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Curious well control situation (using Water based KCL mud).
01 May 2015

Good Day.

I have a question for any folks on drilling fluids or drilling through coal stringers. On a current well we are drilling we are using a water based drilling fluid with a minimum of 3-4% KCL. There are some polymer additives as well but essentially this is a water based mud. There are several coal stringers at a depth of 8300' and we have a shoe depth of 8100'. There was an influx a few days ago and the mud density was increased from 10.9 ppg to 11.4 ppg and well was killed with no issue. We suspect that the influx came from the coal zone after a wiper trip but the strange part is that the influx is acting like it is in Oil based Mud and not water based. An influx can be taken and only a very minor gain is noticed (0.2 bbls)but when it gets close to surface is when it really starts breaking out just like a gas influx in OBM. I have taken numerous kicks with both water based mud and OBM and am aware how gas reacts in each type of fluid. But in this case, we have what we suspect is gas coming from a coal stringer and even though utilizing a water based mud, it is acting like a gas influx would in OBM. Also when we trip out of the hole, gas seems to migrate into the annulus and our hole fill displacement is over what is calculated when we trip out but once we trip back in the hole we've had to shut in the well and circulate out gas. Obvious solution would be to increase mud density but we will incur losses with a higher mud density. I was just wondering if anyone has ran into this before because it's the first time I have seen this. Any info would be a great help.



4 answer(s)
Drilling Consultant
PT Drilling Services
Total Posts: 62
Join Date: 15/09/14
The type of well control behavior is very common in fractured formation such as Basement / Granite and some Carbonate formation. 
As the name described, the formations were fractured and cannot be treated like the normal clastic formation such as sandstone. 
The over-weighted mud would enter the void of the formation and displace the hydrocarbon out of the void, thus eventhough hole taking more than theoretical volume, you still had gas in the well. 
If that was the case, raising MW would make it worst as it would displace more hydrocarbon out of the void. 
We also had the same situation even when drilling with Brine, that the gas "break-out" or expanded rapidly near the surface. We used to suspect it was the condensate or whatever, but that is just the nature of gas which will expand rapidly when nearer to surface. The blessing is that the gas is just a bubble, and not continuous flowing. 

To avoid worsen the situation, try to maintain or even reduce the MW to lessen the amount of hydrocarbon being displaced from the void. You can reduce it slowly and cautiously. 
Like I mentioned before, the bottom up gas (usually comes in earlier due to gas migration) is just a bubble, just circulate the bottoms up gas through the choke and then monitor the well, open BOP and continue with the rest of the operation. 
That was how we drill the Basement wells in Vietnam. Through many years of experimenting and learning curves, we did not bother to raise MW like we did on clastic formation. And we only circulated bottoms up gas through the choke; otherwise we would be spending days circulating through the choke.
Total Posts: 6
Join Date: 13/09/07

Hi Myles

I think the thread comment on C02 is worth investigating.  As an ex coal miner I agree that CO2 levels in coal seams can be high. Is the area known for in-situ thermal or chemical decomposition of the coal - can produce CO2 and all sorts of by products.  On the drilling side, many years ago I was on a job where the decision was taken to acidize pipe stuck in drilled cuttings in a limestone formation, again KCL polymer mud.  We pumped a lot of acid - and then when we finaly got circulation back we had gas break out near surface that blew the slips out and mud to the crown. HCL reacted with the CaCO3 producing CO2 which went into solution and then broke out, the hazard wasn't identified until too late and there you go....


Simon Lucas

AGR Petroleum Aberdeen

Director Drilling
Total Posts: 4
Join Date: 06/02/15

Neil, I don't have recent experience in drilling coal stringers but what you are describing is similar in some ways to what I saw on drilling with high density water based mud / high pressure but low perm formations. We had a number of things going on at once but part of our problem was possibly a "condensate" leaking into the well at considerable depth and pressure (around 11,000psi) and breaking out into a gas near surface. You may be able to diagnose that from the gas analysis and possibly from condensate / oil visibly over the shakers.

We had similar issues to those you describe with high trip gas and even slow migration of gas or condensate (vertical well).

Apologies if I am teaching you how to suck eggs!

Offshore Engineering Manager Western Hemisphere
Noble Energy, Inc.
Total Posts: 13
Join Date: 24/06/14
test the gas for carbon dioxide, it is a common problem for miners in coal seams (black damp), and will go into solution in a water based fluid and behave like gas in WBM, also check if you mud pH is coming down as this is another sign of CO2 going into solution. 


Posted by

Neil Fougere

Operations Manager

Saxon Services

Total Posts: 4
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