for the dynamic adjustment of the water–gas ratio.

Therefore, regarding the necessity of the dynamic adjustment design of the water–gas ratio, this study takes the typical low-permeability reservoir cores as the experimental samples and analyzes the influence of the dynamic water–gas ratio on the development effect of WAG flooding through long-core displacement experiments under different flooding methods. This study also obtains the results of those experiments, using the main evaluation indexes of oil recovery and the average production gas–oil ratio within the development stage. The feasibility and applicability of the dynamic water–gas ratio design of WAG flooding are clarified, with recommendations given for the water–gas ratio design of WAG flooding when elastic development, water flooding, and gas flooding are used as early development methods. This not only clarifies the application of the dynamic and fixed water–gas ratios and solves the problem of whether it is necessary to dynamically adjust the water–gas ratio, but also provides a scientific basis for guiding the reasonable design of the water–gas ratio of WAG flooding. This has important practical significance for further improving the development effect of WAG flooding in low-permeability reservoirs.

2. Experimental Preparation

2.1. Experimental Materials

Short-core physical simulation experiments generally have many problems such as severe injected gas fingering, a small gas diffusion range, and difficulty in replicating both the real displacement process of injected gas in the reservoir and the oil–gas displacement behavior. Therefore, experiments were carried out using long cores with a diameter of 3.8 cm and a length of 90 cm. The parameters of the cores are detailed in

Table 1. Physical properties of core samples.

Core Number	Length/cm	Diameter/cm	Porosity/%	Permeability/mD
1	90	3.8	9.56	3.67
2	90	3.8	8.31	3.33
3	90	3.8	7.24	3.53
4	90	3.8	9.76	3.48
5	90	3.8	8.46	4.24
6	90	3.8	9.63	3.73
7	90	3.8	9.70	3.75
8	90	3.8	9.56	4.34
9	90	3.8	9.23	3.38

.

The actual formation water of a low-permeability reservoir in China was used for saturating the core samples, with a mineralization of 11,562 mg/L, pH value of 7.1, and water type of CaCl₂. The oil used for saturating the core samples was a sample of the actual light crude oil, whose physical parameters are shown in

Table 2. Performance parameters of crude oil samples.

Parameter	Saturation Pressure /MPa	Crude Oil Density /(kg/m3)	Crude Oil Viscosity /(mPa.s)	Solution Gas–Oil Ratio (GOR) /(m3/m3)	Oil Stratum Volume Factor /(m3/m3)
Value	11.23	751.50	2.02	48.10	1.24

. The injected gas used in the displacement experiments was CO₂, which was mixed with a sample of the crude oil. The minimum mixing pressure (MMP) of the light oil–CO₂ system under reservoir conditions (90 °C, 20 MPa) was measured to be 18.54 MPa.

2.2. Experimental Setup

Figure 1 shows a schematic diagram of the core displacement experimental setup, which can handle experiments with up to 100 MPa of pressure and 200 °C of temperature. During the experiment, the core gripper, fluid container, and piping for fluid transportation were located in a thermostatic oven to keep the temperature constant. The core displacement experiments were conducted under reservoir conditions (90 °C, 20 MPa).

2.3. Experimental Cases

For 9 long-core samples in the low-permeability reservoir, 9 cases of displacement experiments were designed to simulate the flooding effects of 3 different flooding methods, namely, continuous gas flooding, WAG flooding with a fixed water–gas ratio, and WAG flooding with a dynamic water–gas ratio, after elastic development, water flooding, and gas flooding. Based on the results of previous research [21,22,23,24,25], the water–gas ratio values of WAG flooding and the dynamic water–gas ratio order were designed. The specific design cases of the displacement experiment are shown in

Table 3. Design cases of displacement experiment.

Core Number	Displacement Experiment	Water–Gas Ratio Design of WAG
1	Continuous gas flooding after elastic development	/
2	WAG flooding with fixed water–gas ratio after elastic development	1:1
3	WAG flooding with dynamic water–gas ratio after elastic development	1:2 for the first 3 cycles, 2:1 for the last 3 cycles
4	Continuous gas flooding after 30% water cut of water flooding	/
5	WAG flooding with fixed water–gas ratio after 30% water cut of water flooding	1:1
6	WAG flooding with dynamic water–gas ratio after 30% water cut of water flooding	1:2 for the first 3 cycles, 2:1 for the last 3 cycles
7	Continuous gas flooding after gas channeling of gas flooding	/
8	WAG flooding with fixed water–gas ratio after gas channeling of gas flooding	1:1
9	WAG flooding with dynamic water–gas ratio after gas channeling of gas flooding	1:2 for the first 3 cycles, 2:1 for the last 3 cycles

.

