Study On Exploitation Characteristics Of Low Permeability Methane Hydrate Reservoir With Underlying Water

Natural gas hydrate is a clean and efficient energy with huge energy reserves and high energy density,and only generate water and methane after dissociation.Because the natural gas hydrate has the necessary stability conditions,so natural gas hydrate resources often buried in permafrost regions and deep sea sediments,and compared with other conventional fossil fuel,natural gas hydrate mining is more complicated.Pressure transfer of low permeability hydrate reservoir is slow,which causes slow gas production rate,so improving the permeability of hydrate reservoir is the key to improve the efficiency of hydrate decomposition.According to the characteristics of hydrate bearing sediments in the South China Sea,Studying the hydrate decomposition of Marine soil can be a reference significance for hydrate exploitation in the South China Sea.In order to study hydrate dissociation under real hydrate sediments of the South China Sea,marine soil of the South China Sea is used as porous medium to conduct methane hydrate generation and dissociation research.On the other hand,a three dimensional model of natural gas hydrate dissociation is used,for low permeability of hydrate reservoirs in the South China sea restricting hydrate dissociation,the influence and promotion effect of fracture technology to low permeability hydrate reservoirs is studied,including the influence of different fracture form and convective heat transfer conditions,which can provide important reference for low permeability hydrate reservoir mining.In addition,through developing simulation program,pressure,temperature and hydrate dissociation of Class Ⅱhydrate sediments is investigated,the effect of step-wise depressurization on gas production rate is also explored.On the basis of step-wise depressurization,heat stimulation is added to explore the mechanism of pressure and heat combined regulation and hydrate dissociation under the condition of heat stimulation.The results show that the hydrate formation process of Marine sediment is different from that of quartz sand,there is no obvious temperature bulge,and the hydrate formation is slow.Gradient depressurization process requires a certain time of pressure stability to effectively reduce the temperature drop in the hydrate decomposition process.Fracture can create additional gas-water flow path in low permeability hydrate sediments,effectively promoting pressure transfer and improving production efficiency.Longer fracture depth is more conducive to the release of deep gas in the hydrate core,but the effect of fracture radius on hydrate dissociation is limited.In addition,the thermal contribution of sensible heat to hydrate dissociation is low,which is always lower than 15.9% in the simulation in this paper,so timely and adequate heat providing is essential of the continuous hydrate dissociation.high water saturation blocks pressure transfer of free water layer of Class Ⅱ hydrate sediments in the early stage.Step-wise depressurization can make gas production stable.Cumulative water production is related to the final production pressure,but independent of the gas production process.