Clay minerals significantly alter the pore size distribution (PSD) of the gas hydrate-bearing sediments and sandstone reservoir rock by adding an intense amount of micropores to the existing intragranular pore space. Therefore, in the present study, the internal pore space of various clay groups is investigated by manually segmenting Scanning Electron Microscopy (SEM) images. We focused on kaolinite, smectite, chlorite, and dissolution holes and characterized their specific pore space using fractal geometry theory and parameters such as pore count, pore size distribution, area, perimeter, circularity, and density. Herein, the fractal properties of different clay groups and dissolution holes were extracted using the box counting technique and were introduced for each group. It was observed that the presence of clays complicates the original PSD of the reservoir by adding about 1.31-61.30 pores/100 μm² with sizes in the range of 0.003-87.69 μm². Meanwhile, dissolution holes complicate the pore space by adding 4.88-8.17 extra pores/100 μm² with sizes in the range of 0.06-119.75 μm². The fractal dimension () and lacunarity () values of the clays’ internal pore structure fell in the ranges of 1.51-1.85 and 0.18-0.99, respectively. Likewise, and of the dissolution holes were in the ranges of, respectively, 1.63-1.65 and 0.56-0.62. The obtained results of the present study lay the foundation for developing improved fractal models of the reservoir properties which would help to better understand the fluid flow, irreducible fluid saturation, and capillary pressure. These issues are of significant importance for reservoir quality and calculating the accurate amount of producible oil and gas.

Investigating the pore structure of shale rocks plays an important role in understanding the mechanism of shale oil accumulation and migration; thus, it has a notable influence on the exploration and development of shale oil resources. In the present study, the microstructure of oil shale samples extracted from the Shahejie Formation in the Dongying Depression was investigated through the joint usage of focused ion beam (FIB) and scanning electron microscopy (SEM). Herein, we selected four representative samples. The SEM images of the sample surfaces milled by FIB indicated that pores were found in both the organic and inorganic matter. Organic pores were less developed in the samples because of the low thermal maturity of the organic matter. Continuous 3D digital rock images of shale samples were generated by FIB grinding and SEM in situ imaging. Based on multithreshold segmentation, the shale rocks were separated into different parts including kerogen, inorganic matter, and pores. The pore connectivity and pore size distributions were analyzed via Avizo software and in‐house developed MATLAB scripts. The pore size and corresponding volume distributions suggested that mesoscale pores (2‐50 nm) were abundant in the samples but they contributed limitedly to the total pore volume. Macropores (>50 nm) accounted for a small proportion but they had a greater contribution to the overall pore space volume. The coordination numbers of the subject four samples were mainly 0 and 1, which means that isolated pores accounted for the vast majority of the pores. These observations are critical to conduct reliable petrophysical simulations in the oil shales and understand their intrinsic properties accurately.

Summary: Petrophysical heterogeneity plays a crucial role in carbon dioxide (CO2)-enhanced oil recovery (EOR) in unconventional reservoirs. To analyze how heterogeneity affects CO2 injection and EOR recovery potential, we designed a suite of cyclic CO2 injection experiments on a group of sandstone samples with spanning permeabilities (K = 0.01 to 10 md). By integrating mercury intrusion capillary porosimetry (MICP) and nuclear magnetic resonance (NMR) measurements, we established relationships between capillary-pore-throat heterogeneity and hydrocarbon recovery with a heterogeneity-based CO2 (HET-CO2) recovery model. Through this work, several important observations were made: The three permeability level samples have average recoveries of 72.75%, 51.24%, and 27.74%. Capillary heterogeneity analyses indicate that weakly adsorbed oil in intragranular characterized mesopores is the predominant producer (>50%) in low- to ultralow-permeability samples (K < 1 md). However, pore heterogeneity analyses further imply that the CO2 injection should focus on recovering more adsorbed oil from mesopores for ultralow permeability rocks with K = 0.1 to 1 md in this area, where the T2 response of these pores at 3–20 ms and pore radius at 0.18–1.20 μm was identified by NMR and MICP. Throat heterogeneity results show that the critical extractable pore radius ranges differently (16–35 nm), mainly influenced by the rock type related to throat tortuosity affecting discharge of oil-soluble gas at the puff stage. The HET-CO2 model predicts that recovery factor in heterogeneous formations with a critical pore radius is related to the saturation of minimum hydraulic flow unit (HFU). This model can be used in optimizing development plans for better extraction of bound oil. The findings from this work can further facilitate EOR efficacy prediction and optimization of CO2 enhanced recovery techniques for heterogeneous reservoirs.