Given the difficulty of water injection and effective displacement system establishment in Changqing oilfield, this research carried out the interfracture injection and production experiment of a large outcrop model with two types of injection fluid medium, natural gas and surfactant, and studied the interfracture injection and production law of different displacing medium and the principal contradiction existing in the different displacing medium. The results show that natural gas drive in tight oil reservoir is beneficial to reduce the Jamin resistance and form a rapid oil production. Still, the gas drive process is easy to develop gas breakthrough, leading to a decrease in gas utilization ratio, and its oil displacement efficiency is 17.25%. Additionally, due to the strong adsorption of surfactant, the seepage capacity of a porous medium in the process of surfactant-oil displacement is reduced, and blockage is formed at the injection end, resulting in the poor oil displacement effect of surfactant. However, compared with natural gas injection, the surfactant has higher oil displacement efficiency, up to 22.05%. Therefore, for tight oil reservoirs, rational utilization of the advantages of different mediums and controlling their disadvantages are essential for the development of such reservoirs.
With the rapid development of China’s economy, the domestic demand for oil and natural gas is increasing, and the exploration and development of ultralow permeability reservoirs and tight oil plays are becoming increasingly significant [1–3]. The petroleum geological resources of Ordos Basin have excellent development potential, and the tight oil reservoirs there are the focus of exploration and development [4, 5]. During the development of tight oil reservoirs, there are many development problems such as complex water injection and the rapid production decline, resulting in low recovery and poor development economics for these reservoirs [6–8]. It is crucial to study the displacement recovery of such reservoirs. Tight oil reservoirs are more accessible than expected to inject fluids under fracturing and other measures, which allows interfracture injection and production in these reservoirs. Additionally, the injection of surfactant, CO2, hydrocarbon gas, and waterflooding in tight oil reservoirs at home and abroad has played a role in enhancing oil recovery (EOR). Still, the effect varies significantly in different regions, even with contradictory results, and the most critical effective displacement system is challenging to establish. The efficient way to replenish energy is the main problem facing the development of tight oil reservoirs [9–14].
To address the problem of the complex development of tight oil reservoirs, many scholars in China have conducted research on interfracture injection and production in tight oil reservoirs, i.e., creating fractures by fracturing in the same horizontal well, as injection and development followed. Since the physical simulation experiments of interfracture injection and production are extremely difficult, many laboratories have difficulty in conducting related studies. Therefore, many scholars mainly focus on numerical simulations [15, 16]. For example, Cheng et al. compared and analyzed various development modes and concluded that interfracture injection and production have an effect on EOR . He et al. established a numerical simulation model of interfracture injection and production and systematically explored the application of interfracture injection and production technology [18, 19].
The above research mainly focuses on the mechanism research. There is lack of research means and corresponding analysis on the mechanical mechanism in the process of different fluid displacement. Aiming at the phenomenon that it is difficult to inject tight oil reservoir, this paper used the high-temperature and high-pressure large-scale physical simulation experimental system to design the interfracture injection and production process. Meanwhile, the research studied the oil production process and pressure distribution in the process of different fluid displacement through outcrop model experiment. Through the research, the difference in pressure dynamic change in two types of fluid (CH4 and surfactant) displacement process and the advantages and disadvantages of different fluids displacement in tight oil reservoirs are put forward, which provides favorable guidance for EOR of tight oil reservoirs.
2. Experimental Design
Due to the variation of pore structure and permeability of tight oil reservoirs, their seepage mechanisms differ significantly from those of conventional reservoirs. The numerical simulation methods based on the seepage theory of medium-permeability and high-permeability reservoirs have been brutal to express the seepage characteristics of tight oil reservoirs. Small size core experiments can reflect the development pattern of low-permeability and tight oil reservoirs. Still, small size core experiments can hardly reflect convection lines, i.e., planar flow characteristics. In this paper, we propose to use the experimental method of the large-scale outcrop model to carry out the analysis of planar seepage characteristics in carrying out different fluid displacement processes to provide a theoretical basis for the development of tight oil reservoirs.
The primary production of Changqing oilfields is tight sandstone reservoirs, and their production law conforms to the seepage law of tight sandstone. Therefore, the tight sandstone outcrops are used as a porous medium to carry out relevant experimental research. The outcrop model size used in the experiment is . In the two types of displacing medium, which are natural gas (CH4) and surfactant, a large outcrop model, interfracture injection and production experiment, is carried out to study the oil displacement law of the two fluids.
