Zhenyuan Yin,Praveen Linga*

1 Department of Chemical and Biomolecular Engineering,National University of Singapore,117585,Singapore

2 Lloyd's Register Singapore Pte.Ltd.,138522,Singapore

Keywords:Methane hydrates Energy recovery Gas production Energy resource Recent advance Future outlook

ABSTRACT Natural gas has been considered as the best transition fuel into the future carbon constraint world.The ever-increasing demand for natural gas has prompted expanding research and development activities worldwide for exploring methane hydrates as a future energy resource.With its vast global resource volume(～3000 trillion cubic meter CH4)and high energy storage capacity(170 CH4v/v methane hydrate),recovering energy from naturally-occurring methane hydrate has attracted both academic and industry interests to demonstrate the technical feasibility and economic viability.In this review paper,we highlight the recent advances in fundamental researches,seminal discoveries and implications from ongoing drilling programs and field production tests,the impending knowledge gaps and the future perspectives of recovering energy from methane hydrates.We further emphasize the current scientific,technological and economic challenges in realizing long-term commercial gas production from methane hydrate reservoir.The continuous growth of the corresponding experimental studies in China should target these specific challenges to narrow the knowledge gaps between laboratory-scale investigations and reservoir-scale applications.Furthermore,we briefly discuss both the environmental and geomechanical issues related to exploiting methane hydrate as the future energy resource and believe that they should be of paramount importance in the future development of novel gas production technologies.

1.Introduction

Due to the ever-increasing population and projected global economic activities,the world primary energy demand is projected to rise continuously by 40.2% from 2016 to 2040 according to World Energy Outlook 2017[1].With the Paris Agreement(COP21)entering into force in 2016,the target is to keep the global temperature rise below 2 °C this century.Thus,the pursuit for cleaner energy has increased significantly.Natural gas (NG)is the cleanest burning fossil fuel and is considered as the best fuel to replace coal and oil to transition us into the future carbon-constrained world.Therefore,the world's demand of natural gas is projected to increase sharply by 44.0% now till 2040 (annual growth rate of 1.7%)reaching a quarter of the primary energy mix (i.e.oil,gas,coal,nuclear and renewables).Thus,rightly so,the International Energy Agency(IEA)terms this century as the“golden age”for natural gas.The supply of NG has been increasingly met by the shale gas boom and the future demand can be met by the prospects of methane hydrates(MHs)adding to the reserves of unconventional natural gas.MH is the most abundantly available resource of CH4in nature.In fact,the energy trapped in MH is more than twice of all the fossil fuels combined[2].

MH is ice-like solid non-stoichiometric crystalline compound,which is stable at favorable low temperature and high-pressure conditions [3].Pure MH forms Structure I hydrate with 2 small cages of 512and 6 large cages of 51262encompassing 46 H2O molecules[4].If all the large and small cages are filled by CH4molecule completely,the hydration number is 5.75.However,the hydration number of MH recovered from geological media varies from 5.90 to 8.20[5,6],indicating that the cage occupancy often is not 100%,especially for the small cages.MH possesses a high energy density and can store concentrated form of CH4:1 m3of MH contains 160 m3of CH4at STP condition along with 0.8 m3of water [7].The predominant gas component stored in naturally-occurring MH bearing sediment is CH4and is estimated about 3000-20000 trillion cubic meter(TCM)worldwide[8-10].Thus,it has attracted significant research interests as an energy source from both academic and industry for the past two decades in countries like,USA,Canada,Japan,China,S.Korea and India.Though significant challenges remain in realizing the commercial production from MH,research and development (R&D)activities and country programs continue to accelerate in the recent years with many significant scientific findings and technological breakthroughs.In this review article,we highlight the recent advances,the impending knowledge gaps,and the future perspectives in recovering energy from MH bearing sediment in the following perspectives:(a)the occurrence of naturally-occurring MH and its resource assessment；(b)the on-going drilling programs,field production tests and production technologies；and(c)the advances in research and development in laboratories.Furthermore,we highlight that with new technologies advancing possible long-term gas production tests from MH-bearing sediments,the assessment on the related environmental and safety issues (geologic hazards,marine ecology,and global warming)should be of paramount importance for future safe exploitation of MH as an energy resource.

