Study On Mining Characteristic Of Multiclass Methane Hydrate Deposits By Depressurization Method

The unreasonable energy structure and serious environmental pollution in our country induce the urgent transformation ofenergy structure.Methane hydrate is an ideal alternative energy source with high energy density and low pollution.Many countries in the world are competing to develop its exploration and mining technologies.At present,the exploitation technonly of natural gas hydrate is not perfect,and further reaserch is needed.Hydrate deposits in the natural are divided into four classes(Class 1,2,3 and 4)according to the difference of distribution of hydrate,free gas and free water in the deposits.Class 4 hydrate deposits lack upper and lower impermeable caprocks and have low hydrate saturation,which are not worth exploiting and researching.There are differences in gas production rate and deposits temperature during the exploitation of different classes hydrate deposits.Current studies mainly focus on the Class 3 hydrate deposits,but lack of experimental comparative studies on different classes of hydrate deposits.In this paper,the mining characteristic for Class 1,2 and 3 hydrate deposit under different backpressure using depressurization were studied by experimental method,which was carried out by changing the way of sand filling and water filling.And the experimental results were compared with the simulation results.It provided data and theoretical support for depressurization mining of multiclass methane hydrates deposits.Class 1,2 and 3 hydrate deposits include gas-excess and water-excess according to the difference of gas and water content.Methane hydrate deposits of gas-excess Class 1,2 and 3 were constructed by three different ways of sand filling and water filling.A total of 11 dissociation experiments were conducted to analyze the effects of different backpressures on the dissocaition characteristics of three classes of hydrate deposits.The results showed that the gas recovery of three classes hydrate deposits was exponential distribution during depressurization,and that of Class 2 hydrate deposits was the highest.It was also found that the gas effective permeability affected mass transfer and the driving force of hydrate dissocaition at lower backpressure,which was the main factor affecting the gas production rate from methane hydrate dissociation.The water-excess Class 2 and 3 hydrate deposits were constructed by multiple water injection.10 groups of dissociation experiments were carried out,and the hydrate saturation in the hydrate layer was about 35%.It was found that under the same backpressure,the water production recovery of Class 2 hydrate deposit was about 10% higher than that of Class 3 hydrate deposit,and gas recovery of Class 2 was lower about 20% than that of Class 3 hydrate deposit.Compare gas-water ratio and gas production,it was found that there was a critical value for the influence of backpressure on the characteristics of depressurization.That is to say,there was not linear negative correlation between backpressure and gas-water ratio of hydrate depressurization exploitation,for example the gas-water ratio of backpressure of 2.0 MPa is lower than 2.3 MPa.In order to study the exploitation characteristics of gas hydrate deposits in South China Sea,the methane hydrate deposits were reconstructed by sediments from South China Sea.The gas hydrate saturation ranged from 10% to 24%.The experiments of hydrate dissocaition characteristics by direct and gradient depressurization methods were carried out respectively.It was found that the hydrate dissocaition rate decreased with the increase of hydrate saturation under direct depressurization method,and hydrate reformation was easy to occur at the initial stage of depressurization when hydrate saturation is low.The deposit temperature changes caused by gradient depressurization process were less than that caused by direct depressurization process,which was conducive to the rapid dissociation of hydrate in the later stage of exploitation.TOUGH+HYDRATE was used to simulate the dissociation process of depressurization in gas-excess and water-excess Class 2 and 3 methane hydrate deposits.The initial state and backpressure selection were the same as the experimental condition.It was found that the gas and water distribution in gas-excess hydrate deposits were evenly after dissociation.The gas and water were ladder distributed in water-excess hydrate deposits.Comparing the simulation and experimental results,it was found that the simulation and experimental result of total gas production by depressurization in water-excess deposits were similar when the backpressure was 2-2.6 MPa,but that in gas-excess deposits were quite different.