Dog Leg Severity

8/11/2019 Dog Leg Severity

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Dog Leg Severity (DLS)

In this section I'll discuss about one of the most important termrelated to Directional Drilling "Dog Leg Severity", abbriviated asDLS.

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 feetof course length. In the metric system, it is usually expressed in degreesper 30 meters or degrees per 10 meters of course length.

Since in a directional well we continuously have changes in inclinationand/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 ishigh when the inclination and/or azimuth changes quickly or occur over ashort interval of course length.

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

Case 1 :MD = MD2 MD1 = 1100 1000 = 100 ft

I = I2I1= 40-20= 20

Thus, Dog Leg Severity (DLS) = I / MD = 20/100 ft

Case 2 :

MD = MD2 MD1 = 1100 1000 = 100 ft
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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 bothare kept in consideration.

Formulae for Dog Leg Severity CalculationIn the above three equations, the 100 changes the dogleg severity toper 100 feet. If metric system is used, the 100 should be changed to30 for dogleg severity in degrees per 30 meters or 10 for doglegseverity in degrees per 10 meters.

Now let me solve an example which can be calculated using aboveequation and is based on calculation of DLS for change in Azimuth.Below given is a table which shows the values of inclination and azimuthfor change in MD to be 100 in each case.
http://1.bp.blogspot.com/-W6ib0Jw1t1g/ThejpoKiGRI/AAAAAAAAAKE/2oO4JDrqPCE/s1600/Dog+Leg+Severity.JPGhttp://4.bp.blogspot.com/-M87tZ_y88EI/TheyjeidqXI/AAAAAAAAAKQ/1NvwPrPlIXQ/s1600/DLS+Formulae.jpghttp://1.bp.blogspot.com/-W6ib0Jw1t1g/ThejpoKiGRI/AAAAAAAAAKE/2oO4JDrqPCE/s1600/Dog+Leg+Severity.JPGhttp://4.bp.blogspot.com/-M87tZ_y88EI/TheyjeidqXI/AAAAAAAAAKQ/1NvwPrPlIXQ/s1600/DLS+Formulae.jpg


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

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

The above equations can be used to calculate dogleg severity for anycombination 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 indirectional well until it is severe !One can easily imagine a situation when DLS is high .. You can evenperform a home based experiment to experience this situation. Take acylindrical 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 fashionand try to feel what you experience when you insert as well as when youpull back the green coloured pipe into and out of the black coloured pipe.
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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 pullout the green coloured pipe if once inserted. You can experience dragwhile performing this experienced.Since the drill pipe is in rotational as well as performs linear motion, thedrill string experiences both TORQUE & DRAG. Torque and drag arecaused by the friction between the drill string and the borehole wall. Moresevere 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 caseof a horizontal well or high inclination well. However, while trippingor reaming, the torque will be greater because the collars are in tensionand increases the overall tension in the drill string. In a horizontal or high

inclination well, the torque may be lower while rotating off bottom. Careshould be taken when tripping after a significant change in hole inclinationand/or direction. The assembly may go to the bottom, but it might notcome back up through the dogleg. An assembly should never beforced to the bottom; it should be reamed to the bottom.

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

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

Below mentioned are some of the methods to reduce torque anddrag acting between the drill string and the wellbore wall:

by keeping the dog leg severity low.
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once a severe dogleg comes into play, its effect can be reduced byusing lubricants in mud system. the lubricant reduces the coefficientof 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-watedrill pipe. The hevi-wate drill pipe is more flexible and reduces theoverall string weight while maintaining the same available bitweight.

Problems associated with dogleg :As drilling continues, the drill string tension in the dogleg increases whichincreases the lateral force. The lateral force causes the drill string to cutinto the wellbore wall at the dogleg (refer figure below). A keyseat isformed 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 jointsand worn spots in the casing which can lead to collapse or a hole in thecasing. Logging tools and drill collars can become stuck in a keyseat.
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Dog Leg Severity

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