Applied Well Cementing Engineering

By Gefei Liu

Applied Well Cementing Engineering delivers the latest technologies, case studies, and procedures to identify the challenges, understand the framework, and implement the solutions for today’s cementing and petroleum engineers. Covering the basics and advances, this contributed reference gives the complete design, flow and job execution in a structured process. Authors, collectively, bring together knowledge from over 250 years of experience in cementing and condense their knowledge into this book. Real-life successful and unsuccessful case studies are included to explain lessons learned about the technologies used today. Other topics include job simulation, displacement efficiency, and hydraulics. A practical guide for cementing engineer, Applied Well Cementing Engineering, gives a critical reference for better job execution.

• Provides a practical guide and industry best practices for both new and seasoned engineers
• Independent chapters enable the readers to quickly access specific subjects
• Gain a complete framework of a cementing job with a detailed road map from casing equipment to plug and abandonment

• Language: English
• Publisher: Elsevier Science
• Release date: Mar 25, 2021
• ISBN: 9780128219485

Book preview

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Chapter One: Introduction to cementing engineering

Gefei Liu    Pegasus Vertex, Inc., Houston, TX, United States

Abstract

This chapter provides an overview of cementing engineering, including what well cementing is, why we do well cementing, how we do it, and illustrates some common problems of well cementing.

The author introduces the concept from ancient drilling practices and brings the readers into the modern world of well cementing. The chapter also maps out the content of the book.

Keywords

Well drilling; Casing; Cementing; Protect water; Wellbore integrity; Zonal isolation; Cement channeling; Centralizers; Slurry

1.1: What is well cementing?

Millions of years ago, algae, plants, and bacteria lived, died, and sank to the seafloor. Over millions of years under high pressure and high temperature, the buried organic substance had slowly broken down and transformed into oil and gas. Well drilling is the only way to extract oil and gas from the earth. Before we discuss about the modern drilling process, let us take a look at a drilling scene 2000 years ago in China, where the Chinese in the Sichuan province originated deep drilling.

The primary motive for deep drilling in China was the search for salt. The ancient percussive cable drilling system involves a derrick with a height of ~ 33 ft and all parts of the rig made from wood (mainly bamboo). A large wooden drum of 16 ft diameter was used to perform round trips. The rocking movement of the balancing beam created the percussive impulses on the bit, which sometimes weighed over 300 lbs. By alternately lifting the bit and letting it fall, they achieved rates of penetration from 1 in. to 3 ft per day. By the beginning of the 3rd century CE, wells were being drilled up to 500 ft deep. Fig. 1.1 shows a typical bamboo rig.

Fig. 1.1

Fig. 1.1 Ancient Chinese drilling rig. (Credit: Tiangongkaiwu, 1637 AC).

The boreholes were lined with bamboo and connected by watertight male-female joints. The bamboo casing prevented water seepage during drilling. They also used straw mixed with Tung oil and lime to make primitive cement and keep the bamboo casing in position. This way, when the brine was reached, the brine would not be diluted by seeping freshwater from shallow zones. This is arguably the earliest record of the casing and cementing.

Deep drilling for brine yielded natural gas (primarily methane) from time to time. The boreholes producing methane were known to the Chinese as fire wells. Drilling for natural gas followed and was developed at the same time. Natural gas was then used to heat evaporation pans of brine to make salt: killing two birds with one stone.

The first successful cementing job was performed in an oil well (Hill no. 4 Spudded September 26, 1905, and completed April 30, 1906) in which a water shutoff was attained by pumping cement through the tubing and behind the casing-forerunner of the modern cementing technique. The well, drilled by the Union Oil Company of California to a total depth of 2507 ft of 10 in. casing and 2237 ft of 8 in. casing, was so securely cemented off that the well subsequently produced for over 45 years. By 1917, oil well cement became commonly available. And by 1948, API code 32 was adopted for oil well cements.

In the modern era, oil and gas wells are drilled using a rotary drilling rig with thousands of feet of drill pipe and a drill bit at the bottom. By rotating the drill pipe from the surface or downhole motor, the drill bit is turned rapidly to cut the formation and create a section of the open wellbore.

A well is drilled and completed in stages. As the well is drilled deeper, formation pressure changes. It is necessary to line the bottomhole with a steel pipe (casing) to keep the wellbore from caving in. Other important reasons for casing and cementing are explained in the next section of this chapter.

