Dog Leg Severity
Dog Leg Severity commonly referred to as DLS is a measure of change in direction of a wellbore. It is described in degrees per 100 feet or per 30 meters. The difference between the directional measurements between two survey points is used for computing the DLS value. In simple terms, the difference in inclination of two survey points is used, but in reality, the directional measurements include inclination (inc) and azimuth (Az) of the well.
The actual computation of DLS is based on the 'Radius of Curvature' method. The formula used for computing DLS is: Dogleg severity (DLS) = {cos-1 [(cos Inc1 x cos Inc2) + (sin Inc1 x sin Inc2) x cos (Az2 – Az1)]} x (100 ÷ MD).
Higher DLS values result in higher torque and drag in drilling & tripping operations. It also risks creating key seats, which increase string wear and the possibility of fatigue failure. Severe key seating can also cause sting stuck-ups.