Fengguang Li,Qing Yuan,Tianduo Li,Zhi Li,Changyu Sun*,Guangjin Chen*

1 Shandong Provincial Key Laboratory of Molecular Engineering,Qilu University of Technology(Shandong Academy of Science),Jinan 250653,China

2 School of Chemistry and Chemical Engineering,Liaocheng University,Liaocheng 252059,China

3 State Key Laboratory of Heavy Oil Processing,China University of Petroleum,Beijing 102249,China

Keywords:Hydrate Exploration/exploitation Recovery Production Stimulation technology

ABSTRACT Natural gas hydrate(NGH)is a highly efficient and clean energy,with huge reserves and widespread distribution in permafrost and marine areas.Researches all over the world are committed to developing an effective exploring technology for NGH reservoirs.In this paper,four conventional in-situ hydrate production methods,such as depressurization,thermal stimulation,inhibitor injection and CO2replacement,are briefly introduced.Due to the limitations of each method,there has been no significantly breakthrough in hydrate exploring technology.Inspired by the development of unconventional oil and gas fields,researchers have put forward some new hydrate production methods.We summarize the enhanced hydrate exploiting methods,such as CO2/N2-CH4replacement,CO2/H2-CH4replacement,hydraulic fracturing treatment,and solid exploration；and potential hydrate mining techniques,such as self-generating heat fluid injection,geothermal stimulation,the well pattern optimization of hydrate exploring.The importance of reservoir stimulation technology for hydrate exploitation is emphasized,and it is believed that hydrate reservoir modification technology is a key to open hydrate resources exploitation,and the major challenges in the process of hydrate exploitation are pointed out.The combination of multiple hydrate exploring technologies and their complementary advantages will be the development trend in the future so as to promote the process of hydrate industrialization.

1.Introduction

Natural gas hydrate(NGH)is widely recognized as one of the new alternative energy sources in the 21st century[1,2].The NGH resources discovered in the oceans and permafrost in the past two decades are particularly large.According to Kvenvolden [3],the global hydratebound methane is about 2.0×1016m3,which is twice the amount of carbon found in proven fossil fuels(coal,oil,and natural gas)all over the world[4].The gas hydrates are mainly found in high-latitude terrestrial permafrost and continental margin marine sediments,and more than 90%of them are distributed in the ocean.According to the different morphologies,hydrate-bearing sediments are classified into three types,pore filling,fractured and massive/noddle [5].NGH exists in many ways in reservoirs,which determines the diversity of hydrate production technology.

NGH resources in China are mainly distributed in the South China Sea,East China Sea,Qinghai-Tibet Plateau tundra and the northeast Permafrost.The South China Sea is the main research area for exploration and development,and many breakthroughs have been made in the research and exploration of NGH[6-10].According to the“China Energy Mineral Geological Survey Report”released in 2016,hydrate resources are about 83.65×1012m3,while the South China Sea resources account for about 80%.

As an energy resource,the international community is highly concerned with the future development and utilization of NGH.The United States,Russia,Japan,Canada,China and other countries have discovered large-scale NGH deposits through investigations,and have also conducted several gas production field tests in permafrost zones and sea areas by using conventional hydrate in situ exploring methods[11-16].Although the trial production has achieved success,it is affected by the sand production in the reservoir.Each field trial has problems such as low gas production and short production time.It indicates that the large-scale commercial exploitation of hydrate resources requires more advanced hydrate exploration technology.

Researchers all over the world are committed to developing effective technologies for the exploitation of NGH reservoirs.NGH resource is different from conventional oil and gas resource,the shale gas and tight sand gas existing in the reservoir in the form of adsorbed or free gas,while the hydrates are solid under the trapped conditions and have certain cementation to sediment particles.This determines that NGH exploring methods are obviously different from conventional oil and gas production methods.Inspired by unconventional oil and gas field production method,researchers have proposed some new methods for hydrate recovery from NGH bearing sediments based on the characteristics of hydrate reservoir distribution.In this research,we summarize the enhanced hydrate exploring methods and potential hydrate mining techniques,emphatically point out the importance of hydrate reservoir stimulation technology for hydrate exploitation,analyze the development trend of hydrate mining technology at present,and put forward the major challenges in the hydrate production process.It will provide important reference for the subsequent research of NGH exploitation techniques.

2.NGH Reservoir Type

According to Moridis and Collett[17,18],there are four main classes of NGH reservoirs in terms of geological characteristics and production strategies as seen in Fig.1.The common feature of class I,class II,and class III hydrate reservoirs is that the hydrate reservoir has an impermeable upper and lower cap layer,while class IV has no upper and lower cap layers.

As shown in Fig.1,class I reservoirs consist of a hydrate-bearing layer and an underlying two-phase zone of liquid water and gas.The upper hydrate-bearing sediment and the lower two-phase region form a stable system cooperatively.It can be divided into two categories based on the presence of components in the hydrate zone:hydrate and gas(Category IG)and hydrate and water(Category IW).Such hydrate reservoirs are currently the preferred target for hydrate production,because hydrate could be decomposed in hydrate reservoirs under equilibrium conditions with very small temperature or pressure disturbance.In addition,the advantage of this type of hydrate reservoir is that even if the hydrate decomposes very little,gas will still be produced due to the presence of free gas in the reservoir.The characteristic of this type of hydrate reservoir is that the gas-liquid-hydrate three-phase reaches equilibrium at the interface where the hydrate layer is in contact with the two-phase region.

Class II hydrate reservoir is a two-layer reservoir consisting of a hydrate-bearing interval and an underlying water-zone.As all components in the hydrate layer have the same temperature and pressure and there is no free gas in the multi-component system,the amount of produced gas and production rate are very low in this kind of gas hydrate reservoir.

