Drilling Fluid | Petroleum Engineering
In this opportunity I want to share with all friends of the community hive especially to all those who are passionate about STEM content all that engineering that is behind the study and description of the drilling fluid in oil well drilling activities.
The beginnings of well drilling were under the percussion drilling technique, this method basically consisted in using a drilling principle in which a sharp drill bit was screwed into steel bars to make them heavier, more rigid and more stable and with all this mechanism the surface was hit to make a hole that was going to get deeper very slowly and precariously.
Since the hole was dry, there was no fluid to extract the gravel (formation cuts), the drill string was removed and water was added to make a watery mixture while a bailer (open tube with a valve at the bottom) was introduced to clean the hole.
This drilling method turned out to be a rather precarious method, since this method had no way to control the pressures at the bottom of the hole, and neither did it have the circulation of any drilling fluid that would help clean the drill cuttings and in general it presented many inconveniences that could be solved once the rotary drilling method was invented that did contemplate the use of a drilling fluid.
Due to the failures presented by the percussion drilling method, the oil industry had to evolve towards a drilling method that could control downhole pressures, raise the gravel that cuts the drill string from the bottom to the surface, and create a smooth and permeable plaster on the walls of the drilled hole.
It was then that with the help of science, engineering and technology it was possible to obtain the technology of rotary drilling.
This new method, which is a little more than a century old, and is the only efficient method used in drilling operations, the implementation of the rotary method brought about important innovations that are totally different from the percussion technique. The fundamental principle of the rotary drilling method is the use of a drilling fluid that is essential for this technique to have and continue to have success in well drilling operations.
Historically comparing the two drilling methods we could say that the rotary drilling method would not be as successful as it has been without giving the drilling fluid the main role of importance, since in the early days of percussion drilling there was a lack of control of pressures as it deepens, while with the incorporation of the rotary technique a drilling fluid can be incorporated that circulates within the well with a high density to create a counter-pressure that can withstand the fluids of the formation (oil and/or natural gas), apart from this main function, there are many other functions that can be mentioned:
Removing the pieces of formation that the wick cuts off as the well is deepened.
Protect the walls of the well so that they do not collapse after being drilled.
I don't want to talk too much about the functions of drilling fluid because I will explain this in more detail in the course of the development.
It is important to mention that any fluid that can meet the minimum requirements of efficiency and safety during the drilling of a well, can enter the range of drilling fluid, in turn any fluid that has a basic composition and then with the help of certain chemical additives can be conditioned for the drilling of a well, can easily adjust to the design requirements in the drilling and construction of a well.
Drilling fluid functions
The general functions of drilling fluids are quite standard. Because the drilling of a well is essentially dependent on drilling fluids, whether the functions of drilling fluids can be fulfilled will be a concern of mud engineers at an oil well drilling location.
This means that the functions that drilling fluids perform will have a capacity to be fulfilled according to the design criteria depending on the supervision that is made to its most important properties, for example just to name some properties that the drilling mud engineer at the location should be constantly monitoring are: density, viscosity, filtration, among others.
Drilling fluids have the primary function of removing debris from the bottom to the surface, lubricating and cooling the drill bit and the entire drill string, so we could say that these are the most basic functions that a drilling fluid must fulfill, but in fact there are other functions of the drilling fluid that are of primary importance in the operations of drilling a well, and that in turn it is very important that any drilling fluid used in the drilling of a well can fulfill all the functions described and explained below:
The drilling fluid lifts and drags the cuts made by the drill string and transports them to the surface, all with the intention of having the well properly cleaned out and thus preventing formation cuttings from accumulating in the space between the pipe string and the hole walls (annular space).
If the drilling fluid does not meet the properties necessary to clean the well and bring the cuttings from the bottom of the hole to the surface, multiple problems will occur in the drilling of the well, such as
Increased tubing string torque.
Increased drag.
Increased hydrostatic pressure.
