What criteria should be considered in the design of casing that is lowered to the bottom of an oil well?

over 3 years ago (Edited)

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Greetings friends of the hive community, especially the StemSocial and OCD communities who have supported me in my articles related to petroleum engineering and the oil industry in general.

On this occasion I would like to explain the importance from the engineering point of view of knowing how to correctly manage the design criteria in the selection of the casing that will go down the well in each of the sections that are built in an oil well.

Introduction

There are many details to take care of when it comes to drilling operations, however having the ability to properly choose the casing to be lowered into a well represents one of the most important aspects to consider in the scheduling, planning and design of an oil well construction.

When we find ourselves planning to drill a well at the site where we are going to drill the well, we may think that it is only necessary to lower casing randomly into the well with nothing to plan or consider, but the reality is that there are many design criteria to consider, so having to design a casing program based on quantity and quality to be lowered into the well is of utmost importance.

If we take into account that at high depths of the well we are going to find high pressures, we must lower to the well the casing that supports those high pressures at the bottom, but always taking into account that we should not work on design, that is to say that it should not be taken into account in the design high cost pipe in a section of the well where perhaps a more economical pipe can support the high pressures, when designing the casing program to be lowered into the well, the economics of the drilling process and the future productivity of the well must be considered. This economic factor is influenced by the correct design of the casing program.

Since drilling costs can increase as wells reach greater depths in length, casing downhole is not something that escapes the elements that make drilling costs rise, so much so that casing represents one of the most expensive parts of a drilling program, averaging 20% of the total cost of a drilled and finished well to be put into production.

Stages of a casing program

For the above reasons it is important to be able to carry out the design in the selection of the casing that will be lowered into the well, the design engineer to achieve that the well is built under the optimization of costs and quality in its construction, the engineer who is going to design the casing program has the responsibility to elaborate the program under a design that contemplates the conditions of the well and the problems that can occur during the construction of the same, based on those problems to be able to design the casing program in function of the most economic and that can support those conditions of the subsoil.

It is necessary to focus basically on three stages that are fundamental to be able to develop a coating program:

[1] Shoe settlement point: The shoe is the guide and anchorage element of the casing, knowing where the casing is going to be anchored or supported implies knowing the pressure that a certain casing is going to endure, so the type of casing is going to determine the point of settlement of the casing.

An example of this is when there is a casing that supports a certain pressure in the subsoil, then a certain casing is lowered to the depth where that pressure is present, and it is in that depth where the casing will be anchored, and it has the same characteristics to support that pressure.

[2] Type of casing: When we talk about the type of casing we refer to many things, for example the nominal grade of the casing, the type of thread, the type of steel, in short practically the structure of the pipe, the type of casing available will give us an idea of what type of casing can be lowered to a certain depth.

The material with which the casing is constructed will determine the pressure that can hold a certain pipe at the bottom of the well where it will be subjected to high pressures, the grade of steel to be more specific is what determines what type of casing should be lowered into the well.

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[3] Wellhead working pressure: It is very important to manage and manipulate these pressure values, especially when casing is being lowered into the well, as these casings are designed to withstand a pressure boundary where the casing may become deformed. Pressure management is important because the casing is lowered into the well having to fill the casing from the inside with drilling fluid, so fluid density management is influential in surface pressure management.

If the design of the casing program to be lowered into the well takes into account the three factors already explained, the risks of this casing collapsing or giving way to the stresses that are experienced during the drilling of an oil well are minimized.

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How are candidate wells classified depending on the type of casing design?

Casing design will depend primarily on the purpose of the well, of course not only on the production purpose of the well, but also on the estimated pressures of the producing reservoir. Taking into account these purposes it will be possible to have a more sustained idea of the casing design, based on these premises the wells in considering the casing design can be classified in

Normal development and advanced wells of a field:

These wells are characterized by having sufficient history of drilling and production of neighboring wells, this history allows making the exact decisions based on the conditions of future drilling operations among which is the casing design. Among the information from neighboring wells that we can count on for our casing design are: pore pressures, fracture pressures, information that allows us to make almost accurate predictions in the selection of casing strings that are quite economical. The information provided to us from neighboring wells in these development fields should eliminate the assumption under which casing strings were designed for exploratory wells, however great care must be taken in these areas to ensure that production and injection have not altered formation pressures in such a way as to force a change in mud densities or the number of casing strings required.

