Liquid-rich shale (LRS) volatile oil plays exhibit several challenges to modeling and analyzing such as flattened produced gas oil ratio (GOR), complex phase behavior, rock properties and fluid flow influenced by pore proximity effects in nano pores. In this study we investigated multi-phase flow and the governing flow mechanisms in these resources using a new procedure to divide shale matrix into different sub-media and a new methodology to model well performance.
We have developed new correlations for modifying PVT properties in nano pores to incorporate the impact of nano-pore confinement on phase behavior. We have subdivided shale matrix into three zones, nano-pores, micro pores, and natural (macro) fractures along with propped fractures with distinctive PVT, rock compaction and relative permeability properties in a compositional simulation model. We have implemented logarithmically-spaced local grid refinement (LS-LGR) to track saturation and pressure changes around the hydraulic fracture. Results from our new method were validated against production data from several LRS oil wells in the Eagle Ford shale.
Our numerical model reproduced anomalous produced GOR's that have been observed in liquid-rich shale oil wells. The study showed that the main impacts of nano-pore confinement on phase behavior, rock properties and fluid flow are reduction of bubble point, enhancement of critical gas saturation and severe permeability reduction due to compaction. Delayed development of two-phase flow as a result of reduction of the bubble point pressure in nano-pores causes the "flat" GOR's observed in early stages of production. 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. We found that ultimate oil recovery could change more than 20% by permeability reduction due to compaction. The study revealed that the period of constant produced GOR depends on the volatility of the reservoir fluid and pore size distribution in the reservoir. For moderate-GOR oil reservoirs, the constant GOR duration is greater than for highly volatile oil reservoirs, as well as in reservoirs with a greater percentage of nano pores.
The methodology introduced in this study allows us to better model LRS volatile oil wells and to estimate EUR more accurately. These new correlations for modifying bulk PVT properties under confinement combined with numerical models enables reservoir engineers to understand better the complicated physics in LRS volatile oil reservoir performance.