After a casing is run into a wellbore, it needs to be secured inside the wellbore, concentrically if possible. The material used to achieve this purpose is cement. Unlike the cements used in construction concrete, well cements do not contain coarse sand or gravel. Finely grounded cement is mixed with water and other chemicals (additives) to make pumpable cement slurry, which has predictable rheological, thickening, and final strength properties.

To hold the casing in position, cement slurry is pumped down the casing and flowed back to the annulus between the casing and borehole. An adequate length of cement column, when set, can firmly bond casing to the borehole. This process of placing liquid cement slurry in the annulus is called primary cementing.

A cement slurry starts its solidification process as soon as it is pumped into casing. Within hours of placement (typically 12–24 h), the cement slurry sets or hardens and forms a solid seal between the casing and borehole. After the cementing job, drilling activity continues to the next depth, and a new and smaller casing is run and cemented. This process repeats until the target is reached.

Well cementing is like a series of bridges that link the drilling phase to the production phase. Today, it is possible to drill through more than 6 miles of rock to reach an oil and gas reservoir. Casing and cementing provide a step-by-step foundation to make deep drilling possible.

The development of oil well cementing was one of the most significant events in the history of petroleum technology. It has increased the productive life of thousands of oil wells and has thereby made available millions of barrels of oil that might otherwise have remained buried. In conjunction with increasing oil and natural gas production, concerns have also mounted in regard to the environmental impacts of these well developments on groundwater quality, public health, and climate due to subsurface gas and fluid migration. Well cementing provides structural barriers consisting of cement and casing strings, to prevent the problems ranging from gas leaks to well blowouts.

1.2: Why do we cement wells?

Even during ancient drilling in China 2000 years ago, water seepage from formation was a serious problem. During brine production, dilution by seeping freshwater from shallow zones was a concern. Bamboo casing and primitive cement that consisted of Tung oil and lime were used to create a seal between the casing and formation. This primitive casing cementing enabled the ancient drilling for salt and gas.

Casing and cementing have become very sophisticated and play many key roles in completing a well nowadays. From a well construction perspective, the roles of casing and cementing are as follows:

•Casing prevents the collapse of a wellbore

A well is normally drilled in stages. At the end of each stage, a casing is run into the end of the drilled section and cemented. Each subsequent casing is smaller than the previous one. The first and widest casing is called the conductor, which prevents the collapse of loose soil near the surface. The next size in the casing string is the surface casing, followed by an intermediate casing. The last type of casing, the smallest in diameter, is the production casing that is run directly into the oil and gas reservoir. Together, those casing strings serve as a major structural component of a wellbore and prevent the rock from caving into the wellbore.

•Cement supports casing

The annual space between casing and rock needs to be filled with cement, which secures casing after the cement sets. Once the cement has set, the sheer strength of the cement column can hold a casing string of several thousand feet in length. This is especially important for the surface casing: the cement supports the surface casing, which supports the blowout preventers (BOPs) for subsequent drilling operations.

•Cemented casing provides a foundation for subsequent drilling

Cementing casing provides a flow passage for drilling fluid and production fluids. It also provides a smooth borehole for deeper drilling. The friction factor between the casing interior and drill pipe is usually lower than that of the open hole and drill pipe. Therefore, the torque-and-drag issues tend to be less severe for cased hole sections.

Drilling is a destructive process of rock cutting and the resulting drill string vibrations and shocks cause unavoidable damage to the drill pipe and downhole tool. Among these, bottomhole assembly (BHA) whirl is the eccentric rotation of the BHA (around a point other than its geometric center). It generates lateral displacements, shocks, and increased friction against the wellbore. The cement provides support to the casing and protects casing from shock loads. Cemented casing serves as a solid foundation for subsequent drilling.

The ultimate goal of the casing and cementing is to provide a hydraulic seal (cement sheath) between the casing, cement, and formation, which prevents fluid communication between the zones. This objective is called zonal isolation.

From a zonal isolation point of view, casing and cementing provide the following functions.

1.Water Protection

Federal, state, and local governments each regulate various aspects of oil and gas operations. Drilling permits protect groundwater by mandating a casing and cementing program for each well. Hardened cementing seals off the annulus to prevent the vertical migration of formation fluids from one layer along the wellbore and polluting the fluids in another layer. Normally, surface casing and its cementing provide the primary barrier against the vertical migration of fluids into freshwater formation.