2.Recent Advances in Recovering MH for Energy

2.1.Occurrence of MH and its resource assessment

Gas hydrates in general consist of water molecules,which form crystal lattice (‘host')encaging the gas molecule (‘guest').Typical guest molecules that can be caged include CH4,C2H6,C3H8,CO2,H2S,N2,H2,etc.[3].MH has CH4as the main constituent and are typically found at permafrost(300-1000 m in depth)and at offshore locations(0-200 m below sea floor)near the continental shelf within the hydrate stability zone [11].The depth of the hydrate stability zone depends on a few factors,including temperature,pressure,geothermal gradient,water salinity,and the species of the hydrateforming gas[12].

The estimate of the total Gas-in-Place(GIP)from MH has varied enormously over the years,ranging from 106trillion cubic meter(TCM)to 104TCM [13].These estimates are based on different models that depend on different evaluation criteria.Because of the wide variety of geological settings and the different modes of MH occurrence,the most critical information in evaluating the GIP is the saturation of the hydrate phase (SH)in the geological medium.Gleaning from the best available data from the MH core samples and using the modest assumption of a global average SH=1.0%,Milkov [8]estimated that the global gas hydrate GIP is 3000 TCM.This represents a substantial share of the carbon resource on earth.Fig.1a shows the amount of CH4stored in MH compared with other conventional and unconventional gas resource.Thus,exploring MH as a resource for energy recovery has continued to attract intense research efforts from governments,industries and research institutions in several countries (e.g.USA,Japan,India,China,S.Korea,Canada etc.).

Fig.1b presents the MH resource pyramid,where the top tier MH reservoirs are identified to be deposits that possess high SH,high T,and high permeability(k)of the sandy medium.Arctic region is ranked higher than marine location because of their close vicinity to existing production facilities.The next challenging tier of recourse includes MH that exist at seafloor as mounds in the shallow water region,and those disseminated MH in the oceanic environment with low saturation(SH=2.0%-4.0%)and no sealing cap.These hydrates represent the majority of the world's global GH in-place resource,unfortunately,the prospects for economic recovery of CH4from these resource are very poor with current technologies [16].However,with disruptive technologies and innovations emerging in the upstream explorations targeting these challenging zones(e.g.hydrate mining,solid-liquid separation technique),there is no doubt that the recoverable MH resource pyramid will continue to expand in the coming years.Future work still need to be carried out to further refine these key factors,including hydrate reservoir area,depth,geological medium porosity and hydrate saturation to accurately assess the GIP resource of hydrate reservoirs.Furthermore,to identify the most promising targets (high-quality hydrate reservoir)is one critical step for future economic-viable longterm production.

In the context of China,the estimation of MH resource is continuously expanding because of the on-going geological surveys led by a number of governmental and regional agencies since 2007.The general consensus is that MH exist abundantly in both permafrost and marine locations [17,18],mainly in the following regions:(a)the Qinghai-Tibet plateau permafrost,Muli region[19]；(b)the Northeast plateau permafrost,Mohe region [20]；(c)the South China Sea (SCS)area [21-23].The estimated resource volume of CH4is claimed to be around 85.0 TCM with major distribution (＞75.0%)in the SCS region [24]and with around 4.0 TCM distributed in the permafrost regions of China[25].Future work still needs to be focused on refining the MH resource volume with the updated logging data from the latest geological surveys in China in order to identify and quantify the producible gas from hydrate deposits and to provide key parameters for future gas hydrate production site selection and numerical simulation.

Fig.1.(a)Volume of CH4stored in MH and other conventional and unconventional gas resource[14]；(b)MH resource pyramid(adapted from Boswell and Collett[15]).

