Experimental Study On The Enhancement Of The Exploitation Of Frozen Natural Gas Hydrate By Heating Method

Natural gas hydrate(NGH)is a new kind of clean energy sources with abundant reserves,and high energy density.The development and utilization of NGH is expected to alleviate the world energy shortage and the global warming from a global perspective,and it is of great strategic significance to guarantee national energy security for China.However,very few studies have been published on the subject of “frozen hydrate” that is frozen by ice in the permafrost,which leads to a relative paucity of in-depth understanding on the decomposition characteristics and exploitation technologies of“frozen hydrate”.Therefore,this study adopts in-situ heating technology to carry out experimental research on the exploitation of frozen hydrate using different mining methods to provide technical support and theoretical guidance for the commercial development of hydrate in permafrost areas in China.The structure characteristics,distribution range,resource amount,reservoir classification of NGH are investigated and the current exploitation methods of NGH are summarized.The effect of field trial exploitation in permafrost area and the laboratory research status of hydrate below freezing point are stated.Based on the above,the in-situ heating technology is used to enhance the development of hydrate in this study.Then,according to the geological conditions of permafrost regions in China,we have synthesized frozen hydrate samples in the porous silica sand,and carried out “frozen hydrate” exploitation experiments by pure depressurization method(PD),pure electric heating method(PH),depressurization combined electric heating method(SDH)and huff and puff method(H&P)respectively below the freezing point.The research results mainly include:(1)In the process of hydrate formation,the system temperature drops rapidly and remains stable after reaching the ambient temperature.Driven by high pressure and low temperature,hydrate is continuously generated.The rate of hydrate formation decreases with time.The formation rate of hydrate is affected by initial pressure,initial gas-liquid saturation and gas-liquid contact area in the reactor.When all the hydrates in the reactor are covered with ice during the cooling stage,the ice film will hinder the contact between gas and water molecules,resulting in the hydrate formation rate almost dropping to 0 even if the system pressure is much higher than the phase equilibrium pressure at this time.(2)In the decomposition process of frozen methane hydrate below freezing point,the hydrate self-protection effect which is caused by the ice makes it impossible for frozen methane hydrate to utilize the sensible heat of the hydrate reservoir.Therefore,it is necessary to inject additional heat to realize the continuous decomposition of frozen methane hydrate below freezing point,and the heat is mainly transferred by the heat conduction among ice,hydrate and porous media particles.And when the ice melts to release gas and water,the heat convection begins to promote the heat transfer rate.(3)The experimental results of the frozen methane hydrate exploitation by different mining methods show that the PD method cannot eliminate the self-protection effect of hydrate,and cannot realize the smooth decomposition of frozen methane hydrate;the PH method and the SDH method which are with continuous heating have more advantages for destroying the self-protection effect of frozen methane hydrate below freezing point;However,the frozen hydrate cannot be completely decomposed by the PH method and H&P method,and the gas production rate and the cumulative gas production in PH is the slowest.In H&P,the obvious secondary formation of hydrate caused by convection-enhanced mass transfer hinders the decomposition of hydrate.Therefore,the production efficiency is better in SDH.(4)Parameter sensitivity analysis shows that under the premise of in-situ heating,reducing the production pressure from above phase equilibrium to below phase equilibrium can promote the decomposition of frozen hydrate,which indicates that depressurization driving force plays a significant role in promoting the decomposition of frozen hydrate.However,when the production pressure is lower than the phase equilibrium pressure,the gas production rate of frozen methane hydrate decomposition is less affected by pressure,which shows that below the quadruple point,frozen methane hydrate is not sensitive to the environmental pressure.In addition,increasing the duration of heating process in H&P is more conducive to the decomposition of frozen hydrate.(5)The energy efficiency analysis of the exploitation experiments involving in-situ heating shows that the H&P method can maintain relatively high energy efficiency in the whole production period,and the shorter heating time in the H&P can obtain greater net energy efficiency.The thermal efficiency of the H&P method is higher than that of the SDH method,and the heat utilization efficiency of the PH is the lowest.The time of obtaining the maximum net energy gain,the energy efficiency,thermal efficiency and hydrate decomposition rate of the H&P and SDH methods are almost the same,and the continuous heat injection mode can obtain the maximum net energy in a shorter time.Therefore,considering the energy efficiency,the thermal efficiency,the time corresponding to the maximum energy and the hydrate decomposition rate,it is considered that the SDH method has more advantages for commercial exploitation of frozen gas hydrate.