Analytical and numerical assessment of the influence of bulk coal strength on drilling operations in unconventional coal seam gas wells

The drilling of horizontal wells in 'weak' coals faces challenges as the equivalent circulating density can easily increase to exceed the formation strength, thus leading to wellbore failure. This paper presents the application of advanced analytical and numerical models for wellbore failure to a case study from the Bowen Basin in Eastern Australia, where the cleating and shrinkage of coal are important considerations. The analytical modelling starts with an effective stress field that incorporates both poroelastic effects and the shrinkage of coal matrix due to gas desorption. An implicit root-finding approach is used to solve for the range of safe mud weights for the full inclination-azimuth parameter space, which greatly improves computational efficiency. Three-dimensional finite element modelling was also conducted to investigate the influence of the heterogeneity introduced by cleat networks. The results were compared to those from the analytical model and typical field observations. The findings of this study highlight the sensitivity of wellbore stability in coals to a range of parameters, including degree of depletion and drilling azimuth.