Increased torque, drag and hydrostatic pressure can trigger other more serious problems such as: sticking of the casing string to the hole walls, loss of fluid circulation, decrease in drilling rate.
A casing string sticking means losing all the drilling work in the well, having to abandon, pump a cement plug and divert the drilling, resulting in increased drilling costs in addition to those budgeted in the drilling program.
A loss of drilling fluid can result in a formation fluid surge which can have drastic consequences such as a fluid burst at the surface, resulting in loss of life.
It is recommended to maintain an ideal mud pump flow rate in order to clean the well, the optimal flow rate is the most recommended since it maintains a drilling fluid velocity where the drilling rubble can be lifted without generating a pressure at the bottom that can overcome the formation fracture pressure.
The pressure to be controlled is the pressure that is generated in the annular space of the hole, that is, in the space between the drill pipe and the hole, for this it is advisable to have a density of the drilling fluid ideal where a circulation pressure is generated, in case the mud pumps are on or a hydrostatic pressure in case the sludge pumps are turned off that is sufficient for to be able to withstand the fluids of the formation such as water, oil or forming gas, but in turn does not generate a pressure so great that it can fracture the formation in the subsoil.
If we want to decrease the pressure we only have to lower the density, for this we only have to add water to the drilling fluid, in case we want to increase the pressure exerted by the drilling fluid we only have to increase the density, for this it is advisable to add a clay called barite.
As we drill the well, the wick and the drill string are heated by the friction generated by the drilling cuts (ripios), the drilling sludge must meet the goal of cooling this system at the bottom of the well and taking out all this heat generated, with this we are extending the life of the drilling wick. It is also necessary that the drilling sludge be able to lubricate the wick and the drill string, for this lubrication certain additives should be used in the drilling sludge as dispersants, friction reducers among others.
While we are drilling the walls of the well can collapse, even if they do not collapse while we drill it may occur when we are lowering the liner. The only way the hole wall does not collapse is that the drilling fluid provides that support, the amount and type of support required depends on the formation that we are going through in the well drilling, for example in formations that are highly consolidated little is required that the sludge fulfills this function, since the same formation supports the walls of the well unrefodied , however already for poorly established formations the drilling sludge must provide by means of a property called filtered enough to form on the wall of the well a thin and firm crust called plaster.
Although the drill wick rotates as the drill string rotates, the drill wick needs hydraulic energy to increase the power at the depth of the drill, and this can only be provided by the drilling fluid.
The energy provided by the drilling fluid to the drilling wick lies in the speed with which the fluid passes through the jets of the wick, making this speed generate a hydraulic force that keeps the hole clean below the wick, making the fuse not have to get stuck by the accumulated old cuts , can even increase the penetration rate in the well.
This may mean one of the functions with which the drilling fluid performs and that is suddenly somewhat ignored when studying and analyzing the various functions that the drilling fluid has, however when records have to be kept in the well to record and store petrofistic information from the subsoil is when we end up realizing that the element serves as a means of transport for the run of many of these records in the well is the drilling fluid.
In the image above we can see a record of mud, which is recording geological and susbasty information for each foot that progresses in the drilling, the main measurement of this record is to show the trend of exponent d corrected towards pressure on the sand
This and other registers can only be made possible by the ability to transmit drilling sludge information (drilling fluid).
There are multiple properties that must be constantly studied and evaluated in the drilling fluid, this monitoring is very well carried out in the location where drilling activities are executed by mud engineers, these properties are called within the engineering study of rheology, among the properties that should be kept strictly in their supervision , so that values are handled according to those emanating from the drilling program are:
viscosity.
Filtering.
Mud Ph level.
Salinity.
Among many other properties, everything depends on the functions we want to be fulfilled in the drilling activities of the oil well, each and every one of the properties that make up the rheology of the drilling sludge can be fully explained, however with the interest of not making the post longer I simply wanted to name them, and that perhaps in the future we will cover these aspects that are very important.