In conclusion, we cannot rely on data provided by neighboring wells, as they may be influenced by the injection of water or gas for wells in production, which in turn may alter the reservoir pressure value, and therefore it would be a mistake to change the mud density value or the number of casing strings.

For engineers responsible for the design of the casing program is more understandable to the design of the casing, as these wells have the historical data of pressures that allow the mathematical calculations associated with the design and to make the design as close as possible to the conditions of pressure and temperature in the subsurface.

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Exploratory wells:

These are wells in which all types of information regarding drilling or production are unknown, so design conditions are more difficult to determine. Due to the lack of information in the exploratory fields, it is necessary to make a set of hypotheses about the pore pressure and fracture resistance of the formations. These hypotheses are possible thanks to the information provided by seismology studies and all the exploratory geology that can be applied to these fields.

It can not be said that for the design of casing in this type of wells are made blind estimates, there are really equations and mathematical correlations that simulate the pressures that we can find in the subsoil and thus able to select the type of casing more consistent with these exploratory conditions in these types of wells.

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Example of casing programs

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In the previous image you can see a mechanical diagram of a well where the description of the diameter of the casing to be lowered is made, in it we can see how the first section of casing is of a diameter of 18.625 inches, this type of casing is called conductive casing.

Then follows the 13.375 inch diameter casing called the surface casing, then a 9.625 inch diameter casing called the intermediate casing. Finally, a 7-inch diameter liner is lowered, which is called a producer liner.

It should be noted that to build the next hole in this schedule, a drill string must be lowered whose diameter can pass through the casing. For example, you can see on the right side of the image that there is a description of the diameter of the wick that can pass through that diameter of casing. Let's take an example to understand, suppose we lower and anchor the surface casing whose outside diameter is 13.375 inches, implying that inside that casing a drill string has to be lowered to drill the next section of the well with a 12.25 inch diameter drill string.

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In the previous image you can see a diagram that illustrates how a well is structured where all the programmed and cemented casing has been lowered, remember that once a section of the well is drilled and the casing is lowered there will be a space between the casing and the walls of the hole that is filled with a cement slurry.

Nothing in drilling is executed at random, everything has a purpose or end, when performing the casing design is fulfilled with a program or casing arrangement that gives answers to the different depths of the well, that is why it is convenient to describe what purpose fulfills the casing:

As much as the Conductor, Surface, Intermediate, Sleeves or liner (production liner).

It is important that the casing program design engineer be aware of the purpose of each length of casing to be lowered into the well, which is why the casing functions are described below:

Conductive Casing: This casing is extremely necessary to protect the unconsolidated sands that exist at shallow depths, to continue to deepen a well without lowering the conductive casing we have problems in the circulation of fluid, to circulate the drilling fluid to the vibrating shale it is necessary that the drilling fluid goes without so much surface sediment that is present in the surface sands, so protecting the well at depths ranging up to 1000 feet is critical.

The conductive casing string also protects the remaining casing strings from corrosion while also serving as structural support for the wellhead, it has virtually many surface functions, as a diverter system or assembly can be installed in the wellhead to divert the flow some distance away from personnel and equipment, should an unexpected influence of formation fluids occur while drilling the surface hole.

Virtually this casing is very functional to support many elements that are important to fluid circulation operations and in turn protect other casing members that are lowered into the well.

Surface Casing: The surface depths harbor freshwater reservoirs (aquifers) that need to be protected from drilling activity and prevent them from being contaminated, which is why one of the fundamental reasons for lowering the surface casing is to protect the aquifers found at this depth from contamination with oil-based drilling mud. Another case of operational problems that occur at surface depths are the collapses of poorly consolidated sands, but by protecting them with the surface casing we avoid the collapse of the weaker formations that are close to the surface.

In the case of an onslaught of undesirable influences on the well, the surface casing allows to contain the influence at the time of closing the valve assembly prevents blowouts (BOP).

