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.
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....
AGR Petroleum Aberdeen
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!