What control methods should be applied when drilling an petroleum well in the event of an influx?
In this opportunity I want to explain two methods for the control of wells that are the method of the perforator and the method of the engineer, in addition to these two methods I want to complement all the mathematical calculations associated with these methods, very necessary to be able to enter the control of the well.
These methods of control of wells are mainly important when the activities of perforation are executed, each one of the methods has its similarities and differences with respect to the other one, also has its advantages and disadvantages, nevertheless by the experience in the field and its applications, without a doubt my recommendation is that within the possible thing the method of the engineer is applied, already later in the development of the present article you will understand the reasons of why I recommend this method of control.
Introduction
There are several scientific and engineering methods that have been developed for the proper control of the well when it incurs an influx of unwanted fluids from the formation to the well, however due to the practicality of drilling operations those that most apply when drilling are the method of the drill and the engineer.
Based on the knowledge acquired in the procedure and essence of application of both methods, I can say based on my experience, that the fundamental principle in the way of acting are the same, that is to say essentially these methods maintain the pressure at the bottom of the well so that the migration of fluids towards the surface is kept under control until finally the well can be circulated and the influx of gas out of the well.
What is the best way to understand the control of a well in relation to the pressures generated at the bottom of the well?
When drilling a well, and if there is a sudden influx of unwanted fluids into the well, it is best not to kill the well.
When I say that it is recommended not to kill the well, I mean that it is not advisable to increase the density of the drilling fluid abruptly and without any kind of control in the mathematical calculations that must be done to apply any of the methods explained below.
If the well is killed, i.e. if the density of the drilling mud is suddenly increased, the hydrostatic pressure generated by the drilling fluid at the bottom of the well can be so great that the formation can be fractured, resulting in a loss of circulation and a possible subsurface burst.
In addition to the uncontrolled hydrostatic pressure that can be generated at the bottom of the well, it is necessary to add the trapped pressure of the formation gas bubble, which is why it is not recommended to kill the well randomly, but rather to control the well, and for this the most advisable is to apply the method of the engineer, as long as you have new drilling mud prepared on location.
Applying some control method is guaranteed by solving mathematical equations, to be able to estimate the pressure value of the influx, the closing pressures in the casing and drill pipe, the exact values of the density of the new control fluid, the values of hydrostatic pressure of the mud, the equivalent density of circulation, among other values more than undoubtedly will help us to control the pressures at the bottom of the well so that a subsurface burst does not occur.
We can conclude with regard to this question, that knowing the pressures at the bottom of the well, allows us to act in a conscious way to control the pressures at the bottom of the well, and not killing the well, where perhaps a fracture may occur in the walls of the formation.
Based on the fact that the two methods perform the function of controlling downhole pressures, what are the differences that can occur from each other?
The two methods mentioned above fulfil the ultimate goal of controlling the influence of the formation and preventing the loss of circulation of drilling mud.
The only difference we can find between these two methods is that in the engineer's method the density is increased, whereas in the drill method the density is not increased but the well is circulated until the formation gas inflow is removed.
This method of control employs circulating the drilling fluid to remove the influx (gas and oil) out of the well.
In this method it is important that we understand in itself which is the fundamental technique to use? It is also important the ideas used in the application of the same one, since other methods of control of wells exist that use many of their fundamental principles.
The disadvantages of the drill method cannot be ignored, since the application of the drill method can cause high pressures in the casing compared to other methods, the other thing to take into consideration is that the drill method technique requires more time to drown and control the well.
Where is it advised to apply the drill method?
[1] The perforator method is ideal to be applied during the maneuvers of removing or inserting drilling pipe into the well, since once it returns to the bottom of the well, the fluid column in the annular space circulates and removes the influx.
[2] It is also suitable for application at times when there is no chemical densifying material available to increase the density of drilling mud. As already mentioned above, the well can be circulated with the same drilling fluid to remove the gas bubble without having to prepare a new mud.
[3] In operational activities where oil wells are drilled or repaired, it is advisable to apply the perforator method as a control method, especially if there is a limitation of equipment and personnel.
[4] The perforator method is not recommended for wells where a loss of circulation is expected to occur, especially if the formations to be pierced during drilling are permeable. This recommendation is made, since with the method of the perforator the pressures handled at the bottom of the well are very high and can fracture the formation, thus causing a loss of circulation, further exacerbating this to provide the environment conducive to the occurrence of an invasion of fluids from the formation into the well.
[5] In conclusion we can see that the drilling method is very applicable when resources are lacking, and apart from that it is very applicable where a quick and timely response is required for well control concerning the migration of fluids such as oil and gas from the bottom of the well to the surface.
How is the perforator method applied?
With the drilling method we were able to remove the influx from the well in two stages, these two stages are summarized as follows:
a first circulation of drilling fluid where we remove the surge from the well. Then if we realize that the existing drilling fluid in the well is below the formation pressures, we proceed to replace the existing drilling fluid with a new, higher density fluid until the well is controlled.
Detailed procedure and summary of perforator method
[1] The well is closed, using soft close or hard close techniques, depending on the operation being performed at the time. This time soft close is recommended as the control methods are being explained when drilling.
[2] The closing pressure in the drill pipe (SIDPP) and the closing pressure in the casing (SICP), already stabilized, are recorded.
