ASSESSING THE IMPACT OF TRAJECTORY ON WELLS DRILLED IN AN OVERTHRUST REGION

Drilling in the Cusiana field, located in the tectonically active foot hills of the Casanare region of Colombia, has proved to be extremely d ifficult. One major contributor to the operational difficulties is poo r hole conditions, which often lead to stuck pipe. However, experience has shown that performance improves when drilling updip of the major faults and bedding; downdip and crossdip well trajectories are the mos t problematic. Stress modeling of the geological cross sections indica tes that the principal stresses in the Cusiana field may be rotated si gnificantly from the vertical and horizontal. After adjusting a conven tional wellbore-stability analysis to allow for stress rotation, a rea sonable match is obtained between prediction and field experience on t he issue of stability variation with hole trajectory for wells drilled in the region.