Class III includes a single hydrate-bearing layer.The entire hydrate layer is in a stable region of temperature-pressure equilibrium.In the exploitation process,the same problem as class II will be encountered,and the early gas production rate is slow.

Class IV hydrate reservoirs are widely distributed in the marine seabed environment,which are characterized by the distributed diffusion and low saturation.The reservoir is short of impermeable upper and lower caprocks.It is generally believed that the exploitation value of this kind of reservoir is very low[20].

3.Introduction of Current Main Production Technology for Hydrates

Gas hydrates are fundamentally different from conventional resources,and the techniques of gas production from NGH are also destined to be different.To recover natural gas from NGH reservoirs,hydrate dissociation is the first step,most scientists and institutes currently focus on the dissociation of the NGH in situ.The most commonly proposed and studied methods for dissociating NGH are depressurization,thermal stimulation,inhibitor injection and gas replacement.Among these approaches,depressurization is regarded as the most promising technique due to its high energy efficiency.

3.1.Depressurization

The principle of the depressurization technique is to decompose the hydrate by lowering the pressure of the hydrate reservoir below the equilibrium pressure of the hydrate at the reservoir temperature[21].The depressurization method has the characteristics of low cost and is suitable for large scale recovering of hydrate reservoirs with high permeability,especially for the exploitation of class I hydrate reservoir containing underlying free fluids.It is considered to be the most promising exploring technology.A large number of experimental and simulation studies have been carried out to explore the effect of different depressurization method at home and abroad [13,22-38].Initially,researches only focused on the relationship between the variations of temperature and pressure parameters and the changes of water production and gas production.Gradually,the gas production law and theoretical analysis are carried out with comprehensive consideration of formation and depressurization conditions.With the advancement of detection technology,the research has shifted from macroscopic to microscopic levels,and great breakthroughs have been made in the microscopic decomposition mechanism[39].

Fig.1.Classification of NGH reservoirs[19].

Compared with other techniques,the approach of depressurization is simple in operation and has obvious advantages in economy and environmental protection.Therefore,this approach has received significant research attention over the years from experimental simulations to short term field production tests[22-29,40,41].The experimental results show that the main factors affecting the efficiency of depressurization are depressurization amplitude,ambient temperature,initial hydrate saturation and reservoir structural properties [26,27,41-44].However,due to the limitations of the depressurization method itself,there has not been much progress in the study of this method.For the class I hydrate reservoir with underlying free gas and liquid water,the pressure of the hydrate reservoir is effectively reduced by extracting free gas and liquid water,thereby realizing the exploitation of the hydrate reservoir.Hydrate reservoir in Mesoyaha gas field in West Siberia of the former Soviet Union has become the only one successfully exploited by depressurization in the world so far[45].

Hydrate decomposition is an endothermic process.The limitation of depressurization method is obvious,mainly because it cannot provide energy for hydrate decomposition,so energy supplement has become one of the most critical control steps for hydrate dissociation.Therefore,the application of depressurization has shown a tendency of combining with thermal stimulation in order to obtain higher recovering efficiency and economic benefits[36,46,47].

3.2.Thermal stimulation

The technical principle of thermal stimulation is to break the hydrate stable condition by raising the temperature of the hydrate reservoir above its equilibrium temperature.The main technical means of the thermal stimulation are traditional wellhead heat injection methods such as hot water,steam,and hot brine[48-56].With the development of technology,some scholars have proposed a variety of new ideas for downhole heating,such as direct heating by electromagnetic(microwave)technology,autogenous heating fluid heating method,solar heating method,and underground dry hot rock heating method[57-61].

The advantage of thermal stimulation method is that the decomposition process of the hydrate can be controlled,and the gas production rate can be regulated by the heat of the injection；the disadvantage of this method is the large energy loss,and the one from the pipeline during the process of injecting hot water from the ground or sea level to the reservoir results in the decrease of energy efficiency.When electromagnetic waves,microwaves,etc.,are used to directly heat the hydrate reservoir,the heat loss of pipeline transmission is avoided,and it is suitable for the exploitation of different types of hydrate reservoirs under different geological conditions[61].The self-generating hot fluid heating method and the underground dry hot rock heating method will be described in detail in Section 6.1.Limited by the injection heat capacity and technical means,the volume of the heatable hydrate reservoir is limited and the cost is extremely high because those technologies are still immature.The main problem is that the heat injected into the formation is not only heating the hydrate,but also more used to raise the temperature of the sediment particles,clay and underlying water,so the energy efficiency is extremely low.Studies have shown that heat loss can reach 10%-75%of the total energy injected from the hot fluid[62,63].

Although the hydrate heating method has made remarkable progress and many new heating methods have been put forward in recent years,these new methods still remain in the theoretical stage and need to be further studied to verify the feasibility of the technology.

3.3.Inhibitor injection

Technique of inhibitor injection works by shifting the equilibrium curve of NGH towards higher pressures and lower temperatures,thereby destabilizing hydrate at local conditions[64-68].The advantage of the approach is that it could greatly enhance gas recovery of hydrates by effectively improving the rate of hydrate dissociation in a very short time.However,the inhibitor injection method is not economical due to high expenses of chemical inhibitors,and may cause environmental problems.

