With the advent of the scientific and technological revolution, the world is witnessing a new wave of transboundary development in various energy revolutions, including the clean coal revolution, unconventional oil and gas revolution, new energy revolution and intelligent revolution. The usage of energy sources by man is evolving from high carbon trend to low carbon and non-carbon trends. “Underground coal gasification” will likely be a new force in this wave: in light of China’s resource reserves characteristics, i.e., “rich in coal but insufficient in oil and gas”, it uses idle underground coal resources to produce gas such as methane and hydrogen, which is of great strategic significance to the development of China’s natural gas industry.

At present, China’s energy resources are facing the severe challenge of high dependence on foreign oil and gas. In 2018, China’s oil and gas external dependence reached 71% and 43%, respectively. With the advent of peak energy demand, it is estimated that the external dependence will go up further. Underground coal gasification has a large energy density and a strong connection with the petroleum and petrochemical industry. Building up petroleum and petrochemical circular economy demonstration areas with net zero emission featuring the industrial chain of “underground coal gasification - hydrogen application by petrochemical refineries - CO₂ enhancing oil recovery and storage” can not only realize the clean utilization of a large number of idle coal resources deep in the strata and relieve the tension in gas supply, but also effectively solve the environmental problems caused by CO₂ emissions from coal combustion in China. What’s more, it can reserve resources and technologies for the coming “hydrogen economy” era, and open up a new path for the construction of a “clean, low-carbon, safe and efficient” modern energy system in China.

In this work, based on the development status and trend of underground coal gasification technology in China and abroad, the basic concept, mechanism and mode of underground coal gasification are introduced, and the challenges, development potential and path in China are analyzed.

Underground coal gasification (UCG) refers to the process of producing CH₄, H₂ and other combustible gases through controlled combustion of coal in situ in the strata with appropriate engineering technology^{[1,2,3,4,5,6]}. UCG can effectively avoid safety and ecological environment problems caused by coal mining, and improve resource utilization efficiency. It can change the coal mining into gas recovery and effectively relieve the contradiction between the “rich coal” and the “insufficient gas”. If successful, it will start “natural gas revolution” and realize a leap-type growth in natural gas production in China.

A UCG unit is mainly composed of one injection well, one production well, an underground gasification furnace, ignition system, and a monitoring system (Fig. 1). In the operation process of a single unit or multiple units, ground monitoring room, ground injection equipment, and produced gas treatment system are also needed^[7].

UCG is a complex physical and chemical process consisting of a series of continuous phases. The gasification process takes place mainly at the gas-solid interface. According to the intensity of the chemical reaction, the whole area along the gasification channel axis can be divided into oxidation zone, reduction zone and dry distillation zone^[8] (Fig. 2).

(1) Oxidation zone. After being injected through injection well, O₂ in the gasification agent will be ignited and make the coal burn, which will produce CO₂, release a large amount of reaction heat, and form a planar combustion space, namely gasification surface. The combustion zone is called oxidation zone. When the concentration of O₂ in the injected gasification agent is close to zero, the combustion reactions stop and the oxidation zone ends. The reactions in the oxidation zone are all exothermic reactions with the reaction temperature of 800-1 200 °C.

(2) Reduction zone. The reaction heat generated in the oxidation zone makes the coal bed in the reduction zone reach an extremely hot state. Reduction reaction occurs between the CO₂ generated in the oxidation zone and the hot carbon, thus generating CO; reduction reaction also takes place between the water vapor and the hot carbon, thus generating CO, H₂, etc. As the reduction reaction is an endothermic reaction, the coal bed and air flow temperature will gradually fall with the reduction reaction going on. When the fall of temperature makes the reduction reaction weak, the reduction zone will end. The reaction temperature of the reduction zone is 600- 900 °C.

(3) Dry distillation zone. After the reduction zone ends, the temperature of air flow is still high, which can heat the coal bed in the adjacent dry distillation zone, making it release pyrolytic combustible gas and methanation reaction happen at the same time. The reaction temperature is 200-600 °C.

