Underpressure, Overpressure, and Sand Pack Experiments

Nelson, Philip H.
U. S. Geological Survey, Denver, CO

One of the attributes of basin-centered gas systems is the presence of abnormal pore pressure. Both underpressured (pressure less than hydrostatic) and overpressured reservoirs (pressure greater than hydrostatic) have been documented in various Rocky Mountain basins. The question arises, how can both underpressured and overpressured rock formations exist over large horizontal distances?

I review two laboratory experiments with sand packs that were undertaken to address these questions. The first uses a column filled with layers of coarse and fine sand to create an underpressured sand layer (Gies, 1984). Air was injected into the bottom of a vertical water-saturated sand pack, displacing water. Pressure in the column was then found to be considerably less than hydrostatic pressure. The existence of underpressure was attributed to disruption of continuity of the water phase and creation of a continuous gas phase.

The second experiment uses layers of clay-sized quartz grains sandwiched between layers of coarser-grained quartz (Shosa and Cathles, 2001). Water flowed through the system in accordance with Darcy’s law. Carbon dioxide gas was introduced throughout the sand pack by dropping the pressure of carbon-dioxide-saturated water, exsolving the carbon dioxide. Flow stopped almost immediately. Pressure drops of approximately 30 psi were determined to exist across the layers of clay-size quartz grains. The induced no-flow pressure regime was stable for days thereafter. 

These two thought-provoking experiments may be relevant to the existence and stability of basin-centered gas systems. The first experiment shows that loss of buoyancy is possible even in high-permeability systems. The second experiment shows that pervasive gas saturation can create seals on a small scale that are additive and could produce substantial no-flow pressure seals separating overpressured compartments in low-permeability rock formations.