Kick tolerance is defined to ensure that we can safely circulate out an influx without breaking down the formations. But it is a completely static scenario, well is closed and the flowing formation is contained by the pressures in the wellbore.
Kick tolerance has two parameters, influx intensity and influx volume. It does not change if we are using managed pressure drilling or underbalanced drilling. The wellbore has two parameters that set the limits, one the fracture pressure at the weak point, the other one is the pore pressure of the flowing formation.
Circulation rate or inflow rate are not part of the kick tolerance calculations. Some people do not even consider intensity in the kick tolerance, swab kicks only. The inflow rate is basically providing a volume.
In an open wellbore we have a wellbore pressure controlled with the mud density. The influx intensity is purely based on the pore pressure in the flowing formation. If the wellbore pressure is 12 ppg and the flowing formation is 14 ppg the intensity is 2 ppg. The volume is based on how fast the influx is detected and the wellbore is closed to contain further inflow. Intensity = SIDP volume = SICP. The faster the influx is detected and the well bore is closed the smaller the volume.
With a closed wellbore in managed pressure drilling we have a wellbore pressure that is a combination of mud density and applied surface pressure. The intensity of an influx remains the same (same pore pressure). We can have a mud weight of 11 ppg and a 1 ppg equivalent surface back pressure but drilling into the same 14 ppg pore pressure flowing formation still provides an intensity of 2 ppg.
The big advantage with MPD is the rapid detection provides a smaller influx volume. The well is already closed and adding surface pressure stops the inflow.
If we then close the BOP to circulate out the influx we are back to the same scenario as with conventional drilling.
An influx from a formation with a pore pressure higher than the weak point pressure still results in an underground blowout. Kick tolerance does not change for MPD or conventional drilling the pressure limits in the well are still defined by the weak point and the pore pressure.
In underbalanced drilling we know the pressure from the flowing formation. The well is full of reservoir fluid and the surface pressure is controlled to minimize the flow of hydrocarbons. If the well is closed in and the weak point fracture pressure is lower than the formation pressure of the reservoir formation we still end up with an underground blowout.
If you really want to make this complex look at kick tolerance in aerated drilling operations where we pump nitrogen in the drilling fluid and then take an influx.
The challenge with the kick tolerance is that there is no standard way to calculate it. Different approaches are taken, some consider the intensity other companies look at swab kicks only.
The inflow rate from the reservoir and the pump rate disperses the influx in the drilling fluid which is not factored in the calculations.
Kick tolerance calculations are based on a defined block of gas in the well bore with a density of methane. That volume is taken to the weak point depth where pressures are re-calculated based on simple gas laws. If the gas is dispersed or even in solution in an oil based mud system the density of the block of gas is no longer gas, but it is drilling fluid.
A great subject for a well control class discussion and in MPD operations a topic that is raised at almost every project.Looking forward to some more answers.