From the perspective of the chemical reaction, there is no strict boundary between the 3 zones. Pyrolytic reaction of coal also occurs in the oxidation zone and reduction zone. The division of the three zones only indicates the intensity of oxidation, reduction and pyrolytic reactions. After going through the 3 reaction zones, combustible components generated are mainly H₂, CO, and CH₄ with the gasification going on. Therefore, the combustible gases mainly come from 3 reactions: coal’s combustion and pyrolysis, reduction of CO₂, and decomposition of water vapor. The temperature of reaction zones and reaction specific surface area control the intensity of these 3 reactions, and also determine the components and heat values of the mixed gas produced. Depending on rank and petrology and quality, one ton of coal can generally produce 1 490-2 470 m³ of mixed gas with heat value of 4187-7 117 J/m³ through underground gasification^[9].

The process of UCG can generally be divided into 6 stages, which involve 8 series and 25 technologies (Fig. 3).

The first thing is to select the site of gasification furnace.

Technical and economic feasibility of UCG projects depends on many geological and non-geological factors. Scientific site selection can minimize risk. The geological evaluation process should consider the interaction of many influence factors such as coal petrology and quality, permeability, water content, strength of roof, and collapse rule as well as their impact on gasification process, so as to provide basis and guarantee for the engineering design and smooth operation in the later stage.

Then, a gasification furnace is built in the area selected, i.e., furnace construction. Generally, drilling technology of CBM exploitation well is adopted. One of the important issues needs to be considered is the method for constructing a channel between the injection well and the production well, i.e., the technology of connection by directional well. The site experimental results show that among the 4 connection methods, i.e., electric connection, explosive fracturing, hydraulic fracturing, and reversal burning, only reversal burning is feasible.

The furnace construction is followed by ignition. Similar to ignition in thermal recovery of heavy oil, it uses ignition compound, then hot coke is dropped and oxygen-enriched air injected into the wellbore, making the coal spontaneously burn under the pressure. After the coal is ignited, a series of exploitation parameters will be optimized as required, to ensure that the system can operate in the optimal state.

Control is the core link of underground gasification. UCG is a thermochemical reaction process. Its core is the controllability of combustion. Besides the internal factors such as geological conditions, coal petrology and quality, external factors such as gasification agent ratio, gasification reaction temperature and pressure, and production pressure difference also control the gasification and combustion process. The directly controllable parameters of UCG process mainly include gasification agent injection pressure, rate, components, and temperature as well as injection point location in the structure of liner CRIP (Controlled Retraction Injection Point), and production wellhead pressure. The injection pressure controls the combustible gas components of the synthetic gas. The higher the pressure is, the higher the proportion of CH₄ and the lower the proportion of CO and H₂ will be. According to the site experiment experience in EI Tremedal of Spain, when the pressure was 5 MPa, the proportion of combustible components would be 55%^[10].

After temperature underground is reduced, the produced gas enters ground treatment link. The produced gas is a mixture of CO, CH₄, H₂, CO₂, and other impurities, mainly including particles, tar, sulfur compound, etc. Therefore, it should be purified before use. The purification has four purposes, i.e., temperature reduction, dehydration, recovery of useful by-products, and removal of detrimental impurities.

The produced gas after ground treatment can be used comprehensively, including the utilization and storage of CO₂, and comprehensive utilization of combustible gas. For example, it can be used to generate electricity for gas turbine engine, or as chemical materials to produce methanol or other chemical products.

Pollutants will be prevented and controlled throughout the process of UCG. They need to be predicted, controlled and treated in different process stages. The monitoring materials provide basis for the control. Monitoring is synchronized with furnace construction, operation, control and ground process. Necessary monitorings during the UCG operation include production performance monitoring, gasification cavity change monitoring, product composition monitoring, and pollutant monitoring, etc.

