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Even in the carbon-neutral age, natural gas will be valuable as environment-friendly fuel that can fulfill the gap between the energy demand and supply from the renewable energies. Marine gas hydrates are a potential natural gas source, but gas production from deposits requires additional heat input owing to the endothermic nature of their dissociation. The amount of fuel needed to produce a unit of energy is important to evaluate energy from economic and environmental perspectives. Using the depressurization method, the value of the energy return on investment or invested (EROI) can be increased to more than 100 for the dissociation process and to approximately 10 or more for the project life cycle that is comparable to liquefied natural gas (LNG) import. Gas transportation through an offshore pipeline from the offshore production facility can give higher EROI than floating LNG; however, the latter has an advantage of market accessibility. If the energy conversion from methane to hydrogen or ammonia at the offshore facility and carbon capture and storage (CCS) can be done at the production site, problems of carbon dioxide emission and market accessibility can be solved, and energy consumption for energy conversion and CCS should be counted to estimate the value of the hydrate resources.
Unlike conventional oil and gas that can be produced by natural flow, combustible hydrocarbon gas extraction from naturally occurring gas hydrates, ice-like solid in the earth crust and stable under high pressure and low temperature, requires some sort of production technology that can consume energy. To bring fuel gas from gas hydrates in geological formation, the solid form hydrocarbon should be dissociated into fluid phase (gas and water) that can move in porous geological formation and in the tubing, but gas hydrate dissociation is an endothermic phenomenon, and thus, the process needs continuous thermal energy input (
Naturally occurring gas hydrate is found in several forms, such as pore-filling hydrates in sandy sediments, bulky hydrate nodules on the seafloor or near the surface, and fault- and fracture-filling hydrates in clayey sediments. Among these, the hydrate deposits in sandy formation in either onshore permafrost or offshore deepwater environments could be the closest target for commercial gas production because currently available technologies for the conventional petroleum productions can be applied (
The in-place volume of naturally occurring gas hydrates has been estimated worldwide or regionally. The early estimation counted 1015–1018 m3 in the methane gas volume under the normal condition (
Even in the carbon-neutral age, natural gas will be valuable as an environment-friendly fuel that can fulfill the gap between the energy demand and supply from the renewable energies. In this article, how much EROI can be expected for each type of production method and system, and its ecological value in the future carbon neutral society are discussed.
To produce methane from the marine gas hydrate in sandy sediments,
Phase equilibrium line and PT changes during the application of the “depressurization method
Thermal energy injection for the “thermal stimulation method” (
The 1-kg structure I methane hydrate [
If the thermal stimulation method is applied, the necessary thermal input is not only the dissociation heat but also the heat to increase the formation temperature that contains sand particles and pore fluids as well as gas hydrates. The heated region can extend beyond the targeted gas hydrate-bearing sediment, and the heat can escape to the surrounding formations by conduction and advection, so more heat input should be necessary. The necessary energy input
The volume of the influenced domain
Possible thermal stimulation options with heat and fluid flows involved. To minimize the heat that is not involved into the gas hydrate dissociation is the key to maximize EROI of the depressurization method.
From 1-kg gas hydrate (methane hydrate), 0.95 × 10–3 m3 of water is produced, and if the degree of the drawdown is 10 MPa, the energy necessary to transport the water from the reservoir to the surface is
In the case of the depressurization method, methane productivity from gas hydrate deposits solely depends on its natural condition and is hardly controllable.
In both thermal stimulation and depressurization cases, the real energy efficiencies in the reservoir are dominated by the heat transport in the formations and highly dependent on the employed technologies and reservoir characters, and detailed numerical modeling works are required to obtain the values.
Estimated recovery rate and energy return on investment of each of the production technologies (depressurization, combination method, and thermal stimulation) at three Nankai Trough gas hydrate reservoirs with different temperatures and pressures evaluated by numerical simulations (
Because the calculation of the energy input necessary for the production is complicated coupled phenomena of mass and heat flow happening in heterogeneous geological formation, the reliability of the result depends on the accuracy of the numerical method, kinetic and reservoir characteristics models applied. Intensive model comparison efforts have been conducted as international efforts (
The world’s first intended gas production from gas hydrate deposits was made in the Mallik site, Mackenzie Delta, Northwest territories, Canada, in 2002, and 468 m3 of gas was produced during the five-day thermal stimulation operation with hot fluid circulation (
Two onshore gas production projects by depressurization were conducted in the arctic area of North America. One was done in the same site of 2002 project in Canada in 2007–2008 (
Energy return (combustion heat from the produced gas,
The main reason for the difference between the onshore and offshore cases is different GWRs. In the offshore test cases, GWRs were less than 100, but onshore test data gave from 100 to 200 of GWR so that works needed to bring water to surface were relatively small. The fact shows that the energy efficiency of gas production from the gas hydrate reservoir relies on how water production volume is limited.
