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.
Kim, Jonghoon (Heesong Geotek. Co.,Ltd.) | Kim, Kiseog (Heesong Geotek. Co.,Ltd.) | Jeong, Wansoon (Heesong Geotek. Co.,Ltd.) | Lee, Seungho (Sangji University) | Kim, Yongsoo (Korea Infrastructure Safety & Technology Corp.) | Kim, Jaedong (Kangwon Nat’l University)
Several rock deformation mechanisms are involved in the oil and gas reservoir formations and the overlaying strata. These deformations may occur during drilling, production, fracturing, stimulation, or enhanced oil recovery (secondary and tertiary). The evaluation of Mohr-Coulomb failure criterion as well as other mechanical properties for reservoir rocks is essential for well planning, development and characterization of oil and gas reservoirs. This is because the understanding of the rock-stress relationship can solve many reservoir problems and avoid cost of remedial work. For example, a Mohr-Coulomb failure criterion may be used for borehole instability analysis, water injection design, production optimization techniques, compaction and sand production prediction, etc. A Mohr-Coulomb failure criterion is a function of the apparent cohesion and the angle of internal friction. The evaluation of these two parameters requires testing of many rock samples using an expensive and time-consuming triaxial testing set-up. In this study, a correlation between the apparent cohesion and the unconfined compressive strength was developed based on laboratory measurement data of more than 300 rock samples of different types obtained from the literature. The correlation coefficient of the developed correlation equals to 0.88. Verification of the developed correlation using literature data from sources other than those used in the correlation development has shown average error of estimation around 10%. Therefore, the Mohr-Coulomb failure criterion’s parameters can be roughly estimated using the developed correlation based on fast and cheap measurements of the unconfined compressive strength.
This study demonstrates the importance of considering anisotropy in borehole stability of transversely isotropic rock. Both analytical and numerical methods are conducted in order to analyze the horizontal borehole stability in transversely isotropic rock. The data for transversely isotropic rock are based on laboratory experiments on shale. The boundary condition is assumed as in situ stress based on the stress ratio of Pohang, Korea. The critical plane approach is applied to describe the anisotropic Mohr-Coulomb failure criterion. A various scenarios with depth in transversely isotropic rock are considered in order to examine the borehole breakout.