Assessing the Impact of Trajectory on Wells Drilled in an Overthrust Region
Authors: Last N.C. , McLean M.R. 
Published: June 2014

Abstract
Drilling in the Cusiana Field, which is located in the tectonically active foothills of the Casanare region of Colombia, has proved to be extremely difficult. The geological setting has provided a most challenging drilling environment where all aspects of drilling have been tested to the limit. One major contributor to the operational difficulties is poor hole conditions often leading to stuck pipe. However, experience has shown that performance improves when drilling up-dip of the major faults and bedding, with down-dip and cross-dip well trajectories being the most problematic.
Computational stress modelling of the geological cross sections indicates that the principal stresses in the Cusiana Field may be rotated significantly from the vertical and horizontal. After adjusting a conventional wellbore stability analysis to allow for stress rotation, a reasonable match is obtained between prediction and field experience on the issue of stability variation with hole trajectory for wells drilled in the region.

Introduction
In partnership with ECOPETROL, TOTAL and TRITON, BP has been drilling in the foothills of the Casanare region of Colombia (Figure 1) since the late eighties. Both the Cusiana and Cupiagua Fields have now been declared commercial. Development well drilling in Cusiana commenced in late 1993. Appraisal wells are currently being drilled in Cupiagua. An aggressive drilling schedule is now being pursued with ten rigs active. From the earliest well, operational difficulties were encountered. Possibly the biggest problem associated with drilling wells in this region has been wellbore instability. Considerable effort has gone into tackling the problems and improvements have been made, as reported in a companion paper. Here the particular aspect of instability variation with well trajectory is discussed.

Background to Wellbore Stability
Wellbore instability (Figure 2) can result in lost circulation where tensile failure has occurred, and spalling and/or hole closure in the case of compressive failure of the rock. In severe cases hole instability can lead to stuck pipe and eventually loss of the open hole section. Due to the large costs associated with wellbore instability it has received considerable attention, as is apparent from the number of publications on the subject. No more so is the extent of the problem greater realised than the drilling experiences encountered in the foothills of the Casanare region of Colombia. For example, Figures 3a and 3b show the caliper traces from a Cusiana development well. Figure 3a shows a hole drilled with a 12 1/4" bit that has enlarged to nearly 45" in places. The in-gauge section is a sand interval. Also, as seen in Figure 3b, the large over-gauge reading in one caliper pair accompanied by a relatively in-gauge reading of the other pair suggests that the stresses may be highly anisotropic.
Selection of mud types and weights to eliminate hole instability of wells drilled in Cusiana has been found to be impossible. There are frequent occurrences of loss zones, which necessitate a low mud weight, being adjacent to collapse zones, which require a high mud weight. It is not realistic to incorporate sufficient intermediate casing strings to isolate each individual problem zone. Hence, mud weight selections are made in an attempt to minimise the problems, without eliminating them, and it has been necessary to live with a certain degree of hole instability.
Wellbore stability theory shows there are many factors, other than mud weight, that are likely to influence the competency of the wellbore wall. Examples are formation pressure, depth, formation strength, stress state, mud cake efficiency and time. Of parameters that can be controlled by the operator, theory shows that well trajectory has a significant influence on stability. Field experience from wells in the Cusiana Field also indicates that trajectory has a significant influence on hole stability. In cases where optimising mud weights cannot alone reduce instability to acceptable levels, then attention to optimising well trajectory to further reduce or at least limit the extent of problems is necessary.