Numerical Study of Production Mechanisms and Gas-Oil Ratio Behavior of Liquid-Rich Shale Oil Reservoirs

Abstract

Abstract This paper summarizes our investigation of production mechanisms and well production performance in liquid-rich shale (LRS) oil wells and introduces a new methodology for modeling LRS oil well performance. It discusses the impact of nanopore confinement on PVT properties, transport properties, rock compaction, how these phenomena affect produced GOR of LRS oil wells, and how to incorporate these phenomena into reservoir modeling studies. The proposed methodology has been applied to construct a compositional simulation model using a hydraulic fracture reservoir model with logarithmically-spaced local grid refinement (LS-LGR grids). We incorporated the impact of nanopore confinement on phase behavior by using new correlations for modifying PVT properties in nanopores. The model was divided into three zones, hydraulic fracture, nanopores, and micro pores; each zone had different PVT, rock compaction and relative permeability properties. Production data from several LRS oil wells in the Eagle Ford shale were used for history matching and model calibration. Our numerical model is able to simulate producing GOR behavior of LRS oil wells accurately. The “flat” GOR's in early stages of production are caused by delayed development of two-phase flow as a result of reduction of the bubble point pressure in nanopores. Enhancement of critical gas saturation delays mobilization of gas molecules in nanopores and could extend non-intuitive GOR behavior further when reservoir pressure drops below the bubble point. We found that the impact of permeability reduction due to compaction on ultimate oil recovery could be more than 20%. The study reveals that the period of constant produced GOR depends on the volatility of the reservoir fluid and on the pore size distribution in the reservoir. For moderate-GOR oil reservoirs, the constant GOR duration is greater than for highly volatile oil reservoirs. Several unique phenomena in LRS oil reservoirs were explored in this study. As a result, we were able to introduce new correlations for modifying bulk PVT properties under confinement. These new correlations and the effects of nanopores can be combined with numerical models to simulate the performance of LRS oil reservoirs and to estimate EUR more accurately. We concluded that numerical models with PVT model based on fluid bulk properties are not able to accurately simulate unconventional LRS oil reservoir fluid flow. Confinement impact on PVT and rock-fluid property of nanopores play key roles in complex production behavior of these resources. We also concluded that, for numerical models to capture accurately fluid distribution and compositional variability in nano Darcy LRS oil reservoirs, it is necessary to have correct knowledge of pore/pore throat size distributions in the reservoir.