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Shenmu Gas Field in Ordos Basin shows the characteristics of low quality gas reservoir, and its deve-lopment effect is quite different from that of surrounding gas fields, which brings some problems to the exploration deployment and sustainable production of the gas field. In order to reveal the origin of low-quality gas reservoirs, the microscopic characteristics and formation mechanism of Taiyuan Formation reservoirs in Shenmu Gas Field were studied based on microscopic experiments at different scales. The results show that the reservoir of Taiyuan Formation in Shenmu Gas Field is characterized by "rich in quartz, poor in feldspar, and high content of rock detritus". The reservoir of Taiyuan Formation is a low porosity and ultra-low permeability tight sandstone reservoir with small pore-fine throat combination, poor pore throat connectivity, and weak reservoir permeability. The formation of low-quality reservoirs in Taiyuan Formation is affected by the matrix, the content of rock detritus in eruptive rocks and diagenesis. During the late Carboniferous to early Permian, the formation of the reservoir was significantly affected by the volcanic activity of the Inner Mongolia ancient uplift in the north of the Ordos Basin. The content of matrix and rock detritus of the eruptive rock in the sandstone was generally high. The rock detritus of the eruptive rock provided the main material basis for the development of secondary pores. While the matrix blocked the pores, it also produced a certain number of matrix dissolved pores, which had a dual impact on the reservoir. The dissolution during diagenesis is critical to the formation of Taiyuan Formation reservoir, and the increased porosity accounts for 64.3% of the current porosity. At the end of the Late Cretaceous, the late Yanshan movement led to the structural inversion of the Ordos Basin, the readjustment of gas and water, and the escape of natural gas along the fault in the eastern part of the basin. Finally, the low quality gas reservoir of Taiyuan Formation in Shenmu Gas Field is formed. The next exploration focus of Shenmu Gas Field should be based on the clear macro distribution law of diagenesis and favorable lithologic traps, and further search for the development area with high content of eruptive rock detritus and low content of matrix.

In order to solve the problem that the pore structure and the microscopic occurrence characteristics of fluid change with stress in real time during the depletion development of tight gas reservoirs, the pore structure, water production and irreducible water distribution characteristics of tight reservoirs in the eighth member of Permian Shihezi Formation, Shenmu Gas Field, Ordos Basin under different stress conditions were quantitatively characterized, and the influence of stress on gas-water relative permeability was analyzed by using nuclear magnetic resonance (NMR), high-pressure mercury injection (HPMI) and displacement experiments. The results show that when effective stress increases from 7 MPa to 17 MPa, reservoir porosity decreases from 8.39% to 7.65%, and the maximum pore radius decreases from 38.3 μm to 35.2 μm among which, the number of pore throats in the range of 0.3-18.9 μm decreases the most. The number of pores involved in seepage decreases due to the decrease of pore throat size, the bound water saturation increases from 51.3% to 59.7%, and the distribution range of the size of bound water increases from 0.012-17.4 μm to 0.012-20.1 μm. The changes of pore structure and bound water distribution characteristics further reduce the relative permeability of both gas and water phases, the relative permeability of gas phase decreases from 0.481 to 0.283, and that of isotonic point decreases from 0.157 to 0.09.

The soil behind a wall is in a stress state with self-weight stress and horizontal stress as the main stress for the Rankine retaining wall with horizontal fill. However, there is a certain wall-soil friction and bonding force in practice. The friction and bonding force may cause the principal stress deflection in the fill near the retaining wall, and the classical Rankine earth pressure theory is not applicable for this case. In the current study, the principal stress deflection of the fill near the retaining wall is processed both by the rotation and by the analysis of the stress from the Mohr circle in the fill behind the wall. The active and passive earth pressures of the retaining wall filled with the cohesive soil is derived, considering the wall-soil friction and bonding force. The influences of the fill friction angle and the wall-soil friction angle on the earth pressures are analyzed. The examples are further used to compare the results from this study with those with the improved Coulomb method. The results show that the Rankine earth pressure is a special case of the formula presented in this study. With the increase of the wall-soil friction angle and internal friction angle, the passive earth pressures gradually increase. The active earth pressures decrease with the increase of the internal friction angle, and the decreasing rate gradually decreases. When the internal friction angle is small, the active earth pressures decrease continuously with the increase of the wall-soil friction angle, and the decreasing speed gradually decreases. When the internal friction angle is large, the active earth pressures first decrease and then increase with the increase of the wall-soil friction angle. The effect of the fill friction angle is more obvious than that of the wall-soil friction angle. Compared with the results from other methods, the active earth pressures obtained with this method are larger, but the passive earth pressure is smaller, and their differences increase with the increase in the wall-soil friction, especially when the cohesion is considered. Because the deflection of the principal stress in the soil on the back of the wall is more consistent with the actual situations, the results from this study will be more suitable to compute the earth pressure in practice.