2.2.Drilling programs,field tests and production technologies

Since the first report of MH contributing to gas production from the Messoyakha gas field(Russia)in 1970s,there have been several worldwide geological surveys and a limited number of field production tests.The main objectives of geological surveys are threefold:(a)to identify the locations of gas hydrate reservoirs；(b)to estimate the GIP in the methane hydrate bearing sediments(MHBS)；and(c)to extract cores of MHBS from field to characterize their geomechanical and thermophysical properties.With the advancement in pressure-coring technology that is capable to preserve MHBS core under high pressure,the physical appearance of MHBS extracted from field has been revealed showing that these naturally-occurring hydrates are associated with certain type of sandy medium(e.g.sandstone,sands,clay,silt)and exhibit various morphologies (e.g.thin veins,nodule,mound,pore-filling in unconsolidated sands).Understanding the occurrence of hydrates and their spatial distribution characteristics in sediment will further refine our estimation on MH GIP resource discussed in Section 2.1.

In terms of recent drilling and coring programs,pressure core analysis from the samples collected in the 2015 National Gas Hydrate Program of India's second expedition,NGHP-02 in the Krishna-Godavari Basin,offshore India has just been released[26,27].The hydrate saturation (SH)of the core samples extracted is in the range of 50%-90% with morphologies of both pore-filling and fracture-filling identified.The effective permeability of the hydrate core is in the range of 0.01-10 mD with the horizontal/vertical permeability ratio determined to be 4 [28].The reduction of permeability with the increase of effective stress is also identified during the test.In addition,analysis of the overlaying seal sediment shows physical properties similar to clayey sediment with low permeability and high compressibility [29].The detailed pressure-core analysis of the MHBS and the log data of the MH interval allow numerical simulation to access the potential gas production behaviour of the hydrate reservoir under different scenarios.Such study predicts that the gas production rate is in the range of several thousand cubic meters per day and recommends a bottom-hole pressure of 10.0 MPa or less [30].However,great precautions should be taken on sand control,excess water production and pressure drawdown rate when lowering pressure for this site at ultra-deep(＞2500 m)water environment.The pressure core studies from NGHP-02 provide a wealth of petrophysical information and valuable findings on the gas-hydrate-bearing sediment,which could allow researchers to construct a state-of-theart numerical model to simulate the potential reservoir response in a depressurization-based scenario.

With respect to China,four geological surveys have been successfully conducted in the South China Sea region during the past decade.In 2007,the Guangzhou Marine Geological Survey(GMGS)completed the first expedition,GMGS1,in the Shenhu drilling area of the Northern South China Sea with 8 logging sites.The drilling results from GMGS1 marked the first time that concentrated (SH=25%-55%)and disseminated gas hydrate have been observed in fine-grain sediments [31].The second expedition,GMGS2,took place in 2013 in the eastern part of the Pearl River Mouth basin in the South China Sea,which lies northeast of the Shenhu site.5 out of 13 drilling sets were selected for further analysis by coring by the Pressure Core Analysis and Transfer System(PCATS).Gas hydrate cores extracted shows a variety of morphologies in both fine-grained and coarse-grained sediments.It is further concluded in GMGS2 that gas hydrate exits abundantly within the first 200 m below the seafloor in the drilled region[32].The third expedition,GMGS3,was completed in 2015 with a total of 19 sites drilled in the Shenhu area adjacent to the sites in GMGS1.Drilling results confirmed that concentrated gas hydrate exists in clay-rich silt layers 20-90 m thick demonstrating the production potential from this area.In addition,for the first time,Structure II hydrate and shallow marine hydrates below the base of the methane hydrate stability zone were observed in some regions of Shenhu [33].Expedition GMGS4 was completed in 2016 with a total of 21 drilling sites.Four sites at Shenhu and two sites at Xisha were selected for coring and in-situ testing.The permeability was relatively low at all sites and varied between 0.4 and 40.0 millidarcies as measured by the in-situ test.Comparing the drilling results with previous nearby site in GMGS3,it is identified that there is a good lateral homogeneity within the Shenhu region and no gas hydrate was identified at the Xisha sites [34].Complimentary to the drilling and coring expedition,a series of laboratory tests on the characteristics of gas hydrates recovered from Shenhu area were performed[35-37]elucidating the possible nature of coexistence of both sI and sII structure of gas hydrate in marine sediment.A brief summary of the past GMGS expeditions and major findings on the concentrated gas hydrate system in the Shenhu area is presented by Yang et al.[38].

