Having worked through this chapter the student will be able to
General
• State the functions of Casing
• Define the terms: conductor; surface; intermediate; and production casing
• Describe the advantages of using a liner rather than a full string of casing.
• List and describe the loads which must be considered in the design of the casing.
Properties of Casing
• Describe the specific meaning of the terms used to describe the properties of casing: casing size, weight and grade
• Describe the various types of connection used on casing.
Wellheads and casing hangers
• Describe a conventional wellhead assembly
• Describe the sequence of operations associated with the installation of a spool type wellhead assembly
• Describe a compact spool wellhead and its advantages over the conventional wellhead
• Describe a conventional christmas tree and its function
• Describe the different types of casing hanger that are available and when each would be used.
Casing Running Operations
• Write a step by step program for a casing and liner running and landing operation
• Explain the reasons behind each step in the casing running operation.
Casing Design
• Describe the steps involved in the casing design process.
• Describe the main considerations in selecting the casing size and setting depths.
• Describe and calculate the internal and external loads which are considered when calculating the burst and collapse loads on a casing.
• Describe the source of tensile loads on casing and the way in which they combine during installation, cementing and production operations
• Describe the Bi-axial and tri-axial loads which the casing will be subjected to and the way in which these loads are accommodated in the design process.
It is generally not possible to drill a well through all of the formations from surface (or the seabed) to the target depth in one hole section. The well is therefore drilled in sections, with each section of the well being sealed off by lining the inside of the borehole with steel pipe, known as casing and filling the annular space between this casing string and the borehole with cement, before drilling the subsequent hole section. This casing string is made up of joints of pipe, of approximately 40ft in length, with threaded connections. Depending on the conditions encountered, 3 or 4 casing strings may be required to reach the target depth. The cost of the casing can therefore constitute 20-30% of the total cost of the well (Ј1-3m). Great care must therefore be taken when designing a casing programme which will meet the requirements of the well.
There are many reasons for casing off formations:
• To prevent unstable formations from caving in;
• To protect weak formations from the high mudweights that may be required in subsequent hole sections. These high mudweights may fracture the weaker zones;
• To isolate zones with abnormally high pore pressure from deeper zones which may be normally pressured;
• To seal off lost circulation zones;
• When set across the production interval: to allow selective access for production / injection/control the flow of fluids from, or into, the reservoir(s).
One of the casing strings will also be required:
• To provide structural support for the wellhead and BOps.
Each string of casing must be carefully designed to withstand the anticipated loads to which it will be exposed during installation, when drilling the next hole section, and when producing from the well. These loads will depend on parameters such as: the types of formation to be drilled; the formation pore pressures; the formation fracture pressures; the geothermal temperature profile; and the nature of the fluids in the formations which will be encountered. The designer must also bear in mind the costs of the casing, the availability of different casing types and the operational problems in running the casing string into the borehole.
since the cost of the casing can represent up to 30% of the total cost of the well, the number of casing strings run into the well should be minimised. Ideally the drilling engineer would drill from surface to the target depth without setting casing at all. However, it is normally the case that several casing strings will have to be run into the well in order to reach the objective formations. These strings must be run concentrically with the largest diameter casing being run first and smaller casing strings being used as the well gets deeper. The sizes and setting depths of these casing strings depends almost entirely on the geological and pore pressure conditions in the particular location in which the well is being drilled. some typical casing string configurations used throughout the world are shown in Figure 1.
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In view of the high cost of casing, each string must be carefully designed. This design will be based on the anticipated loads to which the casing will be exposed. When drilling a development well, these loads will have been encountered in previous wells and so the casing programme can be designed with a high degree of confidence, and minimal cost. In an exploration well, however, these loads can only be estimated and problems may be encountered which were not expected. The casing design must therefore be more conservative and include a higher safety margin when quantifying the design loads for which the casing must be designed. In addition, in the case of an exploration well, the casing configuration should be flexible enough to allow an extra string of casing to be run, if necessary. A well drilled in an area with high pressures or troublesome formations will usually require more casing strings than one in a normally pressured environment (Figure 2). |