A Microfluidic Experiment on CO₂ Injection for Enhanced Oil Recovery in a Shale Oil Reservoir with High Temperature and Pressure

Abstract

In recent years, CO₂ huff and puff has become one of the most important methods developed for unconventional shale oil reservoirs and has been widely used in all major shale oil fields. However, the microscopic mechanism of CO₂ contacting with crude oil is complex, and the change law of the residual oil occurrence after CO₂ injection is unclear. In this paper, a micro visualization fluid flow simulation experiment (microfluidic experiment) under high temperatures and high pressure of a shale reservoir was conducted to reveal the micro mechanism of CO₂ and crude oil after contact at the microscale. This allows conclusion of more precise results than any experiment conducted in a room environment. Combined with gas–oil two-phase micro flow characteristics, the production mechanisms of crude oil by CO₂ huff and puff at the pore scale are clarified, and the change characteristics of the remaining oil occurrence state after CO₂ injection are quantified. The results show that CO₂ mainly produces crude oil in macropores and microfractures in the injection stage of huff and puff, improves the mobility of crude oil through diffusion dissolution in the soaking stage, and that the driving of dissolved gas is dominant in depressurization production. The major micro-mechanisms for CO₂ to improve shale oil are extraction and dissolution expansion, accompanied by a variety of secondary mechanisms, such as the miscibility effect, oil expansion, viscosity reduction and other contact effects, as well as the improvement of crude oil properties. The simulation results of huff and puff development show that soaking is an important stage to enhance oil recovery. With increasing soaking time or the soaking pressure, the recovery degree of crude oil will increase positively.

1. Introduction

Compared with conventional gas injection huff and puff, CO₂ huff and puff mechanism of shale reservoir is more complex, due to the strong heterogeneity of the shale reservoir and the development of microfractures, pores and throats [1]. To improve CO₂ huff and puff recovery, it is urgent to determine the mechanism of crude oil production of fluids in a shale reservoir, the flow law of oil and gas and to clarify the changes in the occurrence state of crude oil during huff and puff. Reasonable injection and production parameters are formulated [2], which refers to the influence law of huff and puff parameters on shale production.

Conventional reservoir huff and puff injection gas can directly enter the matrix to achieve the production of crude oil [3]. However, shale develops a large number of micro- and nano-scale pores, which makes it difficult for gas to directly enter the matrix to produce crude oil. Existing studies show that the mobilization mechanism of crude oil in the huff-and-puff process mainly occurs in the soaking stage [2]. CO₂ enters the matrix from the fracture through diffusion in the contact between oil and gas and separates the crude oil on the rock wall through competitive adsorption with alkanes [4]. In addition, CO₂ extracts the light components in crude oil, reduces the oil–gas interfacial tension [5], resulting in oil–gas miscibility and the increase of oil fluidity caused by the reduction of crude oil viscosity [6]. Relevant studies show that the miscibility can increase CO₂ huff and puff by 18%, thereby greatly improving oil recovery [7]. During the production process, gas can be released due to the pressure reduction, which triggers the driving of dissolved gas [8].

At present, the experiments of CO₂ huff and puff mechanism of shale reservoirs have been carried out, but the traditional methods for studying CO₂ huff and puff shale oil are mostly core scale experiments [9,10]. This proves that CO₂ huff and puff technology can effectively improve the recovery of shale oil [11]. However, there is less research relating to the microscopic mechanism of crude oil mobilization in CO₂ huff and puff process. Previous methods used to study the remaining oil are generally done through environmental scanning electron microscopy, laser focusing and micro fluorescence methods [12]. These methods can only be used for qualitative analysis through static observation. In contrast, the micro visualization experimental technology has the advantages of dynamic description and quantitative analysis, which can accurately and quickly improve knowledge on the seepage law of oil, gas and water and enhance the oil recovery mechanism of CO₂ in fractures and micro- and nano-scale matrices.

