Phase transformation affects multiphase flow in geothermal and gas/condensate reservoirs owing to the same substance occurring in different phases. These effects change the phase behavior and the flow characteristics. The goals of this research were to compare the flow behavior and relative permeability differences between two-phase flow with and without phase-transformation effects in smooth-walled and rough-walled fractures. During this research, an experimental apparatus was built to capture the unstable nature of the two-phase flow in fractures and to display the flow structures in real time. Two-phase-flow experiments with phase-transformation effects (steam/water flow) and without phase-transformation effects (nitrogen/water flow) were conducted. The porous-medium approach was used to calculate two-phase relative permeabilities. From the results in this study, steam/water relative permeabilities are different from nitrogen/water relative permeabilities. The enhanced steam-phase relative permeability is caused by the effects of phase transformation. This shows consistency with some earlier studies in porous media. The nitrogen/water relative permeability is described most appropriately by using the viscous coupling model. However, steam/water flow in the rough-walled fracture, which is coupled with strong phase-transformation effects, seems to be represented better by Brooks-Corey relative permeability functions for fractured media (lambda right arrow infinity). The results from this study suggest that relative permeabilities accounting for phase-transformation effects must be used in simulations of geothermal and solution-gas reservoirs to represent two-phase interactions adequately.
Two-phase flow has long been of interest in earth-fluid and energy production, such as in petroleum reservoir and geothermal-reservoir engineering. Simulations of these reservoirs need knowledge of relative permeability functions, which have been studied theoretically and experimentally in porous media for two-phase, two-component systems (i.e., oil and water). However, the relative permeability properties of (1) fractured media and (2) flow with phase-transformation effects are of great importance but are poorly understood. Fractured reservoirs are not only the major reservoirs in geothermal fields, but they also represent more than 20% of the world's oil reserves (Saidi 1983). The phase-transformation effects are a characteristic of two-phase flows in geothermal reservoirs (steam/water flow) and gas/condensate reservoirs (gas/oil flow). In spite of considerable theoretical and experimental efforts during the last 2 decades for both of these issues, there are still no general models or approaches to describe relative permeability in fractures, either with or without phase-transformation effects.
There have been several studies conducted experimentally and theoretically for the steam/water relative permeability. These studies have been performed in consolidated or unconsolidated porous media. The results of these studies fell generally into two contradictory populations. Several studies suggested that in porous media, the steam/water relative permeability functions behave similarly to the nitrogen/water (or air/water) relative permeability functions (Sanchez and Schechter 1990; Piquemal 1994). However, another set of studies suggested that steam/water relative permeability functions behave differently from nitrogen/water in porous media (Arihara 1976; Counsil 1979; Verma 1986; Satik 1998; Mahiya 1999; O'Connor 2001). Most these studies showed that the steam-phase relative permeability is enhanced in comparison with nitrogen-phase relative permeability. To the best of our knowledge, no steam/water relative permeability results in fractured media have been reported yet because of the difficulties of the steam/water experiments and poor knowledge of fracture modeling for multiphase flows.