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In this paper, a method is proposed to prepare rock-like materials with different thicknesses of hidden joints. Then, permeability and porosity of the self-prepared jointed specimens under different pore pressures during confining pressure loading and unloading are measured. The experimental results indicate that the gas permeability of the jointed specimens gradually decreases with the rise of pore pressure due to the existence of Klinkenberg effect, and Klinkenberg effect gradually decreases with the rise of hidden joint thickness. As the main seepage channels, hidden joints govern the seepage characteristics, and due to the existence of hidden joints, the intrinsic permeability is improved significantly. Besides, due to the existence of hidden joints, the intrinsic permeability and porosity are more sensitive to confining pressure loading than that of the intact specimen, and the sensitivity increases with the rise of hidden joint thickness. During confining pressure loading, there is a permanent deformation of the hidden joints and pores in the specimens, which results in both the intrinsic permeability and porosity being always lower than those in the loading process. Meanwhile, the permanent deformation rises with the increases of hidden joint thickness, which leads to the increases of gap of intrinsic permeability and porosity under loading and unloading processes. Additionally, after comparison of the fitting results, the sub-cubic law can reflect the relationship between flow rate and the thickness of non-persistent joints better than the cubic law. [ABSTRACT FROM AUTHOR]

Monzonitic granite is a low-permeability rock. Monzonitic granite formations are ideal for underground storage of oil due to their low permeability and high mechanical strength. In this study, a series of coupled hydromechanical triaxial tests are carried out using monzonitic granite specimens. The influence of confining and fluid pressures on stress, strain, and permeability is investigated. Failure characteristics under different confining and fluid pressures are discussed based on the analysis of macro fracture planes and micro scanning electron microscopy (SEM). The test results show that the change of permeability with stress and strain reflects the deformation stages of compaction, compression, crack propagation, coalesce, and failure of cracks. Due to the low porosity, the change of permeability is small in the initial phases of compaction and compression, whereas there is a significant increase in permeability when new cracks start to develop and coalesce. Confining pressures have a significant impact on the strength and permeability, particularly the crack damage stress of the rock. Compared with confining pressure, the effect of fluid pressure on rock strength and crack damage stress is small. For the monzonitic granite specimens tested, changing the confining pressure results in different failure modes, whereas the fluid pressure has a relatively small effect on the failure modes. [ABSTRACT FROM AUTHOR]