The process of UCG generally includes heterogeneous reaction and homogeneous reaction. The former is the interaction between gasification agent or gaseous reaction products and solid coal or coke, while the latter includes the reactions between gaseous reaction products or between gaseous reaction products and gasification agent. The process of UCG is, in essence, the thermochemical equilibrium between the solid-phase carbon in coal and the gas-phase O₂, water vapor, CO₂ and H₂. Main factors influencing the chemical equilibrium include gasification agent, contact mode and process conditions, etc.

According to coal reaction processes under different conditions and the differences in product composition, UCG can be divided into three depth sections according to their buried depths, namely the shallow formation with the buried depth less than 500 m (the gasification reaction pressure is generally <4.0 MPa, and temperature is >1 000 °C), the medium-deep formation with the buried depth of 500-2 200 m (the gasification reaction pressure is generally ≥4.0 MPa and <22.1 MPa, and temperature is >1 000 °C), and the deep formation with the buried depth above 2 200 m (the gasification reaction pressure is generally ≥22.1 MPa, and temperature is >1 000°C). There are three exploitation modes corresponding to the different buried depths (Fig. 4). The first mode is hydrogen-rich mode in shallow formation, in which the gasification agent reacts with coal to generate gas through three reaction zones under low pressure. The characteristics of this mode are dry distillation and hydrogen-rich product. For example, in the Barbara field test in Poland, coal bed with buried depth of 20 m was selected, in the produced gas, CH₄, H₂, CO, CO₂ and N₂ accounted for 2.5%, 36%, 32%, 15% and 13% in volume respectively^[10]. The second mode is methane-rich mode in medium-deep formation, in which, with the increase of pressure, the reaction goes to a smaller gas volume until achieving a balance. The methanation of both CO and CO₂ are reaction with reduced gas volume, so the production rate of CH₄ now will quickly increase with the increase of pressure. The characteristics of this mode are domination of methanation and methane-rich product. For example, the Swan Hill field test in Canada, in which the coal bed at a buried depth of 1 400 m was selected, CH₄, H₂, CO, and CO₂ accounted for 37%, 15%, 5% and 41% of produced gas volume respectively^[10]. The third mode is supercritical hydrogen-rich mode in deep formation, in which, when the pressure keeps increasing to 22.1 Mpa, the water vapor in gasification agent enters into the supercritical state (374.3 °C and 22.1 MPa), and the chemical reaction is beyond the above-mentioned common heterogeneous and homogeneous reactions and reaches supercritical gasification with the supercritical water as the gasification medium, achieving the simultaneous pyrolysis, gasification, purification, conversion and separation of coal^[11]. The characteristics of this mode are supercritical reaction and extremely hydrogen-rich product. According to the experimental result of surface supercritical water gasification, CH₄, H₂, CO and CO₂ accounted for 3-4%, 55%-62%, 1% and 32%-39% of the produced gas volume resepectively^[12].

In 1868, German scientist William Siemens proposed for the first time moving the surface gasification furnace to underground coal mine to directly gasify the coal, who was also the first person proposing the possibility of coal gasification in place^[13]. In 1888, Russian chemist Mendeleev advanced the basic process of UCG^[14,15]. From 1906 to 1910, American chemist Betts obtained three technical patents for UCG respectively in the US, Canada and the UK, marking that UCG technology was basically mature^[16].

Industrial tests on UCG in the western industrial countries, despite the ups and downs with the fluctuations in international oil prices, never stopped. Since the 1930s, the former Soviet Union, the US, Belgium, Germany, the UK, Australia and other countries carried out UCG field tests for gasification, hydrogen production, power generation and other purposes^[17], kept improving UCG processes and technologies, developed the GRIP technology and continuously extended UCG to medium-deep layers. From 2009 to 2011, Canada employed the CRIP technology in the UCG project in Swan Hill of Alberta, which is so far the industrial UCG field test in the deepest target coal bed (at buried depth of 1 400 m)^[10].

