Abstract
Abstract Laboratory measurements of gas permeability are used in many petroleum engineering studies, from core analysis for reservoir formation characterization to research work on reservoir engineering, formation damage, and various enhanced recovery techniques. The value of this data depends on how well they translate from one measurement condition to another. In this study, several factors affecting laboratory permeability measurements are investigated. Experimental results show that negligence of these factors may result in errors exceeding fifty percent. Permeability measurements are performed for limestone and sandstone samples using a conventional steady-state technique. The effect of variations in core length, gas type, pressure of gas supply, and the flow rate and pressure of the flowing gas is systematically studied. An accurate mass flow meter for gas is built in-house to eliminate the calibration error of commercially available flow meters that depend on the gas type. Nitrogen, helium, and ethane are used in these measurements. Temperature variations of the flowing gas are recorded, and gas slippage and inertia effects are also taken into account. Our study shows a significant reduction, bordering 25%, in permeability values measured for some shorter cores compared to the permeability value of the original whole core. This difference is more pronounced in highly permeable sandstone samples but is till significant in low permeability carbonates, bordering 20%. Permeability values measured using helium are higher than those measured using nitrogen. The difference is more pronounced in shorter cores, bordering 50%. It is important to note that this variation in permeability between nitrogen and helium exists even when corrections for the gas slippage or Klinkenberg effect are considered. Gas slippage effects are more pronounced for helium compared to nitrogen and for shorter samples compared to longer ones. The effect of length on the non-Darcy permeability and the high-velocity coefficient can also exceed 50% even when the same gas type is used. To the best of our knowledge, almost all laboratory permeability measurements neglect the temperature effect by considering an isothermal process. The effect of length on measured permeability is hinted in a few publications but no systematic study is reported. Our data show that models accounting for the effect of gas type are not accurate. We conclude that permeability measurements need to be corrected for the core length and flowing gas type effects, which vary with rock type and experimental conditions.
Published Version
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