Experimental Study Of Natural Gas Hydrate Exploitation Through Large Scale Three-dimensional Device

Natural gas hydrates(NGHs),which are usually found in continental margins and permafrost regions,have been recognized as a promising source of energy for environmental protection and sustainable development of national economies.a variety of methods,such as depressurization,thermal stimulation,chemical inhibitor injection,and CO2 replacement,have been proposed.In this paper,Methane hydrate exploitation via depressurization and inhibitor injection for a reservoir with a bottom gas-rich zone was simulated using a large 196-L reactor,experiment that below the freezing point was also performed.The main research contents are as follows:(1)The largest hydrate simulation device in China was rebuilt,so that it can be fully suitable for the experimental study of various modes of hydrate exploitation,and the hydrate formation test was carried out,and the distribution rule of hydrate was deduced by using the temperature distribution in the reactor during the process of hydrate formation.(2)The experimental simulation on the exploitation of natural gas hydrate by single well depressurization method was completed,the relevant laws of hydrate decomposition were derived with the temperature distribution in the reactor;and the results were compared with the calculated hydrate decomposition curve and gas production curve.Meanwhile,the gas production was simulated by TOUGH+HYDRATE.The experimental results show that: the areas closer to the well and the bottom of the reactor had a greater J-T effect compared to other areas of the reactor owing to the faster local gas flow rate,and the bottom low-temperature area had a bad effect on the hydrate decomposition during the exploitation process.Exploitation via the depressurization method above the freezing point requires considerable time;the numerical simulation result from TOUGH+HYDRATE is more suitable for qualitative analysis of the gas production law and the theoretical gas production behavior.However,for analysis of hydrate uneven decomposition through different position of the well and the real decomposition rate,the error is relatively large.(3)The experimental simulation of gas hydrate production by dual well method was carried out.The temperature distribution and hydrate decomposition law in the reactor of dual well depressurization method were compared with that of the single well experiment.The experimental results show that: for gas rich system,the production efficiencies of horizontal well and vertical well were similar.Increasing the pressure drop can accelerate the decomposition rate of hydrate,thus shorten the time of complete decomposition of hydrate,however,increasing the pressure drop can easily aggravate the J-T effect and the cold effect caused by hydrate decomposition,which was easy to cause pipeline blockage.This thesis puts forward the method of fracturing technology to solve the problem of production well plugging.(4)The experimental simulation of natural gas hydrate production by inhibitor injection method was carried out.And experimental results show that: increasing injection volume and increasing injection times may have better production effect in theory,however,in practice,large injection volume may cause well plugging,and multiple-time injection was easy to cause plugging since injection wells was repeatedly eroded by the sand.Therefore,single injection method may have better mining effect.Compared with the simple depressurization exploitation method,the inhibitor injection mining method can significantly solve the problem of hydrate uneven decomposition,therefore,it can speed up the efficiency of hydrate reservoir mining.The soak time needs to be controlled within a reasonable range.At the beginning of the soak,the hydrate will continue to decompose until after 40 minutes when the hydrate no longer decomposes.At this stage,if the soak process continues,it may even lead to the secondary formation of hydrate in the far well area due to the increase of pressure.Therefore,the soak time should be no more than 40 minutes.(5)The experimental simulation of gas hydrate production by depressurization with single well below freezing point was carried out.The experimental results show that: for gas hydrate production above freezing point,the temperature in the area near the well and near the lower gas-rich zone decreases significantly,which restrains the further decomposition of hydrate.As a result,exploitation via the depressurization method above the freezing point requires considerable time.However,the exploitation below freezing point can effectively solve this problem which was mainly due to the following two reasons.Hydrate exploitation below the freezing point can significantly reduce the exploitation time for two reasons.1)The heat released from icing in the bottom low-temperature area can promote hydrate decomposition.2)The salt precipitation driven by freezing of the free water can also promote the decomposition process.