Changes of pore structures and permeability of the Chang 7³ medium-to-low maturity shale during in-situ heating treatment

To evaluate the thermally induced permeability enhancement potential and stress sensitivity during in-situ heating (ISH), real-time measurement of pulse-decay gas permeability (PDP) of medium-to-low maturity shale cores from Chang 7³ sub-member shale oil reservoir was carried out in our self-developed high-temperature pseudo-triaxial core holder at different temperatures and effective stresses. Stereo light microscope (SLM), computerized tomography (CT) scan, and nuclear magnetic resonance (NMR) tests were conducted to quantitatively characterize the microstructure development in the shale cores. Based on the lithology and thermal analysis, the primary mechanisms of thermal cracking, nanopores evolution, and physical property improvement were discussed in detail. The experimental results show that the porosity and permeability can be effectively increased to 3.13 times and 624.09 times on average above the threshold temperature of approximately 400 °C, but this also results in a stronger stress sensitivity of permeability up to 30.21%. Furthermore, the coverage, density, complexity, and connectivity of the fracture network can be dramatically improved. A large number of secondary nanopores with diameters in the range of 2–50 nm dominate. There is a good correlation between microstructure development and physical property variation. We found that the kerogen pyrolysis was the most critical mechanism for the significant enhancement in microstructure and physical property.