Experimental Study of Heavy Oil Upgrading of In-Situ Hydrogen

It was reported that hydrogen production from heavy oil reservoirs was feasible. To study the effect of in-situ hydrogen on heavy oil upgrading, a static experiment of hydrogen upgrading heavy oil under different conditions was designed with nitrogen as the control group. The composition and molar content of the produced gas was analyzed by gas chromatography (GC), the microstructure of coke was analyzed by scanning electron microscope (SEM), and the change of carbon number of oil was analyzed by gas chromatography-mass spectrometry (GC-MS). The results showed that in-situ hydrogen made heavy oil produce more light hydrocarbons. The porous coke produced by heavy oil in a hydrogen atmosphere was favorable for combustion. In addition, under the action of hydrogen, the hydrocarbon in heavy oil will be accelerated to transform from high-carbon number alkanes to low-carbon number alkanes. And the viscosity reduction effect was remarkable, the highest viscosity reduction rate reached 99.79%.


Introduction
With the depletion of conventional oil and gas resources and the increase in world energy demand, heavy oil resources are becoming more and more concerned.As a rich unconventional oil resource, heavy oil accounts for 70% of the world's crude oil reserves [1][2][3][4][5], However, the high viscosity and low fluidity of heavy oil make its development a challenging task [6].Therefore, it is imperative to develop new heavy oil recovery methods.
Recently, the in-situ modification technology of crude oil has been widely applied due to its advantages of high efficiency, low carbon, and energy saving.The methods mainly include a solvent method, in-situ combustion (ISC), and catalysis [7], the principle of which is mainly carried out by increasing the ratio of hydrogen to carbon (H/C) and hydrogenation process, as an inorganic hydrogen donor.Hydrogen plays an important role in heavy oil upgrading.Ng et al [8] developed a new process for refining heavy oil emulsions, which uses in-situ hydrogen produced by the water-gas reaction for insitu upgrading.J. Lam-Maldonado et al [9] showed that the API of heavy oil was increased from 13.1 to 18.3 by hydrocracking under the condition of 372 °C an hour.The crude oil was upgraded, and the residuum produced 8.6% naphthalene, 51.4% middle fraction, and 9.8% VGO.Guillermo Felix et al [10] analyzed the in-situ upgrading kinetic model of heavy oil in a hydrogen atmosphere with liquid or nanocatalyst.The CAPRI downhole heavy oil upgrading experiment proved that hydrogen has obvious upgrading effects on heavy oil [11][12][13].Therefore, more and more scholars pay attention to the hydrocracking and hydrogenation of heavy oil.However, in the present published papers, in-situ hydrogen is mainly used as an auxiliary, with nano-materials and liquids as the main body to improve heavy oil.The effect of in-situ hydrogen will be underestimated, which will limit the development and the application prospects of in-situ hydrogen upgrading technology for heavy oil.
The paper, the crude oil L of Xinjiang oilfield was used as the experimental object, and the static experiment was carried out by a high-temperature and high-pressure reactor to study the actual effect of in-situ hydrogen on heavy oil, the effect of in situ hydrogen on heavy oil was revealed by gas chromatography (GC), Scanning electron microscope chromatography (SEM) and GC-MS.

Materials
The crude oil used in the experiment was from Xinjiang Oilfield located in the northwest region of China.Before each experiment, we dehydrate the crude oil to obtain accurate characteristics.Its basic properties and SARA components (saturate, aromatics, resins, and asphaltenes) are summarized in Table 1.The cylinders used in the experiment are high-pressure compressed nitrogen (purity of 99.99%) and high-pressure compressed hydrogen (purity of 99.99%) from Chengdu Keyuan Gas Co, Ltd (Chengdu, China).

Experimental Set-up
The experimental apparatus consists of a reactor, box furnace, six-way valve, and cylinders gas flow controller as shown in Figure 1.Static experiments were conducted using a reactor with a maximum temperature of 700 ℃ and a maximum pressure of 40 MPa.The volume is 0.16 L. The box furnace with a max temperature of 1200 ℃ was purchased from Tianjin Zhonghuan Box Furnace Co, Ltd (Tianjin, China).The gas flow controller controls the gas flow rate from 0 ml/min to 270 ml/min under standard conditions, purchased from Bronkhorst, Netherlands.
Figure 1 The schematic of experiments

Experimental methods
As shown in Figure 1, the experimental procedure is as follows: (1) Nitrogen was injected through the flowmeter to check the air tightness of the experimental device.(2) 20 ml of oil sample was put into the reactor and hydrogen was injected into the reactor until 3 MPa.(3) The pressurized reactor was taken into the box furnace, and the temperature was set to 300 ℃, 350 ℃, 400 ℃, 450 ℃, and 500 ℃ to carry out the experiments.The reaction time were 8 hours, 12 hours, 20 hours.The gas, oil, and residual solids were collected after the reaction.As shown in Figure 2, these instruments were used to analyze the products.The gas analysis using GC (Agilent 7890 B, Santa Clara, CA, USA), the oil composition analysis using GC-MS (THERMO-FISHER, USA), The coke sample was mashed into powder and processed by sample preparation and gold spraying, and then was analyzed by SEM (Gemini SEM 300, ZEISS, GERMANY).