2.Circulation Loss Prevention

Wells, especially deep ones, often encounter abnormally pressured formations, troublesome lost circulation zones, and incompetent shale formations. Casings, often intermediate casings, are required to drill these zones trouble-free.

3.Casing Corrosion Protection

Oxygenated waters, acidic waters, chemical salts, and other highly corrosive subsurface fluids can corrode the casings' steel surface. Elevated casing temperatures accelerate the corrosion rate. The consequences of corrosion in casing include the reduction of wall thickness, overall strength, and ductility, which could eventually lead to casing failure. Cementing will effectively protect the casing from any corrosive formation fluids contacting the casings' steel surface.

4.Blowout Prevention

Zonal isolation prevents oil and gas at high pressure from escaping the formation and traveling up the wellbore. The uncontrolled release of crude oil and/or gas, called blowout, can lead to a catastrophic fire and oil spill. Cementing problems are associated with many well blowouts. Although modern wells have BOPs designed to cut through and seal the drill pipe, casing cementing is the frontline defender to prevent blowouts. In addition to providing the seal, the cement supports the casing which supports the BOPs which provide a last line of defense against blowouts.

Fig. 1.2 illustrates the reasons why we cement wells.

Fig. 1.2

Fig. 1.2 Why do we cement wells? (Credit: Pegasus Vertex, Inc.).

1.3: How do we cement wells?

Cementing is one of the most critical steps in the drilling and completion of an oil and gas well. With successful cementing jobs, subsequent drilling operations and well production are likely to be successful. Because cement remediation is costly, difficult, and not always successful, cementing is usually a one-shot deal.

Cementing jobs in a well can either make or break the financial goals of the well. This fact makes the time spent on designing and executing a cementing job a good investment.

The German philosopher Friedrich Nietzche once said: He who has a why to live can bear almost any how. People realized the importance of a cementing job and in turn, they developed a series of steps to standardize the cementing procedure. Here is how cementing engineers perform their jobs in steps.

1.Run casing into a well

At predefined depths, the drill pipe is removed, and the casing is run into the wellbore. Compared to a drill pipe, a casing is a relatively thin-walled steel pipe that comes around 40 ft (13 m) long with a threaded connection at each end. Centralizers and scratchers are often installed on the outside of the casing to keep it centered and to clean the wall.

A guide shoe, made of steel and concrete, is installed on the bottom joint of the casing to protect the lower edge of the casing. The tapered or bullet-nosed shape helps guide the casing through narrow, deviated hole sections.

A float collar is installed near the bottom of the casing. The check-valve assembly, fixed within the float collar, prevents drilling mud or cement slurry from entering the casing when pumping is halted. It also serves as a receptacle for cement plugs.

Running a casing into a wellbore is accompanied by a displacement of mud in the hole, leading to surge pressure. The accurate prediction of surge pressure is of great importance in wells, where the pressure must be maintained within narrow limits to ensure trouble-free drilling and completion operations. Another concern associated with casing running is the torque and drag loaded on the casing string. The upward frictional drag on the casing may cause casing buckling. And the torque, if the casing is rotated especially in an extended well, may twist the string or damage the liner top.

2.Condition the mud by circulating

After casing running, the condition of mud clears the way for a good cementing job. It is normally recommended to circulate two-hole volumes of the drilling fluid. This conditioning cleans out cuttings or other debris, breaks up mud gels, and reduces the bottom hole temperature.

3.Pump spacer

To separate drilling fluid from cement slurry and to avoid cement contamination, a spacer fluid is pumped ahead of cement slurry to wet the borehole and make the mud displacement more complete.

4.Pump cement slurry

The cement and additives are mixed with water to form a cement slurry and pumped down through the casing and up in the annulus to create a desired cement column. The cement slurry is followed by the drilling or completion fluid pumped after the cement to push the cement to its planned location.

To further prevent the drilling fluid from contaminating the cement, a 2-plug system is used. The one released ahead of cement slurry is called the bottom plug. It stops at the float collar, and pump pressure ruptures the diaphragm so that the cementing slurry can pass through it. A top plug, similar to the bottom plug but without the diaphragm, isolates the displacement fluid and cement slurry. The top plug stops on the bottom plug, and an increased pump pressure signals the completion of mud displacement.

