Pretesting with MDT oriented to high side demonstrates 20x greater permeability compared to low side pretests, proving formation fluid gradient
In an offshore Vietnam well, pressure tests conducted on the low side of the wellbore with XPT were invalid because of low mobility and supercharging.
In deviated wells the weight of the drill pipe is borne on the low side of the wellbore. Suspended cuttings are ground into the pore spaces by the rotating drill-string. Further damage is done as the drill pipe scours the sealing mudcake from the wellbore wall, thereby allowing repeated invasion of drilling fluids on the low side. Sampling from the high side of the wellbore results in faster clean-up due to improved permeability and less filtrate invasion.
On the subsequent run, a 170ft long MDT sampling string was oriented probe-up by Tool Taxis in order to sample from the undamaged high side of the wellbore.
The permeability on the high side of the wellbore was 20x greater than the low side. This allowed the operator to prove hydrocarbon content from pressure gradient alone. On the same run the operator successfully took 12 samples in low permeability reservoirs (~5mD) with an average clean up time of less than an hour.
Further details of this operation can be found in SPE paper SPE-184773-MS.
Petromac wheels orient the probe to the high side resulting in higher pretest success rate and improved permeabilities.
CHALLENGE
The operator expected the reservoir to be low permeability, which was confirmed by the XPT pressure tests. It was not possible to prove hydrocarbon from gradients in many zones. Permeability indicated that sampling would not be possible on the MDT run.
SOLUTION
Petromac Tool Taxis were run on the MDT sampling string to orient the MDT probe up in 12.25” hole. All zones of interest were in-gauge, and as such the large hole kit was not required (hole 13.5”).
RESULTS
The operator obtained high quality pretests which proved the formation fluid gradient and was able to take 12 high quality samples.