In this paper, a microscopic visualization experiment is conducted to describe the real dynamic huff and puff displacement process and the mechanism of oil–gas seepage at the pore scale. The process of oil upgrading with various mechanisms is analyzed respectively and the influence of pore structure on oil movement of shale oil is concluded. The mechanisms of oil mobilization in different situations are quantized by the shape of the oil and the degrees of exertion by different mechanisms. The migration characteristics of fluids with different properties in micro- and nano-scale channels and the enrichment law of remaining oil under the formation conditions are explored from the microscopic pore structure. Sensitivity analysis, including soaking pressure and soaking time, are conducted. The variation of the oil occurrence state is qualitatively and quantitatively characterized. This has a theoretical significance for the application of CO₂ huff and puff technology in shale reservoirs. As the huff and puff development proceeds, the phase of oil transforms from the continuous phase to the discontinuous phase. The flow of crude oil becomes more and more dispersed with intensified discontinuity. The sensitivity analysis shows that cumulative oil recovery has a positive relationship with both soaking pressure and soaking time.

2. Materials and Methods

2.1. Material

In this experiment, after extraction, noise reduction, threshold segmentation and correction of field emission scanning electron microscope (FE-SEM) images of shale cores, the pore throat structure of shale cores was divided, representative pore throat structures were selected, and a micro visualization model that can fully reflect the pore throat distribution of shale reservoirs was produced.

2.2. Specific Experimental Parameters and Conditions

• Microscopic visualization model (shown in Figure 1): the material is glass with hydrophilic characteristics, high temperature and high-pressure resistance, changeable wettability (hydrophilic/oil film plating), strong bonding ability, good transmission imaging effect, etc.; Model size (L × wide × Thickness) is 75 mm × 75 mm × 3 mm, with a minimum diameter of 10 μm. The overall hole throat diameter of the chip is distributed in the range of 10–200 μm. Crude oil viscosity is 12.58 MPa·s and crude oil density is 0.86 g/cm³.
• The experimental crude oil was ground-degassed crude oil mixed with dye. According to the experimental results, Sudan III was selected to dye the transparent yellow crude oil red. The experimental temperature was 110 °C, and the well-plugging pressure and time at different points were set. This allows the identification of the status of oil and gas at different times. This technique allows a dynamic description of CO₂ injection process at shale oil reservoir condition, in comparison to the static observation such as environmental scanning electron microscopy, laser focusing and micro fluorescence methods.

2.3. Instrument

The high temperature and high-pressure microfluidic experimental device is shown in Figure 2. It included a high-pressure injection and production system, high temperature and high-pressure environment loading system, high-precision image recognition, and a data acquisition system. The experimental temperature that the device can reach was 150 °C, and the pressure could reach 70 MPa.

The injection pump injected the displacement medium into the microchannel chip at a constant speed and pressure, simulated the temperature and pressure conditions of the real reservoir through the confining pressure tracking pump, circulating pump and temperature control system, and used a high precision back pressure pump to control the outlet pressure. The image of developing time elapsing in the displacement process in the micro-scale channel was recorded by video microscope, and the experimental results were analyzed quantitatively by image processing technology.

2.4. Methods

The research process was as follows:

• The model was vacuumed and saturated. The model level was put in the supporting system. The kerosene was saturated after vacuuming the model, and the model temperature and pressure were gradually raised to the original formation conditions. Then, the target reservoir crude oil to the model was saturated;
• Gas injection process. Gas was injected into the inlet of the model by the same injection method to capture and record the microscopic interaction time and the change process of gas and shale oil;
• Soaking process. After the gas enters, the inlet end and outlet end were closed. Well-soaking was conducted for 15–45 min. Meanwhile, microscopic changes in the gas and shale oil during the well plugging process were recorded;
• The above steps were repeated to carry out CO₂ huff and puff experiments under different conditions of well-soaking pressure and time;
• The experiment ended. The displacement medium was replaced, either with petroleum ether or toluene. The visualization model was cleaned to the fluid in all pore structure was removed. The image and video of fluid flow in the channel were recorded with the matching software of the microscope, and the flow behavior of fluid in the model was observed in real time on the computer.
• The remaining oil was estimated by image processing and the crude oil was classified and concluded directly by shape including angular, dropwise, membranous, columnar and cluster.
• Through image processing, the chip after oil saturation was scanned, and the number of oil pixels in the saturated oil area was identified. After CO₂ huff and puff, the number of remaining oil pixels was identified again. The number of eventually remaining oil pixels was divided by the number of original saturated oil pixels, and the remaining residual saturation was obtained.