5.Wait on cement

This period, ranging from a few hours to several days, is for liquid cement slurries to solidify and develop sufficient compressive strength. Meanwhile, pipe-cement and formation-cement bonds begin to develop, ensuring the ultimate goal of zonal isolation.

6.Evaluate cement job

After the cement is set, we want to confirm that the cement is in place to provide the desired performance. The first indication that the cement is in place is when cement job measurements including fluid volumes, cement density, surface pressure, and returns all match pre-job expectations. If there are any concerns with the top of cement (TOC), cement quality, or if it is required, a temperature survey or cement evaluation log should be run.

A temperature survey, run several hours after cement placement, takes advantage of the heat generated by the cement column in the annulus. It is very useful to determine the location of TOC. Cement bond log (CBL) is obtained using acoustic waves sent by a transmitter and received by a receiver. The variation of the acoustic wave amplitude represents the quality of the cement bond. Variable-density log (VDL) is another method to evaluate the quality of a cement job, which presents the amplitude of acoustic waves in various shades of a greyscale to identify the cement bond quality.

One of the purposes of the casing and cementing is to provide a solid foundation for subsequent drilling operations. To ensure the quality of a cementing job, it is common to pressurize inside the newly cemented casing until the pressure at the shoe reaches the maximum anticipated pressure at that depth during the next drilling operation. If the pressure declines significantly or if there are other indications of leakage, the casing should be recemented using squeeze cementing operation.

Fig. 1.3 shows major casing accessories and the displacement of drilling fluid in a cement job.

Fig. 1.3

Fig. 1.3 How do we cement wells? (Credit: Pegasus Vertex, Inc.).

1.4: Avoid common cementing problems

The effectiveness of zonal isolation achieved through the casing and cementing depends on the casing integrity, the quality of cement, the casing-cement bond, and the formation-cement bond. Even if the displacement process of a cementing job is a success, it does not guarantee a healthy well life. Cement sheath may fail due to vibration, temperature and pressure changes, and shock loads induced by subsequent drilling, stimulation, and production operations.

Fig. 1.4 shows some common leakage pathways found around the cemented casing, together with some causes.

Fig. 1.4

Fig. 1.4 Common cementing problems. (Credit: Pegasus Vertex, Inc.).

Failure of set cement is usually attributed to contamination, either from the mixing water or drilling fluid during pumping. Water is used to wet the cement solids and create a pumpable cement slurry. Contaminants may accelerate or delay the setting of cement. Thus, the water used to mix cement should be kept as pure as possible.

Besides water contamination, poor mud removal is a major factor of cement quality. When drilling fluid is displaced by another fluid, such as cement slurry inside a pipe or in an annulus, the displacing fluid sometimes flows in a channel on certain sides of the casing or wide side of the eccentric annulus and fails to rise uniformly in the annulus. This cement and mud channeling weakens the bond of cement to the formation and the cement to the casing. It also creates a weak cement sheath.

To minimize cement channeling, a spacer or chemical wash is pumped before the cement slurry to leave the casing and formation water-wet (free of oil), preparing the wellbore for a good bond.

Centralizers are strategically installed on the casing at optimal intervals to maintain a good casing standoff profile. The eccentric annulus is one of the most detrimental factors in limiting the efficiency of displacement. Without centralizers, sections of the casings in a directional well may touch the wellbore. The fluid will flow through the least resistant path on the wide annular side, leaving a mud channel on the narrow annular side.

Fluids are specially designed with rheological properties. Pumping rates are carefully selected to control the bottomhole pressure and to achieve a high displacement efficiency. Often, special chemicals, called additives, are blended with dry cement or measured into the mixing water properties. Pumping to optimize properties of cement slurry such as the thickening time, density, or viscosity.

Applying a rotation or reciprocation to the casing during cementing is an effective way to improve job quality. Casing movement breaks up areas of stagnant mud, which can cause cement channeling.

As soon as slurry is mixed, it starts thickening. Thickening time, the duration that the cement slurry remains pumpable, is affected by downhole temperature and pressure. Of the two, the temperature has the most significant influence. As the downhole temperature increases, the cement slurry sets faster. The pumping time needs to be adjusted accordingly. Therefore, it is desired to predict the dynamic temperature profile along the wellbore during cementing by calculating the transient heat transfer between wellbore fluids and formation.

Cementing is the last step in drilling operations and the first step in completion. Throughout the life of the well, the cement sheath not only supports subsequent drilling operations but also withstands the temperature and pressure changes induced by well testing, simulation, and production operations.

