1. Electric wireline logging8
The master plan
Lessons learnt
It is possible to avoid MDT tool plugging in silicate muds – a good flushing procedure needs to be used.
CMR tools have a higher risk of diff sticking than MDT's (Schlumberger experience).
Overbalance confirmed as 72 bar with the Rogn depleted to 0.98 sg.
Silicates are excellent for logging due to
negligible invasion, very low diff sticking risk
and long open hole times.
Can be used when invasion/unstable hole causes problems in getting good logs or cores
MDT pump times should be based on data from similar reservoir/mud systems (e.g. can use shorter pump times in this situation)
Do not need to run existing fluid analysers (LFA/OFA) for WBM/water sampling.
Commercial considerations should be involved in decisions for the logging programme (e.g. SLB recommend filling all possible sample bottles... 2 for each depth, but only one ea was used). The log witness should be aware of these issues.
Experienced ex-logging engineer consultant log witness very valuable.
Toolstrings
Three runs planned (PEX – minimum success criterion, MDT, CMR). Run in decreasing order of value & increasing order of sticking risk.
Perceived high diff sticking risk
Detailed decision tree for contingency formulated.
Fusible weak point run for max possible pull.
(note CaCO3 added to mud prior to drilling reservoir)
Risk of MDT tool plugging due to silicate precipitation
Experimental flushing procedure used involving surface and downhole flushing with water filled bottles.
PEX
MDT flow diagram
CMR
Run in OFA mode… and not used at all
For internal flushing
Note entire tool eccentralised against borehole… including stationary times when tuning or taking MRF stations
Points to ponder
Excessive pumping time for MDT?
Time for MDT was about 4 hr each… even though resistivity logs showed practically NO invasion into reservoir.
Water samples in WBM meant no clear parameter could be used to differentiate the two – pump time chosen was a “safe” value.
Sensors to distinguish water samples in WBM are under development (based e.g. on pH)
Until then, time will depend qualitatively on data value, level of invasion & mobility
Replace PEX run with LWD?
If the quad combo is run on LWD, the PEX run may be eliminated (sonic run on CMR run). Depth correct with wireline GR.
Advantages – reduce risk in case not able to run wireline, less chance of sticking sources, eliminate one wireline run. Huge upside if only quad data reqd & wireline can be eliminated entirely.
Issues – resolution of LWD, “software incompatibilities” with LWD and Shell software.
Downtime report…
Unable to pass 1130 m on first run with logging string #1. Wiper tripped… see 8 ½" section.
Schlumberger QAQC of cables
Separate failures on three successive cables sent out! 1. Short circuit; 2. Corroded – not checked before sending; 3. Short circuit (had to cut off length). If the 8 ½" section had been drilled in the normal time, this would have caused substantial down time.
See logging AAR minutes for further actions from Schlumberger re better tracking of cables
Hung up at 1130 m first run in the hole.
After wiper trip, ran logs as per plan (wireline TD 1802 m).
EMS caliper did not close after up log across reservoir. Used down log as repeat.
Took 12 pressure readings & 3 sample depths (2 bottles each). Pump time hr. No overpulls after sampling. No sample contamination at all.
Overbalance confirmed as 72 bar, Rogn depleted to 0.98 sg.
Performed bound volume and full polarisation passes.
Took 4 MRF station points (using faster CMR "Plus")
No indications of diff sticking.