Pressure redistribution and free phase gas formation during and after undrained unloading of dissolved gas-charged sediment

ABSTRACT: Free-phase gas (FPG) formation and mobility in gas-saturated waters in the subsurface have geological and engineering implications, including topics as diverse as hydraulic fracturing and slope stability. FPG can form in submarine slopes and in groundwater where thermogenic or biogenic gasses are produced. Gas exsolution occurs when total dissolved gas pressure (PTGD) exceeds pore water pressure (PW), forming FPG (bubbles). We measured the responses of PTGD, PW, and volumetric water content during stepwise unloading of gas-charged sediment in a uniaxial cell filled with glass beads with varying grain sizes (2000 µm, 500 µm, 300 µm, 150 µm, 65 µm) to create FPG-water characteristic curves during FPG production of initially dissolved gas- and water-saturated materials. The transition from discrete FPG bubbles to gas mobilization occurred at higher saturation values in smaller grain size material.