What Does
Dogleg Angle Mean?
Dogleg severity (DLS) is a measure of the change in the direction of a wellbore over a defined length, normally measured in degrees per 100 feet of length. Dogleg severity measurements are typically done between two consecutive points on a wellbore in a directional survey.
In the oil and gas industry, well alignments are rarely perfectly vertical. In practice, the well angle constantly experiences changes in inclination and azimuth. As such, DLS is always present. Extreme DLS can have adverse effects on drilling operations; therefore, engineers typically calculate DLS to measure the maximum dogleg allowed during drilling.
The following formula expresses dogleg severity in degrees/100ft and is based on the Radius of Curvature method.
Dogleg severity (DLS) = {cos-1 [(cos I1 x cos I2) + (sin I1 x sin I2) x cos (Az2 – Az1)]} x (100 ÷ MD)
Where:
DLS = dogleg severity in degrees/l00 ft
MD = Measured Depth between the two survey points in ft
I1 = Inclination (angle) of the first (upper) survey point in degrees
I2 = Inclination (angle) of the second (lower) survey point in degrees
Az1= Azimuth direction of the first (upper) survey point in degrees
Az2 = Azimuth direction of the second (lower) survey point in degrees
Trenchlesspedia Explains Dogleg Angle
Drilling operations are usually planned for a certain amount of dogleg severity between any two points on a well trajectory. Severe doglegs most often occur due to changes in the geology of the wellbore. For example, hard rock may cause the drill string to deviate from the predetermined path in the horizontal or vertical direction.
If the DLS of a wellbore is low, this means that the changes in inclination or azimuth are small over the two points of the survey. Conversely, if the DLS is high, the inclination or azimuth of the bore changes rapidly over the course of its length. The higher the DLS, the greater the potential for problems during drilling.
Is Dogleg Severity Always a Problem?
In short, no. As mentioned previously, DLS is almost always present during drilling operations, and drilling operators typically plan for a certain amount of allowable horizontal or vertical well deviation. However, DLS can be problematic if deviations are severe.
What Are Some of the Adverse Effects of Excessive Dogleg Severity?
The first, and most apparent, problem with excessive DLS is that the wellbore is no longer on the intended or planned path. This can lead to operators having to make time-consuming corrections, which can affect drilling schedule and cost. If corrections are not made, the resulting curvature can be such that the planned casing string no longer fits in the shape of the wellbore.
Another significant problem with high dogleg measurements is repeated abrasion of the drill string in the dogleg section of the wellbore. Repeated abrasion in itself can give rise to several other issues. For instance, excessive abrasion of the string on a particular section of the bore can create a worn spot known as a keyseat. This can result in the drill and bottom hole assembly becoming stuck as they are pulled through the section.
Excessive abrasion is also associated with high friction. Increased torque and drag caused by the friction between the drill string and the borehole can place additional stress on the drilling assembly and lead to excessive wear of mechanical components.