Just to feedback to all your kind replies almost a year ago! we never did get fully get to the bottom of it... however eventually managed to make good progress by using a more stabilised BHA and more controlled and measured drill off tests... we actually continued to see the issue whenever tripping and back on bottom but learnt to be patient and eventually would drill through whatever it was until normal parameters returned. We called this BOB (clue above what it stands for!).
as with all unexplained events downhole... I blame the mud! :)
An extract from a TIR "the search for a silicate lubricant" in 2007
"Silicate mud has proven over the years to be a very effective mud system in our XXX operations for overburden intervals and can be considered to be our preferred High Performance Water Based Mud (HPWBM) system. However, at present we limit the use of silicate mud to wells with inclination below 45 degrees due to torque and drag concerns.
Despite exhaustive testing of commercially available lubricants and development chemicals in either liquid or solid form, there is currently no competent lubricant available for this type of mud. Friction coefficients for both unweighted and barites-weighted silicate mud are typically in the range of 0.25 - 0.30
Identifying an effective lubricant for use in silicate mud would significantly expanded the potential scope of use of this mud system.
I paid for a round of testing of potential lubricants in 2007 as part of our R&D budget the work was completed in BTC Houston. The equipment description and example test results are included in the report.
In this round of testing we did not identify a potential outstanding effective silicate lubricant.
Of the eight mixtures/products tested only one product F, was identified, as a possible candidate for consideration as a lubricant for future drilling operations.
The report "Report on Silicate Lubricant Testing conducted at BTC June 2007" was done by Joe Cartwright working in the Houston laboratory using the large scale Alkco Services LUBE tester we had there. You should be able to get a copy from Ron Rock.
Some products not tested at the time were Techlube 2 (used in Netherlands) and Cosmolubric ETL (China). At the time I also identified solid lubricants such as glass beads and particularly graphite with which we were having good secondary success reducing torque and drag in NAM water based mud operations. As far as I am aware no additional testing has been.