China had explored UCG tests in developed coal mines during 1958 and 1962, and shaft-type UCG tests were carried out in Heilongjiang and Henan for many times afterwards. In the gasification test of “long channel, large section and two stages” process in Xuzhou during the early 1990s, the produced gas had a maximum H₂ content of 60%-80%. From 2009 to 2015, shaft-less gasification furnace tests of L-shaped gasification furnace, V-shaped gasification furnace and unit planar mining gasification furnace in Ulanqab of Inner Mongolia were completed, during which the moving-device retraction gasification without ignition was developed. Such technology realized oxygen-rich continuous production and stable operation lasted for 5 months, in which the effective composition content in the produced gas was above 50% and the control parameters for continuous operation of gasification furnace were obtained. This technology won multiple patents and technological achievements^[18,19]. Influenced by many factors, the Ulanqab pilot project failed to be commercialized. In recent years, some Chinese private enterprises dabbled in UCG industry for different purposes like hydrogen production or gasification and have carried out preliminary work in shallow-layer UCG respectively in Inner Mongolia and Xinjiang. In addition, large-scale oil companies are also intended to carry out deep-layer UCG business in line with their own gas industry chains to improve their gas supply capacity. All these will actively promote the development of China’s UCG industry.

According to the major project “National Coal Resources Potential Evaluation” of Ministry of Land and Resources, the total quantity of national coal resources at buried depth of less than 2 000 m are 5.9×10¹² t, which are mainly distributed in coal measures of four main coal-forming periods, Late Carboniferous-Early Permian, Late Permian, Early-Middle Jurassic and Late Jurassic-Early Cretaceous in North, Northwest and Northeast of China, with proved coal resources of 2.02×10¹² t and predicted resources of 3.88×10¹² t^[20]. So far, the mining depth of coal mining enterprises is mainly less than 1 000 m. Fine evaluation hasn’t been carried out on coal resources at buried depth between 1 000 m-2 000 m due to lack of mining technologies and economic conditions, and large-scale prospection hasn’t been carried out on coal resources with buried depth of over 2 000 m at all. Coal resources at buried depth of more than 1 000 m are the main target for UCG, with huge gasification potential.

According to prediction, China’s onshore quantity of coal resources at buried depth of 1 000-3 000 m is 3.77×10¹² t^[20,21], which are mainly distributed in petroliferous basins like Ordos, Junggar, Tarim, Erlian, Hailar and Songliao, in which the medium-deep coal resources can generate huge amount of methane and hydrogen after UCG. To calculate by UCG producing rate of 40%, the amount of methane resources (natural gas) from UCG is (272-332) × 10¹² m³ (putting aside coal ranks and surface conditions, see Fig. 5), which is about 3 times the conventional natural gas resources, or equivalent to the total unconventional natural gas resources, indicating huge development potential.

The Ordos Basin and the Erlian Basin are the favorable targets for UCG due to the well-developed coal beds and advantageous geographical positions. There is huge resource potential for UCG in the Ordos Basin. Two coal strata, the Upper Paleozoic carboniferous - Permian and the Mesozoic Jurassic are widespread across the whole basin, with a total coal-bearing area of over 29×10⁴ km², and the total predicted quantity of coal at buried depth of less than 4 000 m is 6.92×10¹² t^[22]. The quantity of coal resources at buried depth of 800-2 200 m suitable for UCG in the CBM mining right block in the eastern margin of the basin is 183×10⁸ t, and the quantity of producible coal resources for UCG is preliminarily estimated at 73.2×10⁸ t, which is equivalent to 1.46×10¹² m³ of pure methane resources. To calculate by recovery efficiency of 50%, it is roughly equivalent to a large gas field with annual gas production of 150×10⁸ m³ that can be continuously produced for 50 years.

The Erlian Basin has late coal forming time, and a coal-bearing area of 0.90×10⁴ km². The quantity of coal resources at the buried depth of less than 2 000 m is 6 819×10⁸ t, that at buried depth of 500-1 000 m accounts for more than 90% and the quantity of producible coal resources for UCG in the basin is 754×10⁸ t, which is equivalent to 12.5×10¹² m³ of pure methane resources. To calculate by recovery efficiency of 50%, it is roughly equivalent to a super-large gas field with annual gas production of 1 000×10⁸ m³ that can be continuously mined for 50 years.

