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Transport model of buoyant metamorphic fluid by hydrofracturing in leaky rock
Author(s) -
NAKASHIMA Y.
Publication year - 1995
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1995.tb00255.x
Subject(s) - geology , fluid dynamics , buoyancy , hydrostatic equilibrium , fluid pressure , mechanics , hydrostatic pressure , dilatant , porosity , geotechnical engineering , physics , quantum mechanics
Aqueous fluid released in metamorphism is transported upwards from depth to the Earth's surface. I propose a hydrofracturing model for the fluid transport. In the model, fluid is transported by the upward propagation of a two‐dimensional vertical fluid‐filled crack from a fluid reservoir (e.g. overpressured compartment under a seal) at depth to the Earth's surface; fluid is injected consecutively from the reservoir into the crack at a given (but not necessarily constant) injection rate; some of the injected fluid is lost by infiltration from the crack walls into the surrounding permeable rock. An approximate solution of the crack propagation is obtained using fluid dynamics for turbulent film flow and linear elastic fracture mechanics. The solution shows the transition from a regime in which the excess pressure of the fluid in the reservoir drives the propagation to a regime in which the buoyancy of the fluid in the crack drives the propagation. For example, if the net injection rate of H 2 O is 1 m 2 /s, the regime transition occurs when the vertical crack length becomes 280 m; after the transition, the propagation velocity and average aperture are constant: 21 m/s and 4.8 cm. If the injection rate is lower than a critical value, hydrofracturing cannot be an effective mode for the fluid transport because of the significant fluid loss by infiltration from the crack walls into the surrounding permeable rock. Assuming a fluid‐saturated crust with hydrostatic pore fluid pressure, a lower limit can be estimated for the injection rate required to transport H 2 O by hydrofracturing without significant fluid loss. For example, the lower limit for transport from a depth of 15 km to the Earth's surface is estimated at 0.2 m 2 /s if the crustal permeability is 10 ‐17 m 2 . The lower limit decreases with decreasing crustal permeability.