3. Results and Discussion

3.1. Microscopic Interaction Mechanism between CO₂ and Crude Oil

Based on the high temperature and high-pressure micro-scale flow experiments, CO₂ huff and puff shale oil experiments (15 MPa/20 MPa/25 MPa/35 MPa, 110 °C) were conducted to characterize the microscopic visualization process of CO₂ huff and puff shale oil.

In the process of gas injection at the closed outlet end, as the pressure and the injected gas increases, the gas gradually enters the etching area of the pore structure (as shown in Figure 3). With the increase of pressure, more gas enters the crude oil [13]. CO₂ can then displace a portion of the crude oil into the macropores and microfractures, which could also cause gas channeling in the macropores.

In the soaking stage, the color of crude oil becomes lighter as the amount of gas dissolved in shale oil gradually increases through diffusion [14,15]. This reduces the viscosity of crude oil and increases the fluidity of crude oil [16]. CO₂ molecules diffuse into the oil phase and light hydrocarbons are released into CO₂ from the oil phase [15,17], a very slow process. Therefore, there must be sufficient time for CO₂ molecules to fully diffuse into the oil phase (as shown in Figure 4).

With depressurization production begins, CO₂ escapes from the liquid, generating a gas driving force in the liquid, which improves the oil displacement effect [18]. However, due to the barrier of the rock matrix, it is difficult for gas to enter all the small pores along the normal track. Once the pores with a small pore throat radius receives pressure from other directions, they will be blocked at the throat and enter the throat in the form of bubbles. At the same time, the divided bubbles quickly gather, gradually expand and fill the whole pores, by squeezing the crude oil to induce outflow. At the same time, some CO₂ displaces oil, occupying a certain pore space and becomes bound gas.

Under the experimental pressure of 35 MPa, CO₂ reduces the relative density of the crude oil by extracting and vaporizing the light components of different components in the crude oil, and the color of the crude oil gradually becomes lighter. Then, heavy hydrocarbons are vaporized and produced [19], and finally reach stability; with the increase of pressure, the oil–gas interface contact angle increases, the interfacial tension decreases [20] and gradually becomes miscible, forming a miscible zone of CO₂ and light hydrocarbons. The movement of the miscible zone is the most effective oil displacement process, which can greatly improve oil recovery. Asphaltene precipitation and extraction are more obvious when heavy crude oil mixes with CO₂ and light components are extracted. The miscible and extraction phenomena are shown in Figure 5.

3.2. The Mechanism of Oil Mobilization

Based on the microscopic visualization experimental research, the four mechanisms of CO₂ mobilization were explored. As shown in Figure 6a, the residual oil adhering to one side of the matrix experiences shear force of CO₂ flow on both sides, which focuses in one place, and the drop-like residual oil is motivated [21,22]. There is a certain amount of water in the development of shale after fracturing. As shown in Figure 6b at the pores with poor fluidity, funnel-shaped CO₂ extracts the light components of crude oil and CO₂ flows to carry out crude oil. As shown in Figure 6c, for deep crude oil with large molecular weights, CO₂ and crude oil will be mixed by multiple contacts at the void with good fluidity, and a plume-like stepwise mixing state of CO₂ crude oil and crude oil will appear. Figure 6d CO₂ contacts with the formation fluid and surrounds the crude oil, causing the volume of the crude oil to swell and increasing the formation energy.

The remaining oil can be classified into two types, according to differences in the shape and size: the remaining oil in flake form and the remaining oil in the dispersed phase, mainly including cluster, porous, columnar, membranous and drop-like remaining oil. Cluster residual oil is essentially a large oil block surrounded by small throats, including several pore throats. During gas injection, due to the heterogeneity of rocks and other factors like the large mobility ratio of the displacing phase to the displaced phase, the gas first passes through the large pores which later forms channels. This causes the original oil in the small pores to remain in the original pores. The film-like residual oil is located in the inner walls of the pore and throat, where there is relatively high resistance to flow. The columnar residual oil is equivalent to the pore throat characteristics, either with one end closed or with one end in which it is very difficult to have flow. In the late stage of huff and puff, a small oil block can deform and pass through the large throat under the driving force of gas. Due to the viscosity of oil and the effect of oil–gas interfacial tension, this small oil block cannot pass through other small throats, even under the driving force of gas [23]. This causes the small oil block to stay in the pores and form drop-like residual oil. Cluster residual oil is in the continuous phase with strong production capacity, while the other four are in the discontinuous phase residual oil with weak production capacity.

