24.1 Illustrating tortuosity

This has been variously defined as the variability in dogleg severity or the unnecessary undulations in the wellpath. Essentially tortuosity describes a lack of smoothness in the well trajectory and the greater the tortuosity the greater the likelihood of problems when running casing or excessive drillpipe stress when rotating in hole. One simple presentation of tortuosity is to plot the dogleg severity against measured depth.

However, on its own, this is hard to interpret in a measurable way. Clearly, the well is tortuous but how can we define acceptable tortuosity? If the wellplan had prescribed a 4 deg/100 build rate, it would be inappropriate to penalise a dogleg severity of 4 deg/100. The real problem is when the dogleg severity varies too much and a 4 deg/100 build is constructed with dogleg severities of 0 to 12 varying wildly during the build section. A well which smoothly followed the wellplan would have very little variation in dogleg severity. See well plan above.

The planned dogleg severity should have consisted of a short 2 deg/100 nudge and drop then a simple 2deg/100 build to 27 degrees inclination then a hold to TD. The actual inclination against md graph is plotted in figure below.

Clearly things did not go according to plan. However, the directional driller used a standard technique for the drop section of the nudge by replacing a motor (smooth build) with a pendulum rotary assembly (slow natural drop) which is an acceptable and economic method of returning to vertical. On closer examination, the build section is rapid at first then slower later. This is likely to be an assembly with a bent sub above the motor which is less aggressive at higher inclinations but assembly was capable of over 5 degrees of dls when only 2 was needed, suggesting a slide ratio of less than 40%. This explains the rapid changes in dls in the build section but even that is not unusual. The main problems here lie in the hold section where the directional driller has great trouble holding inclination and tends to steer a great deal in an attempt to place the surveys ‘on the line’. He is over correcting and creating unnecessary doglegs when a simple ‘aim at the target from where you are’ approach would have created a smoother well path..