Four different production techniques have been proposed so far targeting energy recovery from MH,including thermal stimulation,depressurization,the use of chemical injection,and gas-exchange method [2].The working principle of these methods is based on(a)altering the reservoir P/T condition to a region outside the MH stability zone,or (b)by shifting the hydrate equilibrium curve away from the original MH stability region.Depressurization method has been considered as the most efficient and has been tested in the most recent field tests in Eastern Japan Nankai Trough and Shenhu Area in the South China Sea in 2017(see Table 1).Of particular interest is the approach of using CO2injection to substitute CH4originally stabilized in the MH,which realizes energy recovery and carbon sequestration at the same time [39].However,due to our limited understanding of the fundamental behaviour of the slow-kinetic exchange process and the likely need to separate CO2/CH4after production,the concept of using low-SHMH reservoirs as locations for CO2storage and sequestration can be more attractive in the near short term[40].However,technical feasibility of storing CO2in hydrate-form under marine sediment and its long-term stability still need to be demonstrated[41].

Up to the present,eight field production tests (summarized in Table 1)have been carried out at both permafrost and offshore marine locations.In terms of production technology,depressurization has been demonstrated to be the most commonly used approach to destabilize the hydrate system for gas recovery.This is attributed to its higher energy efficiency and faster propagation of pressure front within the hydrate reservoir [42].Recent findings from the 1st offshore methane hydrate production off the coast of Japan in 2013 reveals that under a back-pressure of P=4.3 MPa,the dissociation front of MH expands laterally with a rate of 4.2 m per day from the production well.In addition,it is suggested from numerical simulation that the production rate of gas will increase over time.Moreover,the highly hydrate-saturated layers improves the gas to water ratio of the production fluids and is critically important to increase the energy efficiency for depressurization approach[43].

These field tests clearly demonstrated the technical feasibility of producing gas from hydrate reservoirs by existing petroleum production technologies,and these valuable production data obtained from field also serves as good data point for future numerical validation and prediction study.However,several major engineering challenges encountered during the production (e.g.excess water production,uncontrollable downhole pressure and sand production)coupled with the hostile environment of the production field (arctic and deepwater location)and high capital cost constrain these production field tests practically short in terms of duration.Thus,long-term field production test with sustainable gas production rate(on the scale of105m3per day)is desired and should be demonstrated to enable commercial-scale production activities[14].

Table 1 Summary of the eight MH field production tests up to the present

2.3.Advances in research&development

Our fundamental understanding of the behaviour of naturally occurring MHBS and the associated production of reservoir fluids have improved tremendously because of the aforementioned geological surveys and field tests.However,significant knowledge gaps persist,and these cannot be easily addressed by field tests because of their inhospitable locations of natural hydrate occurrences,long planning period,and very high cost.Given the considerable challenges of extracting undisturbed hydrate cores and the general lack of prior knowledge on naturally occurring MHBS,the associated knowledge gaps need to be addressed in laboratory studies involving synthetic MH samples under controlled conditions.Such laboratory studies are necessary to characterize the thermodynamic and phase equilibrium of MH in various salt solution within different types of porous media[44,45].In addition,laboratory studies also target on the thermophysical and geomechanical properties of MH and of MHBS(e.g.morphology,hydration number,phase equilibrium,density,thermal conductivity,specific heat,heat of formation and dissociation,stress,strain,Poisson's ratio,Young's moduli,and stiffness,etc.)[46].Table 2 summarizes the type of gas hydrate,gas component,the type of sandy medium and the related key thermophysical properties of gas hydrate cores extracted from various sites around the globe.Accurate quantification of these properties will in turn help describe the kinetic behaviour of heat and fluids flow,phase change,and structural change in relation to their host sandy matrix during the MH formation and dissociation processes[47].