The experimental steps were as follows:

(1)

The core was placed in the core clamp using the Huppler standard, and then the no-leakage test was carried out. The pore volume and porosity were measured using the imbibition method, and the permeability was tested by injecting saline water with different flow rates. The permeability was calculated according to Darcy’s law.

(2)

After the core cleaning, leakage test, and saturated oil preparation, three types of tests were carried out: continuous gas flooding, WAG flooding with a fixed water–gas ratio, and WAG flooding with a dynamic water–gas ratio.

The experimental process was as follows:

(1)

The pore volume and porosity were measured using the imbibition method, and the core permeability was tested by injecting saline water with different flow rates. Then, the core was saturated with formation water and the water permeability was measured.

(2)

The oil was saturated with simulated oil, bound water was created, and the oil permeability under bound water was measured.

(3)

The flooding fluid was injected into the core plug at a constant flow rate of 10 mL/h, and the pressure at the outlet end was maintained at 20 MPa. The oil flooding experiment was carried out according to the design standard.

• Elastic development, water flooding with a water content of 30%, or gas flooding with gas after turning to continuous gas flooding (core numbers 1, 4, 7)—after saturating the formation crude oil at 90 °C and 20 MPa, continuous gas flooding was carried out for displacement.
• Elastic development, water flooding with a water content of 30%, or gas flooding with gas after turning to WAG driving (core numbers 2, 5, 8)—after saturating the formation crude oil at 90 °C and 20 MPa, WAG1:1 (0.1 HCPV injected water + 0.1 HCPV injected gas) was carried out for displacement.
• WAG flooding after elastic development, water flooding with water cut of 30%, or the appearance of gas flooding with gas (core numbers 3, 6, 9)—after saturating the formation crude oil at 90 °C and 20 MPa, WAG1:2 (0.1 HCPV water injected + 0.2 HCPV gas injected) for three cycles and then WAG2:1 (0.2 HCPV water injected + 0.1 HCPV gas injected) for three cycles were conducted for displacement.

The pressure difference data, cumulative oil, and gas production were automatically measured and recorded at preset time intervals, before storing them on a personal computer.

3. Experimental Results

3.1. Experimental Results of Different Displacement Methods after Elastic Development

Through the displacement experiments of cases 1~3, the experimental results of the oil recovery, water cut, and production gas–oil ratio were measured under three types of displacement methods, namely, continuous gas flooding, WAG flooding with a fixed water–gas ratio, and WAG flooding with a dynamic water–gas ratio, which were carried out after elastic development, as shown in Figure 2 (HCPV: Hydrocarbon Pore Volume).

(1)

As shown in Figure 2a, the oil recovery of WAG flooding after elastic development is significantly higher than that of continuous gas flooding. In addition, there is not much difference between the oil recovery of WAG flooding with fixed and dynamic water–gas ratios. This indicates that the formation energy decreases more after elastic development, and WAG flooding is more effective in supplementing the formation energy compared with continuous gas flooding.

(2)

As shown in Figure 2b, there is no original formation water production in continuous gas flooding after elastic development, and there is injected water production in WAG flooding. Specifically, WAG flooding with a fixed water–gas ratio produces water earlier than that with a dynamic water–gas ratio, but the water cut of the latter rises faster, which is consistent with the similarity in the water–gas ratios of both WAG flooding methods.

(3)

As shown in Figure 2c, after elastic development, WAG flooding with a dynamic water–gas ratio produces gas the earliest. There is also a greater and increasing fluctuation in the production gas–oil ratio. WAG flooding with a fixed water–gas ratio produces gas later, and there is a small production gas–oil ratio that rises in a wavelike manner. Continuous gas flooding produces gas the latest, but the production gas–oil ratio increases fastest and almost linearly after producing the gas.

In summary, for low-permeability reservoirs that develop through early elastic development, to obtain a higher oil recovery, WAG flooding is more helpful in replenishing the formation energy in early elastic development compared to continuous gas flooding. Compared with WAG flooding with a fixed water–gas ratio, although water is produced later, gas is produced earlier, and there is a higher increase in the water cut and the production gas–oil ratio. It can be seen that WAG flooding after elastic development is more effective in controlling the water cut and suppressing gas channeling with a fixed water–gas ratio, while the dynamic adjustment of the water–gas ratio is not a good method for suppressing water channeling and gas channeling.

3.2. Experimental Results of Different Displacement Methods after Water Flooding

Through the displacement experiments of cases 4~6, the experimental results of the oil recovery, water cut, and production gas–oil ratio were measured under three types of displacement methods, namely, continuous gas flooding, WAG flooding with a fixed water–gas ratio, and WAG flooding with a dynamic water–gas ratio, which were carried out after the development of water flooding, as shown in Figure 3.

