A Study on Generation and Feasibility of Supercritical Multi-Thermal Fluid

Abstract

Supercritical multi-thermal fluid is an emerging and efficient heat carrier for thermal recovery of heavy oil, but the generation of supercritical multi-thermal fluid and its feasibility in thermal recovery are rarely discussed. In this paper, generation and flooding experiments of supercritical multi-thermal fluid were carried out, respectively, for the generation and feasibility of supercritical multi-thermal fluid. During the experiment, the temperature and pressure in the reactor and sand-pack were monitored and recorded, the fluid generated by the reaction was analyzed by chromatography, and enthalpy of the reaction product and displacement efficiency were calculated, respectively. The experimental results showed that the change in temperature and pressure in the reactor could be roughly divided into three stages in the generation process of supercritical multi-thermal fluid. The higher the proportion of oil in the reactant, the higher the maximum temperature in the reactor. When the proportion of oil and water in the reactant was constant, the temperature rise in the reactor was basically the same under different initial temperature and pressure conditions. Compared with the initial temperature and pressure, the oil–water ratio of the reactants had a significant effect on the generated supercritical multi-thermal fluid. The higher the proportion of oil, the more gas that was generated in the supercritical multi-thermal fluid, and the lower the specific enthalpy of the thermal fluid. Under the same proportion of oil and water, the gas–water mass ratio of the supercritical multi-thermal fluid generated by the reaction of crude oil was lower, and the specific enthalpy was higher. Through this study, it was found that supercritical multi-thermal fluid with a low gas–water mass ratio had higher oil displacement efficiency, higher early oil recovery rate, a larger supercritical area formed in the oil layer, and later channeling. The results of this study show that the optimal gas–water mass ratio of supercritical multi-thermal fluid was about 1, under which the oil displacement efficiency and supercritical area in the oil layer reached the maximum. Correspondingly, the optimal proportion of oil in the reactant when generating supercritical multi-component thermal fluid was about 10%. In oilfield applications, because the gas–water ratio in supercritical multi-component thermal fluid has a significant impact on oil displacement efficiency, the optimization of supercritical multi-thermal fluid should not only consider the generation process but also consider the oil displacement effect of the thermal fluid. The findings of this study could improve our understanding of the characteristics of generating supercritical multi-thermal fluid and the feasibility of supercritical multi-thermal fluid generated under different conditions in the oil displacement process. This research is of great significance for field applications of supercritical multi-thermal fluid.

1. Introduction

The world is extremely rich in heavy oil resources, and its geological reserves far exceed conventional crude oil. The thermal recovery method is the most effective method to recover heavy oil at present, mainly including cyclic steam stimulation, steam flooding, SAGD, in-situ combustion, hot water flooding, etc. [1,2,3]. China’s offshore heavy oil resources are mainly distributed in the Bohai Bay, but the utilization degree of these heavy oil resources is less than 20%. This is because the conventional thermal recovery technology of heavy oil has poor adaptability on the sea, which is embodied in four aspects. Firstly, there is serious heat loss during steam injection, which makes the heat insufficient after steam injection into the formation. Secondly, the steam generation process requires a large amount of fresh water. Sea water desalination and oilfield production sewage treatment devices on offshore platforms are bulky, requiring a large amount of platform space and high treatment costs. Thirdly, the steam generation process of offshore platforms is complex; there are many steam generation and injection pipelines and there are prominent pipeline leakage and corrosion problems. Finally, the steam preparation process of offshore platforms is highly dependent on diesel, with high costs and a large amount of carbon dioxide emissions. The whole process is not low-carbon or environmentally friendly. In view of these problems, Zhou et al. [4] combined supercritical water technology with heavy oil thermal recovery technology, and creatively proposed the heavy oil recovery technology of supercritical multi-thermal fluid.

The main components of supercritical multi-thermal fluid are supercritical water, CO₂, and N₂. After the technology was proposed, some scholars carried out several studies on the generation of supercritical multi-thermal fluid. These studies suggested that the generation process of supercritical multi-thermal fluid could be divided into two steps. First, with the strong dissolution and diffusion properties of supercritical water, various organic waste liquids are fully dissolved. Then, they are burned with oxygen-containing gas dissolved in supercritical water to produce supercritical multi-thermal fluid [4,5,6].

As supercritical multi-thermal fluid is produced by the reaction of supercritical water with organic matters such as oil, the research on the reaction of supercritical water with organic matters is also of reference value. These studies focus on the oxidation process of organic matters by supercritical water. According to research results, supercritical water has the effects of solvent and dispersion, hydrogen transfer, and acid catalysis, which can improve the conversion rate of raw materials, reduce the coking amount of the system, and has obvious effects of desulfurization, denitrification, and heavy metal removal [7,8]. In the process of supercritical water oxidation, the organic waste liquid is completely degraded by a rapid oxidation reaction with oxidants such as O₂ in the supercritical water medium, which can realize self-heating and reduce energy consumption [9,10,11,12].

When the concentration of organic matter reaches a certain value, the heat released by the reaction can be used to maintain the heat balance of the process, so as to realize the energy self-compensation reaction and improve the economy of the whole process. Experimental research found that when the oxygen margin was greater than 10%, any substance could be completely separated, and the increase in the amount of oxygen had no effect on the reaction [13]. When the concentration of organic matter in the supercritical water oxidation reaction was greater than 2%, the autothermal reaction could be realized due to the release of heat during the reaction process [14,15,16,17,18]. These studies mainly focused on the supercritical water treatment of wastewater, and research on the supercritical water reaction mechanism and products after the reaction are relatively mature.

