Evaluation of mechanical, chemical, and thermal effects on wellbore stability using different rock failure criteria
Journal of Natural Gas Science and Engineering
School of Engineering
During drilling operation, the occurrence of borehole instability is one of the major problems in which many parameters can affect the wellbore stability. In many cases, wellbore instability is because of a combination of mechanical and chemical instabilities. This problem may lead to complicated states, and in some cases, can cause costly operational problems (e.g. lost circulation, sidetracks, and pipe sticking). Therefore, choosing an optimum rock failure criterion for predicting safe mud window based on all mechanical, chemical and thermal effects would be of great interest to industry. The goal of this study is to comparatively evaluate six practical rock failure criteria such as modified Lade, Mogi‒Coulomb, Mohr‒Coulomb, three-dimensional (3D) Hoek‒Brown, Drucker‒Prager, and modified Wiebols‒Cook criteria considering all mechanical, chemical, and thermal effects. Through the obtained results, it is possible to determine which model is more accurate to be used in unconventional formations such as shale to effectively plan future drilling operations in the field to improve drilling efficiency and future field development. To determine the impact of poroelastic, thermal, and chemical effects on shale stability, three variables including critical mud weights, collapse stress, and pore pressure were examined. In addition, this study investigated the effects of temperature variation, changes in mud salinity, and cohesion alteration on the determination of safe mud window to stabilise the borehole in shale formations using the six rock failure criteria. The results show that all these parameters had a significant effect on the minimum mud pressure predicted by various rock failure criteria and should be considered in addition to other parameters. Lastly, the models suggested the safe mud pressure window for drilling the reservoir safely with and without considering of all mechanical, chemical and thermal effects and represented the results using stereographic projection. Also, the models represent that drilling to the orientation of minimum horizontal stress can minimise the wellbore instabilities. The results of this study can be employed for further improvement in drilling operations, openhole completions, and hydraulic fracturing treatments. © 2020 Elsevier B.V.