Determining permeability of tight rock samples using inverse modeling
Stefan Finsterle and Peter Persoff
Water Resources Research 33(8), 1803-1811, August 1997
Lawrence Berkeley National Laboratory, Earth Sciences Division
University of California, Berkeley, CA 94720
Abstract. Data from gas-pressure-pulse-decay experiments have been analyzed by means of numerical simulation in combination with automatic model calibration techniques to determine hydrologic properties of low permeability, low porosity rock samples. Porosity, permeability, and Klinkenberg slip factor have been estimated for a core plug from The Geysers geothermal field, California. The experiments were conducted using a specially designed permeameter with small gas reservoirs. Pressure changes were measured as gas flows from the pressurized upstream reservoir through the sample to the downstream reservoir. A simultaneous inversion of data from three experiments performed on different pressure levels allows for independent estimation of absolute permeability and gas permeability which is pressure-dependent due to enhanced slip flow. With this measurement and analysis technique, we can determine matrix properties with permeabilities as low as 10-21 m2. In this paper we discuss the procedure of parameter estimation by inverse modeling. We will focus on the error analysis which reveals estimation uncertainty and parameter correlations. The impact of systematic errors due to potential leaking and uncertainty in the initial conditions will also be addressed. The case studies clearly illustrate the need for a thorough error analysis of inverse modeling results.
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