Chemical inhibitors can be divided into two types:thermodynamic inhibitors and kinetic inhibitors.Thermodynamic inhibitors make the conditions of hydrate formation more severe by lowering the activity of water,that is,higher pressure or lower temperature.The thermodynamic inhibitors mainly include organic alcohols (like methanol,ethanol,ethylene glycol,glycerin)and inorganic salts(like NaCl,KCl,CaCl2).Kinetic inhibitors work by slowing down the growth rate of hydrate formation and are mainly used to inhibit the formation of NGH during oil and gas transport in pipelines[69,70].The application of the thermodynamic inhibitors in hydrate production promotes the hydrate decomposition and increases the rate of hydrate dissociation,thereby improving natural gas recovery[71-76].

According to the experimental results,the influencing factors of inhibitor injection include the type and concentration of inhibitor,injection temperature,injection rate and system pressure[67,77-82].Yuan et al.[53]investigated the gas production behavior at different injected solutions (hot water,saline solution,and ethylene glycol)with continuous injection mode using a three-dimensional reactor.It was reported that the ability to promote hydrate decomposition was NaCl ＞ethylene glycol ＞Na2SO4.In addition,as the volatility of the inhibitor increases,the inhibitory effect decreases.Therefore,ethylene glycol is more favorable for the decomposition of hydrates than ethanol and methanol.According to Makogon's findings[83],system pressure has significant impact on the inhibition performance of inhibitors.By comparing the gas production of double wells in continuous injectionproduction mode with that of single well in huff and puff injection production mode,Yuan et al.[53]found that the energy efficiency and gas production of the double wells were higher than that of the single well.

3.4.CO2-CH4replacement

The mechanism of CO2-CH4displacement technology is that according to the hydrate phase equilibrium condition difference between CO2and CH4,that is,under the same temperature/pressure,it is easier for CO2to form hydrates than CH4,CO2is injected into NGH reservoir to replace CH4.In the process,injected CO2replaces CH4and extracts methane to the ground.So far,the researchers have carried out a series of systematic studies on the CO2-CH4displacement from the experimental point of view.The feasibility of thermodynamics and kinetics has been verified[84-86].Different CO2phases and the various factors affecting the displacement reaction have been studied by a large number of macroscopic experiments.With the help of advanced techniques such as laser Raman spectroscopy,X-ray diffraction and neutron diffraction,significant breakthroughs have also been made in the micro-displacement mechanism[87-89].

In theory,the formation enthalpy of CO2hydrate is about 58.96 kJ·mol-1,and the dissociation enthalpy of methane hydrate is about 55.01 kJ·mol-1[90].Thus,it can be seen that the enthalpy of CO2hydrate formation can fully satisfy the enthalpy of methane hydrate dissociation,and ensure the automatically continuous displacement reaction.During the process of replacing NGH with greenhouse gas CO2,the formation of CO2hydrate can consume the water produced by the decomposition of methane hydrate,and release heat which is conducive to the continuous dissociation of methane hydrate.Under the cementation of CO2hydrate,the formation instability caused by the decomposition of NGH is avoided,and the greenhouse gas CO2is sealed in the sea floor,which serves double duty as a means of both sustainable energy source extraction and greenhouse gas sequestration.Therefore,gas replacement technology has attracted more and more attention and becomes a hot spot in the field of hydrate recovery from gas hydrate-bearing sediments[91].However,a large number of experimental results show that the replacement efficiency is low and it is difficult to meet the needs of commercial production.

In order to increase the replacement efficiency of CO2-CH4displacement,many researchers had tried to carry out experiments by using pure CO2gas,liquid CO2,CO2emulsion and gas mixtures of CO2/N2,CO2/H2[92-101].Experimental studies showed that the replacement performance of gas CO2,liquid CO2,and CO2emulsion is gradually increased.However,the overall replacement effect of these three methods is not ideal,the reaction time is long and the replacement efficiency is low,and the highest replacement efficiency of the experimental results is not more than 64%.The main reason is that methane hydrate is encapsulated by a newly formed CO2hydrate film,which seriously hinders the mass transfer and diffusion of gas molecules.Therefore,the biggest disadvantage of the gas replacement is the slow displacement rate.The displacement efficiency is related to sample volume,hydrate saturation,sediment permeability,hydrate formation,and temperature and pressure of CO2injection[93,102].If the gas hydrate saturation in the reservoir is higher,fluid injection will be difficult due to the lower permeability[103].Compared with gaseous or liquid CO2replacement,the displacement efficiency can be increased to 90%by using the mixed gas(CO2/N2,CO2/H2),which will be described in detail in Sections 5.1 and 5.2.

4.Field Production Test From Hydrate Reservoirs

According to the research results obtained by years of laboratory experiments and numerical simulations,the research of hydrates has shifted from resource exploration to field test.So far,seven trials of NGH have been carried out in the world,the main purpose of which is to discover the possibility of commercial exploitation of NGH resources,including 4 times in the land permafrost region and 3 times in the sea area[12,14-16,104-110].The hydrate field test sites are mainly distributed in the Mackenzie Delta of Canada,the North Slope of Alaska,the Nankai Trough of Japan and Shenhu area in the South China Sea.The details of the field test are shown in Table 1.

Moridis and Collett[17]pointed out that in the current global NGH resources,the amount of hydrate resources in the continental permafrost zone is about 100 Tcf(1 Tcf=28.317 Gm3),and the amount of sediment resources in the marine is about 110000 Tcf,and 90%of them are distributed in seabed clay silt or silt deposits.According to Gas Hydrate Resource Pyramid theory proposed by Moridis and Collett[18],there is a positive correlation between the potential of NGH reservoirs and the difficulties of exploration.From the top to the low end of Gas Hydrate Resource Pyramid,as the type of hydrate reservoir changes,the amount of hydrate resources increases,but the proportion of resource taste and recoverable reserves gradually decreases,so the technical difficulty of the exploitation of the NGH resources is also increasing rapidly.The location of NGH resource test in Gas Hydrate Resource Pyramid is shown in Fig.2.