Types of drilling fluids
The drilling fluids most commonly used in the oil industry are those that can circulate inside the well to carry the ripios from the bottom of the well to the surface through the annular space, among the diversity of drilling fluids commonly used are:
Air-gas: Gaseous fluids are normally used when the formation and type of lythology permits, apart from being economically cost-effective to be applied. Within the combination of gaseous fluids that can be implemented are compressed air, natural gas, inert gas, air and water mixtures especially to be used where there is little possibility of finding large amounts of water.
The other thing to take into account is that these fluids are very little dense, so the condition with which it is drilled is that of low balance (when the pressure of the fluid at the bottom is lower than the reservoir pressure), it is very important to have all the surface safety equipment to drill under this condition , as oil and gas inflows can occur.
This low-balance condition means that drilling can be given favorably accordingly to high drilling rates than those achieved with any other type of fluid, thus minimizing drilling time and costs.
Other benefits are also obtained when drilling with gaseous fluids, which is that more accurate and less deviated hole diameters are obtained, therefore better work can be performed in the cementation of the clars, there are no circulation losses since the fluid pressure will always be less than the training pressure.
There are notorious disadvantages when drilling with gaseous fluids, which is that in a gaseous fluid there are no fundamental properties of the fluid that can be highlighted for the effective transport of the drilling ripium from the bottom to the surface. When handling these fluids there is a possibility that they will react with other fluids at the bottom and explosions and fire occur. In general, many of the functions already explained above are not fulfilled and only have the use of conventional fluids.
Aerated fluids: they are called aerated fluids, since aerated fluids are obtained by injecting air or gas into a gel-based sludge, practically with these fluids they manage to have some properties that maintain the most basic functions of drilling fluids, while at the same time managing to reduce hydrostatic pressure to avoid circulation losses in areas with low reservoir pressure. Another advantage is that the drilling rate is increased.
Water-based drilling fluids: Water-based sludge uses water that is mixed with clay and other reactive and inert solids as a continuous phase. The water used can vary between fresh or salt water, it all depends on the location, if you are offshore you use salt water because it is easily accessible. Water-based drilling fluids are added chemicals that can control certain properties to fulfill the above functions.
Drilling fluids in oil emulsion: These drilling fluids are prepared using water-based fluid, to which dispersed or suspended emulsified oil is added in a continuous phase of water. The advantage of using this type of drilling fluid is that it has many of the advantages of oil-based fluids, but in turn are much less expensive.
Oil-based drilling fluids: the base fluid in this case is oil, the oil is used as a continuous phase to which clay and other solids are suspended. In my drilling experience I can tell you that this is the drilling fluid that I could see that was most used, especially in certain special drilling operations such as: when there are high temperatures and in general at very deep depths where water-based sludge is incompatible, especially when drilling when it reaches production intervals , where water-based fluids are used, it can damage the formation.
Regarding oil-based fluids it has evolved with the use of drilling fluids based on mineral characteristics that have replaced oil as the basis for these types of drilling fluids. They are very useful as they provide the same properties and advantages as conventional oil base sludge, apart from being environmentally friendly and with the drill staff who have to handle it.
Despite the multiple drilling fluids used in well drilling, there is a simplicity in its use, i.e. fluids very easy in its preparation, by experience in the area of drilling wells as an engineer of drilling operations I can tell you that when you are drilling the conductive hole which is the hole that is drilled in the first 1000 feet , only a water gel drilling fluid is needed, which will only be prepared with water and bentonite, since at these depths no very special fluids are needed.
Can the drilling fluid be well conditioned to comply with the circulation circuit?
The first thing to know is that the drilling fluid is sucked into by the mud pumps from the active tanks to the bottom of the well, where it exits through the wick jets and climbs through the annular space of the well (space between the well walls and the drill pipe) returns by the return line until it reaches the active tanks again , this tour can be simplified as shown to you in the following image:
To summarize the circulation of the mud, it can be said that drilling mud is prepared in the mud tanks [1]. It is sucked by the mud pumps [2] through a set of hoses and pipes that follow the following route: 3 - 8 - 9 and 10.
