What is the significance of underbalanced drilling and what advantages and disadvantages can it have in petroleum drilling?
Hello friends and STEM content lovers, in this opportunity I want to explain the underbalanced drilling technique, the difference it has in comparison with the traditional technique and the advantages and disadvantages it has.
The immediate meaning that we can find the underbalanced drilling technique is to drill with a drilling fluid pressure that is below the formation pressure values.
Who is the key player in making the drilling fluid pressure lower than the formation pressure?
It is the drilling fluid, since it depends on its density variability that the drilling is executed in the underbalanced condition, it is important to know in advance the value of the formation pressure, and in this way the engineers who design the drilling program of the well will be able to design the drilling mud density curve, that is to say, based on the values of formation pressure in relation to the depth of the well, a graph of density versus depth is constructed, the objective is that if the technique to be used is to drill under balance, then the density of the drilling fluid in each section at the depth of the well manages to generate a hydrostatic pressure lower than the formation pressure.
In all conditions can drilling be done using the underbalanced drilling technique?
As the main factors in this technique are bottomhole pressures and the use of drilling fluids, this technique cannot be used in all cases.
Recall that the other technique used to drill wells in the opposite direction to underbalanced drilling is the technique of overbalanced drilling, the overbalanced drilling technique is applied mainly among other factors to generate a condition in the bottom of the well in which the drilling fluid pressure is higher than the formation pressure, this is achieved to avoid that fluids such as oil and gas migrate from the bottom of the well and reach the surface and may cause a blowout.
This makes us think that a candidate lithological formation for drilling using the underbalanced technique is one that contains low pressure, i.e. there are oil fields that have long been producing oil, therefore the pressure of these deposits has declined over time, so it is very unlikely to generate a pressure at the bottom that makes fluids such as gas and oil migrate to the surface and cause an explosion.
Another condition is that there are formations that are highly porous and permeable such as limestones that are highly fractured, these formations can not be drilled with an overbalanced drilling fluid, as we could cause damage to the formation, so it is best to drill underbalanced.
In reservoirs where there are high formation pressures it is not advisable to drill underbalanced, and that there are risks of a blowout of fluids such as gas and oil at the surface.
What advantages and disadvantages do we have when drilling under balanced?
Let's start with the advantages, when we drill under balance a formation that is highly fractured, or has high porosity and permeability, we are minimizing the damage to the formation, therefore when the well is completed and put into production, it will give higher profitability and the pores of the rock are not clogged with particles of the solid phase of the drilling fluid.
When we drill under balance we obtain an early oil production, since while we are drilling we are receiving the hydrocarbon production thanks to the underbalance in favor of the formation pressure.
Among the disadvantages are the high costs associated with the use of drilling fluids that must be used in underbalanced drilling, such as the use of aerated fluids, with foam, nitrogen, among others. Safety equipment such as special valves that must be installed on the surface also generate high costs, and if we are receiving oil while drilling, it is very dangerous since an explosion can occur on the surface.
Underbalanced drilling has the limitation that it cannot be performed under all conditions, while overbalance drilling can be performed under any conditions, only the use of overbalance drilling is discarded to be replaced by the underbalanced drilling technique when we want to optimize the future production of the well to be drilled.
References consulted and recommended
Note: The cover image was created by the author of this post using Microsoft Powerpoint design tools and using the following image source from wikimedia commons.