Attributes of Underpressured Gas Systems

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

Underpressured gas occurrences are characterized by pressure data that fall below the hydrostatic gradient. The data are usually aligned along pressure gradients of roughly 0.1 psi/ft, which reflect the density of gas. In this paper, underpressured gas systems are discussed in the context of a laboratory sand-pack experiment referred to as the Gies experiment. The Gies experiment showed that it is possible to create and sustain an underpressured state in a simple, unsealed, two-phase system. Consisting of layers of fine and coarse sand within a vertical transparent tube, the system is initially water saturated, with the top of water above the top of the uppermost sand. Air introduced from the bottom of the column displaces water from part of the sand pack, as evidenced by a rise in water level at the top of the column and by a reduction in color as dyed water is displaced from pores. Some isolated sand pores remain water saturated. After the air flow is turned off, a manometer at the bottom of the column shows a marked drop in pressure, responding to the removal of a continuous water phase throughout most of the sand column. The system is stable, at least on a laboratory time scale. When a bypass valve is opened and water at the top of the tube is connected to water at the bottom of the tube, gas is driven from the tube, and the manometer level rises to the top of water within the tube.

The experiment lends insight into some aspects of underpressured gas systems:

• The continuous introduction of gas can displace mobile water almost completely from the pore system. As a result, water continuity is disrupted and water cannot transmit pressure through the system.
• As long as water at the bottom of the system is hydraulically isolated from water at the top of the system, the isolated and underpressured gas is stable.
• Because the gas column is hydraulically isolated from any underlying aquifer, there is no buoyant force acting upon the gas. The pressure gradient is determined by gas density just as in a conventional gas reservoir, but pressure continuity with hydrostatic pressure lies at the top of the gas instead of at the bottom.
• The updip boundary of an underpressured system is not a seal in the usual sense, but rather consists of a water-saturated, coarse-grained sandstone overlying a gas-saturated, fine-grained sandstone. No conventional seal is required to cap the system, because no buoyancy force exists to force the gas upward. It follows that in natural systems, the leaky transition zone can be quite large spatially, allowing underpressured systems to be quite extensive.

Other aspects of natural underpressured systems were not addressed by this one-dimensional laboratory experiment. Extrapolations to a few aspects of natural underpressured systems follow:

• Underpressured systems are found in fine-grained, low-permeability, low-porosity rocks whereas the laboratory experiment was conducted in a permeable sand pack. The existence of natural systems in low-permeability rocks can be linked to the rate of gas generation. The disparity between the high relative mobility of gas and the low relative mobility of water in gas-saturated low-permeability rocks can play an important role in the generation and stability of natural systems.
• Active gas generation might or might not be necessary to sustain an underpressured system. The pressure differential is virtually zero within low permeability rocks near the updip limit. It might be possible to preserve an underpressured system for very long times once gas percolation ceases. On the other hand, an active downdip source could provide a pressure drive sufficient to replace gas loss at the updip transition zone.
• Uplift and erosion might play important roles in creating and maintaining underpressured systems. Tilting of beds creates a preferred geometry for continuity between and coexistence of source, reservoir, and release of gas. Erosion of beds creates a path for gas being actively generated to escape from the underpressured system. Uplift and erosion cause the pressure at the upper end of the system to be continually reduced, allowing gas dissolved in immobile water to be exsolved, thereby providing a secondary source of gas throughout the system.

Although no single laboratory experiment can answer all questions regarding underpressured gas systems, laboratory experiments can play a critical role in stimulating thought on various aspects of these systems and in uncovering phenomena that are not readily obvious.