Abstract

How to significantly improve the recovery rate of medium and deep heavy oil and achieve cost reduction and efficiency increase has always been the bottleneck and key direction of heavy oil reservoir development. The chemical viscosity reduction assisted thermal recovery technology has great potential for application in the development of medium to deep heavy oil reservoirs due to its good viscosity reduction effect and high economic benefits. For this purpose, a comprehensive review was conducted on the existing chemical viscosity reduction technologies applied in medium to deep heavy oil reservoirs, and the basic principles, main characteristics, applicability, and limitations of four processes, namely emulsification viscosity reduction, oil soluble viscosity reduction agent viscosity reduction, hydrothermal catalytic cracking viscosity reduction, and nanomaterial viscosity reduction, were elaborated. The development direction of future viscosity reduction technologies was also proposed. 

The research results indicate that: ①.Emulsification viscosity reduction have the characteristics of low cost, good viscosity reduction effect, simple process, and fast effectiveness. However, the universality of emulsifiers for different heavy oils and their own temperature and salt resistance still need to be further improved; ②.Oil soluble viscosity reducer has low energy consumption and can fully contact heavy oil, but its viscosity reduction effect is limited and its application cost is high. Further in-depth research is needed on the viscosity reduction mechanism and other aspects; ③.The hydrothermal catalytic cracking viscosity reduction technology has great potential. Developing efficient, low-cost, highly active, highly selective, and widely used catalysts to adapt to different heavy oils is the right path, and ultra dispersed nano catalysts are the future research focus; ④.Nanoparticles have great potential as adsorbents and catalysts in medium to deep heavy oil recovery, but their practical applications are still not mature enough.

The conclusion is that by reviewing the current development status and research progress of chemical viscosity reduction technology, it is clear that the research and development of viscosity reduction technology suitable for medium to deep heavy oil extraction is of great practical significance for ensuring oil safety in China and improving the recovery rate of medium to deep heavy oil in China.

1. Research Background

Energy is an important support for human society and economic development. In the current energy system, oil occupies a crucial position and a significant proportion in the global energy consumption structure. As the world's second-largest economy, China's oil demand has been increasing year by year with the continuous growth of the economy. However, China's oil production is significantly lower than other major oil producing countries, and its dependence on foreign oil is still relatively high. According to statistics, the total crude oil production in China in 2020 was 1.95×10⁸ tons, imported 5.42×10⁸ tons, with an external dependence rate of 73.5%. Figure 1 shows the oil production of the world's major oil producing countries in 2020. China's oil production is significantly lower than that of other major oil producing countries, mainly due to limited oil reserves, with a relatively large proportion of low-quality reserves, namely heavy oil reserves. According to data released by the Ministry of Land and Resources in 2015, China's heavy oil resources account for approximately 35% of the total proven oil reserves. According to China's classification standards, heavy oil refers to crude oil with a viscosity greater than 50 mPa·s under reservoir conditions and a relative density greater than 0.92 at 20℃, as shown in Table 1. Due to the high density and viscosity of heavy oil, the difficulty of extraction technology, and the low average daily production per well, improving the recovery rate of heavy oil is an important research direction in the field of oil and gas field development, which is of great significance for ensuring China's oil security and achieving energy transformation and upgrading.

The extraction technology of heavy oil mainly includes two categories: cold recovery and hot recovery technology. Cold recovery technology refers to the use of non heating methods to increase the flow rate of heavy oil and achieve heavy oil recovery, mainly including water flooding, chemical flooding, carbon dioxide flooding, microbial flooding and other technologies. Cold recovery technology is mainly suitable for ordinary heavy oil with a viscosity between 50~100 mPa·s under reservoir conditions. Among the proven heavy oil reservoirs in our country, more than 80% of them are buried at depths greater than 800 meters, of which about half are buried between 1300~1700 meters. They belong to medium to deep heavy oil reservoirs, with higher viscosity and density of crude oil. Compared to shallow heavy oil reservoirs, the development of medium to deep heavy oil reservoirs is more difficult. Generally, thermal recovery technology is used to reduce the viscosity of heavy oil and improve the flow rate of heavy oil through heating. This mainly includes steam stimulation (CSS), steam flooding, steam assisted gravity drainage (SAGD), hot water flooding, and fire flooding technologies. However, both hot water flooding and commonly used steam development methods suffer from high energy consumption, low recovery rates, and multiple reservoir limitations. Therefore, currently, chemical viscosity reduction assisted thermal recovery technology is mostly used to overcome the shortcomings of single thermal recovery, improve the recovery rate of medium and deep heavy oil reservoirs, and enhance economic benefits. Based on the mechanism of viscosity reduction in heavy oil, the author of this article summarizes the current development status and research progress of chemical viscosity reduction technology in heavy oil extraction from four aspects: emulsification viscosity reduction, oil soluble viscosity reduction agent viscosity reduction, hydrothermal catalytic cracking viscosity reduction, and nanomaterial viscosity reduction. The shortcomings and future development trends of these technologies are also discussed, providing reference for the work of medium and deep heavy oil extraction in China.

