Characterization of oil shale pore and fractures during pyrolysis using digital rock reconstruction

As the primary conduit for oil and gas seepage, the pore and fracture structure of oil shale undergoes intricate changes during pyrolysis. However, the lack of understanding regarding its variations hinders effective reservoir classification and development. Therefore, it is important to investigate the influence of oil shale pyrolysis on pore structure at different temperatures and organic matter content. This paper proposes a new method based on 3D digital core reconstruction to quantify fractures and pore structures. The methodology employs parameters such as fracture width, fracture orientation, fractal dimension, and porosity to assess changes in pore and fracture structures throughout the pyrolysis process. The findings are as follows: (1) The proposed digital rock processing method and midplane extraction algorithm for fractured rock can accurately calculate fracture width and fracture orientation distribution. (2) As pyrolysis temperature increased, the porosity of oil shale samples rose from 18.3% to 20.9%, with a corresponding increase in fracture width, indicating improvements in the physical properties of the samples. However, the rate of change in pore and fracture structures decreased after pyrolysis (3) For fractured oil shale samples, porosity increased from 12.1% to 29.3% with higher organic matter content at consistent temperatures. However, Fractal analysis shows that pyrolysis fracture complexity is governed by both the content and, importantly, the initial spatial distribution of organic matter. This provides a new perspective for the precise characterization of pore structure after fracturing oil shale.