Once it enters the drill string [19], it circulates the well to the bottom until it exits through the drill string [26], managing to return to the surface through the annular space, that is, through the space between the hole and the drill string, and returns through the return line [28] until it reaches the mud tanks again without first passing through a set of solid control equipment [2].
In this route, the solids control equipment is very important, since it is the equipment that allows the drilling fluid to comply with the circulation of the entire circuit until it reaches the mud tanks without residues (formation cuttings). Therefore, to answer the question of whether it is possible for the fluid to comply with its complete circuit while being conditioned, this is only possible under two circumstances:
1] That there is a solid control equipment that can receive the drilling fluid returning from the well, so that these teams can clean the fluid from the solids that are incorporated into the fluid at the bottom of the well.
2] As the drilling fluid becomes deep down in the well it loses properties, in order to condition it the mud engineers must constantly monitor certain properties such as density, viscosity, filtration, among others. In case some rheological property of the fluid is out of date according to the drilling program then some additives have to be added to condition the fluid and bring it to the rheological property conditions according to the drilling program of the well.
Solid Control Equipment
The solids control equipment are all those equipments and elements that perform a specific function in controlling the solids present in the drilling fluid when it returns from the well, for this case I will make a brief description and explanation of the equipment mostly used.
The objective of this decanter centrifuge is to be able to separate other fluids involved in the drilling mud, for this purpose this piece centrifuges at canned speeds the drilling fluid that returns from the well making a phase separation and if for example the drilling mud comes with other impurities from the well such as gas and other mixtures, the decanter centrifuge separates these components as much as possible with the objective of leaving the drilling fluid as conditioned as possible.
This equipment is very important because if the drilling mud comes with gas it simply separates the gas from the drilling mud.
This equipment is also known as vibrating shaker, trembler or simply desander. When drilling mud returns from the well, it is accompanied by many drill cuttings that get trapped in a set of sieves of different diameters that make the solid parts get trapped and the liquid part go to the mud tanks.
One of the many functions of this equipment is that it allows the geology team present in the drill hole to perform a geological analysis of the drill cuts that are present in the depths of the well.
This team of geologists called Mud logging are responsible for collecting samples from time to time of the cuts that are trapped in this equipment (desander).
Without the desander's equipment it would be very difficult for geologists to examine the geological characteristics of the rock samples resulting from the well drilling cuttings that are dragged by the drilling fluid and trapped in the desander, which are then collected by the geologists and analyzed to make geological correlations of what is known as a masterlog.
Once the geologists take a sample of fluid from the desander and wash it so it can be analyzed under the microscope it would look like this:
In the previous image you can see how there are different cuts of rock, from siltstones, sandstone and shales, almost always when this happens is because the sample that was collected in the vibrating sieve (desander) took a long time to go looking for them, so the drilling in the well had to have gone through several intervals of rock.
The mud cleaner is a set of desanders whose function is to clean smaller solid parts that could not be cleaned by the desander.
In conclusion, if we put together all the equipment mentioned above and some that I left out so as not to make this post more extensive, we would have a complete set of equipment called solids control equipment and it would look like this:
Conclusions and contributions of this publication to engineering and the oil industry
It can be concluded that the functions of the drilling fluid will be fulfilled according to the rheological properties of the drilling program depending on the ability of the mud engineers present at the drilling location to monitor and control those rheological properties.
Drilling fluid is as important to drilling activities as blood in the human body, if the blood in the human body is not right then the human body will not be right, if the drilling fluid or its rheological properties are right then operational problems are likely to occur such as pipe sticking, lunge or possible fluid bursting such as gas and oil on the surface.
The contribution of this publication from the engineering and oil industry point of view is that through the description of the present post it is possible to give a special importance to drilling fluids and extend it to the optimization of the drilling and construction activities of an oil well.
References
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https://twitter.com/CARLOSJB84/status/1300653231786393600?s=20
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