Phase Behavior and Storage in Organic Shale Nanopores: Modeling of Multicomponent Hydrocarbons in Connected Pore Systems and Implications for Fluids-in-place Estimates in Shale Oil and Gas Reservoirs

Abstract

Abstract Although there have been several efforts to quantify storage in shale nanopores, these have largely been based on generalization of the formulations for conventional reservoirs. Additionally, there is a lack of data addressing the effects of pore proximity on multicomponent adsorption and storage at a diverse set of pressures. Because it is nearly impossible with the currently available technologies to assess storage at the nano-scale, our work relies on the use of Molecular Dynamic simulation (to be called as MDS henceforth) techniques as well as a modified version of the Peng-Robinson EOS appropriate for modeling fluid behavior under pore proximity effects. We first describe the modified PR-EOS and demonstrate applications of pore confined methane phase behavior for different pore size distributions. For these chosen pore size distributions that are representative of organic nanopores, we derive an effective pore size that reproduces the composite phase behavior of the distribution of pore sizes. An effective pore size is defined because of the need to employ only one EOS for compositional modeling. Current efforts at modeling pore-confined phase behavior are largely restricted to tubes of a specified radius and may necessitate several fit-for-purpose EOS to model fluid behavior in different subsets of the pore size distribution. We demonstrate the need for careful examination of phase behavior when the pore volume contribution from the smallest of pores (sub-2nm) is substantial. However, our results indicate that for internmediate sized nanopores, an effective pore size representing the entire porous media may be derived. We then extend our modeling work to multicomponent systems and focus on the storage characteristics and phase behavior under confinement of a mixture of methane and octane. These results also indicate that when a substantial percentage of the pore volume is contained in the smallest of pores, the search for an effective pore size can become challenging. We then demonstrate some of the issues associated with fluid storage in organic nanopores by employing the graphene slit pore model. We model a replica of a connected pore system and demonstrate that pore proximity effects can substantially alter our expectations of storativity of methane, especially in the adsorbed layer. Finally, we demonstrate the need for moving beyond monolayer Langmuir adsorption models for describing storage by highlighting observations of multilayer adsorption of methane in organic pores. The key findings from this paper are as follows: Firstly, because the properties of alkanes differ with pore size, this study is the first to demonstrate that with complex pore connectivities, a simple extension of analyses from a single pore to connected pore systems is somewhat inadequate. This has implications for generating adsorption curves for reservoir simulation, to quantify fluids-in-place and to understand vapor-liquid equilibrium under the influence of pore proximity. We finally demonstrate that careful consideration of pore proximity effects in connected pore systems is necessary for a more meaningful quantification of reserves and predictions of well performance.