| Geomechanical and fluid flow behaviors of reservoir rocks under multi-physics coupling(stress, temperature and fluid flow) conditions are of significant importance for evaluating the performance and efficiency of reservoir resource exploitations. Reservoir rocks are under true triaxial stress(?1>?2>?3) conditions as tectonic stresses exist. In this work, the behaviors of reservoir rocks under multi-physics coupling and true triaxial stress conditions are considered to be research objects. Geomechanical and fluid flow behaviors of reservoir rocks are studied through experiments, theoretical analysis and numerical simulations. A novel multi-functional true triaxial geophysical(TTG) apparatus was designed, fabricated, calibrated, and tested. The apparatus was used to simulate true triaxial stress conditions and reveal geomechanical properties and permeability evolutions of reservoir rocks.The effects of the intermediate principal stress and intermediate principal stress ratio on geomechanical properties and permeability evolutions of reservoir rocks were discussed. Based on the consideration of fracture compressibility, a permeability model applicable to anisotropic materials under true triaxial stress conditions was proposed and verified by matching the permeability data in the experiments. A THM coupling model for reservoir rocks was established and used for field simulations. The main results are as follows:(1)The TTG apparatus was designed to investigate the geomechanical properties and permeability of rocks under true triaxial conditions. The apparatus allows continuous monitoring of 3-D stresses, displacements, fluid injection and outlet flow rates and pressures and acoustic emission(AE) signals. The control and measurement of the fluid flow with effective sealing of rock specimens' corners are achieved by the specially designed internal sealed fluid flow system.(2)Compressive strength experiments of reservoir rocks unde true triaxial stress conditions were conducted. The compression strength increased with increasing ?2 and ?3. In addition, the effects of ?2 on the compression strength of rocks were influenced by ?3.(3)Permeability experiments of sandstone unde true triaxial stress conditions were conducted. The permeability of sandstone decreased with increasing ?1, ?2 and ?3. However, the permeability reductions were not significant as it maintained at the same magnitude. The most significant effects on sandstone permeability were of ?3, while the least ones were of ?1.(4)Permeability experiments of shale unde true triaxial stress conditions were conducted. The permeability of shale decreased with increasing ?1, ?2 and ?3. The permeability decreased by two to three orders of magnitude in shale. The permeability variations induced by the variation of each principal stress were distinct which we attribute to the response of anisotropic fractures in the shale. The greatest permeability reductions were observed when increasing stress normal to the bedding planes. The results showed the dominant effects of shale bedding planes on its permeability.(5)Permeability experiments of coal unde true triaxial stress conditions were conducted. The permeability of coal decreased with increasing ?1, ?2 and ?3. The permeability decreased by one to two orders of magnitude in coal, which was between that of shale and sandstone.(6)The initial fracture distribution, aperture, tortuosity and connectivity in shale are strongly anisotropic. This anisotropy is compounded due to differences in morphology and deformation modulus of fractures along the three principal stress directions. Based on the consideration of rock anisotropy, we modified the S&D model to make it applicable to anisotropic materials under true triaxial stress conditions. The model was verified by matching the permeability data in the experiments.(7)A THM coupling model for reservoir rocks was established. The model was used in COMSOL Multiphysics to verify and to conduct CH4 drainage simulations. |
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