Some researchers have shown high interest in the application of supercritical water to heavy oil recovery, and carried out a series of studies. A heavy oil reservoir in northwest China took the lead in carrying out supercritical water huff and puff production tests [19]. On this basis, relevant research has been successively carried out. Through experimental research and theoretical analysis [20,21,22], the process, characteristics, and production of heavy oil recovery by supercritical water huff and puff technology have been discussed. Some studies focused on the process of supercritical water hydrothermal cracking of heavy oil, and discussed its reaction characteristics [23,24]. The research results showed that supercritical water, due to its high reaction characteristics, could split the macromolecular hydrocarbons in heavy oil into small molecules and produce a certain amount of coke. In this technology, research on the mechanism of enhancing heavy oil recovery by supercritical water had gradually emerged [25,26,27,28], and some of them [26] even started to conduct numerical simulations for this process. As supercritical water has already been used in the oil field to recover heavy oil, research on oilfield applications has gradually emerged. These studies mainly focus on the flow parameters and heat and mass transfer laws of supercritical water in the wellbore, and are discussed for vertical wells and horizontal wells, respectively [29,30,31,32,33]. Through research, the flow law and thermal diffusion law of supercritical water in the wellbore have been clarified, and the prediction model of physical parameters of supercritical water in the wellbore has been established.

In the last two years, researchers began to shift their attention to supercritical multi-thermal fluid. Based on the previous research on supercritical water, researchers gradually began to discuss the numerical simulation of supercritical multi-thermal fluid, the mechanism of enhanced oil recovery, and the cracking of heavy oil [34,35,36]. In general, the existing research provides a certain amount of basic knowledge for the research of supercritical multi-thermal fluid technology, and provides references for this research.

However, the existing research has hardly discussed the influence of different reactants and different temperature and pressure conditions on supercritical multi-thermal fluid and its generation process, and has not considered it with the displacement effect. These are the questions that engineers care about in oilfield applications. This paper aimed to address these problems to better apply the supercritical multi-thermal fluid technology to oilfield applications, using generation and flooding experiments of supercritical multi-thermal fluid. The generation and feasibility of supercritical multi-thermal fluid is discussed by analyzing and comparing the generation process, the composition of products, enthalpy value, oil displacement efficiency, and other indicators of supercritical multi-thermal fluid under different conditions. The technical roadmap and organization of this study are shown in Figure 1. The research results of this paper have important reference value and significance in guiding the field applications of supercritical multi-thermal fluid.

2. Materials and Methods

The experiments in this study include supercritical multi-thermal fluid generation and flooding experiments. The experimental workflow is shown in Figure 2. The following will be introduced in accordance with the experimental workflow.

2.1. Material

The diesel used in the experiment was 0# diesel produced by Sinopec (Figure A1), and the crude oil came from the L block in Bohai Bay of China (Figure A2). The viscosity of the crude oil at reservoir temperature (50 °C) was 4372.6 mPa, which belonged to ordinary heavy oil, and its viscosity–temperature relationship curve is shown in Figure 3. The porous medium in the one-dimensional flooding experiment was filled with 80-mesh quartz sand.

2.2. Apparatus and Procedures

2.2.1. Generation Experiment of Supercritical Multi-Thermal Fluid

(1)

Apparatus

Supercritical multi-thermal fluid generation experiments were used to analyze the supercritical multi-thermal fluid generated by different proportions of an oil-water mixture under different conditions and its characteristics. As shown in Figure 4, the experimental apparatus mainly consists of a supercritical multi-thermal fluid generation system and chromatographic analysis system. The supercritical multi-thermal fluid generation system consisted of a high temperature and high pressure reactor (Figure A1), an ISCO pump, a unidirectional valve, and an intermediate container; the chromatographic analysis system consisted of a gas chromatograph and liquid chromatograph.

(2)

Procedures

①

Analyzed diesel/crude oil by liquid chromatography;

②

Checked whether the experimental apparatus was working normally and whether the temperature and pressure sensors were in good condition, and repaired and replaced them in time if there were any problems;

③

Connected each experimental apparatus according to the experimental flow chart;

④

The diesel/crude oil and water were placed in the high temperature and high pressure reactor according to a predetermined ratio, and the reactor was strictly sealed. The high-temperature and high-pressure reactor used in the experiment was made of alloy HC276. The maximum working pressure was 40 MPa and maximum working temperature was 600 °C.

⑤

Opened the inlet and outlet valves, charged nitrogen gas through the inlet with a large flow for more than 10 min to ensure that the air in the high-temperature and high pressure reactor was completely exhausted, and then closed the outlet valve;

⑥

Nitrogen was filled into the high-temperature and high-pressure reactor at the predetermined pressure, then the power supply of the reactor was turned on, and the heating mode was chosen;

⑦

After the temperature and pressure in the reactor reached supercritical condition, the heating was stopped, and the shaking was started. At the same time, oxygen was injected into the reactor until the reaction ended, and the temperature and pressure in the reactor were monitored and recorded in real time during the whole reaction process;

⑧

After the reaction was completed, the reactor was left to cool down, and after the temperature in the reactor dropped to room temperature, the outlet was opened to collect gas samples in the reactor. Then the pressure was released and the reactor was opened to collect the liquid and solids (if any) in the reactor;

⑨

The collected gas–liquid samples were analyzed by the chromatograph.

