Casing and Liners for Drilling and Completion

Ted G. Byrom

• Editore: Gulf Publishing Company
• Anno: 2013
• ISBN:
  9780127999814
• Argomento: Sciences physiques
• Categoria: Géologie et sciences de la Terre

The Gulf Drilling Series is a joint project between Gulf Publishing Company and the International Association of Drilling Contractors. The first text in this Series presents casing design and mechanics in a concise, two-part format. The first part focuses on basic casing design and instructs engineers and engineering students how to design a safe casing string. The second part covers more advanced material and special problems in casing design in a user-friendly format. Learn how to select sizes and setting depths to achieve well objectives, determine casing loads for design purposes, design casing properties to meet burst, collapse and tensile strength requirements and conduct casing running operations safely and successfully.

CHAPTER 1

Oil-Field Casing

1.1 Introduction

We begin with a chapter on the usual obligatory information for a book on oil-field casing. The real book starts in Chapter 2, but in the rare case that the subject is totally new to you, you will find a rudimentary coverage of what casing is in this chapter. The steel tubes that become a permanent part of an oil or gas well are called casing, and the tubes that are removable, at least in theory, are referred to collectively as tubing which are not covered in this book. Oil-field casing is manufactured in various diameters, wall thicknesses, lengths, strengths, and with various connections. The purpose of this text is to examine the process of selecting the type and amount we need for specific wells. But first, a question: What purpose does casing serve in a well? There are three:

• Maintain the structural integrity of the bore hole.

• Keep formation fluids out of the bore hole.

• Keep bore hole fluids out of the formations.

It is as simple as that, though we could list many subcategories under each of those. Most are self-evident. Additionally, there are some cases where the casing also serves a structural function to support or partially support some production structure, as in water locations.

1.2 Setting the Standards

By necessity oil-field tubulars are standardized. Until recent times, the standards were set by the American Petroleum Institute (API) through various committees and work groups formed from personnel in the industry. Now, the International Organization for Standardization (ISO) is seen as taking on that role. Currently, most of the ISO standards are merely the API standards, but that role may expand in the future. In this text, we refer primarily to the API standards, but it should be understood that there are generally identical standards, and in some cases more advanced standards, under the ISO name.

It is important that some degree of uniformity and standardization is in force and that manufacturers be held to those standards through some type of approval or licensing procedure. In times of casing supply shortages, a number of manufacturers have entered the oil-field tubular market with substandard products. Some of these have led to casing failures where no failure should have occurred. The important point here is that any casing purchased for use in oil or gas wells should meet the current standards as set for oil-field tubulars by the API or ISO.

Some casing is not covered by API or ISO standards. Some of this non-API casing is for typical applications, some for high-pressure applications, high-temperature applications, low-temperature applications, and some for applications in corrosive environments. Most of this type of casing meets API standards or even higher standards, but one must be aware that the standards and quality control for these types of casing are set by the manufacturer. It probably should not be mentioned in the same paragraph with the high-quality pipe just referred to, but it should also be remembered that there are some low-quality imitations of API products on the market as well.

1.3 Manufacture of Oil-Field Casing

There are two types of oil-field casing manufactured today: seamless and welded. Each has specific advantages and disadvantages.

1.3.1 Seamless Casing

Seamless casing accounts for the greatest amount of oil-field casing in use today. Each joint is manufactured in a pipe mill from a solid cylindrical piece of steel, called a billet. The billet is sized so that its volume is equal to that of the joint of pipe that will be made from it. The manufacturing process involves

• Heating the billet to a high temperature.

• Penetrating the solid billet through its length with a mandrel such that it forms a hollow cylinder.

• Sizing the hollow billet with rollers and internal mandrels.

• Heat treating the resulting tube.

• Final sizing and straightening.

