Compositional Variation of Natural Gas in Kerogen During Production
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abstract
AbstractOrganic matter in source rocks such as coal and shale hold significant amount of adsorbed gas. Laboratory methos are available to measure the gas amount stored in the source rock samples. However, adsorbed gas release from the organic pore network and its production using wells is an issue under debate. The intricacy in the debate is mainly due to the physical properties of the adsorbed phase adsorbed phase. The adsorbed phase has an unknown composition and its molecules released in a selective fashion controlled by the organic pore walls. The organic matter nanopores bring in added complexity to the analysis due to nano-confinement effects. Our objective in this study is to predict the gas composition in kerogen pores in order to gain insight into the compositional variation with the pore size distribution in the formation. This prediction should be done properly at the initial conditions and during the pressure depletion. Thus, we also would like to investigate gas desorption impact on the adsorbed hase composition due to production.A molecular simulation method is developed using grand canonicalMonte Carlo simulation to study the in-situ composition of natural gas in model matter pores. We used the composition of produced fluid from a Chinese shale gas well. In essence, the method redistributes the fluid composition back into the organic pores at initial reservoir conditions. Then one-by-one the pores are depleted 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 depletion. The mean free path length of the fluid molecules, the fluid density and viscosity are computed once the thermodynamic simulation reaches an equilibrium.