Although we may have a good cement placement, the severe downhole condition in the late stage of completion and production may lead to cement sheath failure as shown in Fig. 1.4. It is desirable to predict cement sheath integrity using a computer model, considering various temperature and pressure changes.

1.5: Overview of the contents

The cementing operation for the production casing is a closure of drilling operations and a beginning of the production of a well for the next few decades. Therefore, cementing quality is closely related to the longevity and productivity of a well. Since the first use of cement slurry in a Union Oil Co. well in California in 1903, well cementing technology has matured and evolved, ensuring greater production and environmental protection. At the same time, well cementing has become one of the most interesting areas of research because it requires an interdisciplinary approach. Talents are desired from scientific and engineering disciplines, including chemistry, mechanical, physics, petroleum, rheology, geology, electrical engineering, and computer science. It is a team effort. Cementing engineers need to interact with different disciplines to perform jobs.

Looking ahead, we see an array of new and potential technology advances, including new tools to run casing, innovative chemicals to make cement for high-temperature and high-pressure wells, fine-tuned best practices to place cement, and artificial intelligence to predict the success rates, etc.

Over the past 30 years, it has been my privilege to be involved in the fascinating well cementing engineering. I had the pleasure of working with many seasoned experts, including Dr. William C. Maurer (Maurer Engineering), the late Larry Moran (ex-ConocoPhillips), Lawrence Weber (ex-Chevron), Craig Gardner (ex-Chevron), Tongyou Wang (COSL), Donald Jagpath (Tucker Energy Services), Jason Schneider (Sanjel Energy Services), and many other people who have encouraged me and showed me the ropes.

My main areas of research and development include mechanical engineering and computer simulation in cementing engineering. Nevertheless, I do not claim that I am an overall cementing expert. That is why we have a team of 11 contributing authors who are experts of their fields. Collectively, these authors bring together knowledge from over 250 years of experience in cementing and condense their knowledge into this book.

The purpose of this book is to provide a practical guide for both new and seasoned cementing engineering professionals. It focuses on the fundamental and applied engineering aspects of cementing operations, with some of the industry’s best practices.

The structure of this book starts from casing string design to plug and abandonment. However, you can select any chapter and study it. The fact that the individual authors write different chapters makes each topic relatively independent. Readers will be able to quickly access the specific subjects.

The introductory chapter is an overview of cementing engineering. Chapter 2 (Casing String and Design) covers casing string and casing design, which is an important part of well construction and well barrier. It covers casing types, their functions, and casing design basics.

Casing equipment is a fundamental technology for successful cementing operations. Chapter 3 (Casing Equipment) focuses on the importance of casing accessories, which is often ignored or misunderstood. In reality, improper selection and utilization of casing equipment can lead to unfavorable financial, environmental, and safety consequences. This chapter provides a detailed description of the most common types of casing equipment and casing centralizers.

Cementing job starts with running a casing into a hole. The challenges associated with running casing include torque and drag (e.g., buckling, casing connection damage) and surge and swab (e.g., loss of circulation, hole integrity). Chapter 4 (Casing Running) discusses these issues, methods, and tools to reduce the undesirable impacts of these challenges.

Chapter 5 (Fluids) discusses how to design cementing fluids and their target properties to achieve the cementing and zonal isolation objectives. Once drilling is completed and the hole is ready to be cased and cemented, the fluid in the hole has the objective to keep well control, as a primary barrier, maintain hole stability, and finally, to have minimum properties to be removed from the wellbore and be thoroughly displaced by cementing fluids. Preflushes and spacers provide a barrier to prevent fluid contamination inside the casing and in the annular space. This chapter describes the connection between the selection of fluid properties, laboratory, and field execution to the success of the cementing operation.

The success of placement of cement in the designed annular space requires fundamental knowledge and understanding of cement slurry hydraulics. Among several issues in cementing is the frictional pressure that can cause cementing job failure due to equipment rupture and loss of circulation in the wellbore. Chapter 6 (Hydraulics) provides basic knowledge of hydraulics of cement slurry which is critical to designing for safety and the success of cementing operations. Applications of the fluid rheology models are demonstrated through illustrative examples and output of computer programs.

