Evolution of Pore and Fracture Structure of Oil Shale under High Temperature and High Pressure

Abstract

In order to study the coupled effect of the temperature and pressure on pyrolysis characteristics and pore and fracture structures of oil shale, a total of 25 groups of pyrolytic reaction experiments have been conducted on 14 mm long and 7 mm in diameter cylindrical oil shale specimens under different temperature and pressure conditions ranging from 20 to 600 °C and 0.1–15 MPa. Further, both X-ray microcomputed tomography (μCT) and mercury intrusion porosimetry (MIP) have been used to comprehensively investigate the network structure, interconnectivity, and evolution of pore and fractures. The results show that the temperature significantly affects the pyrolysis characteristics of oil shale. With rising temperature, both the mass loss and the porosity increase gradually, the number and the maximum aperture of fractures also increase, and the pyrolytic degree intensifies progressively. The increase is most significant from 300 to 500 °C. The maximum mass loss ratio is 20.84%, the largest porosity is 13.52 times larger than that under the room temperature, and the total number and the maximum aperture of the fractures are 813 and 0.383 mm, respectively. Moreover, the pressure has a significant effect on the pore and fracture structures of oil shale. As the pressure increases, both the pore volume and the fracture distributions first decreased and then increased. With the continuous increase of pressure, the porosity and the total number of fractures reach a maximum at the pressure of 15 MPa. Under the coupled effect of temperature and pressure, with both the temperature and pressure increasing, the pores and fractures in the oil shale specimens developed increasingly. Furthermore, using the μCT scan technology, the distribution laws and the connectivity characteristics of the pores and fractures have been investigated. The connected fractures appear when the temperature reaches 300 °C and further extend along the bedding plane or pass through it at 600 °C.

Influence of In Situ Pyrolysis on the Evolution of Pore Structure of Oil Shale

“…Compared with other scholars' conclusions on the Fushun oil shale, the variation trend of the test results with temperature in this study is consistent with the current literature [38,45]. MIP test results show that the total pore volume and porosity obtained by in situ pyrolysis are smaller than those in data from a no-nitrogen pressure test [45]. Specifically, the difference is small in the temperature range 23-300 • C, and obvious in the temperature range 400-600 • C. Oil shale strength is stable and pores are undeveloped under low-temperature conditions.…”

“…Similarly, the stress can also affect the development of pore structure and the transmission capacity of the hydrocarbon product during pyrolysis. Compared with other scholars' conclusions on the Fushun oil shale, the variation trend of the test results with temperature in this study is consistent with the current literature [38,45]. MIP test results show that the total pore volume and porosity obtained by in situ pyrolysis are smaller than those in data from a no-nitrogen pressure test [45].…”

“…Due to the different effective range and testing principles of various testing methods, while the pore size of oil shale has a larger-scale distribution from the nanometer level to the micrometer level under pyrolysis conditions, it is difficult to fully display the distribution of the entire pore space by using a single means. Therefore, Bai and Geng et al [17,45] analyzed the pyrolysis characteristics and pore structure of oil shale by combining MIP with CT or LPNA. In the above studies, as a factor that cannot be neglected in in situ pyrolysis, the role of lithostatic pressure is seldom considered.…”

Study on the Tri-axial Time-Dependent Deformation and Constitutive Model of Glauberite Salt Rock under the Coupled Effects of Compression and Dissolution

“…The micro-CT scan system at Taiyuan University of Technology [30,31] was used to scan all the glauberite specimens before and after the experiment, and the scans were conducted with 400 frames and a superimposed frame rate of 2 fps, and there were 1500 scanned layers. In this experiment, the parameters used in the CT procedure were the scanning voltage and electric current, which were 70 Kv and 90 μA, respectively.…”

“…The code compiled by MATLAB [31] was used to reprocess the rebuilt CT images and obtain the porosity. Table 4 shows the porosity and weight of glauberite specimens after versus before testing; it can be seen that the porosity of glauberite specimens increase after testing versus before testing, while the weight of glauberite specimens decreases after testing compared to that before testing.…”