REASONS WHY WELLS STOP FLOWING

The flow of an oil well may stop for the following reasons: a reduction in the formational pressure, flooding of the well, or clogging of the tubing or discharge lines. Characteristic changes in the behaviour of the well are observed in each case.

A reduction in the formational pressure is accompanied by a gradual drop of me buffer pressure and of the rate of production. It is usually possible to maintain the rate of production for while by installing a choke of greater diameter, but when the buffer pressure becomes equal to 2–4 kg/cm², this measure does not help; the rate of production continues to drop, and operation of the well becomes irregular – the buffer pressure begins to fluctuate owing to an increase in the quantity of gas evolving from the oil in the upper section of the tubing. This is especially noticeable in wells that have opened up thin strata.

During this period of operation it is good to use a choke of a somewhat smaller diameter; this will result in only a slight drop in the rate of production, but pressure fluctuation will be less pronounced, thus facilitating attendance to the well.

The flooding of flowing wells is usually accompanied by the same phenomena observed when the formational pressure drops – a gradual reduction of the buffer and annular space pressures, a drop in the oil production and eventual pressure, fluctuation are observed. These phenomena are caused by the increase of the back pressure on the stratum due to the accumulation of stratum water in the bottom hole, and also to the weight of the gas-oil mixture in the tubing (the oil rising to the well head contains numerous drops of water).

The content of water in oil at which a well stops flowing may vary considerably, from 2–5 to 30–40 per cent, depending on the conditions characterizing me given oil field. Flooded wells sunk in the edge area of a reservoir from which oil is recovered with artificial maintenance of the formational pressure can flow for a long time.

With progressing flooding, the flow of a well becomes periodic with long interruptions. The well stops flowing when the total hydrostatic pressure of the column of water filling the bottom hole and the column of the gas-oil mixture filling the tubing exceeds the bottom-hole pressure. Normal inflow of oil into the well stops, but the column of water in the bore hole is gradually displaced by the oil which penetrates in from the oil-bearing zone of the stratum, and rises to the upper part of the well.

If the well is kept open during this time, the inflow of oil causes the level of fluid to gradually rise to the well head, and ever flow begins.

If the well is kept shut, a gas cushion forms in the upper section of the tubing and annular space, while their remaining sections are gradually filled with oil which displaces water into the stratum. The column of water may be completely displaced by oil if the well is kept shut in for a long time, and if such a well is opened, natural flow may go on for several hours or even days, until water again accumulates in the bottom hole of the well.

Such a process of water replacement by oil sometimes occurs in completely flooded wells whose further exploitation has been found unprofitable. The oil left in the pores of the flooded formation slowly penetrates into the well, during several months, replacing the water in it. This results in that a considerable pressure may appear at the well head with time. To prevent oil blowout from the well, the heads of such wells must be reliably sealed.

The natural flow of flooded wells can be prolonged by removing the column of water from the bottom hole. This is why operators usually do not wait until natural replacement of the water by oil occurs, but wash the well by pumping clean water-free oil into the annular space, which forces the water from the bottom hole into the tubing and then to the surface.

Washing is effective with a slow increase of the water content in the withdrawn fluid and a high formational pressure. After washing natural flow continues for several weeks. But this measure is, nevertheless, a temporary one, since the flooding of a well by edge water is a natural process, and it becomes inevitable with time to have to abandon operation of the well by natural flow and introduce other methods of production, or overhaul the well to isolate the flooded part of the stratum.

Clogging of the tubing, causing wells to stop flowing, is possible in oil fields composed of weakly cemented sandstone, and where paraffin – base oil is produced.

In the first case with a drop in the rate of production or when the well is shut in the sand carried from the stratum begins to settle in the tubing, owing to a decrease in the velocity of the fluid, and forms a plug in its lower section, stopping the access of oil.

The formation of a sand plug is marked by an increase in the annular space pressure and a sharp drop in the buffer pressure, with a simultaneous reduction in the rate of well production.

As soon as such symptoms are observed, measures must be urgently undertaken to ensure the restoration of normal operation- of the well. First of all circulation of the liquid through the tubing must be restored by pumping clean oil into the annular space. A certain increase in the rate of production may also be of help in such cases.

The partial or complete clogging of tubing is also possible in wells producing paraffin – base oil.

Paraffin plugs usually form around scrapers or other devices lowered into the well for paraffin removal, and jammed in the interval where the paraffin deposit is the greatest. The scraper or other tool which gets stuck in the paraffin deposit and compacts it' almost completely closes the free passage of the tubing. This is accompanied by a sharp reduction in the rate of production and the rate of production and the buffer pressure and by a noticeable rise of the annular space pressure.

 

Вариант IV

 

1. Перепишите и переведите следующие предложения, обращая внимание на сложные формы инфинитива (см. образец выполнения № 1).