Different technologies of field production tests have been carried out to exploit the NGH reservoirs.Among them,in 2002,the Mallic 5L-38 of Mackenzie Delta in Canada was subjected to a 125 h production test by using a thermal stimulation method.The mechanical screen was used for sand control,and a total of 466 m3of natural gas was produced.Eventually,the production was stopped due to efficiency and wellbore sand production[111].In 2007,the method of depressurization was used to test the Mallic 2L-38 well in the same area by perforation completion [11].The accumulative of gas production was 830 m3in 12.5 h,and the electrical submersible pump lifting equipment was worn out in the absence of effective sand control treatments and the trial production was terminated.Subsequently,the sand control device was installed in 2008.After 6 days of continuous depressurization test,and the cumulative gas production was 13000 m3.It fully showed that the depressurization method was superior to the injection heat method.In 2012,the hydrate production test was carried out by the combined method of CO2replacement and depressurization in the Mount Elbert Well on the North Slope of Alaska in USA.The cumulative gas production in 5 weeks was 28300 m3.Although some results have been achieved,the simulation analysis showed that most of the gas production is derived from the depressurization and the contribution of CO2displacement to the productivity was very limited.While it showed that CO2exchange efficiency has been the technical bottleneck restricting the application of this technology.In 2013,Japan conducted the first marine gas hydrate test in its Nankai Trough.Within 6 days,the gas production was 119000 m3tested by depressurization method with an average production of 20000 m3·d-1.However,the production test was forced to terminate due to the large amount of sand production[12].In June 2017,the second field test by depressurization was carried out.The pre-expansion GeoForm screen technology was used for sand control.After 12 days of production,the wellbore was buried by serious sand production,and the test exploring was terminated.Then downhole expansion GeoForm screen was adopted for preventing sand,the total gas production was 240000 m3in 24 days,and the sand control effect was better than the first one [15].In May-July 2017,China adopted depressurization method for the first field production trial of hydrate in Shenhu area of the South China Sea,producing 309000 m3of gas in 60 days,which created the longest time record for continuous stable gas production.These efforts have accumulated a large amount of data and experience,and have taken a solid step in the industrialization of NGH development.

5.Enhanced Approaches for Recovering Natural Gas Hydrate

5.1.CO2/N2-CH4replacement

Three common clathrate hydrate lattice structures have been identified,cubic structure I(sI),cubic structure II(sII)and hexagonal structure H (sH).The hydrate crystal lattices of sI and sII have two kinds of different cavities,i.e.,large and small ones,and the sH has three different cavities.For sI hydrate,the ratio of large cavities to small cavities is 3:1.During the CO2-CH4replacement process,the CH4molecules in the large cavities are mainly replaced,so the displacement efficiency is not more than 75%,and the experimental results also prove this.Adding a certain proportion of N2to CO2is beneficial to improve the replacement efficiency[114].Park et al.[99]used flue gas(CO2/N2)for hydrate replacement experiments and found that the replacement rate of sI hydrate can be increased to 85%.The main reason is the replacement of methane in the small cavities with the help of N2.The additional results were that,for sII gas hydrate,the CO2/N2-CH4displacement rate can be as high as 90%followed by structural transition phenomenon from sII to sI[115].This is also due to the replacement of small cavities in gas hydrates.Therefore,the substitution of CO2gas by CO2+N2mixed gas can simultaneously replace the methane molecules in the large and small cavities in the methane hydrate crystal lattice[87].In addition to the gas replacement mechanism,there is a gas purging mechanism in the process,and the improvement of the replacement efficiency is the result of the combination of the two,thereby obtaining a higher recovery yield.The mixed gas replacement is an enhanced exploitation method.This approach was unanimously recognized by the researchers.In 2012,the US Department of Energy injected a gas mixture of N2(77%)+CO2(23%)into target hydrate layer in Ignik Sikumi oil and gas field to verify the application potential of the mixed gas displacement method.Most of the N2injected into the formationwas recovered,and the CO2recovery rate was less than 50%,and a total of 2.83×104natural gas was successfully produced[108].

Table 1 Summary of NGH production tests in the world[112,113]

In the field of engineering applications,although the replacement rate of gas mixture is high,the permeability of NGH reservoir is extremely low,and there may be difficulty in injecting gas into the hydrate reservoir,so the volume of hydrate reservoir for gas production by replacement exploitation is limited.In order to further improve the gas production capacity of single well with high efficiency and stable production,this approach should be combined with optimized drilling scheme and fracturing techniques to maximize the efficiency of NGH production.

Using flue gas(typically N2content more than 70%)derived from the tail gas of power plants to extract NGH shows great economic advantages in the early stage and is of great significance for reducing carbon emissions.However,the produced gas is a mixture of CH4+N2+CO2in the later period of recovery.The existence of N2and CO2will reduce the combustion value of the natural gas,and the cost of separating CH4/N2gas may be higher [116].

5.2.CO2/H2-CH4replacement

Fig.2.Schematic diagram of previous hydrate exploitation test and their position in the hydrate pyramid.