Computer-assisted design of primary cementing is the main tool for the cementing engineer to ensure the success of the zonal isolation objectives. Chapter 7 (Job Simulation and Design) shows how the design engineer can literally play various scenarios and forecast the potential outcome through software simulation. Today’s software simulation has the power to visualize fluid behavior behind the casing, including their intermixing and other placement consequences such as channeling and immovable mud. In this chapter, an explanation of these elements that jeopardize the validity of the model and the predicted outcome, the strategy, and methods to overcome or mitigate the effect of these elements, along with the importance of the leading role of the cementing engineer, is presented.

The main parameters that affect slurry design and performance are down-hole temperature and pressure, which increase with the depth of the well. As soon as we mix cement with water, the slurry starts thickening. The increased down-hole temperature reduces the thickening time of cement. Chapter 8 (Temperature Prediction) highlights the physical explanation and modeling approach of the wellbore transient heat exchange during cementing.

Displacement efficiency determines cement bonding quality. Unfortunately, it is an inadequately studied area involving complicated physics. Chapter 9 (Displacement Efficiency) focuses on using and interpreting computer simulations to optimize the displacement efficiency of cementing jobs. Factors with significant influence on the job performance are examined, such as fluid rheology, density, pumping rate, casing standoff, well inclination, casing movement, and so on. Understanding these factors will help engineers design better jobs through many aspects such as selecting proper fluids, pumping schedule, centralizers, and utilizing cement equipment or tools and other strategies.

Chapter 10 (Job Execution) discusses three main components to each job’s success: job preparation, wellsite execution, and post-job reporting. Each of the topics discussed in previous chapters is integrated with job execution. Job execution requires a holistic view of the service delivery. This chapter closes the link between design and execution, highlighting the significance of job preparation, planning, and on-site QA/QC to the success of the cementing operation.

Chapter 11 (Job Evaluation) introduces many ways of evaluating the qualities of cementing jobs, beginning with the cement job operation itself. Job data is analyzed as the first step in determining success. After that, the well is observed to determine any obvious problems, such as annular flow. Throughout the life of the well, monitoring of the wellbore pressures will continue to evaluate zonal isolation in the wellbore. Acoustic logging is another popular way to evaluate the quality of the cement job. The purpose of the cement job evaluation is to determine if the operation successfully achieves its goal of placing cement across a given interval and achieving zonal isolation.

Chapter 12 (Plug and Abandonment) provides basic design and equipment knowledge to the cement and drilling engineer to assist in successful zonal isolation during temporary and permanent abandonment (TA/PA) as well as remedial cementing.

Legend has it that Chinese philosopher Confucius once stated 2500 years ago I hear and I forget. I see and I remember. I do and I understand. The aim of this book is not just about the knowledge it contains, but the understanding and application of it to the next cementing job.

We sincerely hope that this book builds a pathway for those who enter the field and bridges the gap between knowledge acquisition and field application.

Chapter Two: Casing string and design

Jiang Wua    a Retired from Chevron, Houston, TX, United States

Abstract

Casing string used in oil and gas well is cemented in wellbore to seal and protect the wellbore formation. This chapter presents fundamental knowledge of casing string and design, to help drilling and completion engineers on the planning and operation of oil and gas well construction. More than one casing string is used in an oil and gas well, with different sizes and setting depths for various functions. Casing types, casing performance properties, casing design principle, casing strengths, casing triaxial stress analysis, casing loads, casing design factors, casing connection, and cementing quality and planning consideration to casing design are discussed in this chapter. A casing design example is also provided with using the information provided in this chapter.

Keywords

Casing; Casing strength; Casing load; Casing design; Casing design factor; Casing triaxial yield; Casing collapse under combined loads; Casing connection

2.1: Casing string and design

Casing string is typically hollow steel tube set and cemented in an oil and gas well to ensure safe drilling, completion, and production of the well. This chapter provides the fundamental knowledge of casing string and design to help drilling and completion engineers on the planning and operation of oil and gas well construction. The following aspects of casing string and design are discussed in this chapter:

•Casing types (conductor, surface, intermediate, and production) and functions,

•Casing grades (American Petroleum Institute (API) and non-API grades) and performance properties,

•Casing design (casing design principle, casing strength, casing triaxial stress analysis, casing load, and casing design factor),

•Casing connection (connection types, performance, evaluation, and qualification), and

•Cementing quality and planning consideration in casing design (casing external pressure, top of cement (TOC) in deepwater well).

