Low rate after Nitrogen underbalance
23 July 2013
I have the following situation during testing a new exploration well:
- the well is drilled to 5000m
- during drilling we had good gas shows on several intervals on gas chromatograph
- formation is sandstone and estimated pressure around 650bar
- the well has been perforated by TCP with Connex type gun and the hole underbalanced (the string filled with water)
- after perforation the well was flowing with a rate of 10 liters/minute (the water from tubing)
- after string displacement with Nitrogen down to 2500m the rate decreased dramaticaly to less than 2 liters /minute
This was happened on4 DST's performed on different intervals.
I cannot find any explanation for this behaviour.
Any sugestions will be very appreciated
Hi David, Hubby212 and Ian
many thanks for your replays.
During TCP perf the string was filled with fresh water above Glass Disk and Brine very salty 1.25 SG in the entire annulus.
Packer set and fired the gun. The well started to flow.
Cement bond is very good and I don't expect any water inflow behind the casing.
For Nitogen displacement, DST valve was closed, circulation valve opened and the water inside string displaced into the annulus.
After displcement down to 2300m, closed circulation valve and opened DST valve.
So, we decreased hydrostatic pressure with almost 230 bars.
By measuring time by time fluid level with slick line it was very clear the rate is less than 2 liters/minute against 10 liters before Nitrogen.
Anyway, now the fluid is at surface, is flowing intermitent, the rate has started to increase and some gas started to be observed.
Again many thanks to everyone
How was the displacement to N2 done? If fresh water was squeezed into the formation you may have hydrated some clays causing a reduction on near wellbore permeability. Alternatively poor cement jobs have caused water to channel from upper/lower zones in the past from time to time....
Most likely a heavier fluid coming from the formation. Salt water?
Cristian, If your initial flow is just due to thermal expansion of the water, after you add a cold fluid - Nitrogen - then the rate of thermal expansion will reduce.