2.2.2. Evaluation Experiment of Supercritical Multi-Thermal Fluids with Different Compositions

(1)

Apparatus

The one-dimensional flooding experiment of supercritical multi-thermal fluid was used to evaluate and compare the displacement efficiency of supercritical multi-thermal fluid with different compositions in a single sand pack. As shown in Figure 5, the experimental apparatus was mainly composed of three parts: an injection and production system, model system, and monitoring system. Among them, the injection and production system was composed of an ISCO pump, supercritical water generator, gas mass flow controller, back pressure valve, hand pump, and a group of band heaters; the model system included an air bath, sand pack, and several buffer containers; the monitoring system included a computer and five groups of temperature and pressure sensors.

(2)

Procedures

The one-dimensional flooding experiment mainly included the following steps:

①

The experiment was carried out in accordance with the industry standard SY/T 6315-2017 “Determination Method for High-Temperature Relative Permeability and Oil Displacement Efficiency of Heavy Oil Reservoirs”;

②

The sand pack was filled with 80-mesh quartz sand, saturated with water after vacuuming, and porosity of the sand pack was calculated according to the volume of water inflow;

③

The experimental apparatus was connected according to the experimental flow chart, flooding the sand pack with different flow rates of water, recording the pressure difference between the two ends of the sand pack, and then calculating the permeability of the sand pack according to Darcy’s law;

④

Saturating the sand pack with crude oil at a low flow rate, irreducible water saturation was calculated according to the volume of outflowing water, and the sand pack was placed in an air bath to age for more than 24 h;

⑤

After completing model initialization, the experimental running phase began. We injected supercritical steam or multi-thermal fluid into the sand pack according to the predetermined flow, and controlled the back pressure of the outlet to maintain it above 22.5 MPa;

⑥

During the experiment, the produced liquid was collected in real time, and the production time and produced oil and water volume were recorded, which was convenient for calculating the injected PV and degree of recovery. When the water cut was greater than 98%, the experiment was stopped to calculate and determine the oil displacement efficiency of supercritical multi-thermal fluid with different compositions. On this basis, the reasonable composition of supercritical multi-thermal fluid was determined;

⑦

After the experiment, the sand pack was opened and the distribution of remaining oil was observed.

3. Results

3.1. Generation Experiments of Supercritical Multi-Thermal Fluid

3.1.1. Supercritical Water–Diesel Reaction Process under Different Conditions

Ten supercritical water–diesel reaction experiments were carried out under different oil–water ratios or different temperature and pressure conditions. Based on the experimental process, reaction characteristics were analyzed. The experimental parameters are shown in

Table 1. Experimental parameters of supercritical water–diesel reactions to generate supercritical multi-thermal fluid under different temperature and pressure conditions.

No.	Reactant	Initial Temperature and Pressure
1	45 mL water + 5 mL diesel oil	400 °C/23 MPa
2	45 mL water + 5 mL diesel oil	400 °C/25 MPa
3	45 mL water + 5 mL diesel oil	450 °C/25 MPa
4	45 mL water + 5 mL diesel oil	500 °C/25 MPa
5	45 mL water + 5 mL diesel oil	500 °C/23 MPa

and

Table 2. Experimental parameters of supercritical water–diesel reactions to generate supercritical multi-thermal fluid under different oil–water ratios.

No.	Reactant	Initial Temperature and Pressure
1	45 mL water + 5 mL diesel oil (10% diesel oil)	400 °C/25 MPa
2	28.3 mL water + 5 mL diesel oil (15% diesel oil)	400 °C/25 MPa
3	20 mL water + 5 mL diesel oil (20% diesel oil)	400 °C/25 MPa
4	15 mL water + 5 mL diesel oil (25% diesel oil)	400 °C/25 MPa
5	11.67 mL water + 5 mL diesel oil (30% diesel oil)	400 °C/25 MPa

.

According to the experimental steps, the SARA analysis of the 0# diesel oil used was carried out before the experiments. The analysis results are shown in

Table 3. Results of SARA analysis of 0# diesel oil.

Component	Value
Asphaltenes	/
Resins	4.4692%
Aromatics	9.9341%
Saturates	85.5967%
Total	100%

. During the experiment, the temperature and pressure changes in the reactor were recorded. After preheating to the predetermined temperature and pressure, the heating was turned off, and the reaction was started. After the experiment, when the reactor had been cooled to room temperature, the gas in the reactor was collected for gas chromatographic analysis, and the remaining liquid was collected for separation and analysis.

According to the results of the chromatographic analysis of the gas collected after the reaction, under the condition of sufficient oxygen, supercritical water and diesel were completely reacted, and there was no residual organic matter; the remaining gases were carbon dioxide, nitrogen and, a small amount of oxygen, and the remaining liquid was pure water, as shown in the Figure 6.

Judging from the temperature and pressure changes in the reactor during the supercritical water–diesel reaction experiment with different oil–water ratios, as shown in Figure 7 and Figure 8, changes in the temperature and pressure under different conditions were basically the same, and could be roughly divided into three stages. In the first stage, after oxygen was injected into the reactor, the reactants rapidly reacted, and released a large amount of heat, so that the temperature in the reactor sharply rose, and the pressure in the reactor also obviously increased. In the second stage, with the progression of the reaction, the temperature in the reactor rapidly dropped after reaching the highest point, indicating that the reactants were decreasing at this time, heat release of the reaction was reduced, and heat released by the reactants per unit time was less than heat dissipation of the reactor per unit time. At this stage, the pressure in the reactor significantly fluctuated. In the third stage, the reactants were completely consumed. The reaction was completely finished, no heat was released, the temperature in the reactor dropped to slightly higher than the initial temperature, and the temperature tended to remain unchanged. The internal temperature tended to be stable, and the volume of the mixing system in the reactor tended to be stable as well, so that the pressure in the reactor was kept constant.

