DETERMINING PERMEABILITY OF TIGHT ROCK SAMPLES USING INVERSE MODELING

Data from gas-pressure-pulse-decay experiments have been analyzed by m eans of numerical simulation in combination with automatic model calib ration techniques to determine hydrologic properties of low-permeabili ty, low-porosity rock samples. Porosity, permeability, and Klinkenberg slip factor have been estimated for a core plug from The Geysers geot hermal field, California. The experiments were conducted using a speci ally designed permeameter with small gas reservoirs. Pressure changes were measured as gas flowed from the pressurized upstream reservoir th rough 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 pe rmeability which is pressure-dependent due to enhanced slip flow. With this measurement and analysis technique we can determine matrix prope rties with permeabilities as low as 10(-21) m(2). In this paper we dis cuss the procedure of parameter estimation by inverse modeling. We wil l focus on the error analysis, which reveals estimation uncertainty an d parameter correlations. This information can also be used to evaluat e and optimize the design of an experiment. The impact of systematic e rrors due to potential leakage and uncertainty in the initial conditio ns will also be addressed. The case studies clearly illustrate the nee d for a thorough error analysis of inverse modeling results.