2.2: Casing types and functions

2.2.1: Casing types and functions

Casing string is used in oil and gas well to ensure safe drilling, completing, and production operations, by protecting shallow fresh water zone, isolating high- or low-formation pressure, and preventing wellbore collapse, etc. Several casing strings of different sizes are set to different well depths in oil and gas well by their various functions. Fig. 2.1 shows an example of well construction with four casing strings (conductor casing, surface casing, intermediate casing, and production casing) in an oil and gas well.

Fig. 2.1

Fig. 2.1 Example of well construction with four casing strings.

Conductor casing, surface casing, intermediate casing, and production casing are generally the four casing types used in an oil and gas well, with the following running sequence, application functions, and cement planning, respectively:

•Conductor casing is the first casing, normally in size of 16″ ~ 26″

–Normally drive-in for on-land well or jet-in for subsea well

–Used to prevent the top formation washout in deeper drilling operation

–Cement to surface if the conductor would be set in a drilled hole (not drive-in, not jet-in)

•Surface casing is the second casing, normally in size of 13 3/8″ ~ 18 5/8″

–Used to protect shallow fresh water zone from contamination by drilling mud

–Also used to support wellhead and subsequent casing/tubing string weights

–Cement to surface

•Intermediate (protective) casing is the next casing after surface casing, normally in size of 9 5/8″ ~ 16″. More than one intermediate casing may be used in deep well application.

–Used to protect weaker formations and unstable formations

–Also used to isolate abnormal pressure and/or problem zones

–Cement to a depth above abnormal pressure and/or problem zones or to the surface

•Production casing is the last casing, normally in size of 7″ ~ 9 5/8″

–Used to provide production zone isolation on corrosive fluid, reservoir fluid, and reservoir high pressure

–Also used to serve as the second barrier for the well integrity to contain oil and/or gas reservoir pressure, in case of tubing failure (tubing is the smallest pipe set inside production casing to produce oil and/or gas, whose design is similar to casing but is not a subject of this chapter)

–Cement to a depth at least above the productive zones or to the surface.

An intermediate liner or production liner may be used, which does not extend to the wellhead (or surface) but hang above the preceding casing shoe (typically 300–500 ft. above the preceding casing shoe), like the 7″ production liner illustrated in Fig. 2.2. A liner is always cemented to the liner top. In Fig. 2.2, a 3.5″ tubing is also shown with a tubing packer at bottom of tubing to seal the tubing annulus.

Fig. 2.2

Fig. 2.2 A 7″ production liner in casing program.

When a production liner is used, like the 7″ liner in Fig. 2.2, the preceding casing will also serve as a production casing, as it will be exposed to the production loads in the production phase. For the 9 5/8″ intermediate (protective) casing in Fig. 2.2, it is first exposed to the drilling loads when drilling the next wellbore for setting the 7″ production liner, and it is then exposed to the production loads when the well is put on production. The details of drilling loads and production loads are discussed in the later sections.

Conductor casing, surface casing, and intermediate casing are also called drilling casing, as they are exposed to drilling loads from the drilling of deeper wellbores. However, an intermediate casing may also be exposed to production loads from the production of the well, if a production liner is set below it, as discussed above. This intermediate casing will also serve as a production casing.

•Drilling casing (conductor casing, surface casing, and intermediate casing): exposed to drilling loads from drilling operations

•Production casing: exposed to production loads from production operations

2.2.2: Casing setting depth

Different types of casings are used to set at different well depth in an oil and gas well, and casing setting depth is determined based on the casing application purposes for a safe drilling, completing, and production operation, such as protecting shallow fresh water zone, isolating wellbore pressure/lost-circulation zone/chemical reactive zone, etc.

Setting depth of production casing is generally the well target depth across reservoir formation, serving as the last casing string in a well.

