Water Resources Research 33(8), 1803-1811, August 1997
Determining permeability of tight rock samples
using inverse modeling
Stefan Finsterle and Peter Persoff
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.
