In-situ stress measurement while drilling and stress characteristics at the margin of Ordos Plateau

The traditional hydraulic fracturing and new thermal-expansion cracking methods employed for measuring in-situ stress require a subsequent lagging test after borehole drilling. This post-drilling often leads to deformation, cracking, and stress release. Additionally, determining whether a crack occurs during the test or naturally after drilling is challenging, posing difficulties in ensuring testing accuracy. To address these challenges for determining the borehole wall integrity of the test point and authenticity of the fracture process in the in-situ stress analysis using conventional hydraulic fracturing methods, we used an optical sonic measurement while drilling system to conduct hydrofracturing measurements of rock in-situ stress in the southwest margin of the Ordos Plateau. The stress characteristics at the margin of the plateau platform were revealed, and the variation law for estimating in-situ stress with depth was obtained. The statistical results were then compared with those obtained for the in-situ stress in a typical area of the North China Plain. The results show that the in-situ stress obtained for the southwest platform margin of the Ordos Plateau is lower than the statistical results of the crustal stress obtained for the shallow crust of mainland China. The in-situ stresses increase linearly with depth, and the growth rate of the minimum horizontal principal stress is higher than that of the maximum horizontal principal stress. The maximum and minimum horizontal principal stresses are lower than those observed in the North China Plain. Due to the influence of the regional structure and plateau platform of the Ordos, the direction of geostress is deflected to the east compared with the North China Plain. The lateral pressure coefficients reflected by the ratio of the maximum and minimum horizontal principal stress to vertical stress are lower than those in the North China Plain. However, the difference between the maximum and minimum horizontal principal stresses is higher than that in the North China Plain. This indicates that although the overall level of in-situ stress in the plateau edge zone is lower than that in the North China Plain, the larger difference between the maximum and minimum horizontal principal stresses is conducive to causing shear failure in underground space structures, which is unfavorable for the stability of underground engineering. This shows that when designing underground projects in the plateau platform area, more attention should be paid to the direction arrangement of the shaft and roadway engineering, as well as the negative effects of low horizontal stress and high-stress difference.