Straight Hole Drilling

Frequently it is necessary to drill a straighter hole than was originally planned when the job was started with a certain string of drill collars. Also, it may happen that it is desired to put more weight on the bit without increasing the deviation. The best single proven way to do either of these things with the same string of drill collars is to add stabilizers. For deviation control a single stabilizer is sufficient if it is properly located in the drill string.

The proper location is the first bend in the drill string above the bit. Since the bending point depends on the hole size, drill size and weight on the bit, it may occur at different points. To some extent the placement is also dependable on the formation characteristics.

It has been considered good practice in some cases to space additional stabilizers about one stand apart through a portion or all of the rest of the drill collar string. The additional stabilizers do not help to keep the hole straight. They do help in preventing wall sticking and other problems associated with hole conditions.

With the use of stabilizers, it is important to remember that stabilizers are usually the weakest point in the drill string and, therefore, subject not only to wear but failure. Some of the more modern stabilizers have been designed so that the wings do not create a serious hazard in the hole. For example, some of these are made of drillable material and some may be removed with washover pipe.

By reducing the weight on the bit, the bending characteristics of the drill string are changed and the hole will tend to be straighter. One of the oldest techniques for straightening a hole was to reduce the weight on the bit and speed up the rotary table. In recent years it has been found that this is not always the best procedure because reducing the bit weight sacrifices considerable penetration rate.

The straightening of a hole by reducing bit weight should be done very gradually so that the hole will tend to return to vertical without sharp bends and will therefore be much more safer for future drilling.

Notes:

Bending point - точка перегиба

Penetration rate - скорость проникновения

Space - помещать, размещать

Washover pipe - обурочная труба

Text 3. Well Design[2]

Well design (fig. 1) can be described as pipeline from the ground surface to the oil bearing formation. Oil comes to the surface through this pipeline. Such pipeline includes a number of special pipes connected between each other and called a casing string.

Casing strings (fig. 1) run in the hole have specific purposes and include:

1) сonductor;

2) surface casing;

3) intermediate casing;

4) intermediate liner;

5) production casing;

6) production tubing.

The first and the largest in diameter casing string is called conductor. This string can be installed in the well or driven in soil.

Surface casing is the second string run in the hole and cemented. It is installed to the depth, which is sufficient in order to protect the well from underground waters and collapse of surface soil.

Another smaller in diameter casing string can be applied. It is called intermediate casing. If the well is shallow, intermediate casing, as a rule, is not used. Intermediate casing strings should be run in the hole when it is not feasible to drill the well to the design depth without preliminary isolation of problematic zones (blowouts, collapses). The last casing string called production casing is installed from the surface, passes inside conductor, surface casing and intermediate casing to the formation pay zone or below it.

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