Drying of an Initially Saturated Fractured Volcanic Tuff

Abstract

The isothermal drying of an initially saturated welded tuffaceous rock was studied experimentally. Gamma-beam densitometry was used to measure the material’s effective porosity distribution prior to the drying experiment. It was then used to measure liquid saturation distributions during a 1400 hour drying period. The core selected for study was taken from the Busted Butte outcrop at the Nevada Test Site, part of the Topopah Spring Member of Paintbrush tuff. This specimen contained several microfractures transversely oriented to the direction of the water or vapor migration. These fractures were found to be regions of rapid dryout or low saturation even though they were displaced from the surface over which dry nitrogen was flowing. An imbibition experiment was performed earlier on the same core. In the imbibition experiment the presence of most of these microfractures was detected by discontinuities in the measured saturation curves, which indicated a delay in liquid transport past the microfractures. The mechanism for this “inside out” drying is believed to be capillary action that removes water from the larger-pore fracture zone. Vapor pressure lowering in the fine pore region, which would result in transport by evaporation, diffusion and condensation, is thought not to be important at room temperatures. Modeling of this dryout experiment reproduced some of the overall features of the experiment but underpredicted the saturation near the drying surfaces.