2. Write down the transcription of the following words and practise their pronunciation:

• unfortunately

• to justify

• approximately

• annulus

• to divert

• blooey line

• fire-extinguishing equipment

• reciprocating compressors

• silicon tetrafluoride

• to alter­nate

3. Read and translate the text.

1) In areas where formations do not contain too much water and will not slough, air or gas drill­ing permits the fastest penetration rates. The lack of hydrostatic pressure keeps the hold-down effect to a minimum; in fact, a negative hold-down effect (from formation to borehole) may occur with air circulation. High-speed photographs have shown that chips seem liter­ally to explode from the bottom of the hole after being barely touched by the bit teeth, causing dull bits to drill just as fast as sharp ones. Footage per bit and overall rates of penetration are usually much better with air or gas than with water or mud.

There are other advantages to air drilling. Rig time can be saved. Protective casing can be set at a shallower depth. Expenses for casing, mud, and extra time and money spent as a result of lost circulation can all be minimized. When for­mations become too porous to support a column of water, air may be a suitable choice for circula­tion.

Unfortunately, there are relatively few places where formation properties make air or gas drilling feasible. Also, the special equipment needed for air or gas work may be too expensive to justify the savings made possible by drilling faster hole. In some cases, an air-drilled hole would have to go down twice as fast as a mud-drilled hole to justify the cost of air drilling modifications to a conventional rig. Expensive foaming agents may also be required.

2) The use of air or gas as a circulating fluid requires making certain changes in rig equipment. Among the additional items needed for air drilling are:

• compressors, with approximately the same power output as the mud pumps;

• an air connection to the standpipe;

• a device such as a rotating head to seal off the annulus between the drill pipe and the top blowout preventer to divert the air flow into the flow line;

• a flow line, or blooey line, to carry air returns to the reserve pit, extended some 200 feet from the rig and equipped with a flare;

• a manifold on the air input to permit chemical injection;

• additional fire-extinguishing equipment as a safety measure in case of oxygen-gas mixtures in the well; and

• air-cooling equipment and personnel with experience in air drilling operations to see that air cooling is handled properly.

The addition of air compressors introduces new maintenance problems for the toolpusher. Compressors can overheat, discharge valves on reciprocating compressors may break, intake filters may become clogged, and lubrication problems can become severe.

3) The presence of water does not prohibit air drilling, but it does make it more difficult. And though shallow wells drilled in geologically old formations may pose no prob­lems at all, wells of any considerable depth will encounter water sooner or later.

In air drilling, the cuttings return to the sur­face in a cloud of dust. If the bit penetrates a water-bearing formation, the natural result is that the cloud of dust is con­verted into mud. When there is only a slight flow of water, the mixture formed is not a liquid mud that flows like circulating mud, but rather a plastic mass that may plug up the bit and the annulus.

There are several ways to remedy this situa­tion. More water can be injected into the hole, converting the plastic mass into a more man­ageable fluid that can be blown out of the hole. Foaming agents, which convert the intruding water into a stiff but manageable foam, are sometimes used. A gas such as silicon tetrafluoride may be squeezed into the water-bearing formation, to react with the water and form a seal that inhibits the entry of formation water.

Determining whether to use air drilling depends on the drilling situation. When the pros and cons are weighed, a compromise that alter­nates air and mud for circulation may be used. As in all decisions affecting the drilling pro­gram, total well cost is the important thing to keep in mind.