| Gas hydrates exploitation by pure depressurization has the disadvantage of decreasing driving force for gas release and extraction.Injecting N2,CO2+N2,CO2+H2and some other gases into hydrate reservoir can effectively promote the release of natural gas from hydrate without additional heat input,in addition to promoting gas transfer.Combining the promoting effects for gas release and transfer offered by injected gas and the directional driving force of depressurization,the released gas can be extracted in time and the production efficiency can be improved.Furthermore,it can also reduce the residual amount of the released gas in the formation and the risk of gas leakage.Given the functional shortcomings of current numerical simulators and limitations in experimental simulation,a new simulator for simulating hydrate production by gas injection is developed.In combination with depressurization,according to the hydrates occurrence condition（ocean/permafrost）,the processes of field-scale hydrate reservoirs exploitation by different gas injection methods are numerically simulated.The phase behaviors,mass and heat transfer,and their interaction mechanisms in hydrate production process under different gas injection methods and production schemes are studied.The main research contents are as follows:（1）The most widely used large-scale simulator TOUGH+HYDRATE is redeveloped to describe the process of hydrates exploitation by gas injection.It accounts for the transfer of heat,three gas components,water,water-soluble inhibitor between four possible phases（gas phase,liquid phase,hydrate phase and ice phase）and phase transitions between them.The behaviors of ternary hydrates and the gas exchange process can be described by the new code.The new simulator can be used to simulate gas hydrate production by gas injection methods（such as injecting N2,air,CO2+N2,CO2+H2,etc.）and coupled methods combining them with traditional mining methods.The history match for hydrate production field test in Alaska is conducted to validate the software.（2）Gas production from oceanic hydrates by N2/air injection combined with depressurization is investigated,and two gas sweep modes were proposed.In addition,the productivity evaluation method is established.When the depressurization degree is small,compared with pure depressurization,the net energy production can be greatly improved under continuous gas sweep.The continuous sweep mode is more suitable for the exploitation of hydrates reservoirs with high thermodynamic stability of hydrate or/and with soft formation that only allow mild depressurization.In the stage sweep mode,the quality of the produced gas are higher,whereas the overall production efficiency is largely dependent on the gas production efficiency during the simple depressurization period,that is,the degree of depressurization.Under reasonable production conditions,compared with pure depressurization,the combination of depressurization and gas sweep can greatly increase the CH4 production efficiency and net energy production,in addition to reduce water production.The production process in N2 and air injection is highly similar.In actual applications,air injection can be used to save the cost of feed gas and gas storage and transportation,and the corresponding safety measures must be taken.（3）Gas production from permafrost hydrate reservoir by depressurization combined with continuous CO2+N2 injection is investigated.The macroscopic performance of exploitation process is a propulsive and continuous alternating cycle process of CH4 hydrate dissociation and CO2+N2 hydrate formation.With the increase of N2 content in injected gas,the gas-to-water ratio and injection-production ratio increase,while the CO2 sequestration ratio and produced gas quality decrease.Properly raising up the injection pressure can improve CH4 production efficiency and amount of CO2 sequestered,while excessive injection pressure will result in a undesirable final recovery.Properly balancing N2-induced hydrate dissociation and CO2 sequestration is a key factor to achieve a favorable CH4 recovery ratio and injection-production ratio.The controlling factors of CH4 production mainly include driving forces for CH4 release and transfer and the local concentration of CH4 near the production well,which vary with time and injection gas composition.（4）According to the controlling factors in the process of hydrate exploitation enhanced with CO2+N2 injection,the process optimization direction is determined,and two production methods is proposed:gas injection with variable composition and CO2-rich gas mixture injection with preheating.The reasonable gas injection temperature for CO2-rich gas,and the direction and interval of gas composition adjustment are determined.The simultaneous optimization of production efficiency,produced gas quality,injection-production ratio and CO2 sequestration ratio is achieved（5）Hydrates exploitation by CO2+H2 injection combined with depressurization is studied,and the escape behaviors of released gas and H2 during the production process was investigated.For the hydrate reservoir without low-permeability caprocks,the injection and production pressure has a great influence on the production of CH4 and recovery of H2,which in turn determines the feasibility and rationality of the production scheme to a large extent,and it requires a higher well pattern density to obtain a favorable recovery of H2.The difference of H2 recovery ratio between different injection gas compositions is very small,whereas the net H2 consumption increases with the increase of H2 content in injection gas.When the hydrate reservoir has low-permeability caprocks,especially a low-permeability overburden,the CH4 recovery ratio,H2 recovery ratio and CO2 sequestration are greatly improved.The tentative idea of injection-production-separation-reinjection cycle process for composite technology combining CO2+H2 injection with CH4 steam reforming is feasible. |
