Abstract
As one of the key status of gas in shale reservoir, adsorption gas accounts for considerable percentage of total gas amount. Due to the complexity and nanostructure of shale gas reservoir, it is very challenging to represent adsorption gas through traditional methods. However, the integration of the fractal theory and molecular dynamics (MD) simulation may provide a new perspective of understanding such nanostructure and the micro-phenomenon happening in it. The key purpose of this paper is to investigate the adsorption phenomenon in shale kerogen. By using MD simulation and grand canonical Monte Carlo (GCMC) algorithm, the adsorption of methane in 2, 5 and 10[Formula: see text]nm slit-like pores is simulated for different temperature and pressure status. According to the results, the average gas density in smaller pores is higher than that in bigger pores, and multilayer adsorption presents on some areas of pore surfaces. Then, the simulation results are analyzed using the multilayer fractal adsorption model. The analysis indicates that the number of adsorption layer increases with pressure increase: four-layer adsorption presents in 10[Formula: see text]nm pores while three-layer adsorption shows up in 2[Formula: see text]nm and 5[Formula: see text]nm pores due to pore volume limit. Fractal dimension of pore wall surface generated in this study is in the range of 2.31–2.63. Moreover, high temperature could decrease the adsorption behavior in reservoir condition.
Highlights
With the technology development, shale gas has become the important component of oil and gas resources
The integration of the fractal theory and molecular dynamics (MD) simulation may provide a new perspective of understanding such nanostructure and the micro-phenomenon happening in it
At the early stage of shale gas development, free gas dominates the production; while the adsorption gas which accounts for 20–85% of original gas in place (OGIP)[1] determines gas production in the later stage due to slow desorption mechanism in nanopores.[2]
Summary
Shale gas has become the important component of oil and gas resources. The methane in shale reservoir mainly exists in three status: free gas in pores and fractures, solution gas in kerogen matrix, and adsorption gas on the surfaces of kerogen and minerals. At the early stage of shale gas development, free gas dominates the production; while the adsorption gas which accounts for 20–85% of original gas in place (OGIP)[1] determines gas production in the later stage due to slow desorption mechanism in nanopores.[2] the adsorption status of methane in shale matrix is very important. Molecular dynamics (MD) simulation plays an important role in investigating phenomena in complex systems, and grand canonical Monte Carlo (GCMC) algorithm is a simulation method to analyze the adsorption phenomenon in micro-porous media. The integration of MD simulation and GCMC technique is applied to analyze the adsorption behavior of methane in shale reservoir. A multilayer fractal adsorption model is used to match the isothermal adsorption data
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