A calculation model of adsorbed shale gas contents considering pore size distribution

Shale absorption gas content is an important indicator to evaluate resource potential and development value of shale gas. Pore size distribution in shale gas reservoirs is highly heterogeneous and micropores and mesopores are relatively developed, so the influence of pore space size on gas adsorption capacity is usually taken into consideration when shale absorption gas content is calculated. As a result, the calculation results are more different and even larger deviation generates. In order to solve these problems, this paper constructs an interlaminar structure of organic matter using graphene. Then, the adsorption phase density values under five groups of temperatures, nine groups of pressures and seven groups of pore sizes were simulated based on molecular dynamics. Finally, based on the simulation results, the calculation formula of adsorption phase density for three pore sizes classified based on pore diameter was established, and the isothermal adsorption model was modified on the basis of adsorption phase density. The calculation result comparison between the conventional isothermal adsorption model and the modified model shows that with the increase of pressure, the difference of the adsorption capacity calculated by both models is more than twice. The actual data processing result shows that the modified model is more consistent with the actual test data because it takes into consideration the influence of pore size on adsorption phase density. In conclusion, if shale absorption gas content is calculated directly by using the isothermal adsorption model without considering the influence of the change of pore size in shale on the adsorption phase density, the evaluation on the adsorption capacity of a shale gas reservoir will be greatly deviated. In the modified model, the calculation deviation is reduced, the calculation accuracy is increased and the calculation result is more consistent with the test result. © 2021, Natural Gas Industry Journal Agency. All right reserved.