Rather than going by the letter of API 53, back to basics this is all about monitoring the primary well control barrier which is the fluid column. What sensitivity does one have to ensure that the well is controlled by the primary barrier?
During drilling operation, flow from the well is a primary indicator and the first indication that shows that an influx is occurring. A flow sensor (paddle, or some other flow meter) provides the first indication that flow from the well is increasing over and above of what is being pumped into the well.
Fluid level increase is a second indicator, but that is delayed simply because of the sensitivity of the circulation system. When drilling and circulating, the surface volume in use is relatively large and any increase needs to be a certain size before it can be seen.
When tripping operations are conducted the primary barrier must still be monitored. There is no circulation. Pumping across the top of the well with a small defined volume of fluid provides the indication that the well is static. When tripping out the correct volume is used to keep the well full, when tripping in the correct volume is returned when pipe is lowered into the well. A trip tank, small volume, is used to ensure that the volume can be measured accurately.
This is done the same way when stripping. Now the volume used and measured is even smaller and it allows monitoring of the primary barrier.
This is not about a well giving you a kick when drilling or tripping that flows at 2000 to 8000 bbls/day (60 to 230 gpm) but if the well was flowing at 10 gpm (14 bbl/hr) during a trip what flow meter or flow paddle would see this? A small tank of say 20 bbls would indicate that something is not right.
The basic question of API 53 really is: do we know if the primary barrier, the fluid column, has failed, so that we can activate the secondary barrier, the BOP.
It does not matter how we check the primary barrier, providing that we know the well is static we comply. Now in deep, hot and narrow margin wells with ballooning and breathing and thermal effects this all gets very complex. But remember no one will ask how we did this or what we did when the well gets to TD and reaches its objectives.
If a bolt on a string from the handrails worked, we are heroes. But if a well blows out…a million questions by a thousand experts are asked even if we had all the flow meters and level sensors we could install.
It is not about how or what can we measure, it about what do we see, how do we interpret what we see and what actions are taken.
If you can monitor the primary barrier you are fully
compliant. If the secondary barrier closes, stops the flow and holds the
pressure you are 100% compliant with API 53.