Abstract
Different from the conventional reservoirs, the liquid-rich shale reservoirs are known to possess a broad pore size distribution. In macropores and fractures, the porous geometries are in the size of micrometers. However, in organic matter, a significant amount of porosity consists of nanopores. In the nanopores, the fluid phase behavior deviates from the bulk-scale phase behavior due to the nano-confinement effect. The deviated phase behavior results in significant challenges in evaluating oil and gas in-place and understanding reservoir fluid depletion mechanism.
In this paper, the nano-confinement effect on hydrocarbon phase behavior in shale reservoirs is studied in three steps. Firstly, the bubble point temperatures of hydrocarbons in multiple sizes of nanopores are measured using the laboratory approach of differential scanning calorimetry (DSC). Secondly, a pore-size-dependent equation of state (PR-C EOS) extended from Peng-Robinson equation of state is completed with the experimental data. The PR-C EOS models the phase diagram with an extra dimension of pore size and the modeling results agree well with the experimental data. Thirdly, a multi-scale PVT simulator is developed to calculate the PVT of reservoir fluids in the shale pore size distribution systems. The whole pore size distribution is discretized into specific sizes of pores and PR-C EOS is used to describe the fluid per pore. The simulated multi-scale PVT provides a realistic picture of fluid phase behavior in liquid-rich shale reservoirs with macro-to nano-scale porous geometries and sheds light upon GOR behavior during production history.
