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Unplanned sidetracks while enlarging the hole with a drillbit
21 July 2020


We have a customer who wishes to enlarge the hole from 12 1/4" to 22" with a rotary BHA.The BHA proposed is packed hole assy with tricone bit without a hole opener in the assembly  The trajectories are vertical at this point. Total interval to be drilled is about 700 m.

Top 400m of the lithology is clay/sandstone and bottom part is dolomitic limestone

We are conscious that this represent a risk of sidetracking the well and would would like to minimize the risk by leading a bullnose into the hole with a stabilizer and enlarge with a hole opener. Our customer has opened the hole using the bit in the past and doesn't see the point in a hole opener deployment.

Can anyone share best practices out there with enlarging such hole size in rotary BHA without hole opener and still manage to avoid risk of sidetracking?



5 answer(s)
SPREAD Associates
Total Posts: 164
Join Date: 05/03/08
Hi Ken,

A bit late to the party, but for what it's worth;

If the Operator has done it before and they have the evidence that it works, then don't bother fighting this battle - even if it is not good practice.

Otherwise, I'm with the other posters - I'd prefer to run the previous bit on a short collar c/w stabiliser to minimise the risk of side tracking.

Why isn't drilling a 14 3/4" or even 16" pilot hole an option?

Both of these would greatly reduce the amount of cuttings generated by the 22" hole section compared to drilling a 12 1/4" pilot hole.

All the best

Engineering Manager
Total Posts: 4
Join Date: 17/09/18

Thanks Peter,

Apologies for the late reply.

14 3/4" hole is not an option. Also bear in mind that it does not eliminate risk of sidetrack, but the amount of cuttings generated overwhelms the shakers capacity.

Regards, Ken

Engineering Manager
Total Posts: 4
Join Date: 17/09/18


Apologies for my late reply.

Correct. It is a two-stage process with the 12 1/4" section previously drilled

Thanks. This helps.

Regards, Ken

Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 502
Join Date: 10/01/05


Gather the evidence? 

e.g. does the customer have a before and after survey of the pilot hole and final opened wellbore drilled? i.e. To demonstrate or not, that they have drilled and opened the same hole on previous offset wells?

Where Drilling log trends should evidently inform you if there is a suspected sidetrack of not. Or as stated by the customer this regional way does in fact work.   

One would suspect you have an evident potentially faster drilling sequence (the first 400m) and then a much slower drilling sequence the 300m further (dolomitic limestone?).

I am surmising the reasons section is delivered in two passes is because rig does not have the capability to pump at a high enough flow rate to drill and clean a 22in hole optimally?
Notably silts/sands in first 400m?

To be optimal?
Drilling a 14 3/4" hole first would seem more prudent?
This leaves far less well to hole-open (can be a slower process than the drilling) particularly in the last 300m. 

vs a 12 1/4" pilot hole then a 22in main well adds more risk surely to the 2nd run? 

Where if customer does not wish to run a 12 1/4in bit and/or bullnose (stabilised short lead) and a packed 22in hole opener assembly behind. (Note: This was always our standard in SE Asia in the 80's (when I was the driller doing and would have no issues with this approach.)  

The other big issue for me is the tools in your region?. w.r.t hole-opening 300m of dolomitic limestone.  Where if you do not have suitable hole opener they will likely fall apart. (se file i compiled in 2000 as attached). 

With the larger pilot hole. Then the probability of a 22in bit NOT following the main 14 3/4" pilot well is physically greatly reduced (where it is far more likely to follow the least path of resistance) in the softer formations as long as pipe is kept moving.

In the dolomitic limestone? as long as 22" BHA is suitably stiff and rigid, one would have to be stationary for quite a long time to side-track the well.

The drilling log from the pilot hole should be indicative of exact depth where stingers etc are expected ( and where areas of greatest side-track hazard/risk are presented) So these should be able to be suitably managed.  

The 'best practice' essentials on these sections is always to assure the pipe is kept moving. In both shallower softer and deeper slower drilling sections.  



Documents uploaded by user:
51 Hole opening optimisation.pdf
Managing Director (
Relentless Pursuit Of Perfection Ltd.
Total Posts: 484
Join Date: 10/01/05

Hi Ken

Is it right to assume, because you are referring to a bullnose, that this is a two-stage process with the 12¼" section already drilled?

I would certainly be very uncomfortable without either a bullnose or the original 12¼" pilot-hole bit in the BHA (below the hole-owner)and would recommend a 4-6 point hole opener.

I hope this helps


Posted by

Ken Atere

Engineering Manager


Total Posts: 4
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