Three-Phase Segmentation Method for Organic Matter Recognition in Source Rocks via CT Images: A Case Study On Oil Shale Pyrolyzed by Steam

Abstract

High-resolution X-ray computed tomography (Xray CT) is a noninvasive nondestructive method for studying the internal structure of source rocks such as oil shale. Here, to accurately identify the distribution of organic matter (OM), pores, and fractures in such rocks based on CT images, a three-phase segmentation method for OM recognition of source rocks via CT images was proposed and used to study the internal structure of Xinjiang Balikun oil shale heated with superheated steam. First, the oil shale samples were heated at different steam temperatures and CT images were acquired, to which the three-phase segmentation method was then applied to obtain the distribution of OM, pores, and fractures in the rock. The effectiveness of the method was verified by means of thermogravimetric analysis (TGA) measurements. The results showed that, within the spatial resolution limit of the CT images (pixel size 3.6 × 3.6 μm 2 ), a large number of ellipsoidal OM clusters were distributed in the oil shale at room temperature (25 °C), accounting for 4.29% of the total volume of the oil shale. As the steam temperature increased, the OM inside the oil shale gradually pyrolyzed, with the discharged pyrolysis products leaving behind a large number of ellipsoidal pores. Simultaneously, fractures gradually developed in the rock and connected with the pores, forming tadpole-like pore-fracture structures. TGA measurements showed that the temperature range of the oil shale's maximum weight loss was 400−550 °C and that the weight loss rate reached 10.4%, which is consistent with the changes in the OM obtained from the CT images.

Scale-Dependent Fractal Properties and Geological Factors for the Pore Structure in Shale: Insights from Field Emission Scanning Electron Microscopy and Fluid Intrusion

“…Various methods have been continuously explored and employed to characterize heterogeneous pores in shale reservoirs, including direct imaging, fluid intrusion, and X-ray scattering techniques. Direct imaging methods, such as field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM), enable the direct acquisition of high-resolution pore images. − Fluid intrusion methods, such as the helium pycnometer, high-pressure mercury intrusion porosimetry (HP-MIP), low-pressure gas (nitrogen or carbon dioxide) adsorption (LP-N 2 /CO 2 GA), and nuclear magnetic resonance (NMR), offer a wide range of quantitative measurements for pore structures. ,, In recent years, numerous advanced methods have been utilized to assess the pore characteristics found in unconventional reservoirs, such as small-angle X-ray/neutron scattering (SAXS/SANS) , and micro/nano-computed tomography (micro-CT/nano-CT). , However, these advanced techniques are difficult to apply because of their expensive costs and arduous data processing procedures.…”

“…5,18,19 In recent years, numerous advanced methods have been utilized to assess the pore characteristics found in unconventional reservoirs, such as small-angle X-ray/neutron scattering (SAXS/SANS) 20,21 and micro/nano-computed tomography (micro-CT/nano-CT). 22,23 However, these advanced techniques are difficult to apply because of their expensive costs and arduous data processing procedures.…”

Investigation of Pore-Scale Remaining Oil Dynamic Evolution in Heterogeneous Marine Carbonate Using Real-Time Computed Tomography Scanning

“…The CT scanning images, pore network models, , and pore structure analysis results of three samples are shown in Table . We extracted a cubic (400 × 400 × 400 voxels) from the original CT scanning images for segmentation and analysis based on the pore network model. , First, the images were filtered and de-noised and then, the images were segmented according to the different gray values of pores and throats. Finally, we built the pore network models.…”

Influence analysis of complex crack geometric parameters on mechanical properties of soft rock

“…The black areas represent pores, and cracks can be easily identified and segmented from the solid matrix. In addition, the pores and cracks were assigned a value of 0, and the solid matrix was assigned a value of 1 (Huang et al 2021;Omori et al 2023). The pores and cracks can be divided from the scan slice one by one.…”