Environmentally and economically responsible tight oil reservoir development is accomplished by understanding primary and enhanced production mechanisms. Tight oil reservoir pore networks include significant fractions of nanometer scale pores, which have a large effect on fluid flow and storage. Improved understanding between production and the pore scale will lead to more successful tight oil development. A history matched numerical model based on a cyclic gas injection field study is used to demonstrate the dual pore network model with scale dependent fluid models to describe pore scale production mechanisms.
Nanometer scale pore fluids have suppressed bubble point pressures due to high capillary pressure and increased fluid/pore-surface interactions. To model the pore scale effects, a scale dependent fluid model and a dual pore network model are used. The model is history matched to published production data from a field scale study in the East Texas Eagle Ford. Tracers are placed on pore network fluid components and injection gasses to provide insight into pore network fluid transfer. Fluid sources and pore network production rates are quantified for primary production and production following huff-n-puff gas injection.
The results of this modeling effort show that a significant fraction of additional oil produced following high pressure rich gas injection comes from nanoscale pores. This oil is not produced during primary production because the capillary forces tend to trap the oil. High pressure allows the injected gas to overcome the capillary forces and enter the pores. The injected gas reduces the oil viscosity and gas-oil interfacial tension, allowing the oil to be produced when the pressure drops during the production cycle. These results are consistent with observed field data that show higher production occurs when gas is injected at higher rates and pressures.
The results from this work will enable improved design of tight oil huff-n-puff gas injection processes. The model can be applied to a range of fluid types and pore throat size distributions to determine optimal injection parameters including injection pressures, gas volumes, and gas compositions. This information is critical in compressor sizing and estimating required gas volumes.