Borehole stability in shale formations: Effects of Thermal˗Mechanical˗Chemical parameters on well design

Abstract

Borehole instability in the petroleum industry is mainly because of mechanical, chemical, and thermal factors which result in substantial yearly expenditures. The essential part of analysing borehole stability is choosing an optimum rock strength criterion. Many rock strength criteria have been used for rock failure analysis, however all mechanical, chemical and thermal effects are seldom coupled with rock failure criteria to evaluate borehole stability. This paper presents six rock strength criteria coupled with a poro˗chemo˗thermo˗elastic model to analyse borehole stability in shale formations. This model considers the mechanical, chemical, and thermal effects within the stress distribution around a wellbore to evaluate shear and tensile failure. The effects of time and wellbore configuration (i.e. azimuth and inclination angles) on the stability of borehole in shale formations have also been examined. Furthermore, the minimum mud pressures are estimated by investigating wellbore stability at both the borehole wall and the entire near˗borehole area (i.e. inside the formation). Finally, a sensitivity analysis was performed to quantify and compare the effect of each mechanical, chemical, and thermal parameter as well as their level of importance on wellbore stability analysis. The results of dimensionless sensitivity coefficients indicate that, depending on the applied rock failure criteria, the parameters have different order of importance in analysing borehole stability. The impact of error in determination of input parameters on the magnitude of error in critical mud pressure was also presented. The results of this paper will help to effectively control time-dependent borehole instability in low-permeable shale formations, thereby lowering drilling nonproductive time and costs.