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:

  1. nice illustration

    ReplyDelete
  2. good explanations! Deepak

    ReplyDelete
  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

    ReplyDelete
  4. Good explanation

    ReplyDelete
  5. There's definately a lot to find out about this topic. I like all the points you've made.


    Also visit my blog post; Corporate Movers

    ReplyDelete
  6. I think you are all awesome dads are fabulous too, but that doesn't mean the kickoff keynote wasn't filled
    with Ballmer-isms, a lady strung out on too
    much WP7, and lots of sexcam sales numbers.
    Is it up to be one of the greatest science sexcam fiction films ever to grace
    the big screen.

    Feel free to surf to my web page ... sexcams

    ReplyDelete
  7. Woah! I'm really digging the template/theme of this website. It's simple, yet effective.
    A lot of times it's challenging to get that "perfect balance" between superb usability and visual appearance. I must say you have done a amazing job with this. Also, the blog loads extremely fast for me on Chrome. Excellent Blog!

    My site :: property to Buy

    ReplyDelete
  8. Very nice post. I just stumbled upon your blog and wanted to say that I've truly enjoyed browsing your blog posts. After all I will be subscribing to your rss feed and I hope you write again soon!

    Here is my weblog :: Michael Kors Handbags

    ReplyDelete
  9. Hello there! Do you know if they make any plugins to assist with
    SEO? I'm trying to get my blog to rank for some targeted keywords but I'm not
    seeing very good success. If you know of any please share.
    Cheers!

    Feel free to surf to my blog: mytreemyearth.org

    ReplyDelete
  10. They are in the process, search over the internet, there is fleshlight no way to access the
    application in the early stages of developing a video about the risks of premature birth.

    It may be as simple as swiping all the way to his ideal lives more frivolously and impudently than fleshlight the man without an ideal.

    It has provided us a front-row seat to the trailblazers behind cutting-edge innovation,
    as well as cube shaped don't fight the wind all that well. Now you must be able to eat it too.

    ReplyDelete
  11. I visited various sites but the audio feature for audio songs present at this web site is in fact excellent.


    my homepage - Louis Vuitton Outlet

    ReplyDelete
  12. It's appropriate time to make some plans for the future and it's
    time to be happy. I've read this post and if I could I want to suggest you few interesting things or advice. Perhaps you could write next articles referring to this article. I wish to read more things about it!

    My web page :: Abercrombie and Fitch

    ReplyDelete
  13. Howdy just wanted to give you a quick heads up. The
    words in your article seem to be running off the screen in Internet explorer.
    I'm not sure if this is a formatting issue or something to do with browser compatibility but I figured I'd post to let you know.
    The layout look great though! Hope you get the problem solved soon.
    Cheers

    Here is my blog; free Adult chat

    ReplyDelete
  14. Hello, I need Help, what is this ΔMD ??

    Please explain because I am a novice.
    Thanks for answer :)
    Dominik

    ReplyDelete
  15. WOW just what I was looking for. Came here by searching for live cams and chat

    Feel free to surf to my web-site - sex online live

    ReplyDelete
  16. Hello to every one, because I am actually keen of reading this webpage's post to be updated daily. It consists of nice data.

    Here is my web site; http://wellnesshealthcenters.com/se/blogs/4546/7459/spice-up-your-life-watching-free

    ReplyDelete
  17. This piece of writing is genuinely a nice one it helps new internet people,
    who are wishing for blogging.

    Also visit my page :: Evgeni Malkin Black Jersey

    ReplyDelete
  18. Do you have a spam problem on this site; I also am a blogger,
    and I was wanting to know your situation; many of us have developed some nice practices and we are looking
    to trade strategies with other folks, why not shoot me an email if interested.



    My blog post :: http://www.streetarticles.com/about/SttyineoksHeaqether/400403

    ReplyDelete
  19. I'm not sure exactly why but this site is loading very slow for me. Is anyone else having this problem or is it a problem on my end? I'll check back later on and see if the problem
    still exists.

    Also visit my web-site - porn chats

    ReplyDelete
  20. Hi there to all, how is everything, I think every one is getting more from this web page, and your views are fastidious designed for
    new people.

    Feel free to surf to my weblog - Tory Burch Shoes

    ReplyDelete
  21. Since the admin of this site is working, no hesitation very
    shortly it will be well-known, due to its feature contents.


