22 January 2017
Good morning. I have 2 questions which are referring to underground blow out. First, to have an underground blow out, is If you haven't calculated correct the kick tolerance. For example your well can withstand 40 bbl and you got 50 bbl. My question is, after you have shut in the well, you can have an directly fracture of the formation under the shoe, or during the circulation of the kick out to the surface, when the kick is in the casing shoe, then you fracture the formation (because the kick has the same pressure).
My second question is when you have a pressure of formation a little bit higher than expected, but still you dont have any problems because your mud pressure is still higher, when its time for shift handover are they gonna recalculate the kick tolerance?
Thank you for your time
Alexis, your questions are not totally clear, but I'll do my best:
Underground blow-out: This is the name for the event where you have through your well created a situation where there is an uncontrolled flow from one layer (say gas from a gas reservoir) into another layer (say a permeable sandstone layer higher up in the well). This can happen for instance when you increase the mudweight to control the gas pressure in the gas reservoir. The permeable layer higher up breaks under this higher mudweight. this causes losses, lowering the pressure in the well. Now the gas can enter the well and starts travelling up. The driller closes in the well, but the mud and the gas keep flowing into the weaker formation.
Another scenario could be what you sketch, that you have a kick in the well, and that you break down the formation higher up while circulating out the kick.
In real life, the potential for an underground blowout is far higher in the first example: Formation breakdown and losses due to too high a mud weight occurs far too often. Luckily there isn't always a oil or gas reservoir open to start flowing...
As to your second question: On shift handover the crew should perform a slow-circulation rate test. The kick tolerance recalculation should happen any time the mud weight is changed.