Fluid Flow and Transport in Rocks Mechanisms and effects
This book represents the proceedings of the 9th written by a very active group of physicists at Kongsberg seminar, held at the Norwegian Mining the University of Oslo - physicists interested in Museum located in the city of Kongsberg about complex systems
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FLUID FLOW AND TRANSPORT IN ROCKS Mechanisms and effects Edited by
B. Jamtveit Department of Geology University of Oslo
and
B. W D. Yardley Department of Earth Sciences University of Leeds
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CHAPMAN
120°C) and the pore fluid flow rates lowest, the pore water should be closest to equilibrium with the minerals. In the case of focused upwards flow of pore water, the pore water may be supersaturated, particularly with respect to the silicate minerals. 2.7 QUANTIFICATIONS OF FLUID FLUXES DURING BASIN SUBSIDENCE - IS LARGE-SCALE, EPISODIC RAPID DEWATERING POSSIBLE?
As discussed above the integrated vertical flux due to sediment compaction during subsidence from 3.5 to 4.5 km may typically be of the order of 50-100 m 3 m - 2 (Fig. 2.3). We have also shown that dehydration of clay minerals such as smectite and kaolinite may produce similar fluxes. The amount of fluid petroleum fluid generates from maturation of kerogen depends on the richness and thickness of the source rock. In the example calculated above, the total fluid volume generated is about 10 m3 m -', which is less than the volume of water generated by compaction and dehydration of minerals on average. Locally within and around the source rocks, the relative contribution of petroleum generation to the build-up of overpressure is, however, much higher and it is probably the main cause of hydro-fracturing, allowing primary migration of petroleum out of the source rocks. In basin modelling the burial rate of the rocks due to basin subsidence and compaction must be treated separately from the velocity of the fluid phase. Assuming a basin subsidence of 10 - 4 m a-I (1 km in 10 Ma) the yearly flux is about 10 - 5 m 3 m - 2. If the porosity is about 10% the absolute velocity is close to the subsidence rate. This means that the rocks
30
Lithological control in sedimentary basins
are sinking through a more or less stationary column of water. During migration the petroleum is focused through anticlines and through the tops of faulted sandstones where the fracture pressure is lowest relative to the pressures in the sandstones (Fig. 2.6). In overpressured systems, water will be focused in the same way through fractures produced by hydro-fracturing in shales over structural highs. The average compaction-driven fluid flux may therefore be increased significantly by this mechanism in overpressured parts of sedimentary basins. Roberts and Nunn (1995) assume in their model that fractures in sealing shales open up and maintain a fracture width of 0.25 mm. As discussed above, to maintain such open fractures the pressure must exceed the horizontal stress and if the pressure falls the fracture will close again. We should consider the possibility that fractures develop gradually so that a balance is reached between fracture width and flow rate causing a more gradual flow along microfra