Inspired by the idea that flue gas could significantly improve the replacement rate of hydrate exploiting,Wang et al.[117]proposed a method for recovering energy from hydrate reservoir by CO2/H2mixed gas on the basis of Rice's research[94].The final product of this approach is hydrogen instead of methane gas,and the main process is as shown in Fig.3.Firstly,the produced methane or gas mixture from NGH reservoir is sent to the steam reforming tower where methane gas can be converted into CO2and H2through steam reforming process.Secondly,part of the hydrogen is separated from the reforming gas,and the remaining mixed gas(CO2/H2)enriched in CO2is re-injected into the NGH reservoir to cyclically produce methane gas by the mixed gas replacement and gas sweep hybrid method.The idea of this work is very innovative,transferring the energy of NGH to the hydrogen energy source,and sealing the CO2generated in the process into subsea sediments,realizing zero carbon emissions.

The author verified the core content of the design idea through experiments,i.e.the feasibility of replacing methane hydrate with CO2/H2mixed gas.Due to the smaller size of H2molecule than that of the CH4and CO2molecules and the more severe conditions for the H2hydrate formation,H2does not exist as H2hydrate in the reservoir and is more easily extracted from the formation.The experimental results showed that the maximum replacement rate could reach about 70%.During the replacement process,it was found that the replacement efficiency was in contradiction with the CO2storage efficiency,and the composition of the mixed gas had great influence on the replacement efficiency.The replacement rate could be adjusted by the ratio of H2in the mixture,which had important enlightenment for the future seabed CO2sequestration.The new idea is still in the theoretical stage,and many technical problems still need further research.

5.3.Hydraulic fracturing treatment in NGH reservoir

Hydraulic fracturing has significantly revolutionized the economic recovery of unconventional oil and gas resource,such as tight gas,shale gas and coal bed gas,and the technology has already become and will continue to be one of the most effective engineering techniques for improving single well production [118,119].It has the following three main functions:(1)a fractured channel with a certain conductivity is set to pass through the damage area near the wellbore zone,bypassing the polluted area；(2)extending the crack channel is extended to make its length sufficient to further improve the production by entering the reservoir；and(3)the fractured channel changes the direction of fluid flow within the reservoir.This technology has been widely used in the field of industrial production,and can increase the output of oil and gas wells by multiples,dozens of times or even hundreds of times.It is a good tool for developing low permeability gas reservoirs and unconventional oil and gas reservoirs.

Fig.3.Diagram of producing natural gas from NGH reservoir by CO2/H2mixed gas replacement and steam reforming for hydrogen production.

For different formation properties of oil and gas reservoirs,the hydraulic fracturing process has great differences,and the fracture geometry formed after fracturing varies widely.In the process of tight sandstone reservoir fracturing,at present,the cross-linked guar gel system is used as fracturing fluid and ceramsite or quartz sand is used as proppant [120].After the fracturing operation,a main sand-filled crack with a certain length,width and height is formed within the reservoir,which has high permeability and greatly improved the recovery of the tight sandstone reservoir.However,for shale gas reservoirs,according to the characteristics of shale reservoir brittleness,largescale fracturing treatments can form as many complex hydraulic fracture networks as possible in the reservoir,thereby improving the recovery of shale gas [121,122].Therefore,for shale gas reservoir with natural fractures,the use of fresh water,low sand ratio and high flow rate fracturing operation method to communicate as many natural cracks as possible,and form a three-dimensional space interconnected fracture network is the ultimate goal of reservoir fracturing.However,the factors that determine whether fracturing networks can be formed depend on the development and brittleness of the natural fractures of the local reservoir instead of the fracturing treatment.This can only be achieved when the fracturing technology and parameters match the reservoir conditions.

NGH is different from conventional oil and gas resources,so hydraulic fracturing methods cannot be used for hydrate exploration without improvement.Hydrates have diagenesis,and hydrate reservoirs have strong rock mechanical properties under the cementation of hydrates on sediment particles.Nowadays,the production techniques on NGH mainly include depressurization,thermal injection,inhibitor injection and CO2exchange.These methods are all around how to decompose NGH in situ which is a key step in the process of hydrate production,but there is an equally important issue that needs to be paid attention to.That is,if the permeability of the hydrate reservoir is very low,even if the hydrate can be dissociated in situ,how the recovered natural gas enters the wellbore and flows to the surface.So far,seven short term field production trials have been carried out worldwide,and the recovery approach uses one or two of the mentioned methods.However,the yield of each field test is very low,the main reason may be the lower permeability of the reservoir,the limited gas production area and the limitation of the production technology itself.Only the hydrate around the wellbore is exploited during the hydrate production process,so the single well production rate is usually not high.

Fractured fractures provide a favorable path for changing the flow pattern,which not only has high permeability and is convenient for the recovered gas to enter the wellbore smoothly,but also significantly increases the gas production area of the hydrate reservoir.Therefore,if the fracturing technology is combined with one or several production methods mentioned above,it is expected to obtain considerable natural gas flow.However,the rigid structure of the reservoir after the hydrate decomposed will deform and even collapse without the cementation of hydrate and the fracturing also loses its function.Therefore,the rigid structure of the reservoir must also be maintained after the NGH is exploited,otherwise all efforts will be wasted.It can be seen that the hydrate reservoir modification and protection technology is a major technical problem that needs to be solved urgently for the commercial exploitation of hydrate resources.

The research on the application of traditional hydraulic fracturing to the hydrate reservoir reconstruction is extremely rare at home and abroad.In the process of conducting related studies,researchers should take full account of the characteristics of the NGH reservoir,combine the basic theory with the engineering technology perfectly,and develop technological innovation,in order to further promote the process of hydrate industrialization.

