Petrophysical characterization of low-permeable carbonaceous rocks: Comparison of different experimental methods

Abstract Pore structure and gas transport properties of fourteen samples from the Jurassic Sargelu and the Cretaceous Garau formations in Lurestan province, southwest Iran, were studied under the aspect of shale gas exploration and production. Porosity was determined by helium pycnometry and water saturation (Archimedes principle). Low-pressure adsorption of N2 was used to determine total pore volumes, specific surface areas and microporosity. High-pressure mercury intrusion porosimetry (MIP) was applied to assess pore-size distributions. Permeability measurements were performed with helium and methane at confining pressures of 40 to 10 MPa using steady state and non-steady state methods. Permeability and porosity values determined by the different methods are in a good agreement. For low-permeable samples (less than 1 microdarcy ~ 10−18 m2) the “constant downstream pressure” technique is the most efficient method for slip flow evaluation (Klinkenberg plot), yielding apparent permeability coefficients over a wide range of reciprocal mean pressures (1/pmean) in one single run. A positive correlation was found between TOC content, porosity and permeability coefficients. Samples displaying clear signs of recrystallization tend to exhibit higher permeability. None of the parameters determined in this study, however, showed a correlation with mineralogy. The relationship between Klinkenberg-corrected permeability coefficients and confining pressure could be expressed by an exponential decay function. Stress sensitivity was typically higher for low-permeable samples. Samples containing highly recrystallized carbonates exhibited lower stress sensitivity, indicating an increase in rigidity due to recrystallization. No correlation was found between methane/helium permeability ratios, sorption capacity, and TOC contents. Thus, gas transport properties of this sample set were not affected by sorption and/or swelling of organic matter.