Combined with the results of chromatographic analysis, it was recognized that, under the condition of sufficient oxygen, diesel oil could completely react in each experiment to generate supercritical multi-thermal fluid. However, due to the different amounts of nitrogen that was injected to achieve the same initial reaction pressure under the conditions of different oil–water ratios, the gas composition of the supercritical multi-thermal fluid generated in each experiment was slightly different. This was mainly because the lower the water ratio, the more nitrogen was injected, and the more nitrogen present in the supercritical multi-thermal fluid generated by the reaction.

When the experimental processes of different oil–water ratios were compared, it was apparent that the higher the diesel ratio, the higher the maximum temperature that could be achieved in the reactor. On the contrary, when the proportion of diesel oil was low, the amount of water was relatively higher. As the heat capacity of water is large and the amount of diesel oil was fixed, the heat released by the reaction was fixed as well, and the system needed more heat to raise a certain temperature. Thus, the maximum temperature in the reactor was lower. At the same time, the higher the proportion of supercritical water in the product, the greater the heat capacity, the less condensation during the cooling process, and the longer the pressure could be maintained.

Comparing the experimental process of different initial temperature and pressure conditions, Figure 9 and Figure 10 show that the change laws of temperature and pressure were basically consistent with the experiments of different oil–water ratios, and could also be roughly divided into three stages. However, in experiments with different initial temperature and pressure conditions, the temperature rise in each experiment was very similar. Considering that the proportion of oil and water in each experiment, total amount of reactants, and the composition and total amount of the products were basically the same, it could be inferred that the reaction heat would be similar. Compared to experiments with different oil–water ratios, the pressure change curves in the reactor were very close in each experiment with different initial temperature and pressure conditions, and the final stable values were basically the same. Thus, the initial temperature and pressure conditions had little effect on the reaction.

3.1.2. Supercritical Water–Crude Oil Reaction Process under Different Conditions

Ten experiments of supercritical water–crude oil reactions with different oil–water ratios and different temperature and pressure conditions were conducted, and the reaction characteristics were analyzed on the basis of the experimental process. The experimental method and data analysis were the same as the previous supercritical water–diesel reaction experiments. The parameters determined for the experiment are shown in

Table 4. Experimental parameters of supercritical water–crude oil reaction to generate supercritical multi-thermal fluids under different temperature and pressure conditions.

No.	Reactant	Initial Temperature and Pressure
1	45 mL water + 5 mL crude oil	400 °C/23 MPa
2	45 mL water + 5 mL crude oil	400 °C/25 MPa
3	45 mL water + 5 mL crude oil	450 °C/25 MPa
4	45 mL water + 5 mL crude oil	500 °C/25 MPa
5	45 mL water + 5 mL crude oil	500 °C/23 MPa

and

Table 5. Experimental parameters of supercritical water–crude oil reaction to generate supercritical multi-thermal fluid under different oil–water ratios.

No.	Reactant	Initial Temperature and Pressure
1	45 mL water + 5 mL crude oil(10% crude oil)	400 °C/25 MPa
2	28.3 mL water + 5 mL crude oil(15% crude oil)	400 °C/25 MPa
3	20 mL water + 5 mL crude oil(20% crude oil)	400 °C/25 MPa
4	15 mL water + 5 mL crude oil(25% crude oil)	400 °C/25 MPa
5	11.67 mL water + 5 mL crude oil(30% crude oil)	400 °C/25 MPa

.

SARA analysis of crude oil was conducted before the experiment (

Table 6. Results of SARA analysis of crude oil.

Component	Value
Asphaltenes	25.9861%
Resins	8.1372%
Aromatics	19.6532%
Saturates	46.2236%
Total	100%

), and the experimental apparatus and procedures were the same as above.

Similar to the supercritical water–diesel reaction experiments, under the condition of sufficient oxygen, the supercritical water–crude oil completely reacted, with no organic matter remaining, remaining gases including carbon dioxide, nitrogen, a small amount of oxygen, a small amount of sulfur dioxide, and other gases (Figure 11), and the remaining liquid being pure water (Figure A6). In other words, the reaction of supercritical water and crude oil could also generate supercritical multi-thermal fluid.

By comparing the experimental process of different oil–water ratios, the water–crude oil reaction process was found to be similar to the water–diesel oil reaction process, and the temperature and pressure changes in the reactor (Figure 12 and Figure 13) could also be divided into three stages: ① The temperature and pressure rapidly rose; ② The temperature sharply dropped after reaching the maximum value, and the pressure obviously fluctuated; ③ The temperature gradually decreased and tended to be stable; the pressure tended to be stable. At the same time, the higher the proportion of crude oil in the crude oil reaction process, the higher the maximum temperature that could be achieved in the reactor. On the contrary, the calorific value of crude oil was lower than that of diesel oil, the temperature rise in the reactor was lower than that of diesel oil, and there was less heat release in the water–crude oil reaction.

The experimental process of different initial temperature and pressure conditions indicated that the water–crude oil reaction process was similar to that of the water–diesel reaction, and temperature increases in the reactor were also very close, as shown in Figure 14 and Figure 15. Considering that the proportion of oil and water in each experiment was kept constant, the total amount of reactants was the same, and the composition and total amount of the products were basically the same, it could also be inferred that the reaction heat would be similar, and that initial temperature and pressure conditions had relatively little influence on the reaction.

