Numerical Study of Impact of Nano-Pores on Gas-Oil Ratio and Production Mechanisms in Liquid-Rich Shale Oil Reservoirs
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Copyright 2015, Unconventional Resources Technology Conference. Several anomalous production phenomena have been observed in liquid-rich shale (LRS) oil wells, indicating that they are quite different from conventional oil wells. One of these unique phenomenon is flattened producing gas oil ratio (GOR) due to nano-darcy permeability of shale rocks and effects of pore proximity on phase behavior and fluid flow in nanoscale pores. To understand true production mechanisms and well production performance in these resources, it is crucial to account for the impact of nano-pores in fluid flow studies. This study discusses the impact of nano-pore 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. Also, we have developed a methodology for modeling LRS oil wells that simulates GOR behavior accurately. The proposed methodology has been applied to construct a compositional simulation model using a single stage hydraulic fracture reservoir model with logarithmically-spaced local grid refinement (LS-LGR) grids. The effect of pore proximity on PVT is considered by introducing new correlations for modifying critical properties of fluid components in nano-pores. Production data from several horizontal wells in the Eagle Ford shale were used for history matching and model calibration. The study shows that "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 nano-pores. Enhancement of critical gas saturation delays mobilization of gas molecules in nano-pores and could extend non-intuitive GOR behavior further when reservoir pressure drops below the bubble point. The study reveals that period of flat produced GOR depends on the volatility of the reservoir fluid and the pore size distribution in the reservoir. For moderate-GOR oil reservoirs, the flat GOR lasts longer than for highly volatile oil reservoirs. We found that permeability reduction due to compaction on can reduce ultimate oil recovery by more than 20%. This study explored several unique phenomena in LRS reservoirs and presents new correlations for reservoir fluid properties and phase behavior under confinement. These new correlations and the effects of nano-pores can be combined with numerical models to simulate better the performance of LRS oil reservoirs and to estimate EUR accurately.
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Proceedings of the 3rd Unconventional Resources Technology Conference
