DIRECTIONAL DRILLING "The art and science involving the intentional deflection of a wellbore in a specific direction in order to reach a predetermined objective below the surface of the earth". Today, it is much more science than art... !

Friday, July 8, 2011

9. Dog Leg Severity (DLS)

In this section I'll discuss about one of the most important term related to Directional Drilling "Dog Leg Severity", abbriviated as DLS.

Dogleg severity is a measure of the amount of change in the inclination, and/or azimuth of a borehole, usually expressed in degrees per 100 feet of course length. In the metric system, it is usually expressed in degrees per 30 meters or degrees per 10 meters of course length.

Since in a directional well we continuously have changes in inclination and/or azimuth, thus the presence of DLS is always there. 

The dogleg severity is low if the changes in inclination and/or azimuth are small or occur over a long interval of course length. The dogleg severity is high when the inclination and/or azimuth changes quickly or occur over a short interval of course length.

Let me give you an example to show how the change in inclination affects the Dog Leg :
Let us consider four cases as given below :


Case 1 :
ΔMD = MD2 – MD1  = 1100 – 1000 = 100 ft

ΔI = I2 – I1 = 40 -20 = 20

Thus, Dog Leg Severity (DLS) = ΔI / ΔMD = 20/100 ft

Case 2 :

ΔMD = MD2 – MD1  = 1100 – 1000 = 100 ft

ΔI = I2 – I1 = 60 -20 = 40

Thus, Dog Leg Severity (DLS) = ΔI / ΔMD = 40/100 ft

Case 3 :
ΔMD = MD2 – MD1  = 1050 – 1000 = 50 ft

ΔI = I2 – I1 = 40 -20 = 20

Thus, Dog Leg Severity (DLS) = ΔI / ΔMD = 20/50 ft = 40/100 ft

Case 4 :
ΔMD = MD2 – MD1  = 1050 – 1000 = 50 ft

ΔI = I2 – I1 = 60 -20 = 40

Thus, Dog Leg Severity (DLS) = ΔI / ΔMD = 40/50 ft = 80/100 ft

From Case 1 & Case 2, Case 3 & Case 4 it is clear that : A greater change in inclination yields a larger dogleg severity for the same change in measured depth.

From Case 1 & Case 3, Case 2 & Case 4 it is clear that : for the same change in inclination, a shorter course length will result in a greater dogleg severity.

The above example gave us idea about how the change in measured depth and inclination affects the DLS. But this is not the end. Even the change in Azimuth affects the DLS.

It took us just few steps to calculate the DLS for change in inclination, but unfortunately it is not that easy to calculate the DLS for change in Azimuth .. !

Will a 20 change in azimuth for 100m change in measured depth yield DLS = 20/100 ft ?
The Answer is NO.

This will only be possible when the inclination is 900.
In simpler words, for the above case, DLS will be equal to 20/100 ft when I1 = 90, I2 =90 (i.e., the inclination remains constant at the current and last survey points) and change in azimuth, ΔA= A2 – A1 =20 and change in measured depth, ΔMD = 100 ft.

 At low inclinations a change in azimuth will have a small dogleg severity. As the inclination increases, the dogleg severity will also increase for the same change in azimuth.
The next example which I am going to solve for you will clear this point . 

We have three formulae to calculate DLS if inclination and azimuth both are kept in consideration.

Formulae for Dog Leg Severity Calculation
In the above three equations, the “100” changes the dogleg severity to “per 100 feet”. If metric system is used, the “100” should be changed to “30” for dogleg severity in degrees per 30 meters or “10” for dogleg severity in degrees per 10 meters. 

Now let me solve an example which can be calculated using above equation and is based on calculation of DLS for change in Azimuth.
Below given is a table which shows the values of inclination and azimuth for change in MD to be 100 in each case. 


 Since the inclination is constant (refer rows in the table below) and only the azimuth changes, thus, substituting the values of I, A and MD in Equation 1 and calculating DLS, the result obtained is as tabulated below :


It can be easily seen from the above result that dogleg severity increases as the inclination increases for the same change in azimuth, i.e., 100.

The above equations can be used to calculate dogleg severity for any combination of changes in azimuth, inclination, and measured depth.

Now the question arises is : Is Dogleg always a problem ?
So, I would like to tell you that Dog leg is not always a problem in directional well until it is severe !
One can easily imagine a situation when DLS is high .. You can even perform a home based experiment to experience this situation. Take a cylindrical hollow pipe as shown in figure below (black coloured), inside that pipe, insert another cylindrical pipe of lesser diameter (refer figure). Now keep varying the curvature of the outer pipe in increasing fashion and try to feel what you experience when you insert as well as when you pull back the green coloured pipe into and out of the black coloured pipe.



The same happens with the wellbore and drill string. The black coloured pipe behaves as wellbore and the green one behaves as our drillstring.
As the inclination increases, it becomes difficult to insert as well as to pull out the green coloured pipe if once inserted. You can experience drag while performing this experienced.
Since the drill pipe is in rotational as well as performs linear motion, the drill string experiences both TORQUE & DRAG. Torque and drag are caused by the friction between the drill string and the borehole wall. More severe doglegs will cause higher torque and drag.

The drill string will experience less torque from a dogleg while drilling, because the collars are in compression except in the case of a horizontal well or high inclination well. However, while tripping or reaming, the torque will be greater because the collars are in tension and increases the overall tension in the drill string. In a horizontal or high inclination well, the torque may be lower while rotating off bottom. Care should be taken when tripping after a significant change in hole inclination and/or direction. The assembly may go to the bottom, but it might not come back up through the dogleg. An assembly should never be forced to the bottom; it should be reamed to the bottom.


Now let us consider a situation where a drill string section is present in the wellbore as shown in the figure below. When the drill string is in tension, it tries to straighten while going around a dogleg. The drill string exerts a force on the formation as shown in figure. As tension on the drill string increases (depth below dogleg increases) the lateral force increases; therefore, the torque and drag increase.



So now since the torque and drag has arisen, we need to find the methods to reduce the torque and drag.

Below mentioned are some of the methods to reduce torque and drag acting between the drill string and the wellbore wall:
  •  by keeping the dog leg severity low.
  • once a severe dogleg comes into play, its effect can be reduced by using lubricants in mud system. the lubricant reduces the coefficient of friction b/w the drill string and the wellbore wall.
  • by reducing the tension in the drill string.This can be accomplished by removing excess collars, or replacing the collar with hevi-wate drill pipe. The hevi-wate drill pipe is more flexible and reduces the overall string weight while maintaining the same available bit weight.
Problems associated with dogleg :
As drilling continues, the drill string tension in the dogleg increases which increases the lateral force. The lateral force causes the drill string to cut into the wellbore wall at the dogleg (refer figure below). A keyseat is formed if the lateral force is large enough to cut into the wall. Soft formations require a lower force than hard formations to form a keyseat. 
Other problems associated with severe doglegs are wearing of tool joints and worn spots in the casing which can lead to collapse or a hole in the casing. Logging tools and drill collars can become stuck in a keyseat.


45 comments:

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  3. Definately good explaination! It would be help ful if add info on the max stress on the drill collar or drill pipe at section due DLS especially bending/combined stresses; nummerical example would be nice

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