The essence of enhancing the CO₂ recovery mechanism is extraction and dissolution expansion, which shows in phenomena such as the miscibility effect, expansion [24], viscosity reduction, improvement of mobility ratio and reduction of interfacial tension. As shown in Figure 7, CO₂ gas directly contacts crude oil to extract its light components. CO₂ dissolves into the crude oil and the crude oil expands. The micro experiment of gas flooding after hydraulic fracturing is used to quantitatively characterize the action weights of the two mechanisms. The mobilization mechanism of residual oil in different shapes is divided into extraction and dissolution expansion. After processing with the software ImageJ, different mechanisms of micro residual oil were obtained. As shown in Figure 8 and Figure 9, the production of crude oil by CO₂ extraction reached 75.95%. The dissolution rate was only 24.05%.

CO₂ contacts and dissolves into crude oil, which leads to crude oil swelling. The volume expansion ability of crude oil in different occurrence states is quantitatively characterized, as shown in Equation (1). The expansion areas of corner-1, membrane-2, membrane-3, membrane-4, column-5 and porous-6 at different times were measured, and their volume expansion coefficients were calculated.

$$v=\frac{V_i}{V_0}$$

(1)

The results of the calculation are shown in Figure 10. As residual oil saturation decreases rapidly in the early stage of huff and puff, the frequency will be larger in the early stage, are last about 5 s each time. Then, as the residual oil saturation decreases slowly in the later stage, the frequency changes to 30 s each time. With the increase of time, the volume expansion rate increases linearly. CO₂ can continuously interact with crude oil and dissolve in it. The smaller the storage volume, the more fully the crude oil can contact CO₂, and the larger the expansion coefficient under the action of CO₂. As a result, membrane-3 shows the largest expansion capacity.

3.3. Occurrence State and Influencing Factors of Remaining Oil after Huff and Puff

3.3.1. Soaking Pressure

As shown in Figure 11, when the soaking pressure is 10 MPa, the color change of crude oil is not obvious, the volume of CO₂ dissolved into the crude oil is small and a large amount of CO₂ still exists in the form of gas in the pores. Due to the pressure gap between the inlet and outlet, CO₂ migrates in the pores, and CO₂ migrates between the pores and throats in a “compression–release” cycle, squeezing the crude oil and diffusing around. In the cross-distribution area of multiple throats, the converged CO₂ will plunge along one of the throats with less resistance, resulting in a decrease in the pressure at the converging point. After the pressure is reduced, the CO₂ at the convergence point does not have enough pressure to force CO₂ into other throats, so insufficient pressure will reduce the swept area of CO₂. The process of gas flooding is a step-by-step displacement process. Under the condition of sufficient soaking pressure, gas will enter the next pore step-by-step, which improves the recovery of crude oil. As shown in Figure 12a, the cluster-like residual oil accounts for more than 80% of total residual oil under 10 MPa. During the production stage, non-wetting phase displaces the wetting phase. There is a large interfacial tension between gas and liquid, and there is a large number of Haines jump phenomena occurring [25]. Gas fingers in and out, forming a gas channel. The sweep coefficient is small. After the experiment, the residual oil mostly exists in a sheet shape.