The experiment uses a large outcrop model experiment technique, which includes four processes: model processing, model encapsulation, model vacuum saturation, and displacement experiment. The model-making process includes sample screening according to permeability and other parameters, fracture fabrication and sand filling, experimental interface installation, electrode installation, and so on.
In the process of model packaging, the processing model is placed in the specific mold. The model is encapsulated with epoxy resin to achieve the effect of sealing all the models except the pressure interface. The encapsulated model can withstand the displacement pressure below 25 MPa in the large outcrop model experimental system. The vacuum pumping process of the model mainly uses vacuum saturation equipment to achieve the purpose of 100% saturated kerosene of the model. According to different experiments, the specific displacement experimental process is designed. In the process of surfactant and natural gas injection and production in interfracture, the constant pressure displacement method is adopted. The pressure of the injection fracture of surfactant is 1.45 MPa; the pressure of the production fracture is 0 MPa. For the natural gas displacement experiment, the flooding rate of the injection fracture is 0.2 ml/min.
2.2. Laboratory Equipment
The critical experimental equipment in this research is the large outcrop model experimental system. The system consists of a large outcrop model gripper, fluid injection equipment, ring pressure holding equipment, back pressure control equipment, fluid metering equipment, pressure acquisition system, resistance acquisition system, and vacuum saturation system.
The large-scale outcrop model holder (Figure 1) is the main section of the experimental system. It provides the following two functions in the experiment. Firstly, it supports the experimental model and provides external environmental control pressure for the experimental model to ensure that the packaging layer will not be damaged during the displacement process under high pressure, to achieve the experimental approach of high-pressure displacement. Simultaneously, the various external interfaces are provided for the experimental model, such as pressure measurement interface and resistance measurement interface. The experimental model is connected to the measuring equipment through the large outcrop model holder-related interfaces without affecting the sealing of the experiment or development.
The fluid injection equipment consists of a plunger pump, intermediate vessel, and various valve assemblies. During the displacement, they provide power for fluid injection. The ring pressure holding equipment is composed of a large plunger pump and a pressure stabilizing pump. The primary function of the equipment is to provide stable ring pressure for the model by injecting fluid into the ring control of the large outcrop model holder. Backpressure control equipment is composed of backpressure valve and valve components. In the experimental process, stable backpressure is established at the outlet. The equipment is mainly used in the natural gas flooding process. Fluid metering equipment is primarily composed of test tubes and automatic collection equipment for continuous or interval collection of produced fluids. The pressure acquisition equipment is a 32-channel automated pressure acquisition system, which is used to display the pressure of different measuring points of the experimental model in time and record the pressure value of 32 channels according to the set time interval. The purpose of the resistance acquisition device is to measure and record the resistance values between different measuring points on the model surface.
By using the experimental equipment, two groups of large outcrop plane model interfracture injection and production experiments were completed. The production law, pressure dynamic change law, and critical factors affecting the investigation were studied.
2.3. The Selected Experimental Fluid
The primary purpose of this experiment is to study the mechanism of the displacement of different injected fluids. Therefore, the interaction between injection fluid (CH4 or surfactant) and displaced fluid (oil), as well as the characteristics of different types of fluid, is considered in the experimental fluid design. The viscosity of refined kerosene is similar to the reservoir crude oil. It does not contain impurities or cause blockage during the displacement process, so refined kerosene is used instead of crude oil in the experiments .
When selecting the injection fluids, phase state, and adsorption of the fluid, interfacial tension between the injection fluid and kerosene should be considered. Based on the above factors, natural gas and surfactant are selected as injection fluids, respectively. The natural gas selected in the experiment represents the soluble gas-phase fluid with low interfacial tension with the oil-phase, which has noticeable phase change in the displacement process. The surfactant system means a fluid with adsorption and low interfacial tension with oil, and there is no phase change in the displacement process. Studying the displacement process between two types of fluid is of great significance to fluid selection in the oil flooding process of tight oil reservoirs. The natural gas component used in the experiment is 100% methane (CH4). Furthermore, a 0.3% betaine+SDS system is used as a surfactant, and the interfacial tension between the system and kerosene is 0.0048 mn/m.
2.4. Experimental Model and Process
In the experiment, the model made of the tight sandstone outcrop of Yanchang Formation in Ordos Basin is used to experiment, and the gas measurement permeability of the model is between 0.3 and 0.8 mD, and the porosity is between 9% and 15%. The size of tight sandstone in the plane experimental model is and encapsulated by epoxy resin. Three fractures are designed on the model, with 46 cm length and 18 cm spacing. The model design picture and actual photo are shown in Figure 2. During the modeling process, the fracture width is about 1 mm, which is cut according to the design position. Then, the quartz sand and cement are mixed in a particular proportion and filled into the cutting fractures, which is mainly to change the conductivity of the fracture and plays a role in supporting the cutting fractures. In this experiment, the permeability of fracture filler is about 1000 mD.
