Pore structure characterization of shale at the micro- and macro-scale
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Pore structures within two Barnett shale samples at the microscale and macroscale were constructed based on conventional core analysis techniques (mercury intrusion capillary pressure (MICP), nitrogen sorption, and helium porosimetry). Measurements were performed on both bulk samples (core 2 and core 6) and organic matter isolated from bulk samples. Pore size distributions obtained from both core 2 and core 6 contain a large volume of micropores, while pore size distributions obtained from isolated organic matter do not, indicating that organic matter-associated pores are mesopores and most of the micropores are within the matrix. The organic matter-associated pore volume of core 2 is about 22% of the total pore volume, and the organic matter-associated pore volume of core 6 is about 41% of the total pore volume. A bundle of short conduits model with constraints can explain the measured nitrogen desorption isotherm on organic matter, and this model was used to represent the microscale pore structure within organic matter. Fragment size effect was observed on both MICP curves and nitrogen sorption isotherms measured on bulk Barnett samples: smaller fragment size results in larger mercury intrusion or nitrogen gas sorption. Fragment size effect does not appear in helium porosity measurement on bulk samples. A multiscale pore structure consisting of connected clusters of organic particles was constructed. The clusters have a characteristic length that controls the accessibility of the pore system, and the clusters are superimposed upon a background of intergranular voids not associated with organic matter. Within the individual organic particles, the pore structure consists of discrete, short pore conduits. The concept of characteristic length of the connected clusters can explain the fragment size effect, and the pore system can be fully accessible if the fragment size is close to this characteristic length. The modeled characteristic lengths for both core 2 and core 6 are in the micrometer range. To estimate permeability, the pore structure within organic matter is assumed to be a collection of dead ends which do not contribute to throughgoing fluid transport. Assuming further that the organic matter clusters are connected onto a network within the inorganic matrix, the permeability of the pore structure in micrometer scale is in 1 nanodarcy range.