Effect of the intermediate principal stress on the evolution of mudstone permeability under true triaxial compression

Abstract

The changes in the permeability of mudstone specimens under compression with different intermediate principal stresses (σ2) were tested using a true triaxial testing system. The confining pressure and pore pressure were set based on the caprock conditions in a CO2 geological storage project. The measured permeability initially increased and then decreased before the failure of the specimen and reached a peak in the form of a sudden increase during the formation of the fault. The permeability during compression decreased with increasing σ2. However, the higher σ2 caused the ductility of the mudstone to decrease significantly and led to the formation of a fault parallel to the σ2 direction. The increase in permeability during the formation of the fault was notably suppressed by the increase in the confining pressure and decreased with increasing flatness of the fault; the flatness of the fault increased with increasing σ2. Moreover, an empirical function that considers the compressive and dilatant strains was proposed to predict the permeability before the failure of the specimen, and the parameters of this function are only slightly affected by σ2. The results of this study reveal the effect of σ2 on the variation of the permeability of mudstone and help better assess the risk of caprock leakage in injection projects. © 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

“…Moreover, the porosity of the mudstone is as high as 21.83%. 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.…”

“…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.…”

“…The permeability of the mudstone was measured using the transient pulse method (TPM) [50][51][52][53] with nitrogen as the fluid medium. The apparatus and its reliability, the principles of TPM, and the experimental procedure were detailed in Shi et al [41].…”

“…Many models based on the power law or exponential law have been proposed to predict the permeability evolution during deviatoric compression in terms of stress and damage [41,49,[59][60][61]. These models have been experimentally proven to have good predictive ability under hydrostatic compression but poor prediction ability under deviatoric compression [41,49]. As mentioned above, compaction and dilatancy are two coupled processes that increase and decrease permeability, respectively.…”

“…As mentioned above, compaction and dilatancy are two coupled processes that increase and decrease permeability, respectively. Therefore, Shi et al [41] proposed a permeability-strain relationship that separately considers the contribution of compaction and dilatancy to permeability without using the indirect variable of damage. However, the model is not appropriate for applications where plastic deformation is large because the dilatant deformation weight r d and predicted permeability are still increasing with deformation such as the rearrangement of clay grains, which can cause a decrease in permeability.…”