Simulation of wellbore stability with thermo-hydro-chemo-mechanical coupling in troublesome formations: an example from Ahwaz oil field, SW Iran

Abstract

Wellbore stability is a main concern in drilling op- eration. Troublesome drilling issues are chemically active for- mations and/or high-pressure-high-temperature environments. These are mainly responsible for most of wellbore instabilities. Wellbore failure is mostly controlled by the interaction between active shales and drilling fluid in shale formations. The factors influencing this interaction consist of fluid pressure, tempera- ture, composition of drilling fluid, and exposure time. In this paper, a non-linear fully coupled chemo-thermo-poroelasticity model is developed. At first, a fully implicit finite difference model is presented to analyze the problem, and then, it is verified through comparison with analytical models. For this purpose, a computer code has been written in MATLAB envi- ronment. Results indicate that chemical osmosis and thermal osmosis significantly influence the wellbore stability in shale formations. Cooling and high salinity in mud decrease pore pressure and total tangential and radial stresses in the formation and vice versa. Moreover, CaCl2 solutions result in higher osmotic pressure than NaCl solutions. The type of constitutive model is investigated on wellbore failure. In addition, the proposed model is applied for wellbore instability in Ahwaz oil field, SW Iran. Results show that the proposed model considerably predicts the wellbore instability in this field.