With an increase of pressure, the solubility coefficient of CO₂ in crude oil gradually increases and the liquid color becomes noticeably lighter. As shown in Figure 12b, at 15 MPa, the cluster residual oil forms account for about 80% of the total residual oil, with a small amount of porous and columnar residual oil, but almost no film and corner dispersed phase residual oil. The recovery degree of crude oil is improved compared with that of 10 MPa, but it is still less than 60%. The molecular diffusion dissolution of CO₂ in crude oil is relatively slow and the effect of improving the recovery degree of crude oil is not obvious. As shown in Figure 12c, with the increase of the soaking pressure to 25 MPa, more CO₂ will dissolve into the crude oil, the color of the crude oil will become lighter, the recovery degree of the crude oil will gradually increase and the oil phase will be divided. Due to the large volume of remaining oil in the sheet, though there is still more than half of the remaining oil in the sheet, the remaining oil in the cluster is mainly concentrated in the surrounding corners, and the evolution of the dispersed phase from the cluster to the porous one takes place. The degree of discontinuity will be intensified, which is characterized by a large quantity, small volume and strong dispersion. The CO₂ swept area becomes larger, and the recovery degree of crude oil significantly increases compared with that of 10 MPa and 15 MPa.

CO₂ contacts with the discontinuous residual oil, diffuses and dissolves into the crude oil, which could reduce the viscosity of crude oil and the viscosity between the oil and the rock wall. CO₂ extracts light hydrocarbon components in crude oil to reduce oil–gas interfacial tension. In addition, the gas flow experiences shear and driving forces. A large number of discontinuous phase residual oil is displaced by gas, droplet residual oil is carried out by gas extraction wiredrawing, film residual oil is carried out by gas stretching and stripping and column residual oil is carried out by gas extrusion. The displacement effect is good. The remaining oil after huff and puff mainly stays in the reservoir in the forms of film, corner and column. As shown in Figure 12d, when the multiple miscible pressure reaches 35 MPa [26], the occurrence state of the remaining oil will also change. The porous remaining oil is as high as 40%, while the clustered remaining oil is only 20%. The drop-like remaining oil will change into the film-like remaining oil when it touches the matrix surface during the flow process. The corner, drop-like and film-like residual oil will gather into the column-like remaining oil at the small throat, and the film-like, column-like and corner-like discontinuous phase residual oil will increase significantly. CO₂ and crude oil dissolve each other, eliminating the interfacial tension, extracting and vaporizing the light hydrocarbon components in the crude oil. This forms a mixed zone of CO₂ and light hydrocarbons, which causes heavy component precipitation in the pores. The movement of the mixed zone greatly improves the recovery of crude oil up to 90%.

The enrichment law of residual oil under different well-soaking pressures is shown in Figure 13. With the increase of huff and puff pressure, the residual oil changes from the flaky to the discontinuous phase. The research shows that, when the huff and puff pressure is lower than the miscible pressure, the residual oil is mainly stored in the shale reservoir in clusters and pores, with less residual oil content in drops, films and columns. During the production process, the remaining oil in noncontinuous phases such as porous, membranous, columnar and angular is transformed into a cluster. When the huff and puff pressure is higher than the miscible pressure, the remaining oil saturation is less than 10%, and most of the remaining oil is stored in porous, columnar, membranous and other discontinuous phases. During the production, the remaining oil in the discontinuous phase flows and accumulates to the cluster-like remaining oil.

As shown in Figure 14, it is found that the increase in the soaking pressure leads to the gradual increase of the crude oil recovery. Under the miscible pressure, the soaking pressure is 15 MPa, which only increases the crude oil recovery by 4.94% compared with 10 MPa. When the soaking pressure increases by 10 MPa, the crude oil recovery increases by 12.41%. When the gas injection pressure is higher than the miscible pressure, the recovery of crude oil at 35 MPa is significantly increased by 23.74% compared with 25 MPa, and the recovery of crude oil is as high as 91.22%.

3.3.2. Soaking Time

In this experiment, the soaking pressure was 20 MPa, which does not reach the miscible pressure. Thus, the influence of the miscible phase on the recovery degree of crude oil was not considered.