In Figure 2(b), the interfaces on the model’s surface are pressure measurement point interfaces. Due to the limitation of the experimental equipment, the maximum number of interfaces designed for the experiment is 32, and these interfaces are the well points and pressure measurement points during the experiment. The pressure field during displacement can be obtained through the pressure measurement of different interface positions. The low permeability of the experimental model leads to the failure to obtain resistance measurements during the experiment, and the resistance measurements are not described in detail in this paper.
The experimental process of surfactant injection was referred to . In this paper, there are two injection fractures, and the injection medium is a surfactant. Furthermore, considering that the CH4 is a gas-phase and its flow capacity is much higher than the surfactant system, the experimental process designed is different from that surfactant displacement experiment. The specific method of this experiment is as follows: (1) according to the experimental design processing model and drying and packaging model; (2) the model was vacuumized and saturated with kerosene. (3) The saturated model was installed in the large-scale outcrop model experimental system, and the connection point data were recorded. (4) The pipelines at each measuring point were emptied with kerosene, and the model was completely saturated. (5) In this study, the natural gas is injected into fractures on both sides, and oil is produced from a fracture in the middle. At the initial stage, the outlet pressure backpressure valve of the model was set to 18 MPa. The flooding rate was set to 0.2 ml/min to carry out constant pressure displacement. After the inlet and outlet pressures are stabilized, it is changed to 0.2 ml/min speed for the repelling, and the oil and gas production is recorded during the repelling process. The pressure distribution at different moments is recorded. (6) When the outlet gas-oil ratio reaches above 2500, the experiment is ended.
3. Experimental Results
Physical simulation experiments of interfracture natural gas and surfactant displacement were carried out by using a large-scale outcrop simulation experiment system. Through comparison of the two groups of experiments, it is found that the recovery degree of surfactant flooding is highest, while the recovery degree of natural gas flooding is lowest, and there are apparent differences in the production laws between the two groups of experiments.
3.1. The Surfactant Displacement Results
Figure 3 shows the relationship between production rate and time in the experimental process of surfactant flooding, and Figure 4 shows the relationship between water cut and recovery. As can be seen from Figure 3, the initial liquid production rate overlaps with the oil production rate, reaching a maximum of 3.8 ml/min. At 7 min, the oil production rate and liquid production rate decrease to 2.8 ml/min and begin to produce water. At 2900 min, the oil production rate decreased to 0 ml/min after a long period of slow decline. After the water breakthrough, the liquid production rate continued to drop, and at 1200 min, the fluid production rate dropped to about 0.28 ml/min. During 1200 min to 2900 min, the fluid production velocity fluctuates and falls with the overall trend. When the water content reaches 100%, the fluid production velocity decreases to 0.10 ml/min.
As can be seen from Figure 4, the final recovery of the surfactant displacement experiment is 22.55%. The recovery degree in a water-free period is 1.37%. The recovery is 1.51% before 20.00% water cut and 21.04% after more than 20.00% water cut. The oil is mainly recovered in the middle and high water cut stage, during the water cut up and down. The above data show that the oil displacement law of interfracture surfactant is as follows: under experimental conditions, oil production during interfracture surfactant displacement shows a continuous decline law, with a rapid decline in early stage and slow oil production in the long term after high water cut. The liquid production rate decreases rapidly in the early stage and shocks in the later stage. During surfactant displacement, crude oil is mainly recovered in the high water cut production stage.
Figure 5 shows the dynamic change of interfracture surfactant flooding pressure. It can be found in the figure that displacement pressure expands rapidly and evenly on the left and right sides from 0 to 20 min. After 20 min, the displacement pressure contour starts to retract toward injected fracture, and the high-pressure area gradually concentrates to injected fracture position. After 1,400 min, pressure breakthrough at the upper left side of the fracture is observed, and the contour is pushed toward the middle of the fracture. Short-term pressure breakthroughs at different locations result in shocks in the fluid production rate during the injection. After pressure distribution stabilization, a large area of low-pressure areas exists, leading to a lower sweep efficiency.