(1)

As shown in Figure 3a, the oil recovery of WAG flooding after water flooding is slightly higher than that of continuous gas flooding, and the difference between the oil recovery of WAG flooding with fixed and dynamic water–gas ratios is very small. This indicates that the formation energy is maintained well after water flooding development, and the effect of WAG and continuous gas flooding in supplementing the formation energy is comparable.

(2)

As shown in Figure 3b, the water cut is higher in the early stage of continuous gas flooding after water flooding when the injected water is replaced, and the water cut decreases significantly in the late stage of continuous gas flooding due to the absence of an injected water replenishment. The difference in the water cut change law between WAG flooding with fixed and dynamic water–gas ratios is relatively small.

(3)

As shown in Figure 3c, WAG flooding with a dynamic water–gas ratio produces gas the earliest after water flooding, followed by continuous gas flooding and then WAG flooding with a fixed water–gas ratio. The production gas–oil ratio steadily rises after continuous gas flooding while producing gas, and the production gas–oil ratio increases in a wavelike manner after WAG flooding. The change rule of the production gas–oil ratio of the two kinds of WAG flooding is the same. Until the end of development, there is not much difference in the production gas–oil ratio corresponding to different flooding methods.

In summary, for low-permeability reservoirs that develop through early water flooding, WAG flooding has a higher oil recovery and lower production gas–oil ratio compared with continuous gas flooding. Compared with WAG flooding with a fixed water–gas ratio, WAG flooding with a dynamic water–gas ratio produces water at a similar time and produces gas earlier, but the degree of increase in the water cut and the production gas–oil ratio is slightly higher. It can be seen that in WAG flooding after water flooding, the dynamic and fixed water–gas ratios can both inhibit gas channeling to a certain extent, but the difference in the development effects is not significant.

3.3. Experimental Results of Different Displacement Methods after Gas Flooding

Through the displacement experiments of cases 7~9, the experimental results of the oil recovery, water cut, and production gas–oil ratio were measured under three types of displacement methods, namely, continuous gas flooding and WAG flooding with fixed and dynamic water–gas ratios, respectively, which were carried out after the development of gas (CO₂) flooding, as shown in Figure 4.

(1)

As shown in Figure 4a, the oil recovery of WAG flooding with a dynamic water–gas ratio after gas flooding is slightly higher than that of WAG flooding with a fixed water–gas ratio and continuous gas flooding. This indicates that the formation energy is maintained better after a gas flooding development, but WAG flooding with a dynamic water–gas ratio is more effective in replenishing the formation energy.

(2)

As shown in Figure 4b, there is no original formation water production after continuous gas flooding, and there is injected water production in WAG flooding. WAG flooding with a fixed water–gas ratio produces water earlier, and the water cut rises faster. WAG flooding with a dynamic water–gas ratio produces water later, but the final water cut is comparable to WAG flooding with a fixed water–gas ratio.

(3)

As shown in Figure 4c, continuous gas flooding produces gas earliest after gas flooding, followed by WAG flooding with a fixed water–gas ratio and that with a dynamic water–gas ratio. After producing the gas, the production gas–oil ratios of the three development types all show a slower increase in the early stage and a faster increase in the later stage. However, the production gas–oil ratio of continuous gas flooding is larger than that of WAG flooding, and the production gas–oil ratio of WAG flooding with a fixed water–gas ratio is larger than that of WAG flooding with a dynamic water–gas ratio.

In summary, for low-permeability reservoirs developed by early gas flooding, WAG flooding has a higher oil recovery and lower production gas–oil ratio compared with continuous gas flooding. Compared with WAG flooding with a fixed water–gas ratio, WAG flooding with a dynamic water–gas ratio not only produces water and gas later but also increases the water cut and the production gas–oil ratio more slowly. It can be seen that the dynamic adjustment of the water–gas ratio in WAG flooding after gas flooding can help to control the rising rate of the water cut and inhibit gas channeling, which is better than the development effect of WAG flooding with a fixed water–gas ratio.

4. Experimental Data Analysis

Based on the experimental data of nine cases of core displacement experiments, the calculation results of evaluation indexes such as the oil recovery at the end of development and the average production gas–oil ratio during the whole development stage were obtained, as shown in

Table 4. Calculation results of evaluation indexes.

