Transport capacity of gas confined in nanoporous ultra-tight gas reservoirs with real gas effect and water storage mechanisms coupling

Abstract

It is a well-established fact that mineral matter in actual ultra-tight gas reservoirs presents a strong affinity for water. However, the presence of water within nanopores has not drawn due attention and is generally overlooked by previous literature. Furthermore, the other key physics required to be captured is how to upscale the gas conductance from a single nanopore to a core scale, which greatly aggravates the complexity of the issue. To date, the comprehensive apparent permeability model considering the aforementioned features is still lacking and the intent of this research is to achieve this goal. Firstly, the Beskok’s model is utilized to characterize bulk-gas transport behavior through nanotubes. And Li’s model is employed to quantify water storage mechanisms, which are based on thermodynamic equilibrium theory between liquid and vapor. More features, such as stress dependence and real gas effect are incorporated. Thus, the apparent permeability model for a single nanopore is developed. Secondly, actual core sample is discretized to numerous little cells and the pores within each cell are assumed to possess uniform pore size, which are assigned by Monte Carlo sampling from pore size distribution (PSD). Thus, the transport capacity of each cell can be quantified by utilizing the aforementioned established model for a single nanopore. Finally, based on apparent permeability of each cell, a reliable geostatistical method is employed to calculate the effective permeability of the core sample. And the proposed apparent permeability model for actual ultra-tight gas reservoirs is successfully verified against experimental data collected from published documents. Subsequently, we identify the effect of PSD, relative humidity, real gas effect and stress dependence on the apparent permeability of actual ultra-tight gas reservoirs. Results show that the presence of heterogeneity in 2D will contribute to the increase of effective permeability and the influence of water condensation only works when the relative humidity is relatively high (>0.8). Moreover, the real gas effect has little effect on the gas transport behavior at a certain pressure and can be neglected.