Study On The Experiment Of Gas Hydrate Production In Vertical Wells

Natural gas hydrate(NGH)is a new kind of clean energy in the world.It has the characteristics of wide distribution and high efficiency.Since the discovery of hydrates in 1810,some countries around the world have been exploring the scale of natural gas hydrates in their country and the methods of exploiting natural gas hydrates.At present,the extraction methods of natural gas hydrates mainly include depressurization,thermal stimulation,thermodynamic inhibitor injection,carbon dioxide displacement method etc.Taking into account the effects of natural gas productivity and mining time,the most commonly used mining method is considered to be a depressurization combined with thermal stimulation method.However,we find that few studies focused on the effect of different mining methods on NGH production in vertical wells,especially aiming at the in-situ electric heating without mass injection and coMParison of production effects in different modes.Considering the current research status,on the basis of the experimental equipment designed by ourselves,this paper carry out five sets of vertical well in situ simulation experiments.We adopt the following four methods of extraction: pure depressurization(PD),pure electric heating(PH),depressurization combined with electric heating method(SDH)and Haff and Puff(H&P).In order to find out the best mining method,We analyze the parameters of temperature field,average gas production rate,hydrate decompositionrate,net energy,energy consumption.The conversion of net energy and energy consumption is coMPared to the energy efficiency of each mining method as an energy quality.At the same time,an average net energy index is designed to coMPare the applicability of these mining methods to commercial development.The main results of this work are summarized as follows:(1)In situ electric heating without mass injection,the electric heating can effectively transfer heat to the hydrate deposit.The temperature of the PD,SDH and PH method continue to rise at different growth rates until the heat balance between inside and outside of the reactor.However,the temperature change trend of the H&P method is a cyclic rise with time fluctuation.The temperature change in the reactor is affected not only by mining methods but also by water production during mining.(2)The cumulative gas production curve and the cumulative gas production curve of each mining method are almost symmetrical about a straight line.The line is the connection that undissociation hydrate mass decomposed hydrates is equal to decomposed hydrates in reactor.It is named hydrate dissociation and gas production equilibrium line(LineNDB).It shows that the free gas in the reactor can not contribute to the cumulative gas production,and hydrate decomposition is the only source of gas production.(3)The energy of PH method is absorbed from outside water bath.The front of hydrate dissociation moves from the periphery to the central vertical well.CoMPared with PH method,The SDH and H&P method provide additional heating driving force.Hydrates mainly decompose from the central well to the boundary.The PH method only provides the thermal decomposition driving force provided by electric heating.Hydrates can only be decomposed from the central well to the boundary and PH method can not be fully exploited.The influence range of electric heating is limited in the PH method.(4)In the process of H&P method,the cumulative production of gas increases step by step,and the undissociation hydrate mass decreases zigzag in the reactor.The cumulative gas production and undissociation hydrate mass of the PD,SDH and PH method change with time gradually.(5)Although the different electric heating power provided in SDH method furnish different thermal drive power,the mining effect is the same in the time when the thermal drive can fully satisfy the decomposition of hydrate.Until the low power provided by the thermal drive can not meet the rapid decomposition of hydrate,the two mining effects are different.We can consider using the same mining method to control the rate of hydrate decomposition and mining time by changing the heating power,which provides a means for commercial exploitation.However,increasing the electric heating power has limited influence on the net energy acquisition.(6)In the condition of the same energy consumption,the driving force of the H&P method is great because the heating power is large,The increase of pressure in the reactor during the soaking period increases the pressure decomposition drive.So the time of mining is shorter than the SDH method.Under the same power,the continuous heating of the SDH method is used to maintain the continuous temperature gradient in the reactor.Electric heating consumes more energy.The H&P method does not belong to the continuous heating.During soaking and gas production,the hydrate absorbs the heat of quartz sand in the heated area and decomposes continuously.The energy supplied by electric heating is mainly absorbed by hydrate,and the heat loss is small.Through comprehensive evaluation of net energy,energy consumption,mining time and net energy rate index,it is considered that the H&P method is the best in the four methods.(7)Different mining methods can obviously affect the effect of hydrate mining.The average net energy index of the PD method is very small,and the commercial exploitation value is low.The energy efficiency and average net energy index of pure heating are negative,which is the lowest among the four methods.The pure heating method is not ideal and has no commercial exploitation value.In the study of the SDH method,we found that the reasonable selection of electric heating power during mining can lead to better mining results.The H&P method is ideal for mining.It has the most commercial exploitation value in the four methods.