Study Of Hydrate Sediments Percolation Characteristics Using Pore Network Model

It has been a strategic subject of national economic development that exploration and exploitation of energy and resources and seeking new alternative energy.Natural gas hydrates,as new clean energy,have huge reserves of resources.Recently,many countries around the world promote the relevant scientific research of natural gas hydrate resources exploration and exploitation in succession.The skeleton structure characteristcis of natural gas hydrate reservoirs and fluid properties inside are principal factors that dominate and conduct water/gas seepage characteristics in the natural gas hydrate reservoirs.Meanwhile,they govern the basic stability of natural gas hydrate reservoirs during the gas production,which is crucial for safely and efficiently natural gas hydrate exploitation.For the hydrate sediments skeleton structure characteristcis,X-ray CT visualization techniques was developed to recognize and anlysis the components in the natural gas hydrate reservoirs under the low temperature and high pressure.Using the X-ray CT visualization system,the internal character information of the pore space and reservoirs structure three-dimensional reconstruction,which lay the foundation for gas/water phase seepage charateristics,was obtained.To research the effects of the the hydrate sediments skeleton structure characteristcis and the fluid properties inside on the water/gas seepage characteristics,pore network model,which is widely applied in the oil field was used to study the the percolation properties in the hydrate sediments.Pore network model that has the random distribution characteristics of hydrate was built to quantize the internal character information of the pore space in the hydrate reservoirs.The simulation results of water/gas seepage characteristics by pore network model were compared with that in the reference and by the experments in our research group to verify the feasibility and accuracy of pore network model.Then pore network model was used to accurately calculate pore space feature factors of natural gas hydrate cores,anlysis the effects of reservoirs size distribution and natural gas hydrate saturation on the permeability variation,and study the influences of hydrate deposits wettability and interfacial tension between the each phase inside on the permeability.The effects of structure features in the hydrate reservoirs on the permeability variation was systematic researched.It was found that lower hydrate saturation accelerated the water phase flow,made water relative permeability and capillary pressure lower.Moreover,higher porosities of the hydrate reservoirs,which made the ratio in the pore network model higher,impeded the continuity and stability of gas phase flow,decreased the gas phase relative permeability and capillary pressure in the hydrate sediments.The effects of fluid properties in the hydrate reservoirs on the permeability variation was deeply discussed.It was conclused that in the uniform wetting system,water(water-wetting phase)relative permeability was lower in the wetting system than that in the non-wetting system.In the uniform non-wetting system,gas/water distribution and flow behaviors are contrary to that in the uniform wetting system,the variation trend of gas/water relative permeability are opposite,too.The water displacement efficiency was higher in the in the wetting system than that in the non-wetting system.In the fractional wetting system,the higher proportion of non-wetting partern promotes more fluid flow,results in the residual gas saturation lower(although gas flow slowly,it is still connected).And the higher the connectivity in the porous media,the lower residual gas saturation.In addition,the interfacial tension between water and gas two phase blocked the water and gas flow,and higher interfacial tension increase capillary pressure in the hydrate sediments.