    Feel free to visit my webpage - www.strategic-Workplace-solutions.com

    ReplyDelete
  22. I do not even know the way I ended up right here,
    however I assumed this post used to be good. I do not realize who you are but definitely you're going to a well-known blogger if you aren't already.
    Cheers!

    Take a look at my site Nike pas cher

    ReplyDelete
  23. I am truly pleased to read this weblog posts which contains tons of useful information, thanks for
    providing these statistics.

    Feel free to visit my blog post; www.enstarz.com

    ReplyDelete
  24. Superb blog you have here but I was curious about if you knew of any user discussion
    forums that cover the same topics discussed here?
    I'd really like to be a part of online community where I can get comments from other knowledgeable people that share the same interest. If you have any suggestions, please let me know. Many thanks!

    Visit my web site: Louis Vuitton Outlet

    ReplyDelete
  25. Awesome! Its truly remarkable paragraph, I have got much
    clear idea regarding from this paragraph.

    Visit my weblog; free sex chat online

    ReplyDelete
  26. Inspiring story there. What occurred after? Thanks!


    my web blog: live web cam chat sites :: :
    :

    ReplyDelete
  27. I would like to thank you for the efforts you have put in writing this website.
    I am hoping to check out the same high-grade blog posts from you in the future as well.
    In fact, your creative writing abilities has motivated me to get
    my own, personal site now ;)

    Here is my web-site :: sex chat live for free

    ReplyDelete
  28. A motivating discussion is definitely worth comment. There's no doubt that that you need to publish more about this topic, it may not be a taboo matter but generally people don't speak
    about such subjects. To the next! Kind regards!

    !

    Feel free to visit my page: mature sex cams

    ReplyDelete
  29. If some one wants expert view on the topic of blogging and site-building after that
    i advise him/her to go to see this weblog, Keep up the nice job.


    Also visit my blog ... free live webcams adult

    ReplyDelete
  30. Hello colleagues, its fantastic post on the topic of
    teachingand completely explained, keep it up all the
    time.

    My homepage - camgirl ()

    ReplyDelete
  31. Really when someone doesn't understand afterward its up to other users that they will assist, so here it occurs.

    my web-site: live streaming sex :: ::

    ReplyDelete
  32. Magnificent goods from you, man. I've understand your stuff previous to and you're just extremely fantastic.
    I actually like what you've acquired here, really like what you're stating and the way in which you say it.
    You make it entertaining and you still care
    for to keep it sensible. I cant wait to read far more from you.
    This is actually a tremendous website.

    Look into my page; live video sex cams

    ReplyDelete
  33. I have been surfing online more than three hours these days,
    but I by no means discovered any interesting article like yours.

    It is pretty price enough for me. In my opinion, if all web owners and bloggers made
    just right content material as you did, the net shall be much more useful
    than ever before.

    my blog post - live nude free cam ()

    ReplyDelete
  34. Excellent blog here! Also your web site loads up
    fast! What web host are you using? Can I get your affiliate link to your host?
    I wish my website loaded up as fast as yours lol

    My webpage free cam on cam sex

    ReplyDelete
  35. It's really a nice and useful piece of information. I am happy that you just shared this helpful info with us. Please stay us informed like this. Thank you for sharing.

    my blog post :: Louis Vuitton Pas Cher - -

    ReplyDelete
  36. It is not my first time to pay a quick visit this web site, i am browsing this website dailly and get pleasant data from here every day.


    Look at my blog post free chat sex rooms

    ReplyDelete
  37. I'm not sure exactly why but this website is loading incredibly slow for me. Is anyone else having this issue or is it a issue on my end? I'll check back
    later and see if the problem still exists.

    Stop by my homepage: live porn vids :: www.bigbrother-aktuell.info ::

    ReplyDelete
  38. With havin so much content and articles do you ever run into any problems of plagorism or copyright
    infringement? My site has a lot of unique content I've either created myself or outsourced but it seems a lot of it is popping it up all over the internet without my authorization. Do you know any techniques to help protect against content from being ripped off? I'd truly appreciate it.



    Review my web-site; view live webcams - -

    ReplyDelete
  39. Quality articles or reviews is the important to be a focus for the users to pay a quick visit the site,
    that's what this web page is providing.

    Here is my site - chat with strangers - http://Houseoftaaj.com/ -

    ReplyDelete
  40. Amazing effort in simple language making it easy to understand..
    awesome...

    ReplyDelete
  41. If I know how many feet I need to turn a 90° well-bore over an estimated course length, can I use OPP/ADJ ARC Tan to calculate DLS?

    ReplyDelete