Furthermore,a number of high-pressure reactors with volume varying between 1.0 L-1710.0 L were set up in different research groups around the world investigating the production behaviour of MH by testing out various production technologies with different well configurations[48-59].Table 3 presents a summary of the key features(scales,dimensions,production methods and capability of in-situ phase visualization)of the reactors employed for investigating the formation and dissociation behaviour of MH-bearing sediment along with the associated fluid production behaviour.The applicability of such innovative production techniques that promotes gas production from MH reactor needs to be further demonstrated in field-scale or reservoir-scale to justify its technical feasibility and economic viability.Fig.2 further illustrates the prospect of expanding research activities spanning from different scales from pore-scale,core-scale to reservoir-scale targeting specific properties and behaviour of MHBS for safe and efficient energy recovery.

In addition,it should be noted that forming synthetic MHBS samples in laboratories that mimic those naturally-occurring MHBS is extremely challenging given the difference in time-scale and dimension-scale between laboratory reactor and the geological reservoir.Various hydrate formation methods have been proposed in the past decade claiming formation of analogous sample of MHBS,however,debates on the best representative strategy continue.Recent work employing advanced instrumental techniques(e.g.X-ray CT[60,61],NMR[62],MRI[63,64],and electrical resistivity tomography[52])and the state-of-the-art numerical simulator(TOUGH+Hydrate v1.5)[65]investigating MH formation processes have provided visual evidence that the issue of SHspatial heterogeneity can be significant inside reactor.Much work remains in the field to investigate MHBS considering their pore-scale behaviour,morphologies,and the spatial distributions of various phases instead of generalizing on a bulk basis.

Furthermore,based on chemical engineering principles(heat transfer,mass transfer and reaction kinetics coupled with multi-phase fluids flow),a few mechanistic and phenomenological models have beenproposed by different research groups to explain the kinetic behaviour observed for MH formation[66-69]and dissociation[70-72]processes during the past two decades.A comprehensive review of these models can be found in[47,73,74].Novel research in laboratories should faithfully simulate the geological process of MH formation and dissociation occurred in natural environment (e.g.saline water concentration,types of marine sediment,mixtures of gas components,gas/water flow behaviour),and further understand the scalability and applicability of experimental production results to reservoir-scale field production test.

Table 2 Summary of gas hydrate structure,gas component,type of sandy medium and the related thermophysical properties of gas hydrate cores extracted from various sites.(keffrefers to the in-situ permeability with hydrate and kabsrefers to the absolute permeability of sandy medium without gas hydrate)

Table 3 Summary of key features of the hydrate reactors used for investigating gas production from MH-bearing sediment

The ability to numerically simulate the behaviour of MH under natural and laboratory conditions has improved notably.These simulation studies examine MH behaviour at multi-scale levels:molecular-scale,pore-scale,and reservoir-scale.The role of reservoir simulation is critically important as it is practically the only tool that allows the assessment of the long-term gas-production potential of MH reservoirs[75].Such a robust numerical simulator requires the description of all the dominant physical processes (heat transfer,fluids flow,and hydrate phase behaviour)involved in the formation and dissociation process of MHBS.A number of simulators were developed by various research groups for such need[73]:(a)the TOUGH+HYDRATE code and its earlier version HydrateResSim by Lawrence Berkeley National Laboratory [76,77]；(b)the MH-21 Hydrate Reservoir Simulator (MH-21 HYDRES)developed by a Japanese team [78]；(c)the STOMP-HYD code developed by the Pacific Northwest National Laboratory[79]；(d)a hydrate-specific variant in the commercial simulator CMG-STARS[80].A summary of the key features and capabilities of these various hydrate simulators and codes is presented in Table 4.The 1st international gas hydrate code comparison study (IGHCCS1)has also been conducted using these established simulators with five benchmark problems to understand how different modeling approaches could affect the production scenarios via depressurization and thermal stimulation involving gas hydrates in 2008[81].The coupling of reservoir simulation with geomechanical codes that is capable to describe the geomechanical response during gas production has also been explored and implemented under the platform of TOUGH+HYDRATE and FLAC3D by Rutqvist and Moridis [82],Rutqvist et al.[83],and Kim et al.[84].However,future effort still need to be taken further to validate and verify the constitutive relationships employed describing the strongly coupled heat transport,fluid flow,kinetics and the complex SH-related geomechanical processes.An ongoing international collaborative efforts (IGHCCS2)are working on benchmark and challenge problems on gas hydrate production with geomechanical coupling.

