A laboratory study of stress arching around an inclusion due to pore pressure changes

When the pore pressure in a porous rock changes, stress arching will occur within the rock and the surrounding region. Stress arching ratio is defined as the total stress changes in the porous rock to the pore pressure change in the region. The region may have the same or different elastic moduli with the surrounding rock, which is usually referred to as inclusion or inhomogeneity. Stress arching is responsible for many geomechanical problems encountered during production or injection; in addition, it is a crucial parameter in stress estimation during field development. This paper aims to present laboratory measurements of vertical stress arching ratio in a material surrounding the inclusion (inhomogeneity). To the authors' knowledge, few laboratory experiments have been reported on direct measurement of stress arching. The inclusion is a cylindrical sandstone (44 mm in diameter and 50 mm in height) embedded in a larger cylindrical sandstone (150 mm in diameter and 154 mm in height), both of which are made synthetically. These two parts are separated and sealed by a internal polyurethane sleeve. Vertical stress changes are recorded by a mini hydraulic sensor embedded in surrounding rock. Laboratory results are compared to those obtained by numerical models. These models are checked with analytical formulations. The results of numerical models show a good agreement with laboratory data. The numerical results also indicate that the sensor response is affected by elastic properties of the internal sleeve. According to the sensitivity analysis performed, in the absence of the internal sleeve, properties of the inclusion have significant effects on the surrounding stress arching induced.