Molecular Simulations Of Shale Gas Behaviors In Porous Nanomedia

The shale gas is an unconventional supplementary energy to traditional fossil energy,and is stored in layered rocks with low permeability and porosity,which leads to the difficulty for exploration of shale gas.On the other hand,human beings are also facing the world's difficult problem of climate warming.To this end,capturing and storing greenhouse gases have become even more important.Combining these two problems,the replacement of shale gas with carbon dioxide has become a breakthrough in solving these two problems,and it has also become an important research topic.At the same time,hydraulic fracturing technology has been successfully applied to the commercial exploitation of shale gas in North America.Therefore,gas-liquid-solid three-phase coexistence is formed in shale reservoirs.It is worth exploring the subject of gas adsorption behavior in shale in the presence of water.In order to explore these two topics in detail,we have established corresponding shale models and simulated the gas adsorption behavior using molecular simulation methods.In this work,we use molecular simulations to study the displacement of shale gas by flue gas,in which flue gas is modeled as a binary mixture of CO2 and N2 and the shale model is represented by inorganic Illite and organic methylnaphthalene.CH4 was used as a shale gas model.Compared to the pure CO2,source of flue gas is easily available and the cost of displacement by flue gas would become lower.Results indicate that the pore size of shale is an important factor in the process of displacing shale gas and sequestrating flue gas,while the flue gas N2-CO2 ratio shows a small effect on process of methane displacement,because the high partial pressure of flue gas is the main driving force for displacement of shale gas.Moreover,the geological condition also has a significant effect on the process of CH4 displacement by flue gas.Therefore,we suggest that shale gas displacement by flue gas at a burial depth of 1km is more suitable operation condition.In the second work,we investigated the effects of pore size and water content in clay shale on the adsorption behavior of CO2,CH4,and their binary mixtures.We first established montmorillonite slit pore models with pore size of 12-20 A,and then selected a 20 A model to adsorb different amounts of water molecules in advance,and finally obtained models with water content of 0 to 0.4 g/ml.At different temperatures,the changes in the adsorption capacity and selectivity of the gases under pressure changes from 1 to 50 MPa were simulated.The results show that the pore size is positively related to the amount of gas adsorbed,and the pore size has a greater effect on the amount of CO2 adsorbed at low pressure;the temperature is negatively related to the amount of gas adsorbed,and the effect of temperature on the amount of CO2 adsorbed at low pressure Stronger.Under different pore size and temperature conditions,the CO2/CH4 adsorption selectivity is always greater than 1,which indicates that CO2 is preferentially adsorbed over the CH4 in montmorillonite model.However,both the pore size and temperature are inversely related to the CO2/CH4 adsorption selectivity.The research on water content found that the presence of water occupied the adsorption sites in the clay model and resulted in a sharp decrease in the amount of CH4 adsorbed.When the water content is small,it is favorable for CO2 adsorption.When the water content is further increased,the CO2 adsorption amount starts to decrease.We also found that the CO2/CH4 adsorption selectivity increased significantly with increasing water content.And in the presence of water,the CO2/CH4 adsorption selectivity has a monotonically decreasing relationship with pressure.Therefore,CO2 can still be used to replace shale gas when water is present,and the replacement effect will be better at low pressure.