5.4.NGH recovery below freezing point

At present,seven field production trials have been carried out worldwide,most of which are forced to end due to sand production in the reservoir.The main reason is that a large amount of free water and methane gas are produced after the decomposition of NGH in the deposit,resulting in the sharp increase of net pore pressure of the reservoir.At the same time,the reservoir will become muddy after losing the cementation of NGH.Under the complex gas-liquid migration,the tiny sediment particles in the reservoir would migrate into the wellbore together,which brings serious problems to the natural gas production.In the process of hydrate exploitation,part of water produced by hydrate dissociation becomes ice,the generated ice can not only reduce the amount of water in the pores of sediment,but also consolidate part of the loose mud and sand,which plays an important role in slowing the sand production in the formation[123].

According to the results of Voronov et al.[124],the dissociation enthalpy of methane hydrate below freezing point is(143±10)J·g-1,and the decomposition enthalpy above freezing point is (415 ±15)J·g-1.Thus the heat released in the process of water freezing can provide enough energy for the hydrate decomposition.Konno et al.[33]performed hydrate decomposition experiments on the world's largest hydrate reactor at present.The author pointed out that the decomposition rate was very low when the energy of hydrate decomposition was provided by the latent heat of hydrate reservoir,while the gas production rate of hydrate increased greatly with the increase of depressurization amplitude and ice formation.Therefore,gas recovery of NGH below freezing points is an enhanced mining method.

Under natural hydrate reservoir conditions,it is roughly estimated that if the hydrate with a saturation of 15%is decomposed,the reservoir temperature can be lowered below 0°C.Once the reservoir temperature is below 0°C,the hydrate continues to decompose,turning the water formed on the surface of the hydrate into ice.It is found that hydrate dissociation has an obvious self-preservation effect at temperatures below-3.0°C[34,125].If the wellbore pressure difference is accurately controlled so that the hydrate reservoir can be exploited below the freezing point,the free water generated by hydrate decomposition can be turned into ice in situ,and the icing effect can be used to prevent sand production,reduce the water production of the hydrate reservoir,and thus increase gas production rate,it can be said to kill three birds with one stone.In the later stage of production,CO2is injected into the reservoir to form CO2hydrate to consolidate the reservoir,which plays a dual role of maintaining the stability of the formation and CO2storage under the seabed.

However,because of the narrow operating temperature range(only 3°C)and reservoir heterogeneity,this method is still in the theoretical stage and many technical problems still need further research.

5.5.NGH recovery in solid

The solid exploration method is to mechanically disintegrate the seabed hydrate and lift it to a sea surface platform in solid state.It uses the higher seawater temperature to provide the energy for the hydrate decomposition,and to decompose the hydrate in a controlled manner to obtain natural gas,instead of the direct seabed in-situ decomposition.This method fully utilizes the energy of high-temperature seawater at sea level,and has wide energy sources,which overcomes the disadvantage of low efficiency of hydrate decomposition in the seabed reservoirs[126].

At present,the NGH solid exploration method in a cutter suction manner attracted much attention.Firstly,the spiral cutters on the excavator cut and crush the seabed hydrate.The crushed hydrate particles are quickly sucked into the pipeline under the strong suction of the lifting pump,then pressurized by the lift pump and directly transported to the decomposition tank on the surface.Secondly,as the pressure in the decomposition tank decreases and the sea surface temperature is high,the hydrate decomposes rapidly.The decomposed natural gas is dried and collected at the gas collection station,while the tailings are discharged to the seafloor through the tailing pipes to prevent pollution of the ocean.Because the cutter suction exploration method directly uses the pipeline to transport the seabed NGH particles extracted by the mining vehicle to the sea surface for subsequent treatment such as decomposition,drying and separation,it is not necessary to construct a closed environment during the exploring process.It has the advantages of simple exploiting operation,strong persistence,and high efficiency,and is suitable for exploiting submarine gas hydrate reservoirs without compact overburden.

When the gas hydrate is raised from the seabed to the sea level,it involves a complex three-phase flow.The technical problem of controlling the flow state and flow pattern in the process of three-phase flow transportation must be solved,which needs to be further studied urgently.The main reason is that the gas in the three-phase flow is highly compressible and unstable.If the hydrate is decomposed,the gas component in the pipeline will increase greatly,and the high-pressure gas in the upper part of the pipeline will expand rapidly,then the rising speed of the hydrate slurry will accelerate and impact the pipeline and the offshore platform.This will bring serious potential safety hazards to gas production.At the same time,it should also actively prevent the riser from being blocked due to the secondary formation of hydrate in the riser.Therefore,pressure fluctuations and flow changes in the pipeline must be monitored in real time,so that system parameters can be adjusted in time to ensure safe and efficient upgrade.

Hydrate solids exploring method is a feasible way for hydrate deposits exposed to the seafloor,especially pure hydrate deposits.However,for hydrate deposits occurred in submarine silt,the relationship between hydrate content,hydrate burial depth and commercial mining value still needs further study.

6.Potential Approaches for Recovering Natural Gas Hydrate

6.1.Self-generating heat fluid injection

For in-situ hydrate heating technology,Russian researchers have proposed in-situ heating methods such as radioactive heat generation and acid-base neutralization reaction heat generation in hydrate reservoirs.By injecting radioactive nuclear waste into the reservoir adjacent layer beneath the hydrate reservoir,the hydrate can be decomposed in situ by using the radioactive heat generated by the nuclear waste.However,the main problem of this method is that the nuclear waste has radioactive damage to the operators during the construction process.And the nuclear waste may cause pollution to the marine ecological environment.Large-scale hydraulic fracturing technology is used to inject liquid acid and alkali into the hydrate layer to cause thermochemical reaction in the formation,and to use the heat generated by the thermochemical reaction to decompose the hydrate[127].The technical advantage of this method is that hydraulic fracturing can form interconnected cracks in the hydrate reservoir,increase the thermal interaction surface,improve the heat transfer efficiency,and solve the problem of mass and heat transfer in low permeability hydrate reservoir.It is a practical gas recovery method for NGH reservoir which can improve single well production.However,there is a great difference between the heating mechanism of this method and the ones of the conventional hot water injection and bottom hole heating methods.Further study is needed on the mechanism of exothermic kinetics,the temperature and pressure variation and the appropriate production process parameters.