Setting depth of intermediate casing is basically determined based on controlling the formation pressure and avoiding formation fracturing in drilling operation. As shown in Fig. 2.3, a minimum drilling mud density profile, normally 0.5 ppg higher than the formation pore pressure gradient profile, is planned to safely control formation pressure with allowing a decrease of effective drilling mud pressure in a trip-up operation (swab); and a maximum mud density profile, normally 0.5 ppg lower than formation fracture pressure gradient profile, is also planned to safely avoid formation fracturing with allowing an increase of effective drilling mud pressure in a trip-down operation (surge). A vertical line, starting from the minimum mud density profile at the well target depth (the production casing setting depth in Fig. 2.3), or Point 1, is drawn to reach the maximum mud density profile, or Point 2, and this point is the intermediate casing setting depth, to protect the formation above this depth from being fractured by the drilling mud density used in drilling to the well target depth (Point 1). If there will be a problem zone (lost-circulation, chemical reactive, etc.) deeper than Point 2, or if there will be a high risk on casing differential stuck in the wellbore below Point 2, the intermediate casing will need to set deeper to protect the problem zone or to reduce the risk of casing differential stuck. Casing differential stuck risk is considered high when the differential pressure between drilling mud pressure and formation pore pressure is larger than 2000–3000 psi. This practice of controlling formation pressure and avoiding formation fracturing in drilling operation is repeated to determine the next shallower intermediate casing setting depth until the surface casing depth, which is determined by a different criterion, is reached or passed. In Fig. 2.3, this practice is repeated by drawing a vertical line, starting from the minimum mud density profile at the intermediate casing depth, or Point 3, to reach the maximum mud density profile, or Point 4. If this Point 4 does not reach or pass the surface casing setting depth, this is the next shallower intermediate casing setting depth; and if this Point 4 reaches or passes the surface casing setting depth, like shown in Fig. 2.3 where this Point 4 just reaches the surface casing setting depth, no more shallower intermediate casing is needed.

Fig. 2.3

Fig. 2.3 Intermediate casing setting depth determination.

The determination of intermediate casing setting depth by the above Point 1 → Point 2 → Point 3 → Point 4 approach is called a bottom-up method, while a top-down method may also be used, to start from the maximum mud density profile at the surface casing setting depth, or Point 4, with a reversed Point 4 → Point 3 → Point 2 → Point 1 approach. The top-down method may result in a deeper intermediate casing setting depth and allow a higher drilling mud density to well target. This will happen when the Point 4 in Fig. 2.3 from the bottom-up method reaches a depth shallower than the surface casing setting depth. The top-down method may be used in drilling new exploration well with less confidence on reservoir pressure, so that a higher drilling mud density can be allowed in drilling to the well target for controlling possible higher reservoir pressure, though it may result in a higher drilling and casing cost.

Setting depth of surface casing is normally governed by local government regulation, to set across the shallow fresh water formation zone and protect it from drilling mud contamination of chemical contents, in drilling the deeper formations below the surface casing.

Setting depth of conductor casing relies on the resistance of top formation soil when it is driven into the ground, and the conductor-driving operation will be stopped when the driving rate approaches or reduces to a specified limit (number of driving blows per foot).

2.2.3: Casing size selection

Casing is started with the largest diameter (conductor casing) on top in a well, and the subsequent casing sizes are progressively smaller in diameter as they must pass through the inside of the previous casing. A sufficient clearance between casing and wellbore is needed to achieve good cementing quality, and that is the basic consideration on casing size selection.

Casing size selection starts with the production casing or production liner (the deepest casing string), to allow accommodation of the production tubing (the producing conduct of oil and gas from the oil and gas reservoir to surface), and to perform any work-over/well-deepening operations. The production tubing size is determined through the production planning and optimization based on oil and gas reservoir characters, such as reservoir formation pressure and permeability, by production engineer and not by drilling engineer and therefore will not be discussed further in this chapter.

Once the production casing size is determined based on the above completion and redrilling considerations, the size of the next larger casing (likely intermediate casing) can be determined based on having a sufficient casing internal diameter to allow a drill-bit to pass and drill the deeper open-hole for the production casing. The open-hole wellbore size or the drill bit size needs to be larger than the production casing connection size and have a sufficient wellbore clearance (half of the difference between wellbore size and casing external diameter) for cementing operation. The optimum wellbore clearance is 0.75″ to achieve a good cementing quality based on the previous studies, though a larger or smaller wellbore clearance may also be accepted. Note that casing and drill-bit are normally manufactured by their standard sizes, and the selection of casing and drill-bit size will then need to be based on their standard sizes, unless a special order of nonstandard sizes is needed and achievable.

After the intermediate casing size is determined, the size of the next larger casing (surface casing or another intermediate casing) can be selected by the same approach till the largest casing (conductor) size is selected in a well. Table 2.1 lists two examples of commonly used casing size and drill-bit size combinations in on-land well with four casing strings.

Table 2.1