Shallow-layer UCG test projects haven’t realized scale industrialization either in China or abroad for 3 reasons: The first is area selection, as demonstration on geological area selection was insufficient, accidents like large amount of formation water rushing into the gasification cavity happened, forcing the test to a stop. The second reason is that UCG technology and process have high requirements on geological, engineering and surface conditions and the technology still need to be improved, for example, connection process, the burning accident of coiled tubing and blocking of produced gas channel could cause failure of the test. The third reason is that most shallow-layer test projects are influenced by external environment, national environmental protection policies, surface depression and shallow water pollution could cause the test to stop^{[8, 13, 23-26]} (Table 1).

Field tests both in China and abroad indicate that the shallow-layer UCG technology is basically mature, but both technology and economic benefits need to be improved to realize industrialization development of UCG. In line with relevant technological progress, industrial situation and environmental protection requirements, one development direction of UCG is deep-layer development, avoiding the environment sensitive areas and traditional coal mining areas; another direction is integration with other industries, such as integration with power generation, with carbon capture, utilization and storage, with hydrogen production and with fuel cell industry, etc.^[25]

Medium-deep formations are the main targets for UCG development, which, compared with shallow coal formations, have many advantages. First, the gasification furnace is far from the surface and the source of drinking water, avoiding direct environmental pollution; second, deeper buried depth is conductive to the leakproof of gasification furnace, avoiding the leakage of produced gas caused by too many fractures; third, the temperature increases as the buried depth gets deeper, and so will the gasification reaction rate and caloric value. However, formation pressure also increases as the buried depth gets deeper, and the formation situation is getting more complicated, bringing more difficulties in construction and monitoring control and increase of project cost. To avoid possible underground water pollution and overlapping of business scope with coal mining enterprises as much as possible, the future UCG must develop toward medium-deep, deep and even super-deep formations.

Industrial test of medium-deep UCG still faces three challenges. The core issue is that the medium-deep UCG reaction mechanism is more complicated and has higher requirement on engineering technologies. Influenced by high temperature and high pressure, the chemical reaction mechanism of UCG is dominated by methanation instead of simple combustion and dry distillation reaction, the underground reaction process is more complicated and reaction conditions such as the coal petrology and quality and closeness in the reaction cavity have obvious impact on UCG, presenting higher requirement on the standard of geographical site selection and technologies for precise control of reaction.

The second issue is environmental impact. Environment impacts of UCG mainly include underground water pollution and large amount of CO² emission. Underground water could be polluted in two ways: pollutants spread and permeate to the surrounding strata along with gas through the fissures in the surrounding rocks; pollutants leach out in underground water and migrate with it^[27]. The pollutants include benzene and its derivatives, phenolic compound, polycyclic aromatic hydrocarbon, heterocyclic compound and other organic pollutants^[28,29] and ammonia nitrogen, cyanide, metallic elements and other inorganic pollutants^[28]. These pollutants can be effectively prevented and controlled but can’t be eliminated at a low cost. CO₂ disposal in produced gas is another environmental problem in front of UCG after large-scale production. In combination with the development practice of oil and gas industry, there are three ways for CO₂ disposal (Fig. 6). The first way is using it for flooding in adjacent low-permeability oilfields and burying it, to realize net zero emission of wastes by building demonstration projects of “UCG - hydrogen for petrochemical refinery - enhancement of oil recovery by CO₂ and sequestration of CO₂ simultaneously”. The second way is burying it directly in suitable strata nearby. The third way is utilizing it directly after purification, such as in food industry, where it can be made into dry ice for refrigeration and CO₂ supercritical fluid extraction, etc.

The last issue is economic impact. According to preliminary calculation, under current conditions, unit cost of methane production in the UCG project at a production scale of 20×10⁸ m³/a and buried depth of 800 m is about 1.1-1.3 yuan/m³, which is comparable to the price of imported gas, but the price advantage is not obvious and project economics is greatly influenced by external factors. At the same time, low price of new energy has huge impact on UCG industry. It is inevitable for China to adjust its energy structure from the “one big and three small” (namely coal, oil, natural gas and new energy) to the situation of “tripod” (namely coal, oil and gas and new energy). Once the development of new energy is sped up at low price, the projects of clean utilization of coal such as UCG may be severely impacted. Therefore, how to reduce development cost will be one of the important constraints in the commercialization process of China’s UCG industry.