Fig.2.Schematics illustrating the prospect of multiscale investigation of the fundamental behaviour and properties of methane hydrate bearing sediment for safe and efficient energy recovery.

The application of numerical simulation on predicting the production behaviour of hydrate and the associated fluid production under different production scenarios is powerful [85-87],however,it still requires considerable effort in validation against experimental measurement and field production data to instill confidence.The parameters that describe the composite properties of MHBS(e.g.composite thermal conductivity[88])and the constitutional relationships that quantify the physical processes(e.g.relative permeability,capillary pressure,permeability reduction,kinetic rate model[46])need to be further improved to reflect a realistic hydrate reservoir or laboratory scenario[65,89].In many cases,this critical information can only be obtained from direct measurements from laboratory,drilling programs,field tests,and sometimes determined through a history-matching method.Reliable properties of MH-bearing sediment and constitutional relationships are the fundamental building blocks for a robust hydrate simulator [73].Moreover,sensitivity and optimization studies by means of numerical simulator will also play a key role in future production planning,including drilling site selection,well design and placement,predication and optimization on production of reservoir fluids[90].

It should be well noted that there may be fundamental difference in the controlling mechanism of fluid production in laboratory reactor compared with hydrate reservoir [73].From a number of numerical studies,it has already been elucidated that heat transfer and kinetics are the most dominant processes controlling the fluid production[47,91]；whereas in a hydrate reservoir,multiphase fluids flow through porous medium driven by pressure is the key[92,93].The most notable reason is the difference in both spatial and temporal scale between the two systems.In addition,MH formation and dissociation processes in laboratory reactor is bound to be disturbed by the surrounding ambient environment,especially those related to thermal processes[65,94].To illustrate,an infinite or semi-infinite boundary condition could be generally assumed in the production test of a hydrate reservoir,while most production studies in laboratory reactor(a closed system)employ a fixed temperature boundary,whose effect on fluid production could be significant.These disturbances could also affect the spatial distribution of hydrate inside reactor and so far,only a few advanced experimental [52,61,95]or numerical techniques [65,89]are capable to quantify such spatial heterogeneity.Thus,we should be cautious when extending lab fluid production data to field trails.However,the entire process of hydrate dissociation in reactor could be viewed as a longterm production test in reservoir based on scaling analysis[96].The final gas and water recovery ratio of a hydrate reservoir under different production scenarios could be similar to those obtained from experimental studies；whereas,the rate of fluid production requires a detailed scaling analysis or full-scale numerical analysis considering all geological features[43,97].

Table 4 Summary of the key features of various hydrate numerical simulators and codes

Fig.3.The roadmap of MH recovery activities around the world(adapted from Collett[111],courtesy of U.S.Department of Energy).

3.Challenges and Future Outlook

3.1.Technological and economic challenges

A review of the past field production tests and numerical predictions have revealed that depressurization is the most effective production technique for gas production from hydrate reservoirs.However,engineering challenges remain in continuous production of gas because of sudden surge in the bore-hole pressure,uncontrollable production of sands,and excessive water production[43].The release of gas from hydrates will also result in the production of water(sometimes massive).These are largely related to the unconsolidated nature of hydrate bearing sediment and the permeability change of the formation during hydrate dissociation.The technology advancement in production engineering(e.g.sand production management [98],dewatering and gas-water separation technology[99])need to be incorporated in future hydrate production tests to minimize the risk of borehole plugging,allowing for successful long-term production of gas.

