Greetings friends of the hive community.
On this occasion I want to share with you the present article where I will explain in a clear and simple way the most important aspects in the process of cementing an oil well, taking as a main objective to be able to fix the casing to the walls of the hole.
The primary purpose of the entire thematic series on well cementing is to cover a complete description of this operation and/or process encompassing the primary objectives of cementing, the testing procedures used to determine if the set grout can meet the primary objectives, the additives commonly used to obtain the desired properties under different well conditions, the techniques used to place the cement in the desired location of the well.
Cement is used in drilling operations to protect and support the casing, prevent movement of fluids from the formation to the surface, through the annular space on the outside of the casing. Cement is also used to stop the flow of fluids into fractured formations, outside of purposes clearly associated with the drilling and construction of a well, we can use the cementing process to close and abandon a partial portion of the well, and at other times to completely abandon a well that is in production.
Aspects and generalities in the well cementing process
It is considered cement to any substance or mixture of substances that have adhesive properties, so we can say that cement consists of a fine powder obtained from the process of grinding a mixture of clays and limestone, however this concept is associated with a base composition, which may vary based on the requirements of use. Now then how we can realize, one thing is the cement, and another is the cementation, the cement is the main compound to make a cement slurry necessary to be able to make the cementation process to the well.
The definition described above, together with the experience obtained in these works whose processes involve the cementing of oil wells, gives us an idea to be able to define cementing as an operation that involves a large workforce and active engineers who are not only capable of preparing the cement slurry, but to be able to successfully accomplish this whole process that goes from making a design and cementing program to being able to meet the objectives with an engineering vision. For this purpose, specialized mixing equipment is used, so that this cement slurry can be pumped and moved throughout the annular space: casing - pit walls (formation or pre-set area).
What are the objectives of well cementing in the construction of an oil well?
That of being able to glue and support the casing to the walls of the well, and thus provide continuity with subsequent drilling and completion operations of the well.
When the different holes are cemented in the drilling of wells, the aim is to isolate the different zones, in this way we achieve the isolation of the different fluids present in the different formations, with this we avoid possible migrations of fluids that could contaminate fresh water aquifers, protect the oil producing formations, among other benefits.
Another reason for cementing is that the cement must protect the casing from corrosion and stress (axial loads), in order to be able to continue drilling the next holes as indicated in the drilling program.
What can affect proper compliance in the well cementing process?
During the cementing process there are many factors that are present that can alter or affect the correct performance of both the process and the grout after setting at the bottom of the well, so I present a list of elements that should be considered when performing the cementing:
Grout Design: The grout design must be in accordance with the requirements of the well, and have incorporated a program that meets the demands of the construction of the well, it is very important all the considerations that can incorporate the cementing engineer in the implementation of the design of the grout.
Elaboration of the cement: It is convenient to choose a composition and elaboration of a type of cement that is in agreement with the cementation processes of the oil industry.
Hole geometry: If the hole cannot be built with a suitable drilling rate, using a controlled drilling mud rheology, it is most likely that the hole geometry will be formed with caverns, and in general it will not be the straightest hole possible, therefore we can say that if the hole does not have the most balanced geometry possible without caverns and holes, then the cementing process will not take place in the best possible way.
Equipment failure: It is necessary that the equipment works optimally, if for example the pump fails, the time of cementation is delayed, affecting the time of setting of the mixture (grout).
Personnel with little expertise: If the personnel that will carry out the cementing operations do not have enough experience, mistakes can be made that would delay the time of cementing, and therefore would influence the time of setting of the cement, the properties of the grout would be lost, and even the pumpability would be affected.
These are some factors that intervene unfavorably in the cementing process, however there can be many more, it all depends on the elements that are incorporated, remember that the more cementing works are done, the more experiences are obtained and new recommendations arise based on factors that influence this process unfavorably.