3.2. The Natural Gas Displacement Results
Figure 6 shows the relationship curve between the gas-oil ratio and time in the natural gas displacement experiment. It can be found from the relationship curve between gas-oil ratio and time (Figure 6) that the injected natural gas broke through production fracture at 20 min, and a large amount of gas begins to be seen at the outlet of the model. At 266 min, the gas-oil ratio exceeds 2500 m3/m3. Interfracture natural gas displacement has the characteristics of early gas breakthrough and rapid increase of gas production after gas breakthrough, which is not conducive to the utilization of natural gas. It can be seen from Figure 7 that before the breakthrough of injected gas, the oil production rate increased from 1.98 ml/min to 2.22 ml/min and then decreased rapidly. At 50 min, the oil production rate drops to 0.50 ml/min. After the gas breakthrough, the oil production rate decreases slowly. At 266 min, the oil production rate drops to 0.21 min/min. The oil production rate of interfracture natural gas displacement is much higher than surfactant displacement.
Figures 7 and 8, respectively, show the oil production rate curve at different times and the relationship curve between recovery and gas content at the outlet end. The gas content at the outlet end refers to the ratio of volume corresponding to pressure at outlet end calculated from gas production in different periods to the total oil and gas volume. The significance of this parameter is similar to that of water displacement water cut. It can be seen from Figure 8 that recovery at outlet is about 4.07% when gas breakthrough. When the gas-oil ratio reaches 2500 m3/m3, gas content at the outlet is about 91%, and recovery is 17.25%. It is lower than the recovery of 22.05% in the surfactant displacement experiment. Through the above analysis, it is concluded that natural gas displacement can effectively improve oil production rate, but it is easy to form gas crossflow, resulting in a lower recovery.
Figure 9 shows the dynamic process of experimental pressure of interfracture natural gas injection and production. Because displacement pressure difference is minimal in the later stage, the change of actual pressure distribution cannot be seen by the using same color code, so different color codes are used for each map. After 15 minutes of gas displacement, the pressure distribution is confusing, and there is no regular pressure expansion front. This indicates that the fingering phenomenon is evident in the process of gas injection. The crossflow characteristics are vital, resulting in a reduction in sweep efficiency and low oil recovery. The result is consistent with oil production data (Figures 6–8).
4.1. The Oil Recovery Capacity of the Different Injection Medium
Based on the production data, injection capacity and oil recovery capacity of interfracture fluid (surfactant and CH4) injection and production are analyzed. According to the principle of injection production conservation principle, the dimensionless liquid production capacity is used to evaluate the injection capacity. That is, the more robust the liquid production capacity is, the higher the injection capacity is. The dimensionless oil recovery capacity is used to evaluate oil recovery capacity. The dimensionless fluid production capacity refers to the ratio of fluid production rate under unit pressure difference to that under initial unit pressure difference. The dimensionless oil production capacity refers to the ratio of oil production rate under unit pressure difference to that under initial unit pressure difference. Using the dimensionless data conversion, the influence of model parameters on comparison data can be eliminated and reduced. In the interfracture natural gas injection and production experiments, the gas production data is converted to volumes at 20 MPa and combined with oil production volumes for causeless liquid production capacity calculations.
Figure 10 shows the dimensionless oil production capacity curves of interfracture fluid (surfactant and CH4) injection and production. It can be found from Figure 10 that the oil production capacity of interfracture natural gas injection and production is significantly higher than that of interfracture surfactant injection and production because of the early water breakthrough of interfracture surfactant injection and production.
It can be seen from Figure 11 that the dimensionless liquid production capacity of interfracture surfactant injection and production decreases gradually with time. The dimensionless liquid production capacity of interfracture natural gas injection and production gradually increases with time, and the dimensionless liquid production capacity is much higher than other surfactant fluids. According to the principle of injection production conservation principle, the injection capacity of interfracture natural gas injection and production is the strongest, injection capacity increases with time, and injection capacity of surfactant displacement is poor.
4.2. The Influencing Factors and Mechanisms
In the process of displacement in a tight oil reservoir, the action mechanism of the interfacial tension is different in two types of fluid displacement processes. For tight oil reservoirs with water-wet type, the process of interfracture water injection and production is that the wetting phase displaces the nonwetting phase. After the water enters a larger seepage space, the water in the larger pore will imbibe and replace with the crude oil in the nearby small pore through imbibition. The imbibition and replacement cause many of oil-water dispersion systems to be formed in the seepage channel. In the displacement process, the oil-water dispersion system is under the action of the interface, and the Jamin effect is formed. This leads to a rapid increase in flow resistance and a sharp decline in seepage capacity. However, after water breakthrough, Jamin action tends to be stable, and water injection capacity remains unchanged . In the process of surfactant and gas displacement, due to the decrease of interfacial tension, the imbibition effect is weakened, and the Jamin effect is reduced.