Energy consumption for the gas production stated in the former section (HEAT DEMAND AND OUTPUT BY GAS PRODUCTION METHODS) did not consider any indirect energy demand for exploration, construction of facilities, indirect operational demands such as utility for the production facilities, transport of gas from the production site to shore, and treatment and disposal of produced water, plug and abandonment of boreholes, and decommissioning of the facilities that are required in the entire life cycle of the commercial gas production.
A Japanese research team of MH21 tried counting all those demands to evaluate the life cycle of EROI of the gas production system (
In the study, three offshore gas production and transport systems, namely, 1) offshore platform, 2) floating LNG (FLNG), and 3) long tie-back from the subsea production system to the shore (LTB), were modeled (
Three concepts of offshore gas production and the transport system. (
For the above situation, EROI for each scenario was calculated and was compared with other energy sources. The gas and energy produced by combustion and the EROI numerator were given from the numerical simulator. The energy consumption of the life cycle is the sum of the total energy demand that is divided into the capital energy demand (exploration, manufacturing, and construction of whole facilities such as an offshore platform, subsea devices, pipeline, transportation of the devices from factory to the site, drilling of wells, plugging and abandonment of the wells, and decommissioning of the facilities) and operational demand (which includes electricity to operate pumps and any facilities such as the platform itself and liquefaction facility, and gas compressor to transport gas to shore). Consumed fuels, energy to create materials, and other operational and capital energy demands are counted to produce a unit energy output.
Energy input structure to produce each energy (coal, heavy oil, crude oil, natural gas (NG), LNG imported to Japan, and gas hydrate with offshore platform option). (
Total energy outputs of gas hydrate cases are calculated from the produced gas volume estimated by numerical simulation.
Energy return on investment of each energy source consumed in Japan.
Carbon intensity of each energy—emission from the production process and combustion of products.
Gas production from gas hydrate-bearing underground sediments is an energy-consuming process owing to its endothermic nature, resulting in low EROI. The theoretical value of the EROI of the production process is more than 100, and the actual values measured during gas production tests in the eastern Nankai Trough proved the value. However, along with the energy necessary for gas production, other processes in the reservoir life cycle, such as construction of offshore production systems, processing on the surface and gas transportation to the shore, and produced water disposal, require additional energy input. If the gas production rate predicted by numerical models can be realized in the real reservoir, a higher EROI than the imported liquid natural gas can be expected under certain reservoir conditions. Among offshore production systems, long tiebacks to shore or offshore platforms are a better choice than floating LNG; however, floating LNG has an advantage of market accessibility, so it is an option in the early stage of gas production.
Those values mean that the net CO2 emission from the gas hydrate production can be comparable or even lower than the conventional oil and gas. To bring the CO2 emission lower and close to zero, a combination of gas production with CO2 capture and storage should be considered. Energy conversion from methane to the “blue” hydrogen or ammonia with CO2 capture at offshore facilities and CO2 storage in deepwater gas hydrate reservoirs or aquifers may extend the chance to reduce the net CO2 emissions and contribute to the carbon-neutral society. Moreover, if the gas is transformed into liquid fuel, the problem of market accessibility can be solved; however, such additional process for energy conversion can reduce the value of EROI. As another idea, conversion of methane into electricity on the platform (gas-to-wire, or GTW) is possible, too (
Schematic of the integration of offshore gas hydrate production, energy conversion, and CCS (modified from
Another idea of CO2 utilization is CH4–CO2 exchange for gas production, which is an idea since CO2 is more favorable to form gas hydrate than CH4 in a certain pressure and temperature range (
Methane has another face as a strong greenhouse gas, which has 28 times larger global warming potential than CO2 (IPCC Fifth Assessment Report, 2014). Using the depressurization method, methane cannot be discharged into the sea floor or atmosphere, except in accidental situations. If any trouble or damage of subsea and subsurface production facilities such as the well structure, subsea pump, and manifold and flow line lead to the flow-in of cold sea water into the well that affects the pressure and temperature of the borehole, gas hydrate dissociation will be terminated immediately. However, any injection activities for well stimulation or enhanced recovery may create a gas leakage pathway. Furthermore, gas leakage is possible because of the damaged surface facility and flow line through accidents or natural disasters. Therefore, activities and facilities should be carefully designed to minimize the chance of any unintended greenhouse gas release.
All the values of EROI presented here and field development scenarios are based on the gas production behavior predicted by numerical simulators. Until 2021, there are a few attempts to produce gas from marine gas hydrate deposits in Japanese (
KY had worked for the organization of the study, computation of EROI from physical process, analysis of the field test data, and design of combination system of gas production and CCS. SN evaluated and designed the offshore production system and evaluated the economics and EROI of them from the industrial viewpoints.
SN was employed by the company Japan Methane Hydrate Operating Co., Ltd. (JMH).
The other author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
This paper is a part of the MH21-S research program funded by the Ministry of Economy, Trade and Industry. The authors acknowledge H. Kameda of Tokyo Gas and Mizuho Information and Research Institute Inc. (currently, Mizuho Research and Technologies, Ltd.) for their contribution to this work.