The drilling operations engineer has to do with the selection of the best cement composition and placement technique for each required application. Cementing is a pumping process that is applied for each phase in the construction of the well, the space between the casing and the walls of the well must be cemented, this process must be carried out for the different holes in the well drilling, passing through the hole: conductive, surface, intermediate and production.
For all this process there must be an accuracy between the different components of the type of cement, the volume of grout, the properties of the grout and the technique used for pumping and placement of the grout, all these operations are under the responsibility of the drilling operations engineer in conjunction with the cementing engineer, in that teamwork between the engineers is the success of the cementing, from the design of the grout to the operational procedures carried out in the drill for the cementing in the well.
There are a number of circumstances in which the correct study and analysis of a grout design can be applied correctly to ensure the final construction of the well, for example if you have a deep well where abnormally high pressures are encountered, it may require several strings of casing properly cemented to a suitable depth before the well can be drilled and completed successfully.
All this background leads us to conclude, that the calculations and general design of the cementing can be contemplated in the drilling and construction program of the well since the first exploration data of the reservoir to be explored are available.
The composition of the cement and the placement technique for each job, must be chosen in such a way that the cement can obtain an adequate and quick resistance after it has been placed in the desired place.
Drilling costs become very high as the drilling time rises above the estimated in the program, so it is convenient that the cement setting time be as short as possible, and this will depend on the good technique and criteria handled by the cementing engineer at the time of selecting the cement composition and the most appropriate technique to perform the cementing operation. This setting property must be treated with great interest, since the cement must remain pumpable long enough to allow its placement in the desired place.
Each cementing job must be designed in such a way that the density and length of the unset cement column results in sufficient sub-surface pressure to control fluid flow from the formation into the well without causing fracturing of the formation. In other words, the cementing engineer must be able to design a cement slurry with a density capable of generating a hydrostatic column that will withstand the pressure exerted by the formation fluids, without having to fracture the formation.
What is the type of cement commonly used in the oil industry to carry out the well cementing process?
In the national and international oil industry there are many types of additives for the different types of cement, however there is a type of cement that is the most used for the preparation of cement slurries, this is the portland cement, the portland cement is an artificial cement made by burning a mixture of limestone and clay, the portland cement is the same basic type of cement that is used for the preparation of concrete.
A portland cement and water slurry is ideal for use in the cementing of oil wells, because it can be easily pumped and hardens quickly in an underwater environment. The name Portland cement was chosen by its inventor JOSEPH ASPDIN, because he thought that the solid produced resembled a stone taken from the island of Portland off the coast of England.
It should be noted that of the final composition of a cement slurry, Portland cement is only the base on which the final slurry will be supported, this means that it is necessary to add other additives that will give the desired consistency to the type of slurry.
Cement chemists believe that there are four crystalline compounds in clinker that hydrate to form or assist in the formation of a rigid and compact cement structure, these are
The excess water that does not hydrate, reduces the resistance of the cement, and makes it more porous and permeable. Tricalcium silicate is thought to contribute most to strength, especially during the first 28 days of cure. While the tricalcium silicate contributes to the resistance but in a very slow way, generally this chemical compound acts in the cement after 28 days.
During the first days of setting, the tricalcium aluminate hydrates very quickly and produces most of the heat needed for hydration. The chemical composition of Portland cement is generally evaluated based on oxide analysis. The relative amounts of the four compounds listed above that must be carried in a portland cement are calculated based on an oxide analysis, from which American Petroleum Institute (API) equations are used, which generally provide the weight percent of the four oxides listed above.
What are the tests that can be performed on the grout before starting to pump cement down the well?
Mud balance: The mud balance is an instrument widely used by fluid engineers to measure the density of drilling mud, this measurement can be done in the laboratories where special tests are performed, or otherwise where it is quite useful as it is to measure the density on site (in the field). In the same way that the mud balance is used to measure the density of the drilling fluid, it can be used to measure the density of the cement slurry to be used in the cementing process of any hole, whether conductive, surface, intermediate or production. Cement testing also involves density, so the use of the mud balance can also be used to measure the density of the cement slurry, this measurement can be given both in the laboratories where the tests are run prior to pumping in the hole, and the other measurements are made on site while the slurry is being pumped into the hole.
