Effects of different fluids on microcrack propagation in sandstone under true triaxial loading conditions

Abstract

The mechanical behavior of rock under conditions of multiphase fluids is currently a key scientific issue in the field of geotechnical engineering. In this work, we conducted triaxial compression experiments of initially water‐saturated samples, which were flooded with CO2 using the semipermeable separator method; comparatively investigated the mechanical behaviors of sandstone under the conditions of mono‐phase fluids CO2/N2/H2O and biphase fluid CO2‐H2O; and analyzed the stress‐strain characteristics, internal crack expansion, and macrocrack failure patterns of sandstones. Compared with H2O, CO2 and N2 could lead sandstone cracks to show a pattern of multiple fractures, displaying a double shear failure mode and multiple branch cracks along principal fractures. Moreover, the cracking effect of supercritical CO2 was more evident. Under the conditions of pore pressure of 10 MPa and temperature of 323 K, the ratio of work done by CO2, N2 and H2O during crack expansion was 23.5:24.1:1, with transient pressure drops to 7.82 MPa, 9.25 MPa, and 0.1 MPa, respectively. Overall, the study analyzed the mechanisms of physicochemical interactions of fluid types and the number of phases in sandstone minerals at the microscale and revealed the mechanical mechanism between fluids and rock minerals, as well as the impacts of these fluids on the strength, internal crack expansion and deformation of sandstone. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Investigation of the Effect of Confining Pressure on the Mechanics-Permeability Behavior of Mudstone under Triaxial Compression

“…We have already used the same cores to study the influence of the intermediate principal stress (σ 2 ) on the permeability evolution during true triaxial compression [41]. Although this paper emphasizes the effect of confining pressure (P c ) on the strain-permeability behavior of mudstone, which can be tested by a conventional triaxial apparatus with a cylindrical specimen, the true triaxial apparatus for rocks [41][42][43][44][45] at the Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, which can exert three stresses independently, was still employed in this study because the specimen usually failed with a relatively flat fault parallel to σ 2 [42,[46][47][48]; the flow in the σ 2 direction can still be regarded as a one-dimensional flow. In contrast, the fault of the specimen in the CTC is not only very rough but also oblique to the flow direction [40,49] and the fluid flow inside the specimen near or after failure cannot be approximated as a one-dimensional flow; therefore, the cores in this study were also processed into cuboid specimens of 5 cm square by 10 cm long ( Figure 1) and the permeability in the σ 2 direction was monitored during deviatoric loading.…”

Impact of Chemical Environment on Compaction Creep of Quartz Sand and Possible Geomechanical Applications

“…The observed effects may be extended to the compaction creep of sandstones, when subcritical crack growth controls deformation. Previous workers have shown that microcracking is an important mechanism in sandstone deformation high stresses and strains (Heap et al, ; J. Zhang et al, ) and the presence of water leads to a pronounced weakening effect (Baud et al, ; Hu et al, , ). This suggests that, compared to the behavior of quartz sands, sandstones respond similarly to changes in water content and solution pH and presumably to the introduction of additives such as those studied here.…”

Permeability of Argillaceous Sandstone Modeled with Tortuous Capillary Tubes Using Fractal Geometry

“…The effective shear stress is a dominant factor in controlling crack initiation and expansion within the rock. ,, During the triaxial compression test, the effective shear stress τ eff can be expressed as follows τ normale normalf normalf = false( σ 1 − σ 3 false) 2 cos nobreak0em0.25em⁡ φ − tan nobreak0em0.25em⁡ φ ( σ 1 + σ 3 ) 2 + α .25em tan nobreak0em0.25em⁡ φ P w where φ is the internal friction angle of rock …”

“…The permeability characteristics of rock determine the efficiency of engineering operations and long-term stability of rock infrastructures in the process of oil/gas exploitation, , geothermal energy extraction, , carbon dioxide storage, − water-rich ore mining, and nuclear waste disposal. , For example, reservoirs are gradually depleted owing to the exploitation of oil/gas. During reservoir depletion, , the pore pressure ( p w ) and effective stress inside the reservoir rock decrease and increase respectively, resulting in decreased reservoir rock permeability and exploitation efficiency .…”