Statistical characterization of gaspatch distributions in partially saturated rocks
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Reservoir rocks are often saturated by two or more fluid phases forming complex patterns on all length scales. The objective of this work is to quantify the geometry of fluid phase distribution in partially saturated porous rocks using statistical methods and to model the associated acoustic signatures. Based on Xray tomographic images at submillimeter resolution obtained during a gasinjection experiment, the spatial distribution of the gas phase in initially watersaturated limestone samples are constructed. Maps of the continuous variation of the percentage of gas saturation are computed and associated binary maps obtained through a global thresholding technique. The autocorrelation function is derived via the twopoint probability function computed from the binary gasdistribution maps using Monte Carlo simulations.The autocorrelation function can be approximated well by a single Debye correlation function or a superposition of two such functions. The characteristic length scales and show sensitivity (and hence significance) with respect to the percentage of gas saturation. An almost linear decrease of the Debye correlation length occurs with increasing gas saturation. It is concluded that correlation function and correlation length provide useful statistical information to quantify fluidsaturation patterns and changes in these patterns at the mesoscale. These spatial statistical measures are linked to a model that predicts compressional wave attenuation and dispersion from local, waveinduced fluid flow in randomly heterogeneous poroelastic solids. In particular, for a limestone sample, with flow permeability of 5 darcies and an average gas saturation of ~5%, significant Pwave attenuation is predicted at ultrasonic frequencies.
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Published by the Society of Exploration Geophysicists
© 2009 Society of Exploration Geophysicists
The link to the journal’s home page is at : http://segdl.org/geophysics/
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