The world's energy resources are undergoing the third major transformation from oil and gas to new energy, forming a new pattern dominated by oil, natural gas, coal and new energy, in which natural gas functions as a bridge. In China's primary energy production in 2017, the production of oil, natural gas, coal and new energy accounted for 8%, 5%, 70% and 17% respectively, in which coal production accounted for the largest proportion, showing a pattern of "one big and three smalls". China's natural gas industry has entered a new stage of development. Under the background of ecological environment construction, energy structure adjustment and accelerated new urbanization, natural gas plays an increasingly prominent role in the energy structure, ushering in a "golden era" for China's natural gas development^[30,31].

China's oil and gas consumption is growing rapidly, China’s dependence on external oil and gas goes up. From 2007 to 2018, domestic crude oil production increased slowly from 1.86×10⁸ t to 2.15×10⁸ t and then kept dropping for three years successively, while the consumption increased from 3.78×10⁸ t to 6.25×10⁸ t^[32] (Fig. 7), as a result, the gap between supply and demand became wider, and the external dependence of crude oil in 2018 was as high as 71%. In the meantime, domestic gas production increased rapidly from 695×10⁸ m³ to 1 580×10⁸ m³ in 2018 (107×10⁸ m³ of which was shale gas) and gas consumption also kept increasing rapidly, reaching 2 766×10⁸ m^3[33] in 2018 (Fig. 7), and the external dependence of gas was 43%. Promoted by energy structure adjustment and consumption growth, external dependence of oil and gas will go up further in the future. It is predicted that China’s gas demand in 2020 will reach 3 500×10⁸ m³, accounting for 10% of total primary energy consumption, with a gap of about (1 700-1 800)×10⁸ m³; gas demand in 2030 will be about (5 500-6 000)×10⁸ m³, accounting for 12% of total primary energy consumption, with a gap of about (3500-4 000)×10⁸ m³; and gas demand in 2050 will be about (6 500-7 000)×10⁸ m^3[34], accounting for 15% of total primary energy consumption, with a gap of about (4 000-5 000)×10⁸ m³. Under the background of obvious global multi-polarization and increasingly tense geopolitics, the huge gap in natural gas supply will pose great challenges to China's energy security, and may bring a series of chain reactions in various aspects such as economy and diplomacy. How to increase the natural gas supply quickly and effectively has become a major energy problem restricting the development of ecological civilization.

At the same time, domestic economic development is bearing huge environmental pressure. Internally, extensive and intensive haze weather is reversely forcing the transformation of energy structure. Externally, CO₂ emission reduction is also a tough task. So far, 85% of CO₂ and SO₂ emission, 60% of oxynitride emission and 70% of dust in China come from coal burning. Pollutants from coal burning are major reasons for the combined air pollution such as regional dust and haze weather^[35]. It is difficult to change China’s energy structure dominated by coal in a short term. To realize environment-friendly energy development mode, on one hand, it is necessary to increase the ratio of clean energy in total energy consumption, and on the other hand, it is a must to promote the clean and efficient use and green mining of fossil energy, especially coal, so as to strengthen the transformation of coal from primary energy to secondary energy and provide powerful support to build a “beautiful China”.