6.2.Geothermal stimulation

Geothermal resources are a kind of renewable and clean energy.At present,the development and utilization of deep geothermal resources have been booming all over the world.Dry-hot rock is the most potential energy source of geothermal energy.It has the advantages of stability,high efficiency,safety,and green non-pollution[58,128].Preliminary research indicates that China's South China Sea is rich in geothermal resources[129].According to reports,China has successfully completed the experimental well of dry-hot rock development in Qiongbei area of Hainan in May 2018.If underground dry-hot rocks are used to provide energy sources for gas hydrate decomposition,it is also one of the best solutions for the development of NGH resources.

This approach combines horizontal wells,branch wells and hydraulic fracturing techniques.Firstly,horizontal wells or branch wells are drilled in the target dry-hot strata beneath the hydrate reservoir,and then a large number of interconnected fractures are formed in the dry-hot rock formation by using hydraulic fracturing technology to increase the seawater heat exchange surface and enhance the seawater heating efficiency.Secondly,horizontal wells are drilled separately at the top and bottom of the hydrate reservoir.Hydraulic fracturing is carried out in the hydrate reservoir using cross-fracturing mode.The top well is used as the production well and the bottom one is used as the hot seawater injection well.The high-temperature seawater heated by dry-hot rocks is injected into the hydrate reservoir to provide hydrate decomposition.Through the circulation of seawater in dry hot rocks and hydrate reservoirs,the energy of dry-hot racks is transferred to hydrate reservoir to realize the exploitation of hydrate resources.This method is feasible in principle,but the mechanisms of sand production and gas liquid flow in reservoirs are important scientific problems to be solved.

6.3.Optimizing the well pattern of hydrate exploring

Up to now,all the hydrate field production trials adopt the vertical well at home and abroad.The hydrate production tests show that there is a common problem of low single well production and short production time.In order to improve the production efficiency,dual-well or even multi-well production plan will be the future development trend,directional wells,especially horizontal wells,have also been highly valued by researchers[130,131].And the combination of well pattern and hydraulic fracturing technology is recognized as indispensable in the future hydrate large-scale exploring technology.

The vertical well is the most important drilling method for hydrate production at present.The main reason is that the approach is simple and economical,and can be combined with the existing main hydrate production methods and applied to the field trials of various hydrate reservoirs.Compared to a single well exploring program,a two-well or multi-well production plan is more conducive to hydrate exploitation[53,131,132].Simulation experiments show that the mining effect of the double well is obviously better than that of a single well.Among the hydrate multi-well mining methods,the five-point well netting method is of interest,which is usually implemented in combination with the heating method [133,134].Usually,there are two kinds of well pattern:one is to arrange four injection wells in the periphery and one production well in the middle and the other is to arrange four production wells in the periphery and only one injection well in the middle.In general,the heat injection wells and production wells are arranged in the same hydrate interval for completion,so that the hydrate decomposition and gas recovery can be concentrated in the same hydrate layer,which is more conducive to improving the efficiency of hydrate production.

Regardless of the type of well production method,increasing the permeability of tight NGH reservoir and the range of hydrate decomposition is the most critical issue to increase the single well yield of hydrate.Therefore,the combination of horizontal well and fracturing technology in hydrate exploring will be the main research direction in the future.

7.Challenges in NGH Exploitation

7.1.How to maximize single well productivity

Seven field production trials worldwide have accumulated a large amount of hydrate in-site exploiting experience,and the results show that the current use of hydrate recovering methods cannot obtain commercial gas flow,so the next research focus is how to maximize single well capacity.Horizontal drilling and hydraulic fracturing are necessary to reach economic success in shale gas reservoirs.And techniques such as horizontal well drilling,multi-lateral drilling,radial sidetracking and fracturing treatment are expected to significantly improve the area of hydrate recovery,and expandable screen technology can effectively achieve reservoir sand control,which was confirmed to be more liable in the marine field test in Japan in 2017[15].

Some technical measures in traditional oil and gas well engineering have significantly reference for the exploration and development of NGH,but they cannot be completely copied.In view of the special characteristics of hydrate reservoirs,we should develop safe,efficient and environmentally friendly hydrate resource development technologies based on original technologies through institutional innovation,technological innovation and equipment innovation.

7.2.How to prevent engineering and geological risks

The exploration of NGH,especially marine gas hydrates,must be carried out in a safe,efficient and continuous manner.The identification and prevention measures of engineering and geological risks related to the exploitation of marine gas hydrate must be done in advance.The following three aspects need to be investigated in detail:(1)possible types of engineering and geological risks and their inducing factors in the development of marine gas hydrates；(2)study of the impact mechanisms of different types of engineering and geological risks and the degree of influence on the exploitation of marine gas hydrate production；and(3)exploration of the preventive measures for different engineering and geological risks so as to ensure that various risks are within the controllable range and ensure long-term and safe exploring of hydrates.