Intermediate casing: When previously explained the design system to meet a casing program, is generally designed for deep wells, as we can see in the image regarding the casing program the depths for these arrangements are approximately between 17000 to 18000 feet deep, so it is convenient to emphasize that for these deep wells, that penetrate formations in many cases with abnormal pressures, circulation loss zones or unstable shale sections, to solve an influx in view of the presence of abnormal pressures, or pipe sticking due to shale swelling, it is advisable to cover these intermediate depths (8000 - 15000) feet with the intermediate liner.

Liners: This casing is characterized by not extending to the surface, but is suspended or hung over the terminal depth of the larger diameter previous casing, as shown in the second arrangement from left to right in the casing program image.

Generally, approximately 300 feet of distance between the top of the casing and the casing shoe is used to hang this pipe to ensure a good cement seal.

One of the advantages of liners is that, since they do not come down from the surface, there are cost savings in the design of the liner program. However, when lowering and hanging this type of casing, problems can occur with the hanger seal and poor cementation.

A production liner is similar to the casing intermediate, in that it also serves to isolate problem areas that tend to cause problems with the well during drilling operations.

Operational aspects to be fulfilled for the execution of the liner design and program

For the run of a very important casing a series of considerations from the operational point of view, that will give the happy fulfillment to the design previously made by the design engineers of the program of perforation of the well, for it with the operational experience that concerns me, provided by means of the following article a cumulus of recommendations and operational considerations that help to that the design of casing can be executed in the best way.

The objective of any casing run or downhole is to set the casing at the depth determined by the casing design and program, leaving a depth gap between the casing and the bottom of the well, generally taking a 5 foot gap.

Operational procedure for casing removal

Measure and calibrate the entire casing, before making the casing measurement record.
The operations engineer, team leader, and service company representative in charge of the casing run should verify the dimensions, equipment and accessory features of the production casing, such as the hanger, guide shoes, and all casing joints in the hole.
For the casing run, the ironing of the drill must be conditioned and dressed according to everything that is necessary to work in its descent, such as: force keys, elevators, wedges, collar, light and heavy equipment, among others.
To carry out a pre-operational and safety meeting with all the personnel involved in the descent of the casing to the well, in which the steps to be followed are defined and where all the responsibilities are assigned.
The casing in question is lowered to the depth described in the program and casing design, following the following order:

Floating guide shoe.
Liner.
Centralizers if required so that the casing is well centered in the well, and the subsequent cementation is as optimal as possible.

It is recommended that liquid steel (Baker lock) be placed in the connections of the first 4 casing joints.
It is recommended to test all the equipment and accessories that will be used for the casing run, such as: shoe flotation equipment, hanger, lading collar among others, all in order to rule out possible leaks and obstructions.
It is recommended to use rubber protectors or thread guards when climbing the liners on the ramp, otherwise API grease should be applied to the thread pin of each liner in its connections.
It is recommended that the correct torque be applied to the casing threads as indicated in the drilling program. - It should be filled with drilling mud every 5 casing joints.

Image Source

In the image above we can see all the casing that is accommodated, and that in particular I can tell you from work experience that as an operations engineer many times I had to measure the length of each one of those pipes, all with the objective of having the precision in knowing the amount of pipes that are going to be lowered to the bottom of the hole and the feet of casing length that are going to be exposed on the surface to connect the cementing head.

Conclusions and contributions to engineering

The design of the program in the selection of the liner pipe contemplates the possibility of minimizing the risks that the pipe may suffer deformation resulting from high temperatures and pressures at the bottom of the hole, the other favorable aspect involves selecting the type of pipe that best applies in criteria of economic optimization but without compromising the integrity of the liner pipe and well in general.

The contribution of this article from an engineering point of view is that it concisely provides the criteria for selecting the liner pipe based on the conditions demanded by the hole at high depths such as high pressures and temperatures, so the design of a liner program should be a design studied and analyzed under the strictest safety parameters and implementation of mathematical resources and calculations that facilitate the accuracy of the program based on the type of liners to choose from.