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GL2298-
2007, Juiyu Hsu, Master thesis, Department of Applied Geology, National Central University, Taiwan

Abstract:

Overpressure plays an important role on thrust faulting of fold-and-thrust belt. In petroleum geology, understanding abnormally high pore pressure is also important for studying primary petroleum migration and drilling, as well as for analyzing sedimentary basins. The parameters which have most influential to overpressure are permeability, porosity and specific storage coefficient. Accurate measuring of the stress dependent fluid flow properties is essentially important to explore the fluid percolation process in crust. An integrated permeability/porosity measurement system-YOYK2 was utilized to measure the pressure-dependency of permeability and porosity of core samples from Taiwan Chelungpu fault Drilling Project, Hole-A and Hole-B. The measured permeabilities of sandstone and siltstone/shale are 10-13~10-14m2 and 10-16~10-19m2 under confining pressure of 3~120 MPa. The permeability of siltstone and shale is more sensitive to confining pressure than that of sandstone. The measured porosities of sandstone, siltstone and shale under confining pressure of 3~120 MPa are 15%~20%, 8%~11% and 13%~14%, respectively. Meanwhile, different rock types have almost identical pressure-sensitivity of porosity. Two pressure-dependent models, power law and exponential relation, for describing the pressure-dependent permeability/porosity were used to fit the experiment results. The calibrated porosity sensitivity exponent is estimated to range form 3.26 to 5.47 (loading) and range form 2.34 to 3.08 (unloading) for tested sandstones by using a power law to describe the pressure-dependency of permeability/porosity. The porosity sensitivity exponent is estimated to range from 25.98 to 47.50 (loading) and from 6.91 to 46.43 (unloading) for tested siltstone and shale which is much higher than that of sandstone. The specific storage coefficient (related to the pressure-dependent porosity) also demonstrates more pressure sensitivity for adopting a power law than using an exponential relation. The calculated specific storage is ranged from to for sandstone and from to for siltstone and shale when the confining pressure increasing from 3 MPa to 120 MPa. The pressure-dependent specific storage coefficient, as well as the permeability/porosity, can be incorporated into a non-linear Gibson equation to calculate the overpressure generation and dissipation by finite difference method. The calculated overpressure using power law to describe the pressure-dependent fluid flow properties is less than the one using exponential relation. Proper selection of the pressure-dependent models of fluid flow properties is critical for calculating the overpressure in crust.

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