Бурение грунтовых зондов, установка энергетических колодцев

When the casing configuration (casing size and setting depth) has been selected, the loads to which each string will be exposed will be computed. Casing, of the required size, and with adequate load bearing capacity will then be selected from manufacturer’s catalogues or cementing company handbooks.

Casing joints are manufactured in a wide variety of sizes, weights and material grades and a number of different types of connection are available. The detailed specification of the sizes, weights and grades of casing which are most commonly used has been standardised by the American Petroleum Institute — API. The majority of sizes, weights and grades of casing which are available can be found in manufacturer’s catalogues and cementing company handbooks (e. g. Halliburton Cementing Tables).

Casing is generally classified, in manufacturer’s catalogues and handbooks, in terms of its size (O. D.), weight, grade and connection type:

1.6 Casing Size (Outside Diameter — O. D.)

The size of the casing refers to the outside diameter (O. D.) of the main body of the tubular (not the connector). Casing sizes vary from 4.5" to 36" diameter. Tubulars with an O. D. of less than 4.5” are called Tubing. The sizes of casing used for a particular well will generally be limited to the standard sizes that are shown in Figure 3. The hole sizes required to accommodate these casing sizes are also shown in this diagram. The casing string configuration used in any given location e. g. 20” x 13 3/8” x 9 5/8” x 7” x 4 1/2” is generally the result of local convention, and the availability of particular sizes.

1.7 Length of Joint

The length of a joint of casing has been standardised and classified by the API as follows:

Although casing must meet the classification requirements of the API, set out above, it is not possible to manufacture it to a precise length. Therefore, when the casing is delivered to the rig, the precise length of each joint has to be measured and recorded on a tally sheet. The length is measured from the top of the connector to a reference point on the pin end of the connection at the far end of the casing joint. Lengths are recorded on the tally sheet to the nearest 100th of a foot. Range 2 is the most common length, although shorter lengths are useful as pup joints when attempting to assemble a precise length of string.

1.8 Casing Weight

For each casing size there are a range of casing weights available. The weight of the casing is in fact the weight per foot of the casing and is a representation of the wall thickness of the pipe. There are for instance four different weights of 9 5/8" casing:

although there are strict tolerances on the dimensions of casing, set out by the ApI, the actual I. D. of the casing will vary slightly in the manufacturing process. for this reason the drift diameter of casing is quoted in the specifications for all casing. The drift diameter refers to the guaranteed minimum I. D. of the casing. This may be important when deciding whether certain drilling or completion tools will be able to pass through the casing e. g. the drift diameter of 9 5/8” 53.5 lb/ft casing is less than 8 1/2" bit and therefore an 8 1/2” bit cannot be used below this casing setting depth. If the 47 lb/ft casing is too weak for the particular application then a higher grade of casing would be used (see below). The nominal I. D. of the casing is used for calculating the volumetric capacity of the casing.

1.9 Casing Grade

The chemical composition of casing varies widely, and a variety of compositions and treatment processes are used during the manufacturing process This means that the physical properties of the steel varies widely. The materials which result from the manufacturing process have been classified by the API into a series of “grades” (Table 3). each grade is designated by a letter, and a number. The letter refers to the chemical composition of the material and the number refers to the minimum yield strength of the material e. g. N-80 casing has a minimum yield strength of 80000 psi and K-55 has a minimum yield strength of 55000 psi. Hence the grade of the casing provides an indication of the strength of the casing. The higher the grade, the higher the strength of the casing.

Institute of Petroleum Engineering, Heriot-Watt University 9

In addition to the API grades, certain manufacturers produce their own grades of material. Both seamless and welded tubulars are used as casing although seamless casing is the most common type of casing and only H and J grades are welded.

1.10 Connections

Individual joints of casing are connected together by a threaded connection. These connections are variously classified as: API; premium; gastight; and metal-to — metal seal. In the case of API connections, the casing joints are threaded externally at either end and each joint is connected to the next joint by a coupling which is threaded internally (Figure 5). A coupling is already installed on one end of each joint when the casing is delivered to the rig. The connection must be leak proof but can have a higher or lower physical strength than the main body of the casing joint. A wide variety of threaded connections are available. The standard types of API threaded and coupled connection are:

• Short thread connection (STC)

• Long thread connection (LTC)

• Buttress thread connection (BTC)

In addition to threaded and coupled connections there are also externally and internally upset connections such as that shown in Figure 4. A standard API upset connection is:

• Extreme line (EL)

The STC thread profile is rounded with 8 threads per inch. The LTC is similar but with a longer coupling, which provides better strength and sealing properties than the STC. The buttress thread profile has flat crests, with the front and back cut at different angles. Extreme line connections also have flat crests and have 5 or 6

threads per inch. The EL connection is the only API connection that has a metal to

metal seal at the end of the pin and at the external shoulder of the connection, whereas all of the other API connections rely upon the thread compound, used to make up the connection, to seal off the leak path between the threads of the connection.

In addition to API connections, various manufacturers have developed and patented their own connections (e. g. Hydril, Vallourec, Mannesman). These connections are designed to contain high pressure gas and are often called gastight, premium and metal-to-metal seal connections. These connections are termed metal-to-metal seal because they have a specific surface machined into both the pin and box of the connection which are brought together and subjected to stress when the connection is made up.

Surveys have shown that over 80% of leaks in casing can be attributed to poor make­up of connections. This may be due to a variety of reasons:

• Excessive torque used in making-up the connections

• Dirty threads

• Cross-threading

• Using the wrong thread compound.

The casing string should be tested for pressure integrity before drilling the subsequent hole section. Most of the causes of connection failure can be eliminated by good handling and running procedures on the rig.

The recommended make-up torque for API connections is given in API RP 5C1. These recommended torques are based on an empirical equation obtained from tests using API modified thread compound on API connections. The recommended make up torque for other connections is available from manufacturers.

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