An experimental method was developed to measure both steam-water and air-water capillary pressures using an X-ray CT technique. We conducted spontaneous water imbibition and gravity drainage in ceramic core samples for steam-water and air-water systems respectively. The results were compared, and it was found that the steamwater capillary pressures were significantly less than the air-water capillary pressures in both imbibition and drainage cases.
Steam-water flow is of importance during steam injection into heavy oil reservoirs and water injection into geothermal reservoirs in which steam is produced. In these reservoirs, steam-water capillary pressure plays important role in controlling fluid distribution, transfer of liquid between fracture and matrix, and well productivity. The study of steam-water flow in porous media may also be useful for other single-component two-phase systems or even some multicomponent gas-liquid systems such as solution gas-oil and gas-condensate systems in which there is significant mass transfer between the two phases as pressure changes. However, it is very difficult to measure steam-water capillary pressure due to the phase transformation and the significant mass transfer between the two phases.
There has been some argument regarding the differences between steam-water and air-water flow through porous media in recent years. If there are no differences between the two, we could represent steamwater flow by air-water flow in which capillary pressure can be measured easily. Sanchez and Schechter1 reported that the differences between steam-water and nitrogenwater relative permeabilities were almost negligible in an unconsolidated core sample. However, Horne et al.2 found significant differences in experiments using Berea sandstone with a much lower permeability than that of the core sample used by Sanchez and Schechter1.
Accordingly, there may also be significant differences between steam-water and nitrogen-water capillary pressures. Very few direct comparisons of steam-water and air-water capillary pressures are available due to the scarcity of methods available to measure both steamwater and air-water capillary pressures. Li and Horne3 developed a technique based on the Kelvin equation to calculate steam-water capillary pressure using the data from steady-state steam-water flow experiments. This method is suitable for steam-water systems but not for air-water systems. Therefore, we developed another method that could measure both steam-water and airwater capillary pressures in order to identify the differences between the two. We conducted spontaneous water imbibition and gravity drainage in ceramic core samples for steam-water and air-water systems. The core sample was positioned vertically. The steam-water and air-water capillary pressures were calculated using the relationship between height and water saturation measured by an X-ray CT method. The steam-water and air-water capillary pressures of the core sample were obtained correspondingly.
In this study, the basic theory behind the measurements of steam-water and air-water capillary pressures is the balance between gravity and capillary pressure as a function of height in a core sample positioned vertically. Steam-water or air-water capillary pressure is equal to the gravity force once the spontaneous water imbibition into the core sample has been completed. The equation is expressed as follows: