Differences between Steam-water and Air-water Capillary Pressures

Steam-water flow exists in most geothermal reservoirs where steam-water capillary pressure plays an important role in controlling fluid distribution, transfer of liquid between fracture and matrix, well productivity, and even the reserves. 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 as pressure changes. We would like to know if there are any differences between steam-water and air-water capillary pressures. If not, we could represent steamwater flow by air-water flow in which air-water capillary pressure can be measured easily. To address this question, we conducted spontaneous water imbibition into a vertically positioned ceramic sample saturated with steam and air respectively. The capillary force was equal to the gravity force once the water imbibition was completed. The imbibition steam-water and air-water capillary pressures in the same core sample were obtained correspondingly. The experimental results were compared, and it was found that there were significant differences between steam-water and air-water capillary pressures. The imbibition steam-water capillary pressure was less than the imbibition air-water capillary pressure. The experimental data demonstrated that we would not be able to substitute steam-water capillary pressure simply using air-water capillary pressure. Instead it is necessary to measure steam-water capillary pressure directly.