[3] Circulate and remove the invading fluid from the well. To circulate the well with the existing fluid in the well, it is necessary to follow some guidelines of valve openings and closings, where the choke line will be aligned and the HCR valve will be opened.
[4] Once the well has been circulated and the influent has been removed, it is ready to close the well for the second time, always controlling that the closing pressures do not exceed the maximum admissible surface pressure (MAASP). The closing of the well for the second time must be carried out applying the same methodology as the first closing.
[5] If it is determined that the existing drilling fluid in the well generates a lower pressure than the formation fluids, then the density of the existing fluid is increased, or a higher density fluid is prepared and replaced by the existing fluid in the well. In this case, we would be in the case in which the drilling method is not effective, so we have to think about the possibility of applying the engineer's method, which is the method that contemplates the preparation of a new control fluid.
[6] The well is circulated a second time, this with the intention of removing a new influx that may have remained in the well. This second circulation is done either with the same fluid, this if the sludge has sufficient density to generate a higher pressure than the formation fluids, or can be circulated with a heavier sludge that has been prepared for the second circulation of the sludge.
Next I present an animated image of a similarity where the annular spherical valve closes, the killing line closes, and then the relief line (HCR) opens. The casing and drill pipe closing pressures must be observed so that when opening and closing valves the maximum pressure that the formation can withstand is not exceeded:
Engineer's Method
Just like the perforator method, this method has its advantages and disadvantages, its use just like the perforator method depends on the conditions at the well.
The engineer's method controls the well by killing the emergence of invasive fluids into the well in a shorter time compared to the perforator method, while maintaining the pressure ranges at the bottom of the well and the surface at the proper for control.
Because of the above, the most recommended method to apply when controlling a well before an influx of invasive fluids, especially if drilling, is the engineer's method, however if you do not have all the resources available is not very recommendable its application, as it requires good mixing facilities for the preparation of drilling fluid, complete crews and additional help for monitoring its application.
Practically the control of the well through the application of this method is done in a single cycle, that is the marked difference in comparison to the method of the perforator that its control is given in two phases.
In this method, once the invasion of unwanted fluids has been detected, the well is closed by means of already known methods, the already stabilized closing pressures in both the drill pipe and the casing are recorded, and the volume of gains in the travel tank, produced by the threat of invasive fluids, is also recorded. Before the sludge begins to circulate, its density increases, hence the name "wait and weigh," or better known as the engineer's method.
Then a single circulation is made with the heavy sludge, always maintaining the correct density and pressures, during well control.
The application of the drill method is presented in a systematic and summarized form below:
[1] The well is closed, using soft close or hard close techniques, depending on the operation being performed at the time.
[2] The closing pressure in the drill pipe (SIDPP) and the closing pressure in the casing (SICP), already stabilized, are recorded.
[3] The density of the fluid is increased until the density calculated for the control fluid is reached.
[4] When the sludge is prepared in the active tanks, the fluid circulates.
[5] The drilling fluid pumping is done following a table where for each circulation pressure value a specific volume value of drilling fluid is pumped for well control until the final circulation pressure value is reached.
It is the responsibility of everyone involved in well control to maintain proper circulation pressure as the control sludge is pumped from the surface to the wick, and from the wick to the surface. Therefore, pressure adjustments should be made as needed.
Applied mathematics for well control methods
Basically the calculations used in the control of the well by means of the methods explained above, is due to the need that one has to circulate the well to take out the influence of the well, it is also necessary to maintain the pressure constant in the bottom of the well, and once the total volume of pumping is reached the final pressure of circulation is reached.
There are applied mathematics for the control of the well that deserve to calculate the density of the new control fluid.
Next I present an animated image where some equations involved with the mathematical calculations that must be made to apply the engineer's method as well as well control are presented:
Conclusion, considerations and contributions to engineering
[1] With the application of the control methods we are under the real control of the well in case of a formation fluid surge, especially if we are in drilling or well repair activities.
[2] There are many methods that can be applied for well control, however in the oil industry the most used are the driller's method and the engineer's method.
[3] If the necessary resources are available in the location where the drilling activities are being executed, my recommendation is to apply the engineer's method as a method of well control, since it is a method that involves a series of mathematical calculations, which are necessary to fulfill the primary objective of controlling the well, managing the pressures at the bottom of the hole without exceeding the maximum pressure that the formation can support.
[4] The importance of correctly executing the mathematical calculations involved to employ the engineer's method is that the value of the density with which the new control fluid is prepared depends on those calculations.
[5] It is not possible to obviate all the techniques learned for the closing of wells, considering the soft closing if it is being drilled, and considering the hard closing if it is in a tubed hole, already having closed the well, the mathematical calculations involving the control method must be done in an optimal time to not allow the gas to migrate to the surface, and that even though the well is closed, it may exceed the formation pressure.
[6] Once the well is closed, we must not forget to wait for the pressures to stabilize, and thus be able to record the drill string closing pressures and the casing closing pressures, all with the intention that we do not exceed the maximum pressure at which the formation may fracture.
Bibliography consulted and recommended
[1] Well Control Manual. Well control School. Harvey, Louisiana.
[2] Manual of Procedures for Closing the Well by Will Well Control.
[3] Oil and Gas Well Drilling and Servicing eTool
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Credits of the images used in this article:
All images are my own, and were created using Microsoft PowerPoint editing tools.