Private Forums
11 November 2005
Does anybody have any experience/information/theories/ideas or
knowledge about running two jars in the same drillstring?

I understand that one set is right on top of the BHA (horizontal
section) and the other will be in the vertical part of the hole.

What are the operational considerations when running such a set up?
How are the jars cocked and fired?

Comments appreciated.

Donald Mackay
9 answer(s)
Total Posts: 2
Join Date: 23/09/08
There is the option of running the combination of a mechanical and
an a hydraulic jar in a string for e.g one above and one inside a
horizontal section. The theory is that with a straight pull the
upper (mechanical) jar will fire first and by putting torque in the
string first you can lock the mechanical jar, check manufacturer and
fire the bottom hydraulic jar first, and fire the bottom hydraulic
jar. CHeck with manufacturers since I do not have my notes with me
here at the moment!

Good luck
Addax Petroleum
Total Posts: 1
Join Date: 14/12/05
Hi Donald,

The design for jar placement is such that when the lower jar is at
horizontal ~ TD, the upper jar will be at about 60 deg. While
drilling or otherwise and the drill string gets stuck below 60 deg
then the first jar can be cocked using the BHA mass and firing
velocity to achieve the momentun that may possibly free any stuck
pipe (with luck). However, beyound 60 deg where the first jar is at
horizontal and has 0 effect, the second jar which is still about
vertical takes advantage of the weight of the BHA above it to
possibly free any stuck pipe. [email protected]
Drilling Supervisor
Oil Search Limited
Total Posts: 6
Join Date: 08/12/05
Pete Oakes is on the money with his research. In the late 80's the
Dutch sector had a horizontal boom, Unocal & Conoco being the
leaders. One and then two jars were used, with much debate. In the
late 80's - early 90's some exotic well profiles were drilled and
the wells were so complex that jar placement was too difficult a

Due to the novelty of horizontal drilling and the perceived high
risks, wells were given top engineering focus; T&D studies became
vogue and paid dividends assiting the strong focus on hole cleaning
best practices. Combined with the use of OBM stuck pipe in
horizontal wells was not common.

By '93/94 confidence in well-engineered wells made no-jar strings
more common and to my knowledge no increase in stuck pipe instances
was observed. Jars are a weaklink causing string failures and their
elimination when drilling in well known "low trouble" formations can
be seen as a benefit. Spend your money on "best practice" hole
cleaning, tripping and quality mud systems, not jar rental.
Managing Director (
Relentless Pursuit Of Perfection Ltd.
Total Posts: 395
Join Date: 10/01/05
When i took over my last job i noticed that it was common practice
to run two jars in the string. one in the upper part of the BHA and
one in the vertical section. No explanation was given why. i also
noticed that in case of stuck pipe the chance of getting free was
zero. Personaaly i can not see why jarring in horizontal holes is of
any use. The dampening of the jarring mass is most likely very high.
In other words the actual acceleration of the jarring mass is low,
resulting in a low impact force. We discontinued the use of jars in
horizontal holes completely and concentrated on the prevention of
stuck pipe which resulted in a large improvement in performance.


Gerrit van Wilpe
Drilling Engineer
AGR Petroleum
Total Posts: 4
Join Date: 03/10/05
To jar, not to jar or to run jars at all???

Comments appreciated.

Donald Mackay
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 352
Join Date: 10/01/05

Normally reserved for high angle/erd/horizontal wells?
Suggest you contact a jar specialist who will explain the ins/out of
this, and show you some representative calcs to the

One such I always consult is Euan, who is based here in Aberdeen and
is always happy to discuss technicalities of Jar/accelerators.

[email protected]


Drilling Engineer
Total Posts: 2
Join Date: 12/08/05

This was common practice in the early days of horizontal drilling, in
the UK at least, well records from all the early wells, from the
early 90's will show double jar strings. The theory was that if you
got stuck in the horizontal section the chances are that it will be
above a set of conventionally placed jars. This way you'd have a set
of jars that would work in the casing. It fell out of practice and by
the mid 90's if you have access to well records from that time for
the following reasons. As we did more horizontal wells confidence
grew that the chances of being stuck were just the same as any other
wells and the focus moved to hole cleaning and good drilling

The problem was if you envisaged a long run, where do you place the
jars. If you put them so they were in casing at the end of the run
and got stuck early on then they were no use, and if you put them too
deep they would be in open hole when you needed them.

Other more practical issues were they would close and "fire" as the
DD slid to bottom, due to the string going into compression and
tension, causing confusion. Not such an issue now with RST tools.

At Mobil in about 1993, the decision was taken on Southern Gas basin
6" hole sections that no jars were run, but a safety joint was run
above the expensive BHA jewellery. The idea behind this was if the
worst happened, we would back off the safety joint, POOH and go in
with a fishing assembly based on exact knowledge of what we were
facing. it never had to be used so i can't comment on it's
Managing Director -Relentless Pursuit of Perfection Ltd.
Relentless Pursuit Of Perfection Ltd.
Total Posts: 26
Join Date: 10/01/05
Hi Donald,

Great question !! I recall we ran as you describe when drilling at
Shell in the Gannet field and elsewhere.

I'll pass on to Ray Procter of OCTG-Procter.

Best wishes

Dave Taylor
Consultant [retired Shell staff]
Total Posts: 237
Join Date: 02/09/05
Mixed feelings on the subject.

Prefer a single double-action hydraulic jar near the Bottom Hole

If forced - over my best judgement - to use 2 jars, I would use a
Mechanical Jar + enough Heavy Weight Drill Pipes well above the
Hydraulic jar on the BHA.

The mechanical jar set to fire AFTER the hydraulic one.

Another good question for Gerrit [email protected]
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