Detection of abnormal pressures during drilling: Drilling rate

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Hello friends and STEM content lover.

This time I want to develop this post to explain the detection technique for abnormal pressures using the penetration rate parameter.

The important thing about this technique is that it allows us to know if there are abnormal pressures (high or lower than normal) in the subsurface while we are drilling, and thus be able to take precautions in this regard, high pressures in the bottom of the well can cause a rush of fluids (gas and oil) from the bottom of the well to the surface, causing an uncontrollable blowout at the surface.

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Generally there are several techniques to detect abnormal pressures but they are not applied for detection while drilling, but they are geological studies prior to drilling that give us an idea on how we can design for example the drilling fluid and other drilling parameters.

Although seismic and geological studies give us an idea to design the density of the drilling fluid and maintain the hydrostatic pressure to withstand the formation pressure, it is vital that we can count on other techniques that are real time during drilling as is the case of the drilling rate, rate of penetration or also called drilling speed, since during drilling is where we will really live the experience of a fluid onslaught due to the abnormal pressures of the oil reservoirs found in the subsurface.

It is important for the people involved in the drilling of the well such as engineers and supervisors to be involved with the well control techniques associated with taking pressure readings that can be seen in the set of pressure gauges such as the pressure manifold and the accumulator supply pressure.

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Drilling penetration rate.

It is important to keep in mind that if we do not vary any drilling parameter such as drilling mud pumping pressure, drill string rotation, weight on the bit, it should not change the speed with which we are drilling, however there are other elements that if not varied would make the drilling speed does not vary as are the rheological properties of the drilling fluid such as density and viscosity.

The idea is that the driller should be aware that if he observes a change in the drilling rate while keeping the above mentioned parameters constant, then it is very likely that he is in the presence of abnormal pressures downhole.

The differential pressure is the subtraction of the hydrostatic pressure exerted by the drilling fluid to the formation minus the formation pressure which is the pressure at which fluids such as gas and oil push to enter the vicinity of the wellbore:

DIFFERENTIAL PRESSURE= HYDROSTATIC PRESSURE (Ph) - FORMATION PRESSURE (Pf)

The only way for the differential pressure to decrease as we are drilling and deepening in the well is that the formation pressure is increasing and the fluids such as gas and oil manage to enter the well, once the gas cuts the drilling fluid (drilling mud) it makes it decrease its density, which makes the drilling speed increase since there would not be a restriction of the drilling fluid to continue advancing.

As we continue drilling and deepening the differential pressure curve will decrease as the drilling speed will increase, as shown in the following image:

Conclusion

The important thing that engineering provides for this case an important study to evaluate a decrease or an increase in the drilling rate is that it gives us an indication to know how fast we are drilling, it is essential to educate the driller of the importance in detecting an abrupt change in the drilling rate, since an abrupt change in the drilling rate translates into the possibility of finding ourselves in front of an abnormal pressure.

All detection techniques are important, however, the penetration rate evaluation technique is critical because the penetration rate is an indicator that is constantly accessible during drilling and easily accessible on the driller's control panel.

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In conclusion, there is no drilling parameter that we can read as we drill the well that is in the first place other than the rate of penetration, since the other drilling parameters can only be read once the drilling fluid carries all the cuttings from the bottom of the hole to the surface.

Bibliography consulted and recommended

Downhole procedures and operations. DATALOG Drilling Manual. Version 3.0. March 2001. See page to download book in PDF format