Hello my dear friend @lemouth.

At the beginning of the detection we see a change in the drilling rate when we are in the presence of abnormal pressure, however as we continue drilling two cases of differential pressure can occur:

1. that the formation pressure is less than the pressure exerted by the drilling fluid, in this case there is an overbalance in favor of the drilling fluid so detection is of a low abnormal pressure.

2. That the formation pressure is higher than that exerted by the drilling fluid, in this case there is a differential pressure overbalance, in which fluids such as gas and oil can enter the formation and migrate to the surface and cause an explosion at the surface, for this case the detection is of an abnormal high pressure.

For the case of the graph we can see that at the beginning of the detection we observe a high drilling speed, and as the formation pressure decreases with depth, the differential pressure becomes higher until it stabilizes, this is an indication that we have completely passed the zone of abnormal pressure, therefore the drilling speed is normalized and the differential pressure remains constant.

The important thing in the case of low abnormal pressures is that initially there is no probability that fluids such as gas and oil enter the well and the pressure exerted by the drilling fluid is higher, however if the formation pressure becomes very low, the pressure exerted by the drilling fluid can fracture the formation causing all the drilling fluid to be lost into the formation.

If all the drilling fluid is lost to the formation, the column of drilling fluid is decreasing, this causes the hydrostatic pressure of the fluid to be lower, and at some point the formation pressure will become higher than the pressure exerted by the drilling fluid on the formation, with the danger of a blowout at the surface.

The best solution for the case shown in the graph is to lower the fluid density until a stable differential pressure is reached, thus avoiding the possibility of fracturing the formation.

Greetings friend, actually I am very passionate about this topic, reason for the length of the comment (I apologize for it) thank you very much for reading the post and comment on your doubts, this motivates me a lot to continue sharing relevant information within the best community as it is StemSocial.