3.1.3. Comparison of Supercritical Multi-Thermal Fluid Generated under Different Conditions

Component Composition

As mentioned above, after carrying out supercritical water–oil (diesel oil or crude oil) reaction experiments under different oil–water ratios and different temperature and pressure conditions, the reactor was cooled to room temperature. We then collected gas samples in the reactor and conducted chromatographic analysis, to determine the composition of the supercritical multi-thermal fluid generated by the reaction.

The chromatographic analysis results (

Table 7. Gas chromatographic analysis results after supercritical water–diesel reaction.

No.	Type	Nitrogen,%	Carbon Dioxide, %	Oxygen, %
1	10% diesel	66.52	11.52	21.96%
2	15% diesel	66.70	10.43	22.87%
3	20% diesel	63.53	10.47	26.00%
4	25% diesel	64.90	8.84	26.26%
5	30% diesel	67.33	8.59	24.08%
6	400 °C/ 23 MPa	66.52	11.52	21.96
7	400 °C/ 25 MPa	65.31	10.48	24.21
8	450 °C/ 25 MPa	65.77	11.03	23.20
9	500 °C/ 25 MPa	69.42	12.07	18.51
10	500 °C/ 23 MPa	66.70	13.01	20.29

and

Table 8. Gas chromatographic analysis results after supercritical water–crude oil reaction.

No.	Type	Nitrogen,%	Carbon Dioxide, %	Oxygen, %	Sulfur Dioxide, %	Nitrogen Dioxide, %
1	10% crude oil	63.79	12.40	23.43	0.12	0.27
2	15% crude oil	61.68	11.03	26.81	0.15	0.33
3	20% crude oil	66.57	11.50	21.66	0.08	0.19
4	25% crude oil	63.06	8.04	28.32	0.18	0.40
5	30% crude oil	64.19	8.43	26.86	0.16	0.36
6	400 °C/ 23 MPa	63.79	12.40	23.43	0.12	0.27
7	400 °C/ 25 MPa	66.21	11.74	21.81	0.07	0.17
8	450 °C/ 25 MPa	63.10	11.95	24.37	0.18	0.40
9	500 °C/ 25 MPa	64.19	12.85	22.47	0.15	0.34
10	500 °C/ 23 MPa	62.60	13.84	22.76	0.24	0.55

) showed that the main components of gas products (at room temperature) after the reaction of diesel oil and heavy oil were similar, that is, they mainly comprised carbon dioxide, nitrogen, and oxygen not consumed by the reaction. However, compared with the reaction products of diesel oil, the reaction products of heavy oil contained more impure gases, such as sulfur dioxide and nitrogen dioxide. This was because, compared with diesel oil, crude oil contains asphaltene, which contains non-hydrocarbon substances with heteroatoms such as sulfur and nitrogen, converting into sulfur dioxide and nitrogen dioxide after the full reaction under conditions of sufficient oxygen.

When the reaction products under different initial temperature and pressure conditions were compared, the composition of reaction products were basically unchanged within the temperature and pressure range given in the experiment, whether the reactant was heavy oil or diesel oil. In other words, within the temperature and pressure range of these experiments, the influence of temperature and pressure on the reaction products could be ignored.

In contrast, the proportion of oil and water in the reactant had a more obvious effect on chromatography results. The total amount of oil was controlled to remain unchanged in the experiment, and the oil–water ratio was adjusted by changing the amount of water. The volume of the reactor could not be changed. In order to reach the same initial pressure, we varied the amount of nitrogen charged. The higher the proportion of oil, the less the amount of water, and the smaller the total volume of oil–water; meanwhile, to reach the same initial pressure as the other oil–water ratio experiments, the more nitrogen was charged. Therefore, the chromatogram of the gas collected after the reaction indicated that, with an increasing proportion of oil, the proportion of carbon dioxide decreased and the proportion of nitrogen increased. Due to the amount of oil involved in the reaction remaining constant, the amount of oxygen charged was roughly the same; thus, the proportion of oxygen in the chromatographic results was basically the same.

21)

Enthalpy of Supercritical Multi-Thermal Fluids

Generally speaking, the higher the enthalpy of thermal fluid, the higher the heat it carried, which was more conducive to the recovery of heavy oil. Therefore, the enthalpy of the thermal fluid generated by the reaction was a very noteworthy index. It should be pointed out that, due to the experimental methods and processes used in this study, the proportion of nitrogen and oxygen in the reactor of the experiment was not the same as the proportion of nitrogen and oxygen in the air. To make the enthalpy value of supercritical multi-thermal fluid determined by calculation have universal significance and comparability, we corrected the amount of nitrogen in the reactor based on the chromatographic analysis results of the produced fluid, combined with the nitrogen–oxygen ratio in the air. At the same time, without considering the remaining oxygen in the reaction, the enthalpy h of the supercritical multi-thermal fluid produced in different reactions could be calculated. The calculation formula was as follows:

$$h=\frac{w_i}{{\displaystyle \sum _{i=1}^n}{w}_i}{h}_i$$

Based on the specific enthalpy of thermal fluids, as shown in

Table 9. Enthalpy of supercritical multi-thermal fluids generated by supercritical water–diesel reaction under different oil–water ratios.