Two types of fluid injected into the fracture break through to the middle production fracture earlier, forming a surfactant system and gas crossflow phenomenon. Reducing the interfacial tension can improve the oil displacement efficiency. However, it will increase the impact of reservoir heterogeneity on the flow field and reduce the sweep coefficient [22–24]. Additionally, the betaine+SDS surfactant system is used in the interfracture injection and production. Betaine is an amphiphilic surfactant system, which is easy to form multilayer adsorption in oil displacement. In the tight sandstone core, multilayer adsorption will narrow the seepage channel and reduce the seepage capacity. In the process of surfactant displacement, adsorption will weaken with the increase of injection depth. The adsorption mainly occurs near injection fracture, and adsorption intensifies with time. In the experiment, this effect is reflected in the retraction of pressure isoline and the continuous decline of injection capacity of surfactant flooding after surfactant interfracture injection and production for a while (Figures 5 and 9). Hence, surfactant adsorption is the crucial factor leading to the continuous decline of liquid production capacity in surfactant interfracture injection and production. In the process of surfactant interfracture injection and production, there is no phase change of oil-water two phases with the change of pressure. However, in the process of natural gas displacement, with the change of pressure, the dissolution and precipitation of the gas-phase lead to the drastic change of phase state in the process displacement. The change of phase state seriously affects the oil recovery process of natural gas flooding.
Figure 12 shows the variation of the displacement pressure with time during the natural gas displacement. At the beginning of the experiment, the repellency pressure was increased to 20 MPa, and the recovery outlet pressure was controlled at 18 MPa. And then, the repellency was displaced at a continuous flow rate of 0.2 ml/min. At the beginning of the constant flow rate flooding, with the increase of gas injection, displacement differential pressure decreases rapidly. At 45 min, after the gas breaks through the extraction outlet, displacement differential pressure changes to a slight decrease.
Under displacement pressure, injected natural gas forms a dissolution equilibrium with crude oil. In this process, gas phase volume decreases, liquid phase volume increases, and liquid phase viscosity decreases, which is conducive to improving the recovery of natural gas displacement . In the process of pressure drop, the dissolution equilibrium of the oil-gas system is broken, and the gas phase is separated from the crude oil. Many oil and gas bubbles come into being in oil displacement channels. The difference in seepage capacity in different positions of displacement channels leads to oil and gas bubbles formed by degassing, mainly concentrated at the edge of displacement channels. This leads to the smaller contact surface between injected natural gas and crude oil with difficulty increasing sweep efficiency. In natural gas displacement, the injection capacity keeps growing, but it is difficult to improve the sweep coefficient due to the influence of phase changes. The final recovery degree is lower than that of surfactant displacement.
Interfracture injection and production of surfactant and natural gas can establish an effective displacement system in tight oil reservoirs. The displacement efficiency and seepage mechanism of the two types of fluid are different. The displacement efficiency of surfactant injection is about five percentage points higher than natural gas injection in tight reservoirs
The advantages of natural gas displacement are low seepage resistance and easy effective the displacement establishment. The disadvantage is that gas crossflow is easy to form in the displacement process, and the depressurization in displacement process will cause the change of phase state. If it is difficult to establish an effective well pattern for interfracture water injection and production, a natural gas displacement model can be considered
The injected surfactant is a targeted development method by reducing oil-water interfacial tension and seepage resistance. The disadvantages of this method are easy to form water crossflow and adsorption blockage, while the advantage is solving the problem of capillary force in the process of water injection, and there is no phase change with pressure. Additionally, surfactant suitable for tight oil reservoirs needs to be studied. By controlling the range of interfacial tension, surfactant displacement can be combined with interfracture injection and production under the condition of the low capillary force, so as to realize the effective use of tight oil reservoirs
The data, except the information presented in the manuscript, used to support the funding of this study is restricted by the safety law of Petrol China, while the source data in the manuscript is not open.
Conflicts of Interest
The authors declare no conflicts of interest.
This work is supported by the Major Program of PetroChina (2020DJ2201), the Study on the Seepage Law of Typical Low-Grade Oil Reservoirs and New Methods for Enhancing Oil Recovery (2021DJ1102), and the Research on Tight Oil Physical Simulation and Production Mechanism (2021DJ2204).