This time I want to share with you some information related to STEM content that deals with the generalities associated with the drill string in the processes of drilling and construction of an oil well.
Many of you probably know that in order to build an oil well it is necessary to drill a hole, for this it is necessary to use a set of pipes called drill string. The purpose of this post is to describe from an engineering approach everything related to the drill string, especially describe the components that make it up and emphasize the importance it has in the operations of drilling and construction of the oil well.
Introduction
The drill string is very varied, i.e. the whole set of pipes are not all the same, it is necessary that the set of pipes (drill string) have certain distinctive features that provide a different function and benefit at the time of drilling the hole.
However, even though the whole drill string has different components, there are general functions, for example, the whole drill string has to transmit power through the circulation of the drilling fluid to the drill bit.
Another function that all the components of the drillstring share in common is to apply torque through the top drive, in short, there are functions that are shared by all the components of the drillstring, and there are very particular functions in each of the components that make up the drillstring. The functions that differ in their components is due to the fact that the drillstring has certain requirements such as:
Flexibility of the drillstring.
Ability to be released in case it sticks to the lithological formation.
To have enough weight on the drill bit so that the drilling can be executed correctly especially in the sections where the lithology of the formation is quite hard.
That is why the objective of this article is to explain and link the functions of the different components with the possibility that the drilling of the well will be perfectly executed, since for example if the tubing string does not have the components to detach the string once it sticks to the lithological formation, there is the probability that when different attempts are made to tension the string to detach it, the tubing may break at the bottom of the well, this has negative repercussions in reference to the additional economic expenses in the drilling and construction of the well.
What components make up the drill string?
Depending on the philosophy of study that you have, some people may structure the drillstring in different parts, however the most commonly seen in regard to the structure of the drillstring as we are taught in college when we studied petroleum engineering and when we managed to have work experience in the oil industry is that the drillstring is divided into three parts which are:
Bottom hole assembly (BHA).
Transitional drill pipe, which is commonly heavy drill pipe. (HWDP).
Drill pipe.
It is called downhole assembly because all the elements that make up this part of the drillstring is composed of a special downhole assembly. Under this description and of the first components that we find from bottom to top is:
The drill bit which is used to cut the rock, then threaded over the bit are the drill collars which have a larger diameter and are used to give more weight to the bit so that drilling can be performed correctly.
Above the drill collars are generally placed the stabilizers, its main function is to stabilize the whole set of the BHA, as sometimes the tubing string will seek to deviate its trajectory, however with the use of stabilizers the trajectory is maintained avoiding or correcting that deviation by means of the stabilizers.
The previously named components are the most common when assembling a simple BHA to drill vertically, however if you are in a deep part of the well and want to drill directionally, you have to add other components to the drillstring, among which are:
- Motor de fondo: el motor de fondo es llamado también motor de lodo, es un motor ubicado en el BHA con la intención de que el fluido de perforación (lodo de perforación) circule por dentro del motor de lodo y haga girar en el fondo la mecha de perforación, con esto se logra un mayor control en la perforación direccional.
I only wanted to give an example of the possible elements that can be added to the BHA, it all depends on the requirements at the moment that the well is being drilled and constructed, however while drilling a vertical section the conventional BHA that is designed is that of a drill bit, drill collars and stabilizers.
I am going to explain very briefly the two missing components, which are the heavy drill pipe and the drill pipe.
The heavy drill pipe is a transition pipe, it is said to be a transition pipe because the only function of this pipe is to work as a drill pipe but being heavier, when the balance point or center of mass of the drillstring is found (usually it is in the heavy pipe) then above the transition pipe the drill pipe is placed, which is the one that will finally end up giving the depth to the well as the drilling of the well progresses.
Conclusions and contributions of this publication
There are two very important points that should be highlighted in this publication which are to consider a string design for each type of well condition and to evaluate the possibility of the tubing string getting stuck, we must take into consideration that the two points can be perfectly explained taking into consideration the points that have been previously explained.
In the first place we must keep in mind that not all the string designs must be the same, everything will depend on the type of well we are going to drill, that is to say it will depend on the existing lithology in the subsoil, the depth of the well, it will also depend on the directionality of the well, that is to say if the hole is going to be built vertically, horizontally, type s, among other types of directionalities.
Even from work experience in the oil industry I can tell you that the design of the drill string will depend on the way the field supervisors and engineers work.
In the case that the drill pipe string gets stuck for whatever reason, there must be some element in the BHA to make the pipe string to take off at the bottom of the hole, this element that make the pipe string to take off is called drill hammers, the driller simply must activate multiple times these hydraulic hammers to try to take off the string, generally the drill hammers are activated by applying tension to the drill string without exceeding the yield values of the drill pipe by tension.
Below I show an image of examples of hydraulic hammers that as mentioned above are activated in drilling when the pipe gets stuck at the bottom of the hole with the objective of getting the pipe to come unstuck:
The content described in this publication is of great use in petroleum engineering because it describes the components of the drillstring and its usefulness in drilling wells, it can also satisfactorily orient people (engineers and workers) who are starting to drill oil wells at the operational level.
References consulted and recommended
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