Can we predict the pressure increase for petroleum well flow?

Image source

When we drill a well we simultaneously pump inside the drill pipe a fluid called drilling fluid or also called drilling mud, if for some reason the drill pipe has to be pulled out of the well, the well is filled with drilling fluid.

In the case that the drill pipe is introduced into the well or when the casing pipe is introduced a phenomenon called exceptional flow occurs, and occurs when once the pipe is introduced, either the drill pipe or the casing, the drilling fluid (drilling mud) moves from inside the well to the surface, one of the most important aspects to consider within this phenomenon of exceptional flow is that inside the well there are pressures called overpressures.

Perhaps we could think that any type of pressure originated at the bottom of the well can generate problems, however the only one that generates problems are overpressures, because when we analyze other types of pressures, such as when we take out drill pipe from the well, negative pressures or also called underpressures are generated, although this type of pressures stimulate the well to rebound, that is to say that the formation pressures are higher than the pressure exerted by the drilling fluid on the walls of the hole, but if a good design of the drilling fluid density can be made, there may be a margin of gain in favor of the pressure generated by the drilling fluid, and thus prevent a fluid blowout at the surface.

In relation to designs, what must be designed correctly is the maximum allowable velocity with which the pipe must be lowered into the well, since it must be a maximum allowable velocity that does not generate an excess of overpressure of the exceptional flow on the walls of the well, since if it is exceeded, the formation can be fractured, if the formation is fractured, all the drilling fluid can go into the formation, and therefore lower the hydrostatic pressure values, which can cause the formation pressure to be higher than the drilling fluid pressure, and at the same time this can cause a blowout at the surface.

Then observe the following image in which we can see how the overpressure phenomenon occurs:

When the pipe is run into the well when it is filled with drilling fluid, the drilling fluid returns through the annulus and may generate a backpressure, however this will depend on the speed at which the pipe is run into the well, as shown in the figure below:

Image source

As a conclusion, I can say that of the pressures generated downhole the one that can generate some kind of problem that exceeds the solutions provided by the drilling design are the overpressures generated when pipe is introduced into the well when it is full of drilling fluid, however the prevention in the speed of lowering the pipe can be a great solution to avoid this type of problem.

References

Fluid mechanics for drilling

Fluid friction

Dynamic wellbore pressure prediction