No.	Type	Nitrogen Quantity, g	CO2 Quantity, g	Total Water Quality, g	Total Mass of Fluid, g	Total Enthalpy of Thermal Fluids, J (400 °C, 25 MPa)	Specific Enthalpy of Thermal Fluids, J/g (400 °C, 25 MPa)
1	10% diesel	55.821	16.446	49.928	122.195	217,958.660	1783.70
2	15% diesel	53.435	15.743	33.227	102.405	163,223.870	1593.91
3	20% diesel	52.437	15.499	24.868	92.804	136,095.959	1466.49
4	25% diesel	55.981	16.493	19.931	92.405	124,608.843	1348.51
5	30% diesel	53.768	15.841	16.402	86.011	111,136.352	1292.12

and

Table 10. Enthalpy of supercritical multi-thermal fluids generated by supercritical water–diesel reaction under different initial temperature and pressure conditions.

No.	Type	Nitrogen Quantity, g	CO2 Quantity, g	Total Water Quality, g	Total Mass of Fluid, g	Total Enthalpy of Thermal Fluids, J (400 °C, 25 MPa)	Specific Enthalpy of Thermal Fluids, J/g (400 °C, 25 MPa)
1	400 °C/ 23 MPa	54.749	16.130	49.914	120.793	181,454.782	1502.20
2	400 °C/ 25 MPa	55.126	16.241	49.919	121.286	181,830.813	1499.19
3	450 °C/ 25 MPa	54.219	15.974	49.908	120.101	180,928.838	1506.47
4	500 °C/ 25 MPa	53.737	15.832	49.901	119.470	180,446.440	1510.39
5	500 °C/ 23 MPa	52.726	15.534	49.888	118.148	179,438.812	1518.76

, the lower the proportion of diesel oil and the higher the proportion of water, the higher the specific enthalpy of thermal fluids, because the thermal fluids were mainly carried by supercritical water, and the higher the proportion of supercritical water, the higher the enthalpy of supercritical multi-thermal fluid. According to the previous method, the enthalpy of supercritical multi-thermal fluid generated by the reaction in experiments under different initial temperature and pressure conditions was calculated. As mentioned above, the initial temperature and pressure basically had no effect on the composition of reaction products, so the total enthalpy and specific enthalpy of thermal fluid generated under different initial temperature and pressure conditions were very similar after conversion to the same temperature and pressure (400 °C, 25 MPa), and the average values were 180,819.937 J and 1507.402 J/g, respectively.

As with the previous method, the enthalpy of supercritical multi-thermal fluid produced by the reaction of supercritical water–crude oil was calculated on the basis of the composition of the produced fluid, the results are shown in

Table 11. Enthalpy of supercritical multi-thermal fluid generated by supercritical water-crude oil reaction under different oil-water ratios.

No.	Type	Nitrogen Quantity, g	CO2 Quantity, g	SO2 Quantity, g	NO2 Quantity, g	Total Water Quality, g	Total Mass of Fluids, g	Total enthalpy of Thermal Fluids, J (400 °C, 25 MPa)	Specific Enthalpy of Thermal Fluids, J/g (400 °C, 25 MPa)
1	10% crude oil	42.856	12.673	0.312	0.512	48.888	105.241	167,995.845	1596.30
2	15% crude oil	43.188	12.879	0.248	0.408	32.282	89.005	125,462.946	1409.62
3	20% crude oil	43.777	13.244	0.137	0.224	24.064	81.446	104,784.173	1286.55
4	25% crude oil	42.410	12.396	0.397	0.652	18.803	74.658	90,029.995	1205.90
5	30% crude oil	42.695	12.573	0.342	0.562	15.358	71.530	81,393.320	1137.89

and

Table 12. Enthalpy of supercritical multi-thermal fluid generated by supercritical water–crude oil reaction under different initial temperature and pressure conditions.

No.	Type	Nitrogen Quantity, g	CO2 Quantity, g	SO2 Quantity, g	NO2 Quantity, g	Total Water Quality, g	Total Mass of Fluid, g	Total Enthalpy of Thermal Fluids, J (400 °C, 25 MPa)	Specific Enthalpy of Thermal Fluids, J/g (400 °C, 25 MPa)
1	400 °C 23 MPa	43.544	13.100	0.181	0.297	49.019	106.141	168,931.956	1591.58
2	400 °C 25 MPa	43.868	13.301	0.119	0.196	49.081	106.565	169,373.536	1589.39
3	450 °C 25 MPa	43.050	12.793	0.275	0.452	48.925	105.495	168,260.094	1594.96
4	500 °C 25 MPa	43.338	12.972	0.220	0.361	48.980	105.871	168,651.679	1592.99
5	500 °C 23 MPa	42.810	12.644	0.321	0.527	48.879	105.181	167,932.908	1596.61

. Based on the specific enthalpy of thermal fluid, the law was also similar to that of the reaction of supercritical water–crude oil; thus, the higher the proportion of water, the higher the proportion of supercritical water in the product, and the higher the specific enthalpy of thermal fluid.

According to the above method, the enthalpy of supercritical multi-thermal fluid generated by the reaction was calculated in experiments under different initial temperature and pressure conditions. Similarly, when converted to the same temperature and pressure (400 °C, 25 MPa), the total enthalpy and specific enthalpy of thermal fluid generated under different initial temperature and pressure conditions were very close, with average values of 168,630.035 J and 1593.106 J/g, respectively.

