Dog Leg Severity

Changes in hole curvature are often referred to as dog-legs

The severity of a dog-leg is determined by the average changes in angle and/or direction calculated on the distance over which this change occurs. For example, if there is a 3° change in angle (no direction change) over 100ft of hole, the dog-leg severity is 3° per 100ft.

Until a dog-leg reaches some threshold value, no drill stem fatigue damage occurs. This threshold value is called the Critical Dog-leg. The critical dog-leg is dependent upon the dimension (size) and metallurgy of the drill pipe and drill pipe tension (pull) in the dog-leg.

The planning of directional wells should include a 'Dog-leg control programme'. Critical dog leg limits should also be considered for drill collars.

Dog-leg limits are established to prevent drill pipe fatigue, but when those limits are maintained, there is also a reduction in associated hole problems. Excessive dog-legs cause key seats, casing wear, rotating torque, trip drag, etc. Overall drilling rate can be greatly improved by a carefully planned and executed dog-leg control programme (Refer to Section 8.1).

1) If extreme torque is encountered during drilling deviated holes, consider the following

  • in order of priority):
  • Improve hole cleaning verifying the flow velocity in the drill pipe/open hole annulus and adjusting mud properties (high instantaneous gels, low viscosity and, conditions permitting, high mud weight).
  • Add a non-polluting torque reducer additive or, if possible, diesel oil (to a maximum of 10% diesel) to the mud.
  • On the subsequent trip leave out some drill collars and stabilisers. Replace with HWDP. Replace the near bit stabiliser with a near bit roller reamer (if available).
  • Check alignment of derrick over the centre of the well.
  • Check the wear bushing on the first trip.
  • Use lowest practical rpm and consider the use of downhole motor.
  • Use the minimum practical weight of bottom hole assembly.
  • Do not rotate with the bit off bottom.
  • Install protectors at less than 3ft (1m) above worn tool joints and, if so required, also 3ft (1m) below tool joints in any section of the top hole where the hole curvature makes it necessary. Minimum OD of the protectors is 7".

Set casing through the build up section to 200-300ft (60-90m) in the tangent section, if possible.

The use of hard-banded drill pipe is not allowed inside casing. Check the DP tool joints every trip with a fixed caliper.

It is recommended to have a magnet placed in the flow line to collect metal cuttings coming out of the hole. Excessive metal cuttings may indicate casing wear or collision with another well (on a cluster). In case of indications that the drill pipe and casing are eroding , the following actions are suggested:

ENI S.p.A. Agip Division

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Responses

  • roan
    What does a dogleg refer to in directional drilling?
    8 years ago
  • balbo
    How casing is installed in dogleg drilling?
    7 years ago
  • martha
    How is dog leg severity measured?
    7 years ago
  • katie
    How to prevent dogleg severity?
    6 years ago
  • Hagosa
    What is a high dog leg severity?
    4 years ago
  • Eliisa
    What is consider excessive casing wear?
    3 years ago
  • Jaxon
    What dogleg can drill collars rotate thru?
    3 years ago
  • prisca romano
    Why a directional driller should avoid dogleg?
    1 year ago
  • Mathilda
    What is considered excessive dogleg?
    2 months ago

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