## 1 Introduction  
Unconventional reserves are of strategic importance to replenish the resource base in Russia and worldwide, and their successful development can play a crucial role in increasing oil production and repeatedly compensate for the decline in production from depleted traditional reservoirs [1; 2; 3]. Recently, thermal enhanced oil recovery methods have attracted a high level of interest because of their possible application to shale reservoirs [4; 5]. One of the effective techniques is high-pressure air injection (HPAI), which involves the initiation of an oxidation front in the formation which does not only displace the oil by combustion gases, heated fluids, and steam but also causes an increase in the reservoir pressure and temperature [6; 7]. The necessary critical conditions in the formation can be achieved artificially, in the laboratory by an external impact on the rock, for example, by thermal exposure. The most critical experiment for evaluating the effect and obtaining technological parameters is a combustion tube test [7, 8]. The combustion tube (CT) test is the physical modeling of thermal exposure on porous media in formation conditions. It is conducted to assess the HPAI potential for a particular reservoir type as well as to obtain parameters for further upscaling of the technology in the field. High-temperature exposure results in a significant evolution of the shale porosity and permeability due to kerogen transformation. This affects the reservoir properties and leads to a change in the rock's porous structure and the expansion of the stimulated reservoir area. The change in petrophysical properties during thermal conversion is often evaluated by defining the porosity and permeability change [9]. Multiple studies are devoted to the study of the pore evolution during the controlled pyrolysis of the shales [10, 11, 12]. According to the published studies, heating (pyrolysis) of organic-rich shales generally leads to an increase in porosity and pore connectivity [13, 14, 15]. [PERSON] et al. [16] investigate the oil shale before and after the pyrolysis and demonstrates changes in the pore structure of raw and pyrolyzed shale using X-Ray microcomputed tomography. They showed a porosity increase of 20-25% as a result of heating to a temperature of 500 \({}^{\circ}\)C. [PERSON] et al. demonstrated the first micro-fractures nucleation at a temperature of 350 \({}^{\circ}\)C using advanced real-time 3D X-Ray tomography [13]. Also, in the latter study [14], they stated that the reason for this fracturing is the expansion of organic matter (OM). Using a similar approach of 3D X-Ray tomography, [PERSON] et al. noticed that a significant increase of porosity occurs at a temperature of 390\({}^{\circ}\) C (up to 20-25%); further heating mainly causes the growth of the pore connectivity [15]. Different essential factors in rock pyrolysis, such as confinement, are considered in the papers by [17, 18, 19]. [PERSON] et al. and [PERSON] et al. reported intensive fracture evolution in Woodford Shale samples detected with X-ray tomography during hydrous pyrolysis with uniaxial confinement and absence of fractures without external forces [17]. The authors confirmed their conclusions in [18] and demonstrated minor lateral extension and absence of vertical fractures under uniaxial confining stress during the heating. [PERSON] et al. and [PERSON] et al. showed an increase of liquid components with increasing thermal stress using NMR in [19]. To assess the changes in shale pore structure, a number of advanced laboratory techniques can be applied. For instance, [PERSON] et al. presented the results on the evolution of oil shale pore structure from 23 to 650 \({}^{\circ}\)C by simulating in-situ pyrolysis using a TGA, MIP, and LPNA [20]. To estimate the organic matter (OM) transformation, pyrolytic chromatography of rock samples by the Rock-Eval method is required before and after the combustion. The parameters may serve as engineering guidelines for in-situ combustion or other HPHT processes [21].