Permeability jail for two-phase flow in tight sandstones: Formulation, application and sensitivity studies

Abstract

Permeability jails in tight rocks correspond to saturation values where no phase is mobile. This can lead to severe impacts on hydrocarbon production. However, although this is a severe and significant problem, there is a limited understanding of the mechanisms behind the concept of a permeability jail, its dependence on pore size distributions, saturations and pressure. In a previous work, we describe a model to compute the flow resistance during two-phase flow in tight rocks. In this paper, we extend our work to propose a model for the permeability jail in tight rocks to calculate relative permeabilities during two-phase flow. The rock is assumed to be characterized by a bundle of capillary tubes with non-uniform radii where the fluids are present as discontinuous phases. The pore-scale flow resistance is calculated which takes into account the capillary pressure induced by the Jamin effect as well as the water and gas viscous forces. The model is validated by experimental data of gas-water relative permeability tests and Nuclear Magnetic Resonance (NMR) experiments on tight cores from the Sichuan Basin in Southwest China. According to the model, the pressure drop between the inlet and outlet sides of the core should be higher than the maximum capillary pressure to maintain flow during the experiments. We also observe that the relative permeability curves are pressure-sensitive. An increase in the pressure drop leads to a reduction of gas relative permeability and an increase of water relative permeability. However, the influence of this pressure drop is somewhat limited. Once trapped in a permeability jail, fluids generally cannot regain flow capability by changing the pressure drop over a small range of values. This necessitates a careful consideration of the factors contributing to the permeability jail in order to avoid loss of well productivity.