Case Number	Oil Recovery/%	Increased Ratio of Oil Recovery/%	Average Production Gas–Oil Ratio/(cm3/mL)	Decreased Ratio of Average Production Gas–Oil Ratio/%
Case 1	76.1		1257.7
Case 2	81.6	7.2	1063.4	15.5
Case 3	82.2	8.0	1721.1	−36.8
Case 4	80.2		1226.8
Case 5	82.6	3.0	959.0	21.8
Case 6	83.1	3.6	1034.0	15.7
Case 7	81.5		2204.2
Case 8	82.5	1.2	2099.4	4.8
Case 9	84.1	3.1	1633.6	25.9

.

The comparison results of the oil recovery and the increased ratio of the oil recovery (WAG flooding relative to continuous gas flooding when the early development methods are the same) for nine cases of core displacement experiments are shown in Figure 5.

As can be seen in Figure 5, when the early development methods are the same, the subsequent displacement method with WAG flooding has a higher oil recovery compared with continuous gas flooding, and the increased ratio of the oil recovery of WAG flooding with a dynamic water–gas ratio is higher than that of WAG flooding with a fixed water–gas ratio (1~2%). By contrast, with different early development methods, the increased ratio of the oil recovery of WAG flooding after elastic development is highest (7~8%), the increased ratio of the oil recovery of WAG flooding after water flooding is the second highest (3~4%), and the increased ratio of the oil recovery of WAG flooding after gas flooding is the lowest (1~3%).

It can be seen that, when the early development methods are the same, the oil recovery of different subsequent displacement methods is as follows: WAG flooding with a dynamic water–gas ratio > WAG flooding with a fixed water–gas ratio > continuous gas flooding. With identical subsequent displacement methods, the increased ratio of the oil recovery of different early development methods is as follows: elastic development > water flooding > gas flooding.

The results of the comparison of the average production gas–oil ratio with a decreased average production gas–oil ratio (WAG flooding relative to continuous gas flooding when the early development methods are the same) during the whole development stage for the nine cases of core displacement experiments are shown in Figure 6.

It can be seen from Figure 6 that, when the early development method is elastic development, during the whole development stage, WAG flooding with a fixed water–gas ratio can reduce the average production gas–oil ratio (15.5%), whereas it increases with WAG flooding with a dynamic water–gas ratio (36.8%). When the early development method is water flooding, the two WAG flooding methods used in the subsequent displacement method can reduce the average production gas–oil ratio during the whole development stage, with a relatively small difference. WAG flooding with a fixed water–gas ratio has a larger reduction (21.8%), and WAG flooding with a dynamic water–gas ratio has the second largest reduction (15.7%). When the early development method is gas flooding, there is a clear difference between the two WAG flooding methods in the average production gas–oil ratio during the whole development stage. The decrease in WAG flooding with a dynamic water–gas ratio is largest (25.9%) and the decrease in WAG flooding with a fixed water–gas ratio is smaller (4.8%).

It can be seen that the average production gas–oil ratio during the whole development stage corresponding to the early development method is as follows: water flooding < elastic development < gas flooding. When the subsequent displacement method is WAG flooding with a fixed water–gas ratio, the decreased average production gas–oil ratio corresponding to the early development method is as follows: water flooding > elastic development > gas flooding. When the subsequent displacement method is WAG flooding with a dynamic water–gas ratio, the decreased average production gas–oil ratio corresponding to the early development method is as follows: gas flooding > water flooding > elastic development (negative value).

5. Conclusions

(1)

WAG (alternating water and gas injection) flooding is a CO₂-enhanced oil recovery (EOR) displacement technology that has been intensively developed in terms of research and application nationally and internationally in recent years. In this study, the development effect of different displacement methods was investigated through long-core displacement experiments in low-permeability reservoirs. Using the oil recovery and average production gas–oil ratio in the development stage as the main evaluation indexes, we compared the development effects of different subsequent developments (WAG flooding with a dynamic water–gas ratio, WAG flooding with a fixed water–gas ratio, continuous gas flooding), with the early development methods such as elastic development, water flooding, and gas flooding. The experimental results show that WAG flooding is better than continuous gas flooding, but the effect of the WAG flooding development depends on the early development method.

(2)

The feasibility and applicability of the dynamic design of WAG flooding were clarified based on the results of an experimental analysis. The recommendations for the water–gas ratio design of WAG flooding are as follows: when the early stage is elastic development, the dynamic water–gas ratio is not conducive to the control of the rise in the water cut and gas channeling, so it is more suitable to use the fixed water–gas ratio. When the early stage is water flooding, the oil recovery of the dynamic water–gas ratio is higher, so the use of the dynamic water–gas ratio is recommended. When the early stage is gas flooding, the development effect of the dynamic water–gas ratio is significantly better than that with the fixed water–gas ratio; therefore, the use of the dynamic water–gas ratio is recommended. This has important practical significance for further improving the development effect of WAG flooding in low-permeability reservoirs.