Na Wei,Ruiling Bai,Jinzhou Zhao,Yao Zhang,Jin Xue

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,No.8 Xindu Avenue,Xindu District,Chengdu,610500,PR China

ABSTRACT To achieve the goals of Peak Carbon Dioxide Emissions and Carbon Neutrality,China's energy system will continue to accelerate the transition to a clean and low-carbon one.As the cleanest fossil fuel,natural gas is regarded as an inevitable choice for China to build a clean,safe,efficient,and low-carbon energy system and fulfill the goal of“double carbon”.However,the domestic conventional natural gas supply remains rigid while the stimulation of unconventional natural gas is still limited.If we have a firm grip on the principal line of“understanding the ocean–developing resources–ensuring security”to realize the large-scale development of 85 trillion square meters of NGH in the South China Sea,then we could not only greatly reduce China‘s foreign dependence on natural gas,but also guarantee the safety of China‘s natural gas multi-path supply and safeguard the sovereignty of the South China Sea.Thus,the goal of Peak Carbon Dioxide Emissions and Carbon Neutrality can be achieved in no time.

Keywords:Natural gas hydrate Peak carbon dioxide emissions Carbon neutrality Natural gas Carbon emission reduction

1.Introduction

According to the World Meteorological Organization(WMO),the global average temperature in 2020 was 14.9°C,1.2°C higher than the pre-industrial period(1850-1900)[1].Since the 1980s,the global average temperature has been rising in a turbulent way,and climate problems such as rising sea level and melting glaciers caused by rising temperature have become increasingly prominent[2](see Fig.1).In order to effectively combat global warming,the international community has actively promoted carbon emission reduction and adopted the Paris Agreement in 2015,which has made arrangements for the post-2020 international regime for combating climate change and further proposed to achieve the 1.5°C warming control goal.This marked a new stage of global response to climate change[3].

On September 22,2020,President Xi delivered an important speech at the 75th Session of the United Nations General Assembly,saying that China will strive to achieve Peak Carbon Dioxide Emissions by 2030 and Carbon Neutrality by 2060[4],Fig.2.This is the first time that China has made a solemn commitment to the world on a long-term carbon emission reduction.It demonstrates China's determination to follow the path of green and low-carbon development,and shows China's responsibility as a major power in tackling climate change and building a community with a shared future for mankind.

2.Energy development trend under“Double Carbon”target

China is the world's largest carbon emitter,which has emitted 9.826 billion tons of carbon dioxide in 2019,accounting for 28.76%of the world's total carbon emissions,according to BP[5],see Fig.3.From Peak Carbon Dioxide Emissions to Carbon Neutrality,developed countries in Europe and America have spent 50-70 years.But China has only 30 years to go through it.Considering China's more than 10 billion tons of annual carbon emissions and still in the stage of rapid growth,China's economic development growth demand and carbon emission reduction pressure will exist at the same time.To finish the heavy task in a short time is the prominent feature of China realizing the“Double Carbon”target(Fig.4).

According to the 2020 Statistical Communique on National Economic and Social Development released by the National Bureau of Statistics,as a major energy consumer,China's total energy consumption in 2020 was 4.98 billion tons of standard coal.It is expected that by 2030,China's energy demand will increase to 6 billion tons of standard coal,with an average annual growth rate of 2%.Per capita energy demand rose from 3.4 tons of standard coal in 2019 to 4.1 tons of standard coal[6].Therefore,Peak Carbon Dioxide Emissions and Carbon Neutrality are systemic issues dominated by energy,the core of which is to transform into a low-carbon energy system.From the perspective of the energy structure and various energy consumption,replacing fossil energy with nonfossil energy is a fundamental measure for China to achieve Carbon Neutrality by 2060,while it is difficult to achieve in a short time.At the power generation sector,limited by resources,safety issues and environmental protection requirements,the increment of nuclear and hydro energy is insufficient.The photovoltaic power generation and wind power generation are greatly affected by the weather and the location and its peak load regulation capacity is inadequate.At the heating sector and industrial sector,the cost of hydrogen energy and renewable energy power generation and heat recovery technologies is too high these technologies are and not yet in right conditions for large-scale promotion.Therefore,there is a long way to go to increase the proportion of non-fossil energy.Thus,increasing the proportion of low-carbon energy consumption in fossil energy consumption is an effective way to achieve Carbon Neutrality in China.

Fossil energy is divided into oil,natural gas and coal.According to statistics,in 2020,the main source of carbon emissions from energy combustion in China is coal,which accounts for more than 67% of carbon dioxide emissions[7],Fig.5.In terms of carbon emissions per unit of energy,carbon emissions per unit calorific value of oil are 70% of coal,while natural gas are only 50%,Fig.6.Compared with coal and oil,natural gas has the advantages of higher calorific value,less carbon emission and no waste residue or waste water after combustion.It is clean and safe to use[8].According to the forecast chart of energy consumption in the BP Statistical Review of World Energy in 2020,it can be found that under any type of energy transformation pattern,the proportion of natural gas consumption grows rapidly on a large scale,which will play the role of“anchor”of world energy consumption.Therefore,before non-fossil energy becomes the major source of energy,natural gas will be the most realistic choice for China to realize the transition from high carbon energy to low carbon energy.The 14th Five-Year Plan for National Economic Development also clearly points out that a new energy consumption structure system dominated by green and low-carbon energy should be gradually constructed,and the consumption proportion of natural gas in primary energy should be increased(Fig.7).

