Three-phase flow simulation in ultra-low permeability organic shale via a multiple permeability approach

This paper proposes a novel approach to model multiphase flow of hydrocarbons in unconventional shale reservoirs. Understanding the storage/flow mechanisms from such reservoirs as in the Eagle Ford, Woodford, and Bakken, is crucial in the overall effort to increase the ultimate hydrocarbon production. Based on the geological evidence, three different pore systems with distinctive storage/transport characteristics have been recognized in the shale reservoirs - inorganic medium, kerogen (or organic matter), and natural/hydraulic fractures. Our simulation results show that compared to oil, gas recovery in shale reservoirs can be more efficient due to a dual Darcy/diffusive flow mechanism of the gas phase (as a result of molecule-wall collisions). Darcy flow is considered as the only flow means for the liquid phase transport in such reservoirs. Although oil-wet kerogen is considered to be a rich source of hydrocarbon, nano-darcy permeability of the rock hinders oil production in organic-rich shale. Our sensitivity analysis shows that considering diffusive flow will increase gas recovery by facilitating gas transport in kerogen. On the other hand, although increasing adsorption capabilities in the rock means more hydrocarbon is stored in the organic carbon, the oil-wet nature of the hydrocarbon-saturated kerogen will control the depletion process in this media. Therefore, the limited pressure decrease in the kerogen reduces the effects of hydrocarbon desorption on oil and gas recovery.