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Directional Drilling of MTC Structures
02 September 2015
Does anybody have experience with directional drilling offshore (deepwater) in Mass Transport Complex (MTC) structures.
Should we be concerned about wellbore stability (more that normal) when doing directional drilling in these structures?
I would be pleased to learn of any experience people have with directional drilling these formations or of development-projects that have drilled these formations. 

BTW MTCs are the result of slumping of deepwater sediment. The slumped formations no longer display their orginal bedding but have turned into a jumbled mess.
3 answer(s)
PhilippeFornage
Operation Geologist
SPREADAssociates
Total Posts: 2
Join Date: 18/10/14
Seismic while drilling could be interesting to manage properly the trajectory.
mhayes
Director
Stanfield-Hayes Consulting
Total Posts: 31
Join Date: 25/03/11

We do a lot of light directional work in Mass Transport Deposits in top hole, 26 in. section to 20° inclination in West Africa. These are conventional slumped MTD's (Mulder & Alexander 2001), and though less turbiditic than I believe you are suggesting.  What we have found is that a shallow hazard assessment in invaluable as the sandier sections in particular can be particularly unstable as the inclination increases. For this reason we tend to only go to a maximum of 20° inclination in the section, and have worked on procedures to reduce stuck risk if we have these larger sand units in the previous stand (circulation / hi-vis pill whilst making connection) and in what we case we pre-define a higher risk well we will mobilise an eccentric shoe for the 20 in. casing.

One of the big issues we have with the MTD is the faulting, particularly approaching the decollement with the 'conventional' formation. Since the surface casing shoe is placed in this region if we are above the decolloment we have to perform detailed fault analysis when placing the casing shoe in 3-D space. And though setting this shoe below the decollement allows more flexibility due to less faulting, we find a very small difference in experienced LOT strength if we correctly place the shoe above, with the main driver being overburden.

One last note is that thought everything here describes a shallow top hole sequence, our deeper sequences are turbiditic, through which we drill at very high angle. These can challenge any wellbore stability modelling since small lateral variations can result in quite large changes in fabric and structure (we have seen this in close offset boreholes in some of the turbidite sequence that are drilled through). But this is quite common in turbidite sequences, along with defined sections of weak rock.

(For any drillers interested in geology there is an online resource around deepwater sediment systems and their classification on the Society for Sedimentary Geology website at: http://www.sepmstrata.org/page.aspx?pageid=37 . And there is a lot more info available on the website.)

Tim.Harrison
Wells Team Leader
British Gas
Total Posts: 6
Join Date: 03/10/07
I believe Exxon pblished something a few years back in SPE.  Worth searching the archives.
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