Borehole stability analysis at the Coporo-1 well, Colombia

Abstract

Coporo-1 is an 18,000-ft dry hole located in the tectonically active foothills of the Eastern Cordillera of Colombia. A mechanical stability analysis allowed explaining most of the drilling stability-related problems and elaborating an optimal mud-density window for Coporo-1 and future wells in the area. Repeated sections of the Carbonera formation, high in-situ stresses, abnormal pore pressure in some intervals, high temperature, and micro-fractured formations make drilling in this region both technically and economically challenging. Shear failures in shales caused frequent pack-off, stuck pipe, reaming, and back reaming. Tensile failures in sands generated lost-circulation incidents. Log-derived mechanical properties, leak-off test and lost-circulation data, borehole geometry, estimated pore pressure based on the Eaton's method, and borehole images from Coporo-1 and partial information from the Cumaral-1AX well were available for this research to develop a geomechanical model for Coporo-1. Further calculations based on the Mohr-Coulomb criterion were applied to elaborate a mud-density window designing mud-densities that allow controlled breakouts in the formations for its lower limit and mud-densities equivalent to the magnitude of the minimum horizontal stress in its upper limit. The Carbonera formation C7 unit at 13,620 ft is one of the most difficult formations to drill in the area. There appears to be no optimal mud-density window to simultaneously avoid serious borehole deterioration and lost circulation risks in this formation. Additional research is required to analyze the Carbonera formation C5 unit that caused casing to collapse in several intervals in the Coporo-1 well. The C5 unit is likely to be a potential problem during the drilling and production operations of future wells in the area. Conclusions and recommendations are provided for future wells in the area.