Since rock formations are always saturated by fluids, the coupled poroelastic models were developed for improved wellbore stability analysis. Poroelastic models allow to consider phenomena such as well communication with rock formation and pore pressure redistribution. This paper presents the analytical solution of one coupled poroelastic model and then departs to consider two common drilling scenarios, under-balanced and over-balanced drilling. Obtained results show that time-dependent delayed failure and failure initiation taking place inside the formation rather than at the wellbore wall can be predicted by virtue of the coupled model. Moreover, these two phenomena are different for the two drilling scenarios mentioned above. For the under-balanced drilling borehole stability will get enhanced with time due to poroelastic effect in the formation. On the contrary, the borehole stability status will get worse and time-delayed failure may happen for the over-balanced drilling.
The deformation behavior of Tage Tuff during desiccation was investigated. Strain as well as compressional (P) and shear (S) wave velocities were measured concurrently in a set of cylindrical Tage Tuff specimens cored across and along the bedding plane. Two saturated specimens were prepared in one-dimensional drying conditions (50°C, 50% relative humidity) at a time. The strain and weight of one specimen was automatically measured using strain rosettes and an electronic scale. The ultrasonic wave velocity and weight of the second specimen were measured using transducers and an electronic scale and manually recorded.
When the degree of saturation decreased by no more than 70%, P wave velocity decreased with two distinct trends depending on the two main textural directions. However, S wave velocity and strain remained constant. This was due to the inter-bedding and intra-bedding pore water characteristics within the specimen. When the saturations dropped below 30%, both velocities increased with a decrease in strains due to the desiccation-driven hardening. The relationship of incremental P wave velocity and incremental volumetric strain followed the shape of a parabola which can explain by the theories of linear poroelasticity. Different height specimens (5cm and 3cm) exhibit similar strain magnitude and wave velocity behavior.
Tanaka, Tatsuya (Obayashi Corporation) | Uyama, Masao (Obayashi Corporation) | Ishida, Tomoko (Obayashi Corporation) | Nakanishi, Tatsuro (Japan Atomic Energy Agency (JAEA)) | Ohnishi, Yuzo (Kyoto University)