Despite certain engineering challenges in the field,another major aspect to consider related to when gas hydrate will become a contributor to global energy supply is the economic viability of gas hydrate production.Information on the cost of past shortterm production tests is very limited and not yet representative for the economic evaluation of long-term production test.Based on the long-term production prediction from simulation,the estimated lowest gas price that would allow economically viable production from gas hydrates is 7.8 USD·Mbtu-1(2016)[100].Given the worldwide liquefied natural gas import price,Japan and China with high import price of 7.0-13.0 USD·Mbtu-1(2016)[101]and with the support of governing funds and favorable governmental policies will likely to expedite their country programs and first realize commercial-level production around 2025 based on their well-established country programs as shown in Fig.3.Rigorous economic viability analysis and life-cycle analysis of gas production from MH reservoir based on state-of-the-art design of production system is strongly desired with the additional acquisition of production experience and technological gains.

3.2.Environmental and geomechanical issues

While we discussed in detail about the future energy potential of MH,the environmental impact of MH is also an often-discussed topic of interest.The“clathrate gun hypothesis”postulates that the abrupt release of CH4(a potent greenhouse gas that is 21 times more potent than CO2)from MH could cause runaway global warming on a time scale less than a human life.However,a recent study from observation of gas seepage from an arctic shallow marine gas hydrate reservoir have found that ocean warming will have limited effect towards hydrate dissociation and gas hydrate could act as a temporary methane reservoir with methane release dominantly controlled by large-scale earth system changes (e.g.geology,oceanography and glaciology)[102].Though CH4release from MH is not a near-term detrimental environmental issue,close monitoring of gas seepage from shallow hydrate reservoirs and establishment of environmental baselines is strongly recommended to fully understand the impact of MH dissociation on climate change [103].Moreover,life-cycle analysis from drilling to production to well abandonment for gas production from largescale MH reservoirs will provide more insight on the critical stages to reduce the environmental impact.Nonetheless,safe design of production system considering all environmental and geomechanical issues in future field tests are required to avoid any possible catastrophic accident or release related to gas production from MH reservoirs.

Geomechanical behaviour of MHBS during gas production is complex,involving coupled processes pf multiphase fluids flow,heat transport,thermodynamic and geomechanical behaviour of MHBS.The complexity is more pronounced when hydrate dissociation occurs at geological media with different layers of strata,like the 1st and 2nd field tests at Eastern Nankai Trough,Japan[43,104].Submarine geohazards,such as marine landslides due to hydrate dissociation,and wellbore stability during methane gas production from hydrate reservoir,are two of the most outstanding issues that warrants investigation of the mechanical properties of MHBS [105].Triaxial tests have been conducted extensively to measure the bulk modulus,shear modulus,Young's modulus,stiffness,Poisson's ratio of both synthetic MHBS [106-108]and naturally-recovered MHBS[109]in various research groups worldwide.However,much uncertainly still exists about the relationship drawn using the non-uniform synthetic MHBS,where the corescale morphology of hydrate and its interaction between hydrate and sandy medium need to be considered[110].In addition,validation between numerical simulator coupling all the thermo-hydromechanical-chemical processes and experimental measurement on the geomechanical response of MHBS during gas production is yet to be demonstrated and warrants further investigation.The application of such numerical simulator to predict the stability of MHBS for both short-term drilling and well completion as well as long-term gas production from hydrate reservoir is much desired[82].

4.Summary and Conclusion

The continuous growing demand on natural gas and the preponderance of CH4stored in naturally occurring MHBS ascertain the role of MH as a potential energy resource.The past successful field expeditions and production tests have proved the technical feasibility of gas production from hydrate reservoirs and break new ground for future long-term commercial-level field activities of MH.Ongoing R&D activities across academic and industry are targeting specific challenges to understand the fundamental behaviour of MH,to develop cost-effective and environmentally responsible technologies to exploit MH resource efficiently.Understanding how MH interacts with other important geological,biological and chemical processes in the earth system and how MH production activities affect the global carbon cycle are still in the early stage and should be the emphasis for a safe exploration of MH as tomorrow's energy resource.

Acknowledgements

The financial support from the National University of Singapore(R-279-000-542-114)is greatly appreciated.Zhenyuan Yin would like to thank the EDB and LRS for the industrial postgraduate programme(IPP)scholarship.