An experimental model of episodic gas release through fracture of fluid confined within a pressurized elastic reservoir. Issue 2 (26th January 2017)

• Record Type: Journal Article
• Authors: Rocco, Stefano; Woods, Andrew W.; Harrington, Jon; Norris, Simon
• Is Part Of: Geophysical research letters. Volume 44:Issue 2(2017)
• Page Start: 751
• Page End: 759
• Publication Date: 2017-01-26
• Subjects: fracturing -- clay seal -- episodic -- pressurization; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1002/2016GL071546
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: We present new experiments that identify a mechanism for episodic release of gas from a pressurized, deformable reservoir confined by a clay seal, as a result of the transition from bulk deformation to channel growth through the clay. Air is injected into the center of a thin cylindrical cell initially filled with a mixture of bentonite clay and water. For sufficiently dry mixtures, the pressure initially increases with little volume change. On reaching the yield stress of the clay‐water mixture, the lid of the cell then deforms elastically and an air‐filled void forms in the center of the cell as the clay is driven radially outward. With continued supply of air, the pressure continues to increase until reaching the fracture strength of the clay. A fracture‐like channel then forms and migrates to the outer edge of the cell, enabling the air to escape. The pressure then falls, and the clay flows back toward the center of the cell and seals the channel so the cycle can repeat. The phenomena may be relevant at mud volcanoes. Plain Language Summary: When gas is produced in a confined system in the subsurface, and trapped by a clay seal, then the gas pressure gradually builds up over time. Eventually, the pressure can become sufficiently large to mobilise the clay and this eventually leads to formation of a pathway along which the gas can escape. After decompression, the pathway becomes re‐sealed by the clay, and the generation of gas leads to a further increase in pressure, causing the cycle to repeat. In this way, short lived gas explosions may occur at the surface even if the gas is produced continuously at depth in the confined system. This may explain the process of cyclic explosions at mud volcanoes, and is relevant for using clay as a seal in geological waste repositories. Key Points: When clay is pressurized, there is a transition from plastic flow to fracture The rheological transitions lead to episodic gas release from a source with constant supply of gas The model provides an analogue to the process of intermittent mud volcano explosions