The threads may be cut on the joints by the manufacturer or the plain-end tubes may be sent or sold to other companies for threading. The most difficult aspect of the manufacture of seamless casing is that of obtaining a uniform wall thickness. For obvious reasons, it is important that the inside of the pipe is concentric with the outside. Most steel companies today are very good at this. A small few are not, and that is one reason that API and ISO standards of quality were adopted. Current standards allow a 12.5% variation in wall thickness for seamless casing. The straightening process at the mill affects the strength of the casing. In some cases, it is done with rollers when the pipe is cool and other cases when the pipe is still hot. Seamless casing has its advantages and also a few disadvantages.

• Advantages of Seamless Casing.

• No seams to fail.

• No circumferential variation of physical properties.

• Disadvantages of Seamless Casing.

• Variations in wall thickness.

• More expensive and difficult manufacturing process.

1.3.2 Welded Casing

The manufacturing process for welded casing is quite different from that of seamless casing. The process also starts with a heated steel slab that is rectangular in shape rather than cylindrical. One process uses a relatively small slab that is rolled into a flat plate and trimmed to size for a single joint of pipe. It is then rolled into the shape of a tube and the two edges are electrically flash welded together to form a single tube. Another process uses electric resistance welding (ERW) as a continuous process on a long ribbon of steel from a large coil. The first stage in this process is a milling line in the steel mill:

• A large heated slab is rolled into a long flat plate or ribbon of uniform thickness.

• Plate is rolled into a coil at the end of the milling line.

The large coils of steel “ribbon” are then sent to the second stage of the process, called a forming line.

• Steel is rolled off the coil and the thickness is sized.

• Width is sized to give the proper diameter tube.

• Sized steel ribbon is formed into a tubular shape with rollers.

• Seam is fused using electric induction current.

• Welding flash is removed.

• Weld is given an ultrasonic inspection.

• Seam is heat treated to normalize.

• Tube is cooled.

• Tube is externally sized with rollers.

• Full body of pipe is ultrasonically inspected.

• Tube is cut into desired lengths.

• Individual tubes are straightened with rollers.

This is the same process by which coiled tubing is manufactured, except coiled tubing is rolled onto coils at the end of the process instead of being cut into joints. Note that, in the welding process, no filler material is used; it is solely a matter of heat and fusion of the edges.

Welded casing has been available for many years, but there was an initial reluctance by many to use it because of the welding process. Welding has always been a matter of quality control in all applications, and a poor-quality weld can lead to serious failure. Today, it is both widely accepted and widely used for almost all applications except high-pressure and/or high-temperature applications. It is not used in the higher yield strength grades of casing.

• Advantages of Welded Casing.

• Uniform wall thickness.

• Less expensive than seamless.

• Easier manufacturing process.

• Casing is inspected during manufacturing process (ERW) and the defective sections removed.

• Uniform wall thickness is very important in some applications, such as the newer expandable casing.

• Disadvantages of Welded Casing

• High temperatures of welding process.

• Possible variation of material properties due to welding.

• Possible faulty welds.

• Possible susceptibility to failure in weld.

Welded casing has been used for many years now. Many of the so-called disadvantages are perhaps more a matter of perception than actuality.

1.3.3 Strength Treatment of Casing

When a cast billet or slab is formed into a tube it is done at quite high temperature. The deformation that takes place in the forming process is in a plastic or viscoplastic regime of behavior for the steel. As it cools its crystalline structure begins to form. Once the crystalline structure forms or begins to form, any additional plastic deformation to which we subject the tube will change its properties. The change may be minor or significant, depending on the constituents of the steel, the amount of deformation, and the temperature. Heating a tube above certain temperatures and cooling slowly allows the crystals to form more uniformly with fewer structural imperfections, called dislocations, in the lattice structure. The properties of the steel can modified by the addition of certain constituents to the alloy and, to some extent, by controlling the cooling rate. One common process for enhancing the performance properties of casing is to heat the tube above a certain temperature then quickly cool it by spraying it with water or some other cool fluid to change its properties, especially near the surface. This process is call...