The above four tables indicate that the total enthalpy of supercritical multi-thermal fluid generated in the reaction experiment of diesel and supercritical water was higher than that of supercritical multi-thermal fluid generated in the reaction of heavy oil, but the relationship between the specific enthalpy of the two was just the opposite. Taking the reaction experiment of 10% diesel oil and 10% heavy oil as an example, both reactions started at 400 °C and 25 MPa. It was evident from the experimental results that although the volume of oil was 5 mL, more carbon dioxide and water were generated after the reaction of diesel, and the difference in the carbon dioxide generated was greater than the difference in the water generated. According to further calculations, as shown in

Table 13. Gas–water mass ratio of supercritical multi-thermal fluid generated by supercritical water–diesel reaction under different conditions.

No.	Type	Total Gas Mass, g	Water Mass, g	Gas–Water Ratio
1	10% diesel	72.267	49.928	1.3
2	15% diesel	69.178	33.227	1.8
3	20% diesel	67.936	24.868	2.4
4	25% diesel	72.474	19.931	3.2
5	30% diesel	69.609	16.402	3.7
6	400 °C/ 23 MPa	71.4	49.919	1.4
7	400 °C/ 25 MPa	71.3	49.919	1.4
8	450 °C/ 25 MPa	67.9	49.884	1.4
9	500 °C/ 25 MPa	69.1	49.897	1.4
10	500 °C/ 23 MPa	71.4	49.922	1.4

and

Table 14. Gas–water mass ratio of supercritical multi-thermal fluid generated by supercritical water crude oil reaction under different conditions.

No.	Type	Total Gas Mass, g	Water Mass, g	Gas–Water Ratio
1	10% crude oil	56.353	48.888	1.2
2	15% crude oil	56.723	32.282	1.8
3	20% crude oil	57.382	24.064	2.4
4	25% crude oil	55.855	18.803	3.0
5	30% crude oil	56.172	15.358	3.7
6	400 °C/ 23 MPa	57.122	49.019	1.2
7	400 °C/ 25 MPa	57.484	49.081	1.2
8	450 °C/ 25 MPa	56.570	48.925	1.2
9	500 °C/ 25 MPa	56.891	48.980	1.2
10	500 °C/ 23 MPa	56.302	48.879	1.2

, the density of diesel oil was lower, and the mass of 5 mL diesel oil was slightly less than 5 mL heavy oil, but more than 95% of diesel oil was made up of hydrocarbons, and the content of saturated hydrocarbons accounted for 90% of all hydrocarbons, whereas the hydrocarbon content of crude oil used in the experiment was less than 70%, and the content of saturated hydrocarbons was about half of the saturated hydrocarbons in diesel oil. Compared with non-hydrocarbon substances, hydrocarbon substances consumed more oxygen, which made diesel oil consume more oxygen in the reaction process, and correspondingly generated more carbon dioxide and water. As oxygen came from the air, the amount of nitrogen in the thermal fluids would also increase, so that the gas mass, total mass, and total enthalpy of the thermal fluids generated by the 5 mL diesel reaction were greater. On the contrary, crude oil had low oxygen consumption, and correspondingly less nitrogen in thermal fluids and less gas mass in the thermal fluid generated by the reaction of 5 mL of crude oil. Thus, crude oil reactions had a lower total mass of thermal fluids, and lower total enthalpy. Due to the fact that thermal fluid mainly relies on supercritical water to carry heat, and the heat capacity of the gas is far lower than that of water, the higher proportion of water in the supercritical multi-thermal fluid generated by the crude oil reaction increased the specific enthalpy of the supercritical multi-thermal fluid generated by the crude oil reaction than that of the supercritical multi-thermal fluid generated by the diesel oil reaction. This characteristic not only appeared in the reaction experiment of 10% diesel oil and 10% heavy oil, but in the other experiments.

3.2. Evaluation Experiment of Supercritical Multi-Thermal Fluid Flooding

The previous analysis of the supercritical multi-thermal fluid generation experiment showed that the gas–water mass ratio in the supercritical multi-thermal fluid generated by the reaction of different oil–water ratios was different. Existing field practice has confirmed that the more gas there is, the better the process of heavy oil. Therefore, it is necessary to further optimize the proportion of non-condensable gas and supercritical water in the thermal fluid injected underground, and determine the appropriate gas–water ratio that can better use supercritical multi-thermal fluid for heavy oil recovery.

Combined with the composition of supercritical multi-thermal fluid generated in the previous experiment and the mass ratio of gas–water, seven experiments with the mass ratio of gas–water in a range of 1–4 were carried out. The experimental parameters are shown in

Table 15. One-dimensional experimental parameters.

No.	Porosity (%)	Permeability (D)	Oil Saturation (%)	Gas–Water Mass Ratio
1	36.8	3.95	86.2	1
2	38.4	4.59	84.3	1.5
3	37.3	4.24	85.1	2
4	36.7	4.45	87.2	2.5
5	36.9	4.29	83.7	3
6	35.4	3.91	84.2	3.5
7	36.3	4.19	86.3	4

.

During the experiment, with an increase in the gas–water mass ratio, the supercritical area was found to be narrowed (Figure 16), and the existence of non-condensable gas made gas channeling appear in the experiment, which then led to steam channeling. The higher the gas–water mass ratio, the earlier and more serious this phenomenon appeared. At the same time, the emulsification of the produced liquid was serious when the supercritical multi-thermal fluid flooding with a low gas–water mass ratio was used. With the increase of the gas–water mass ratio, the liquid emulsification of the products decreased. After opening the model and observing the oil sand, it could be seen that the further the quartz sand was from the injection end after flooding, the smaller the sweep multiple and the darker the color (Figure A7).