3.The bright prospect of NGH

Although the domestic natural gas production has increased rapidly in the past ten years,it is far from catching up with the growth of its consumption.The Party and the state require that oil and gas security should be hold firmly in our own hands.China must tackle the difficulties such as ensuring energy security and improving energy self-sufficiency.At present,the annual production of natural gas in China is 190 billion m3,while the annual consumption is 320 billion m3,resulting in foreign dependence ratio as high as 43%.It is predicted that the foreign dependence ratio will exceed 60%in 2025[9,10].Therefore,the security issue of natural gas in China is very severe.

4.2.China's world-leading NGH research

On May 18,2017,China Geological Survey successfully exploited NGH from the reservoir 203-277 m below the mud line of 1266 m water depth by depressurization method in Shenhu area,and had produced 30.9 × 104m3gas continuously for 60 days[33].This is the first time in the world to successfully achieve the safe and controllable exploitation of argillaceous-siltstone NGH,which is the most difficult type to develop and accounts for more than 90%of the global NGH resources.This has made China realize the historic leap from“following”to“leading”in this field( Table 1).

Table 1 Overview of NGH trial production projects around the world[25 32].

In November 2012,at the Shuangqing Forum in Qingdao,China,Southwest Petroleum University(SWPU)and CNOOC first proposed the principle of solid fluidization exploitation technology,and established“the world's first large-scale physical simulation experimental system and state key laboratory for solid fluidization exploitation of marine NGH”within 5 years[34,35].On May 25,2017,CNOOC and SWPU successfully developed NGH with the solid fluidization method in Shenhu area.Compared with the conventional methods,the recovery rate of solid fluidization method increases by 46%,which verifies its scientific principle is correct,the technical method is feasible,and the project implementation is effective[36].The target exploration,drill sampling and solid fluidization test production are all based on the domestically developed technologies and equipment.For the first time,new exploration has been carried out in the safe and green trial production of marine NGH in the world.The further upgrading of the simulation experiment system is expected to promote the solid fluidization exploitation technology to become a disruptive technology leading the frontier research of NGH in China and accelerate the commercial development of NGH in China and even in the world.

On March 26,2020,for the first time in the world,China successfully extracted natural gas from the reservoir buried the expression of depth should be consistent,237-304 m deep via horizontal well technology in Shenhu area,forming 32 key technologies in six categories and developing 12 core equipment[37-46].Among them,China's independent innovation of atmosphere,water,seabed and underground“four-in-one”environmental monitoring system ensures that there is no leakage of methane and no geological disasters in the trial production process,which further confirms the feasibility of green development of NGH[47,48].

In general,since the track study from 1980s to 1990s,China's the depressurization method has surpass other countries and the solid fluidization method is leading the world in NGH research and trial production[49-55].According to the U.S.Geological Survey,China has already outdated these estimates of NGH development of other countries and is the country with the most promising future for commercial NGH development[56](Fig.11).

5.Prospect of development and utilization of NGH

5.1.The development and utilization of NGH are important driving force to achieve the goal of“double carbon”and“double track rightdefending”

China's the depressurization method has surpass other countries and the solid fluidization method is leading the world in NGH development.However,the domestic conventional natural gas supply remains rigid while the stimulation of unconventional natural gas is still limited.If we achieve the large-scale development of NGH,then the natural gas production would increase to more than 80 billion m3by 2035.Thus China‘s foreign dependence on natural gas would be reduced greatly.This will certainly help to change China's energy supply pattern,improve the energy consumption structure and accelerate the realization of Peak Carbon Dioxide Emissions and Carbon Neutrality goals and“double track right-defending".

5.2.The NGH development technology is becoming more and more mature,and large-scale development is around the corner

In recent years,the research and development of natural gas hydrate has been increasing for a long time.Multiple trial production methods(depressurization method,chemical reagent injection method,solid fluidization method,etc.)have been carried on.The trial production areas have been extend from permafrost region in land to land slope under the ocean.The daily production also increases from dozens of cubic meters to 42 consecutive stable gas production days in 2020,with an average daily gas production of 35,700 m3and a cumulative gas production of 1,498,600 m3.A review of the recent process and development of trial production shows that the focus of NGH research and development has gradually shifted from trial production to pre-production.It can be said that the development of NGH has dawned,the hope of breaking through the technical bottleneck is imminent,and the large-scale production is just around the corner.

5.3.The development of NGH is in line with China's national conditions and needs to be accelerated

China's energy development is consistent with the world's energy development,namely,clean,efficient and carbon emission reduction.But only by combining this goal with China's energy reserves and energy security can we promote the sustainable development of the country.In this context,it is urgent to pro mote the large scale development and utilization of NGH as soon as possible,increase China's natural gas production,reduce foreign dependence,and ensure the security of multi-supply of natural gas in China.Therefore,the following suggestions are proposed.First,enhance state support and multi-investment.It is suggested that a policy similar to the Methane Hydrate Research and Development Act of 2000 should be issued as a programmatic document for the development of combustible ice.Second,establish an international standard system in advance.It is suggested that the General Administration of Quality Supervision,Inspection and Quarantine and the Standardization Administration of China should intervene in advance to support Chinese enterprises,alliances and associations to participate in or lead the development of international standards.Third,formulate industrial talent development system and supporting policies.It is suggested that the state should vigorously support qualified universities to carry out the scientific and technological achievements pilot reform of“three rights”,and formulate supporting policies and measures centering on the key tasks of university and local government collaborative innovation such as special economic and industrial fund support,tax reduction and exemption,and individual income tax preference for scientific and technological employees.

Declaration of competing interest

No conflict of interest exits in the submission of this manuscript,and the manuscript is approved by all authors for publication.

Acknowledgments

This study has been supported by the Programme of Introducing Talents of Discipline to Universities(D21025),the National Natural Science Foundation of China(U20B6005-05),and the National Key R&D Program of China(2019YFC0312300).