A “clean, low-carbon, safe and efficient” modern energy system is the goal of high-quality development of China’s energy industry. In the case that natural gas resources can’t fully meet the rapid increase of market demand and new energy of low cost with sufficient supply hasn’t come to replace oil and gas, the key to achieving high-quality development of energy industry and solving energy and environmental problems is to strengthen the efficient and clean use of coal based on national conditions. To this end, the state has published a series of policies to encourage and support the clean and efficient use of coal. UCG, which transforms physical coal mining to chemical gas production, not only can effectively reduce problems in safety and ecological environment caused by coal mining, but also can transfer coal from direct burning to the sophisticated natural gas industry. It is proposed in The plan for Clean and Efficient Use of Coal (2015-2020) issued by National Energy Administration that “we shall promote the construction of UCG demonstration projects and explore the UCG development routes suitable for our own national conditions” and “we shall actively carry out technical researches and build demonstration projects of CO₂ capture, utilization and storage; encourage the cooperation between modern coal chemical industry with petroleum enterprises and other related industries and carry out demonstration projects such as flooding, microalgae absorption and geological storage to accumulate experience for carbon emission reduction of other industries in a wider range^[36] ”. So far, surface coal-based SNG has entered the stage of large-scale development and the productivity, projects represented by Yili project of Xinjiang Kingho, Inner Mongolia Huineng, Hexigten Coal-based SNG Project of Datang International and Fuxin Coal-based SNG Project of Datang International approved by China National Development and Reform Commission (NDRC) are expected to produce 622×10⁸ m³/a, but UCG is still in the preliminary research stage.

Compared with surface coal gasification projects, UCG projects are more environment-friendly and more economical, and breakthroughs have been achieved in key technologies. Due to the higher energy density, gas production rate and efficiency than that of conventional gas, UCG is expected to open up a new strategic path for the rapid and effective gas supply with Chinese characteristics. Natural gas abundance that is technically recoverable in a coal bed of 10 m thick is about 1.5×10⁹ m³/km² (equivalent of pure methane), which is higher than most of the conventional and unconventional gas fields in production now. To calculate by a working face of gasification of 70 m, horizontal interval of 800 m (exclusive of 200 m of the frontal distance from the target point) and daily combustion of 0.4 m of a single well, the maximum daily pure methane production of single gasification furnace (single pair of horizontal wells) is about 3×10⁴ m³, and the cumulative methane production of single gasification furnace for five years successively is about 2×10⁸ m³; nine wells can be deployed in 1 km², with potential production of marketable gas of 10× 10⁸ m³/a and stable production duration of 1.5 years.

H₂ accounts for a great proportion in the produced gas from UCG, therefore, the development of UCG industry can reserve resources and technologies for the coming "hydrogen economy" era. Hydrogen is a clean energy carrier and to build an integrated application platform of hydrogen can realize interconnection between different kinds of energy such as renewable energy, electric energy and oil and gas, thus realizing effective storage and utilization of energy. So far, the hydrogen and fuel cell technologies have been listed by the state as a key direction for future energy development and key field for new strategic businesses. The content of H₂ in UCG products can be controlled by temperature, pressure, and gasification agents etc, reaching up to around 60%, and the proportion can be controlled by technical means as per actual needs in the future. Therefore, the significance of UCG to China’s energy industry is even more than that of shale gas revolution to the energy industry of the US.

As an interdiscipline of utility engineering and basic sciences, UCG involves physics, chemistry, fluid mechanics, solid mechanics, thermodynamics, geology, hydrogeology, automatic control and other disciplines. Interactions of various factors make the underground combustion and gasification process of coal complicated (including some reversible chemical reactions), which presents great challenges to UCG monitoring and control. Therefore, there are high requirements on UCG technologies and processes and it is a multi- disciplinary integrated technology system involving underground geology, well drilling and completion, equipment manufacturing, surface processing and other technologies related to natural gas industry. Compared with coal enterprises, petroleum and petrochemical enterprises have obvious edges in medium-deep underground (down-hole) technologies, natural gas pipeline network, market and integrative development.

First, petroleum and petrochemical enterprises have had deeper understanding of medium-deep coal resources during oil and gas exploration. Coal-bearing strata formed in different periods are widely distributed in the mining right blocks of petroleum and petrochemical enterprises. Wide-range medium-deep coal-bearing strata have been encountered during oil and gas exploration and development in petroliferous basins such as Ordos, Tarim, Junggar and Erlian, and relatively rich data of coal bed geology and analysis test has been obtained. As a result, certain knowledge of medium-deep coal of different coal petrology and quality and gasifiable resources developed in different geological epochs has been acquired. Due to different resource target subjects and long-term accumulation, petroleum and petrochemical enterprises have obvious advantages in deep-layer geological detection theories and technologies compared with coal enterprises.

