Hello,
I was recently preparing a well for a rig over. Initial gas monitoring read 0. On removal of a temporary cover plate by hot cutting.  After removal of cover plate the gas tester was inserted into an open well and recorded an explosive atmosphere. My question is a general estimate of what sort of pressure could have been generated had the atmosphere ignited when hot cutting. Type of gas is uncertain as no reading of methane or hydrogen sulphide was on the testing machine. The well head was 1 meter underground but an open pipe marker was sitting on top of the temp plate. I did consider rust inside the pipe may have depleted the oxygen to a level where a small amount of gas may have produced an explosive atmosphere. I will request a SOP for this situation and would appreciate some advise of my concerns so as not to get shafted by others. I can provide any other relevant info.

Really hard to say with the lack of information - but 5 psi or about 700 psf over pressure might not be uncommon!!

The maximum combustion pressure of a hydrocarbon-air mixture in a perfect fuel-air ratio is about 150 psi (or ten times initial pressure).