As a professional experience I can mention that in several cementing processes carried out in the drill, especially in the conductive hole cementing, I could be present in those cementing processes where I supervised the grout density measurement operation moments before starting with the pumping of the well, The density of the excess grout leaving the well through the annular space, i.e., the space between the conductive casing (20 inches outside diameter) and the walls of the well, was then re-measured to see if, and in what percentage, the grout could have been contaminated with drilling mud.
If the density of the grout after pumping changes significantly, it has most likely been contaminated with drilling mud or other formation fluids such as water, oil or natural gas. This is a fairly normal situation if handled within certain ranges, which do not alter the good cementation of the hole to be cemented.
Filter Press: Like drilling fluid, the cement slurry also has the ability to adhere to the walls of the well to form a crust so that it can form a primary cement that does not warrant future secondary work.
The filter press is the instrument used to measure the filtering property of the grout. Variations in the water content of the slurry can significantly affect the characteristics of thickening time, rheology, compressive strength and other properties.
This filtering ability of the cement is very important, since when a cement grout is subjected to pressure against a permeable medium, it tends to lose water through filtration to the point where interstitial water is reached, resulting in a dehydrated and unburnable cement grout.
Rotational Viscometer: This instrument is in charge of providing the measurement of the rheological properties of the cement slurry, that is to say, just like drilling mud, it is also very important to measure the viscosity of the cement slurry, since it must be such that it is not able to offer a very high resistance to the well pumping process. Why is it necessary to use the rotational viscometer to measure the viscosity of the cement slurry?
First of all we must take into account that one of the most important properties of the cement slurry is viscosity, since the viscosity in the slurry determines the resistance to flow while pumping. In the primary cementing works, that is, in the first cementing work performed in the wells, the grout must have a viscosity that facilitates an efficient removal of the drilling mud and a good adhesion of the cement with the formation and the casing. To achieve these objectives, it is necessary to control the viscosity of the grout according to an optimum water-cement ratio.
Particle size, surface area and additives influence the amount of mixed water required to achieve a given viscosity.
Conclusion and Recommendations
If the casing design is of utmost importance for the construction of an oil well, the cementing process is no less important. This process involves the preparation of a cement slurry to be pumped into the well to fulfill certain functions that are important in the completion of the phase of each hole built in the well, in the mechanical configuration of a well, generally there are different phases in its construction, as is the case of the hole: conductive, surface, intermediate and production.
For each one of these holes, the casing is lowered more according to the design of the target casing for each one of these holes, subsequently the annular space between the hole and the casing has to be filled with a mixture of water and cement (grout).
In the image below you can see a mechanical diagram of the well and the casing design that would be cemented and fixed with the walls of the hole throughout the construction of the well:
The activity in the process of cementing an oil well makes both labor and engineering personnel understand that it is not an isolated process like lowering the casing.
This is done because once all the casing is lowered at the drilling site, the well is immediately cemented. With this we would be saving the time associated with the costs of non-productive time in the cementing of oil wells.
As an important point to conclude with the explanation of this complex but important process in the construction of an oil well as the cementing of wells, I want to say that, the cementing of wells is more than a process where cement is pumped to the bottom of the well, it is an art if you look at all the engineering that is behind the design and cementing program, all the tests that are made to the cement from hydraulic tests, tests to evaluate the compression capacity of the cement and even simulate the pressure and temperature behavior to know if it will support the conditions of the subsoil.
There are so many factors to take into account that even certain mathematical calculations are involved to calculate the volume of the cement slurry that will be pumped into the well, this and other aspects will perhaps be addressed in future articles, for now I just have to say goodbye and see you in a next delivery.
Note: all those images to which no mention is made of their source of image is because they are of my authorship.