SEM GC-MS GC
Figure 2 GC, SEM, and GC-MS system

Gas products analysis
The gas from nitrogen cracking experiments of heavy oil determined by GC was listed in Table 2. Table 2 shows that a small amount of hydrogen was produced in heavy oil cracking under a nitrogen environment, and a small amount of light hydrocarbon was produced at 400 °C.When the temperature increases, the content of light hydrocarbon was higher than that under a low-temperature environment.This is because the high-temperature pyrolysis of heavy oil in this stage of coke, C-C bond fracture, as well as a small amount of hydrogen generated also participate in the reaction of heavy oil, resulting in smaller molecular weight of light hydrocarbons, such as CH 4 , C 2 H 4 , C 2 H 6 .At the same time, due to the deposition or polycondensation reaction of heavy components, the components with higher aromatic degrees and high molecular weight are finally converted into coke.Table 3 shows the gas composition data of crude oil hydrocracking upgrading.As it is shown in Table 3, the cracking reaction of crude oil hardly ever undergoes in a hydrogen atmosphere at the temperature of 300 ℃.When the temperature reached 400 °C, the cracking of heavy oil already occurred, this is because the activation energy of asphaltene hydrocracking is 44.027KJ/mol, which was lower than thermal cracking.When the temperature increases, the hydrogen consumption capacity of heavy oil increased obviously.At this time, hydrogen cracking and hydrogenation reactions were carried out simultaneously.Figure 3 illustrated that the amount of light hydrocarbon produced from the reaction of hydrogen with heavy oil at 300-400 °C was lower than that produced at temperatures above 400 °C.Additionally, higher temperatures facilitate the conversion of hydrogen molecules into hydrogen radicals, which is beneficial for heavy oil upgrading.The light hydrocarbon, which is produced by the reaction of hydrogen the chemical bonds such as C-S, C-C, C-O, and C-N, is broken in crude oil, and the side chains of heavy oil components are cracked.The conversion of higher carbon number molecules into lower molecular weight light hydrocarbon products yields large amounts of light hydrocarbons.

Coke analysis
The coke was not formed between 300 °C and 400 °C.At 450 °C, the coke formation of heavy oil in the hydrogen atmosphere was less than that in a nitrogen atmosphere, but when the temperature reached 500 °C, the opposite phenomenon occurred, which is shown in Figure 4.This is due to the peak inhibition mechanism of hydrogen on condensation reaction, and the existence of H + reduced the condensation between free radicals so that the organic matter cracking reaction had less tar pitch formation, thus increasing oil production.As it is shown in Table 3, When the temperature reached 500 °C, H 2 was largely consumed, and crude oil was fully cracked, increasing carbon content.At the same time, the microstructure of coke formed in a hydrogen atmosphere was like that in a nitrogen atmosphere, as shown in Figure 5.Many pores were formed on the surface of coke, and the results of the pores in the coke formed under the action of hydrogen were smaller and more, which can increase the specific surface area of coke, thereby increasing the contact area between coke and oxygen, it is favorable for coke combustion [14].Coke was the main fuel of in-situ combustion, and its properties determine the heat transfer efficiency in the combustion zone.Therefore, the coke formed by the reaction of hydrogen with heavy oil was beneficial to reservoir upgrading.the composition of crude oil was complex and the carbon number distribution is widespread.The temperature also has a significant impact on heavy oil upgrading.At 350 °C, the resulting products are primarily C 15+ or higher.Similarly, with a reaction time of 12 hours, if the temperature increased to 400 °C, the observation of an increase in C 7 -C 13 hydrocarbons was evident, and as the reaction time lengthens, there was a transfer of high-carbon hydrocarbons to low-carbon hydrocarbons.It was also notable that in-situ hydrogen has an important impact on the upgrading of crude oil.
Viscosity was a crucial factor in determining the fluidity of oil, and a lower viscosity leads to better flow and more efficient oil recovery.Figure 7 demonstrates this relationship.To improve oil recovery, we measured the viscosity of oil samples after subjecting them to various reaction conditions.As revealed by our findings, the viscosity of crude oil decreased significantly with increasing temperature and time.When the reaction temperature was set to 400 °C and maintained for 12 hours, the viscosity reduction rate was 99.97%.This indicates that the use of in-situ hydrogen during heavy oil upgrading was highly effective.Furthermore, the results also suggest that in-situ hydrogen can improve heavy oil even at low temperatures.However, it is important to note that the most significant effects are observed at temperatures above 400 °C.

Conclusion
The hydrogen-supplying agent has been found to have a significant impact on heavy oil upgrading.This is due to its ability to supply hydrogen, which reduces viscosity and enhances oil recovery.In this work, we aim to study the effect of hydrogen on heavy oil upgrading through a static oxidation experiment conducted at high temperatures and high pressure.The conclusions are as follows: (1) The presence of in-situ hydrogen can enhance the production of light hydrocarbons in heavy oil compared to nitrogen.However, this effect was negligible when the temperature was below 400 °C.On the other hand, at temperatures above 400 °C, hydrogen can significantly improve the performance of heavy oil.
(2) The hydrocracking and hydrogen cracking reactions of heavy oil share similarities with nitrogen pyrolysis.The coke produced in a hydrogen atmosphere has a comparable structure to that produced in a nitrogen atmosphere.However, the coke produced in a hydrogen atmosphere offers benefits to coke combustion, ultimately leading to increased heat production and improved reservoir quality.In the presence of hydrogen, high temperatures can cause a small amount of crude oil to completely coke.
(3) In this study, it was found that the reaction between hydrogen and crude oil can speed up converting high-carbon alkanes into low-carbon alkanes.Additionally, the presence of in-situ hydrogen has a positive effect on reducing crude oil viscosity.The viscosity reduction rate was observed to be as high as 99.97% when the temperature reached 400 °C and the reaction time reached 20 hours.
(4) The generation of light hydrocarbon, and the decrease of carbon number, and viscosity is mainly due to the change of molecular structure in heavy oil caused by in-situ hydrogen.The degree of change depends on the reaction temperature and the content of in-situ hydrogen reacting with heavy oil.

Figure 4 Figure 5
Figure 4 Coke formation content under different atmospheres

Figure 6 Figure 7
Figure 6 Carbon number distribution