2. Composition and Viscosity Mechanism of Heavy Oil

Heavy oil is a complex product composed of light components such as saturated hydrocarbons and aromatic hydrocarbons, as well as heavy components such as resins and asphaltenes in a certain proportion. The reason why heavy oil is "thick" is due to its high proportion of gum and asphaltene. There are many studies and speculations in the academic community regarding the structure and viscosity mechanism of gum and asphaltene, and it is generally believed that asphaltene is the main influencing factor. The monomer molecules of asphaltene exhibit strong polarity and contain a large number of aromatic heterocyclic structures. The aromatic heterocyclic structures are stacked through π-π conjugation, which can be further fixed by hydrogen bonding of heteroatoms such as oxygen, nitrogen, and sulfur in asphaltene or chelation of metal heteroatoms such as Ni and V in heavy oil, forming asphaltene supramolecular structures. Glial molecules with multiple polar groups are coated around asphaltene aggregates to form a coating layer, and the asphaltene particles coated by the resin also have strong polarity. Through hydrogen bonding, they further pile up with each other, ultimately forming macromolecular aggregates, resulting in high viscosity of heavy oil. Glial molecules with multiple polar groups are coated around asphaltene aggregates to form a coating layer, and the asphaltene particles coated by the resin also have strong polarity. Through hydrogen bonding, they further pile up with each other, ultimately forming macromolecular aggregates, resulting in high viscosity of heavy oil. Figure 2 is a supramolecular assembly structure of asphaltene aggregates proposed by Gray et al., which suggests that asphaltene aggregates are formed by hydrogen bonding, metal chelation, van der Waals forces, π-π interactions, and acid-base interactions.

In addition, heavy oil can easily form stable W/O type heavy oil emulsions with a small amount of water, leading to an increase in viscosity. In the petroleum industry, processing this stable W/O type heavy oil emulsion is quite difficult and challenging. The presence of asphaltene is beneficial for the stability of this W/O type emulsion, because asphaltene molecules themselves are also natural surfactants, and their polar functional groups such as metal heteroatoms have strong hydrophilicity, while the aromatic ring structure and hydrocarbon chains in the molecules have hydrophobicity. This amphiphilic nature of asphaltene molecules enables them to adsorb at the oil-water interface, forming an interface facial mask with certain mechanical strength, thus preventing water droplets from coalescence and enhancing the stability of W/O emulsion. Liu et al. used molecular simulation to study the interfacial behavior of asphaltene molecules at the toluene/water interface. It was found that asphaltene molecules can self assemble into ordered aggregates at the toluene/water interface, and there is strong interaction between these aggregates, which makes a stable facial mask form at the toluene/water interface. The oil phase surrounds water droplets in the asphaltene boundary facial mask (Figure 3, green, white and pink represent water, toluene and asphaltene molecules respectively).

3. Chemical Viscosity Reduction Extraction Method for Medium and Deep Heavy Oil

3.1 Emulsifying and Viscosity Reducing

Emulsification and viscosity reduction technology began in the 1960s and is a relatively mature technology. It generally refers to the process of emulsifying or inverting heavy oil emulsions into O/W type emulsions by adding a certain amount of surfactant, in order to reduce the viscosity of heavy oil (Figure 4). At present, there are many research reports on emulsification and viscosity reduction of heavy oil, with the highest viscosity reduction rate exceeding 99%. Research on various types of heavy oil emulsifying and viscosity reducing agents has been reported, and the following categories will be introduced.