Impact of Composition on Natural Gas Desorption from Kerogen

Shale gas reservoirs hold adsorbed gas in kerogen. Laboratory techniques have been developed to measure the gas amount in samples. However, adsorbed gas recovery is an unsettled issue. Complexity is mainly due to adsorbed phase having an unknown composition and density, and desorbing in a selective fashion. Kerogen nanopores bring in added complexity to the analysis due to confinement effects. Our objective is to predict gas composition in kerogen pores and measure desorption limit during pressure depletion. A new molecular simulation method is developed to predict in-situ composition of natural gas in model kerogen pores using composition of produced fluid from a Chinese shale gas well. In essence, the method re-distributes the fluid composition back into kerogen pores at initial reservoir conditions. Then one-by-one the pores are blown down in small pressure steps while the compositional variation in the pores is monitored. The recovery is measured by comparing the residual hydrocarbon molecules at different pressure steps during the blow down. Density, viscosity, mean free path of the fluids in model kerogen pores are computed using the trajectories of the adsorbed and free molecules. At initial reservoir conditions we found that the gas mixture in kerogen nanopores becomes heavier and more viscous as the pore size becomes smaller. These compositional effects become significantly more amplified during the pressure depletion. Consequently, we observe that the kerogen pores release only the lighter end of the initial natural gas mixture in the pores, not allowing desorption of the heavier mixtures in smaller pores. The predicted Knudsen number values indicate that the compositional evolution in kerogen with pore size and pressure does not allow flow regime change. For the Chinese shale gas well fluid composition, we predict that the pores smaller than 5 nm has limited gas recovery and the transport in the larger pores stays in the slip flow regime. The paper presents results of recovery from kerogen using molecular simulation of fluids in nanopores. The results bring in new insights into our understanding of the natural gas production limits from kerogen. The results indicate that methane adsorption isotherms do not represent the true nature of multi-component gas desorption from shale samples.