When the miscible pressure is lower than 20 MPa, a large amount of CO₂ dissolves in the crude oil after the well is soaked for 15 min, and the color of the crude oil becomes lighter. The diffusion process of CO₂ molecules is very slow, and the 15 min soak time fails to fully diffuse CO₂ molecules into the oil phase. As shown in Figure 14, the recovery of crude oil only increases by 5.85% compared with 15 MPa and 30 min soaking time. A large amount of clustered and porous residual oil was retained, and a wide range of gas channels were formed through gas fingering and breakthrough. The swept area was small and the remaining oil saturation was high. The soak time was increased to 45 min, as shown in Figure 15. CO₂ was fully diffused into the crude oil. Compared with the conditions of 15 MPa and 30 min, the recovery of crude oil increased by 14.61%. Compared with the conditions of 20 MPa and 15 min, the recovery degree of crude oil increased from 67.22% to 75.98%, with an increase of 8.75%. Therefore, the recovery is positively related with well-soaking time [18].

By comparing the distribution of the remaining oil at different soaking times of 20 MPa, it was found in Figure 16 that, with the increase of soaking time, the remaining oil mainly exists in the shale reservoir in porous, columnar and cluster forms, and the remaining oil content of discontinuous phases such as film and drop is lower. With the increase of pressure, the cluster-like residual oil of the continuous phase has strong mobility, the oil phase is divided, the discontinuous phase increases and the flow of crude oil becomes more and more dispersed, as shown in Figure 17.

3.4. Further Research Plan

For further research, a component test experiment about the contact of CO₂ and crude oil is planned to be carried out to study the CO₂ extraction component. Since the scale of this micro experiment was so small, the oil volume was not satisfactory for the component test. Therefore, core scale CO₂ and crude oil contact experiments are necessary to verify the extraction effect of CO₂ on the oil components. Further, the experimental results show that the distribution of the remaining oil in CO₂ huff and puff is relatively concentrated. This was because it was easy for CO₂ to form gas channeling during the recovery process. The subsequent effective plugging of gas channels can be combined with nano particles to further improve the utilization of CO₂ for remaining oil [27].

4. Conclusions

In this paper, the mechanisms of the oil–gas flow of CO₂ huff and puff in shale reservoirs were studied by microscopic visualization experiments and the effects of different CO₂ huff and puff pressures and different well-soaking times on the degree of crude oil recovery were analyzed. The main conclusions are as follows:

• The mechanisms of CO₂ huff and puff include CO₂ diffusion dissolution, miscibility with crude oil, dissolved gas flooding, extraction, etc. Gas–liquid miscibility can improve the recovery of crude oil. The extraction effect of CO₂ on crude oil is more obvious than that of solution expansion;
• The mobilization mechanisms of crude oil mainly comprise four aspects, and different movability mechanisms correspond to different occurrence of states of crude oil. (1) The residual oil adhered to the matrix transforms from film to drop due to shear force formed by the flow of CO₂ on both sides. The oil peels off the surface of the matrix under the continuous action of the shear force; (2) For the crude oil existing in the corner with poor fluidity, CO₂ is mostly funnel-shaped to extract the light components of crude oil; (3) At pores with good flow capacity, CO₂ and crude oil are miscible after multiple contacts, which decreases the interfacial tension between oil and gas and increases the flow capacity of oil; (4) When the gas content is high, CO₂ contacts with the formation fluid and surrounds the crude oil, causing the crude oil to expand;
• With the increase of pressure, the recovery degree of crude oil gradually increases. When the gas injection pressure is higher than the miscible pressure, the recovery degree of crude oil at 35 MPa is significantly higher than that at 25 MPa by 23.74%, and the recovery degree of crude oil is as high as 91.22%; With the increase of the soaking time, the recovery degree of crude oil increases. Under 20 MPa, CO₂ can fully diffuse into the crude oil with 45 min of soaking time. Compared with 15 min of soaking time at the same pressure, the crude oil recovery increased from 67.22% to 75.98%, an increase of 8.75%;
• With increase of pressure, the oil phase is divided and the remaining oil, in the form of cluster flow, transforms into a discontinuous phase, in the form of porous flow. The flow of crude oil becomes increasingly dispersed and the degree of discontinuity is intensified;
• In this experiment, the minimum pore throat is 10 μm. The results do not cover all type of shale reservoirs because there are shale reservoirs that have pore throats at the nanometer scale. In order to have comprehensive understanding about the mechanism of CO₂, an experiment accurate to the nanometer scale is needed.