Steam-Water and Air-Water Capillary Pressures: Measurement and Comparison

Abstract

Abstract Direct comparisons between steam-water and air-water capillary pressures have been few because of the difficulty in experimental measurement. An experimental method was developed to measure both steam-water and air-water capillary pressures using an X-Ray CT technique. Spontaneous water imbibition and gravity drainage tests were conducted 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 less than the air-water capillary pressures. Introduction 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 an 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 also significant mass transfer between the two phases as pressure changes. However, it is difficult to measure steam-water capillary pressure using routine techniques (for example, semipermeable porousplate and centrifuge methods) due to the phase transformation and the significant mass transfer between the two phases. In situ fluid saturation measurement techniques, such as X-Ray CT, γ-ray, and NMR techniques, may be useful to overcome the difficulties. Many papers(1–4) have been published in oil-gas and oil-water flow, but few in the measurement of both steam-water and airwater capillary pressures. A similar fluid flow to steam-water flow is gas-condensate flow. Although a lot of papers(5–8) have been published on the measurement of relative permeability for gas-condensate systems, reports on the direct measurement of gas-condensate capillary pressure have been few. On the other hand, 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 steam-water flow by air-water flow in which capillary pressure can be measured easily. Sanchez and Schechter(9) reported that the differences between steam-water and nitrogen-water relative permeabilities were almost negligible in an unconsolidated core sample. However, Horne et al.(10) found significant differences in experiments using Berea sandstone with a much lower permeability than that of the core sample used by Sanchez and Schechter(9). 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 them. Li and Horne(11) 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 air-water capillary pressures in order to identify the differences between the two in this study. 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.