Bi-centre bits vs. Under-reamers

25 March 2013 Hi folks

At a very recent DWOP we were reviewing the planned BHAs.  The well design is fairly complicated because it is Deepwater x HPHT and has a very comprehensive data acquisition programme.

The base plan makes extensive use of (ball-activated) under-reamers.  However there is often MWD below the under-reamer and, in some cases, to assure verticality there is also a Rotary Steerable System (RSS) thrown in for good measure. 

This leaves a long distance from the pilot-bit and the under-reamer and there is a school-of-thought that says that this so-called 'rathole' needs to be opened up prior to running casing. The reasons being: wanting to get the casing shoe as near as bottom as possible because a few of the casing seats are very formation (depth-dependent; concern about cement blocks falling in whilst drilling the subsequent hole section.

This makes for very expensive business.

We talked through a few options (see below), for which we'd be delighted to hear your opinions, plus any other 'emerging techniques/technologies' you are working on:

1. Don't bother opening up the rat-hole: pump some viscous fluid (or equivalent) into the rat-hole before pulling out the first assy, so that the cement will not channel through it. That might be OK (we even talked about using fibres in the cement also, to minimise the potential for 'cement blocks' whilst drilling the subsequent hole sections.
2. Use bi-centred bits: That would get an opened hole but "RSS don't work with bi-centred bits".
   

Of course, it would be ideal to have some kind of under-reamer immediately above the bit that could be activated after we drill the pilot-hole.  In effect, drill the section with the unit pinned closed, get to TD then pull back and activate the hole-enlarging device and open the hole .. all with the same assy.  Now you've gotta' figure out how to activate the device without dropping a ball.

Thoughts?

Kind regards

Dave

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