Abstract
Understanding the heterogeneity of the nano scale pore structure is critical for assessment of hydrocarbon flow and storage in porous reservoirs. Multifractal theory is a powerful method to acquire the detailed heterogeneity information of the multiple pore system. The generalized dimension spectrum and singularity spectrum are two forms to depict the multifractal feature. Hurst exponent (H) and the width of singularity spectrum (Aa) are two parameters to effectively assess the pore connectivity and heterogeneity, respectively. In this study, multifractal analysis was performed on over-mature Wufeng-Longmaxi shales and their corresponding isolated organic matter (OM) via gas physisorption (CO2 and N2) test to ascertain the pore heterogeneity and its governing factors. Micropore (diameter<2 nm) structure in bulk shale contains stronger pore heterogeneity but weaker connectivity than meso-macropore (diameter = 2~(-1)00 nm) structure. The comparison of heterogeneity features of shale versus its isolated OM reveals that micropore associated with minerals could enhance the micropore heterogeneity in bulk shale, while meso-macropore existing in minerals could reduce the meso-macropore heterogeneity in bulk shale. Besides, the total organic carbon content has a positive effect on the micropore heterogeneity. The micropore and meso-macropore volume have a certain impact on the pore heterogeneity in the corresponding pore size ranges. Compared with the low-mature Bakken shale, the over-mature Wufeng-Longmaxi shale contains the stronger micropore heterogeneity but weaker meso-macropore heterogeneity, where less difference between the micropore and meso-macropore heterogeneities was also observed. We deduced that the whole shale pore spectrum likely tends to be homogeneous along with the OM thermal maturation.
NSTL