7.3.Low temperature drilling fluid technology

Drilling fluid technology is an important part of oil and gas drilling engineering and plays a very important role in ensuring safe,high quality and fast drilling[135].The main functions of drilling fluid mud are embodied in carrying rock cuttings,stabilizing the borehole wall,balancing formation pressure,cooling and lubrication.Drilling mud contains multi-functional chemical additives.When drilling the hydrate reservoir,the drilling mud temperature is usually higher than the temperature of local hydrate reservoir and the chemical additives in mud,which will destroy the stable conditions of the hydrates and promote decomposition.The decomposed methane gas enters into the mud and causes the gas invasion,wellbore instability and even collapse,which affects normal drilling.Therefore,it is necessary to use the low temperature hydrate drilling fluid before drilling the hydrate reservoir,and the additives in the drilling fluid must also be compatible with characteristics of hydrate reservoir and do not promote the decomposition of the hydrate.

7.4.Cementing technology

Cementing engineering is the key to ensure the life of oil and gas wells,enhance oil recovery and develop oil and gas resources well in oil and gas well drilling[136].Cementing cement consists of minerals with different chemical properties.When mixed with water,complex multiphase chemical reactions would occur.The cementing process is the process of continuous hydration of cement slurry,and a large amount of heat will be released.And the released heat by the solidification of the cement will stimulate the continuous decomposition of the hydrate in the reservoir,resulting in creep and even collapse of the reservoir around wellbore.As a result,the wellbore may be enlarged and the water and gas may be turbulent,which may seriously affect the distribution of the annulus of the cement ring,and eventually the cementing quality is unqualified.So it probably affects the subsequent safety production.By optimizing the cement slurry system and suitable operation technology,the engineering and geological risks in cementing process can be prevented.

7.5.Sand control technology

Sand production in the stratum has always been one of the main problems plaguing loose sandstone reservoirs.The main geological factors of sand production in the reservoir are the loose cementation and low strength of mud.The mismatch of exploring patterns in the exploiting process will also cause sand production.During the exploitation of NGH reservoirs,the sediment particles lose the cementation after the decomposition of hydrates,and the problem of sand production in the formation will be more serious.The practice of several short term field production tests has shown that most of them have to stop testing because of sand production.It can be seen that sand control technology is of great significance for long term recovery of NGH bearing sediments.Chemical sand control is rarely used because it injects a large amount of chemical reagent into the formation,which may cause different degrees of pollution to the reservoir.Therefore,mechanical sand control has gradually become the mainstream,and the sand screen effect of the expansion screen is better.This is also evidenced by the fact that the downhole-expanded GeoForm screen used by Japan in 2017 showed a good sand control effect.

During the NGH production testing process,the sand production in different production modes cannot be well predicted.The main reason is that the coupling mechanism of gas-liquid flow is unclear in the process of hydrate exploring.In order to achieve better sand control effect,it is necessary to combine the geological characteristics of hydratebearing sediments to optimize the sand control method according to the size of sediment particles,focus on the design and development of multi-function expansion screen,and select the appropriate operation process parameters through the investigation of the gas-liquid coupling flow mechanism within the reservoir.

7.6.How to prevent the secondary formation of hydrates

During hydrate production,secondary hydrate formation may occur in reservoir,production string and transportation pipeline.Due to the difference of reservoir permeability and endothermic effect of hydrate dissociation,the decomposed gas may regenerate hydrate during the migration process within the reservoir.This will not affect normal safety production and may have an effect on the output of gas production.On the contrary,the hydrate particles that regenerated in the production string and transfer line can agglomerate into hydrate plugs,which may cause total blockage,and bring serious threat to the safety production[69,70,137,138].

The critical temperature,pressure conditions and location of hydrate formation under actual production conditions should be clarified in advance,and a reasonable hydrate inhibitor injection scheme is established to ensure the normal operation of the hydrate production string and the pipeline.

8.Prospect of Hydrate Industrialization

The field production trials worldwide have shown that the current techniques such as depressurization,thermal stimulation,inhibitor injection and CO2exchange are not likely to be applied independently to the commercial exploitation of NGH.The main reason is that these methods mainly consider how to decompose NGH in situ,without considering the pathway of recovering gas migration during largescale hydrate production.

Traditional oil and gas exploitation methods still play an important role in the development of NGH resources.How to increase the recovery area of hydrate reservoir to improve the productivity of single well and prevent the sand production and stability during the process of hydrate exploitation is the main scientific problems in the future of gas hydrate reservoir exploration.Hydraulic fracturing has been widely applied in the development of unconventional oil and gas reservoirs and has achieved great success.The fracturing method can significantly increase the recovery area of oil and gas reservoirs by creating artificial sandfilling fractures with high permeability and facilitate gas migration in the reservoir.In view of the specific characteristics of hydrate reservoir,it is necessary to continuously carry out innovative ideas,technological innovation and equipment innovation to develop sustainable,safe,efficient and environmentally friendly production technologies.For example,during the drilling process,multiple branches are formed in the hydrate reservoir with the help of techniques of horizontal well drilling,multi-branch drilling and radial drilling and combine with appropriate expansion screen completion technology for sand control and special stimulation technology to further improve the recovery area of the hydrate reservoir.It will be possible to fundamentally solve the production capacity constraints of hydrate production.

The exploitation of NGH is a comprehensive and multidisciplinary work involving basic science,applied technology and engineering technique.Marine gas hydrate exploration involves complex engineering and geological problems,high investment and high risk.The development methods and key technologies of NGH are difficult to verify in the limited number of field production trial.And it is difficult for universities and research institutes to achieve fundamental breakthroughs.Therefore,it is urgent to require reasonable guidance from government agencies.Universities and research institutes are responsible for the laboratory simulation experiment and the development of the theoretical model,and large enterprises with strong strength and technology are responsible for the field application of hydrate production.The two work together and cooperate closely to promote the industrialization process of NGH.