Second, some oil and gas exploration and development technologies and equipment can be used in medium-deep and deep UCG. On one hand, oil and gas field exploration and development technologies can facilitate UCG projects. Technical progress in directional drilling and coiled tubing, in particular, has promoted the great-leap-forward development UCG technology from roadway-type development to shaft- less development. On the other hand, through supporting technologies of oil and gas development, the sophisticated comprehensive geological evaluation technologies (CBM), geophysical prospecting technologies (seismic and well logging), horizontal well drilling and completion technologies, coiled tubing integration technologies, high-temperature heavy oil thermal recovery technologies, real-time online monitoring technologies and surface natural gas purification technologies of petroleum and petrochemical enterprises can lead the development of medium-deep UCG industry. Through pertinent improvement, these technologies can be applied in key links of UCG, such as site selection, furnace construction, gas injection, ignition and production. For example, 3D seismic, VSP (Vertical Seismic Profile) and well logging technologies can be used for fine structural interpretation of coal bed and evaluation of coal petrology and quality and stress field, to provide geological basis for furnace construction. Micro-seismic monitoring technology can be employed to monitor the changes in shape and size of UCG cavity in real time. Some techniques in combustion of oil in-situ such as ignition, injection control and shaft integrity can be used as reference for UCG ignition and control, which are expected to inspire key breakthroughs in medium-deep and deep UCG projects.

Third, petroleum and petrochemical enterprises can give full play to synergistic effect. Due to the attributes of “underground, high temperature and fluid”, UCG is highly integrated with the existing oil and gas industry chain of petroleum and petrochemical enterprises. It can realize coordinated development with natural gas industry chain, refining, energy replacement in mining area, gas storage, CO₂ flooding and burial and hydrogen energy industry chain according to the situation in different regions, thus realizing vertical and comprehensive development and utilization of resources. Moreover, it can also promote related technical service industries of petroleum and petrochemical enterprises to the horizontal expansion of emerging business and in-depth development of high-grade, precision and advanced technologies, thus realizing the highly integrated development of UCG industry and oil and gas industry, and achieving the synergetic benefits of “1+1>2”.

UCG technology has been basically mature, but its industrialization has been slow due to technologies and external environmental impacts such as market, safety and environmental protection. Medium-deep formation development and industrial integration are the future development directions of UCG.

Putting aside coal ranks and surface conditions, it is initially estimated that the amount of natural gas resources from underground coal gasification is (272-332)×10¹² m³, which is about 3 times the resources of conventional natural gas, or equivalent to the total unconventional natural gas resources, indicating huge development potential.

Due to the higher energy density, gas production rate and efficiency than that of conventional gas, UCG can open up a new strategic path for the rapid and effective gas supply with Chinese characteristics and is likely to be a natural gas technology revolution with Chinese characteristics. The significance of UCG to China’s energy industry is even more than that of shale gas revolution to the energy industry of the US.

The attributes of “underground, high temperature and fluid” of UCG have common points with oil and gas business chain of petroleum and petrochemical enterprises. Naturally, petroleum and petrochemical enterprises have integrative advantages in developing medium-deep UCG and can select the technological path from the three development modes of “hydrogen-rich in shallow formations, methane-rich in medium and deep formations and supercritical hydrogen-rich in deep formations” to carry out UCG business as per different demands and corresponding technology readiness.

With consumption market, sufficient resources, development basis and mature technologies, the industrialization of UCG is seeing the first glimmer of dawn, but there are still uncertainties in mechanism, technology, environmental protection, economy and other aspects, which require targeted desk research, pilot field tests and industrial tests. Petroleum and petrochemical enterprises should make full use of their own advantages to guide and push the development of China’s UCG industry, which may bring about China’s natural gas revolution and rapid development of hydrogen industry.