Figure 17 and Figure 18 demonstrate that in the early stage of flooding, the supercritical multi-thermal fluid with different gas–water mass ratios could achieve higher oil recovery rates, and with the progress of flooding, the oil recovery rate gradually slowed down. Moreover, the lower the gas–water mass ratio, the higher the oil recovery rate in the early stage, and overall oil recovery rate was also higher than that of the supercritical multi-thermal fluid with a high gas–water mass ratio. With an increase in non-condensable gas in the supercritical multi-thermal fluid, the oil displacement efficiency decreased. When the water content reached 98%, the oil displacement efficiency was 90.72, 88.8, 86.3, 84.2, 82.71, 80.30, and 71.8%. Therefore, from the perspective of oil displacement efficiency, in these five experiments, the gas–water mass ratio in the supercritical multi-thermal fluid was optimal when it was about 1. In other words, when the proportion of oil was about 10%, the supercritical multi-thermal fluid produced by the reaction had the best oil displacement effect.

4. Discussion

In this study, the generation and application of supercritical multi-thermal fluid were considered as a whole, and then the optimization direction and factors to be considered for supercritical multi-thermal fluid were clarified. Through this study, we clarified the influence of reactants and temperature and pressure conditions during the reaction on supercritical multi-thermal fluid and its generation process. We also clarified the sweep efficiency of supercritical multi-thermal fluid generated under different conditions, and the optimization of supercritical multi-thermal fluid was expounded.

Our study demonstrated that the proportion of oil and water in the generation process of supercritical multi-thermal fluid not only had a direct and obvious impact on the generated thermal fluid, but also indirectly affected the utility of the thermal fluid. In oilfield applications, the above aspects should be considered for the optimization of supercritical multi-thermal fluid.

On the one hand, whether diesel oil or heavy oil was used to react to generate supercritical multi-thermal fluid, an increase in the proportion of oil led to an increase in the proportion of gas in the generated supercritical multi-thermal fluid, that is, the gas–water mass ratio increased. This increase in the proportion of oil brings about an increase in the cost of supercritical multi-thermal fluid, whereas the increase in the gas–water mass ratio has no positive impact on the effect of supercritical multi-thermal fluid. Both previous practical experience and the experimental results of this study confirmed that when the gas–water mass ratio was relatively low, the thermal fluid recovery effect was better than when the gas–water mass ratio was high.

On the other hand, a low proportion of oil does not always result in the most optimal supercritical multi-thermal fluid generation. In fact, the goal of the supercritical multi-thermal fluid generation process is to create a suitable initial reaction condition to promote the reaction of supercritical water and oil under conditions of sufficient oxygen to generate supercritical multi-thermal fluid. The reaction process energy is no longer provided from the outside world, and the heat of reaction requires not only maintaining the high enthalpy value of the supercritical multi-component thermal fluid, but also heating the subsequent incoming reactants to continue the reaction. Thus, the proportion of oil must not be reduced too much in pursuit of a low gas–water mass ratio. When the gas–water mass ratio of thermal fluid is too low, it will become a single supercritical water, which is no longer a supercritical multi-thermal fluid.

The universal oil–water ratio really depends on whether diesel oil, crude oil, or a mixture of the two is used. At the same time, it also depends on the thermal efficiency of the supercritical multi-thermal fluid generator itself. In this study, this issue was not discussed, which is a limitation of this study. This issue should be further explored in future studies. Another limitation of the current study was that some of the experimental research and analysis appeared relatively rough, due to limits in experimental conditions, which is a disadvantage of this study that should be addressed in future research.

5. Summary and Conclusions

In order to study the generation and feasibility of supercritical multi-thermal fluid, generation and flooding experiments of supercritical multi-thermal fluid were carried out, respectively. From the analysis and discussion of the experimental results, the following conclusions were reached:

(1)

In the process of supercritical multi-thermal fluid generation, the higher the proportion of oil in the reactant, the higher the maximum temperature that could be reached in the reactor. When the proportion of oil and water in the reactant was certain, the temperature rise in the reactor was basically the same under different initial reaction temperature conditions. The optimal proportion of oil and water in the reactant should be determined according to the specific oil and supercritical multi-thermal fluid generator.

(2)

Within the scope of this study, the effect of the reaction initiation temperature on the generation of supercritical multi-thermal fluid could be neglected. In contrast, the proportion of oil and water in the reactant had a significant impact on the supercritical multi-thermal fluid generated. The higher the proportion of oil, the more gas in the generated supercritical multi-thermal fluid, the higher the mass ratio of gas–water, and the lower the specific enthalpy of the thermal fluid. Compared with diesel oil, the gas–water mass ratio of the supercritical multi-thermal fluid produced by the reaction of crude oil was lower and the specific enthalpy was higher under the same oil–water ratio.

(3)

By carrying out the supercritical multi-thermal fluid flooding experiment, it was confirmed that the supercritical multi-thermal fluid with a low gas–water mass ratio had higher oil displacement efficiency, higher oil recovery rate in the early stage, a larger supercritical area formed in the oil layer, and later channeling. In this study, when the gas–water mass ratio in supercritical multi-thermal fluid was about 1, the oil displacement efficiency of supercritical multi-thermal fluid was the highest, reaching 90.72%, and the corresponding oil–water ratio of reactants during the generation of supercritical multi-thermal fluid was about 10%.

(4)

The proportion of oil and water in the generation process of supercritical multi-thermal fluid directly affected the composition and specific enthalpy of the generated thermal fluid, which then affected the application effect of thermal fluid. In oilfield applications, the above factors should be taken into account for the optimization of the generation process of supercritical multi-thermal fluid.