1. We know the Soviet oil industry to have halted its slide in 1997.

2. The oil well to be drilled here is very perspective.

3. I am glad to be finishing this work at last.

4. Good luck to all of you out there who would have reason to be reading this page.

5. I can't tell you much about the book because you're supposed to be reading it.

6. The city also encourages employees to consider taking the bus or biking to work.

7. We always have to consider the evidence if we want to know what reality is like.

2. Перепишите и переведите следующие предложения. Помните, что объектный и субъектный инфинитивные обороты соответствуют придаточным предложениям (см. образец выполнения № 2).

1. Reserves of oil offshore Sakhalin are thought to be significant.

2. A large part of the reserves are thought to consist of heavy oil.

3. We know the Timan-Pechora basin to stretch from the Urals in the east to the Barents Sea in the north.

4. Specialists consider the region’s main drawback to be its distance from markets and the lack of transport infrastructure.

5. The future is expected to bring many new applications for ultrasonics.

6. The instruments to be used are made of copper.

7. The students watched the device work in the laboratory.

 

3. Перепишите и переведите предложения, содержащие придаточное условия. Определите тип условного предложения (см. образец выполнения № 3).

1. If somebody wants to design a car of the advanced type, he must take into account the properties of some of the new materials.

2. If oil had not been discovered in Siberia, Baku oil field could not have satisfied all the needs in fuel.

3. It would be impossible to solve this problem unless new computers were used.

4. Should he fail his examination he will be allowed to try again in a month.

5. Had we come to the station a minute earlier we should not have missed the train.

6. You could stay in my place provided you promise to leave no mess.

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SUBSURFACE EQUIPMENT

The Subsurface Equipment of a flowing well consists of the tubing and various devices, serving for bringing the well into operation.

The tubing is lowered into the well, as noted in a previous section to ensure better utilization of the energy of the accompanying gas which evolves from the oil when it rises to the surface. In addition the tubing considerably simplifies and speeds up the process of starting and shutting in of a flowing well. It is possible to force out of the well through the tubing the liquid which previously filled the well and change the pressure exerted on its bottom hole by injecting air, oil water or drilling mud into the annular space.

The lower and of the tubing is arranged directly opposite the stratum which oil is produced from. This arrangement of the tubing makes it possible to wash the bottom hole. This is of especial importance when the latter is clogged with dirt and sand, and also in coning, when a column of water accumulates in the lower section of the well, creates an additional pressure on the bottom hole and prevents the inflow of fluid from the stratum.

The diameter of the tubing is chosen depending on the expected rate of production. Tubes of smaller diameter are used in wells with low production and low formational pressure, where they permit the period of flowing to be prolonged.

In choosing tubes for a well in which paraffin may be deposited in operation, the possibility of using cleaning equipment such as scrapers inside the diameter of the tubes is so selected mat the velocity of the gas-oil flow will not be too high.

The tubes used in flowing wells are also used in wells from which oil is produced by the air – and gas-lift methods, and in pumping wells outfitted with subsurface pumps.

Taper threads with a special profile are cut on the ends of the tubes, which are joined to one another with the aid of couplings screwed on one of their ends at the manufacturing plant.

Tubing is costly equipment, and it should therefore be handled with care, preventing untimely damage and failure.

In transportation bending of tubes and sagging of their ends should be prevented. The threaded ends of tubes and couplings must be protected from damage with the, aid of metal rings and nipples which completely cover the threaded sections.

Tubes are not to be thrown down when loading and unloading then; they must be carried from place to place either by hand, holding their ends, or rolled along inclined ramps, seeing that they do not strike one another.

The threaded joints of tubes kept in storage for a long time must be given a protective coating of grease.

The preparation of tubing for lowering into a well is an important operation. It is necessary not only to measure each tube of the string, but also thoroughly check the condition of the tubes, looking for such faults as camber, the state of the internal surface and threaded joints.

The threaded joints are usually checked by external inspection. The threads must be smooth, and free of burrs, fissures, dents and other faults detracting from the strength and tightness of the joints.

When screwing tubes together see that the threaded joint is sufficiently strong. No more than two open turns of a thread are allowed on a tube screwed into a coupling. Depending on the nature of the faults serving as the cause for rejection, defective tubes are other not used at all, or are installed in less important places, for instance, in the lower section of a flowing well tubing, where the weight load on the tubes and threaded joints is not great.

Вариант V

 

1. Перепишите и переведите следующие предложения, обращая внимание на сложные формы инфинитива (см. образец выполнения № 1).

1. New cities are to be constructed in this region.

2. I am glad to have been invited to this party.

3. There are some problems to be solved immediately.

4. It suggested that I must have read all of the Belgian artist and writer's books.

5. A cracking noise made me think we might have brought home an unwilling creature.

6. Both experiences have made me want to experiment and challenge myself even more.

7. They made me wait 2 and half hours before a doctor finally agreed to examine me.

 

2. Перепишите и переведите следующие предложения. Помните, что объектный и субъектный инфинитивные обороты соответствуют придаточным предложениям (см. образец выполнения № 2).

1. We know the geological prospecting to be developing now.

2. Decreasing of volumes of oil extraction is said to continue till 1996.

3. Samotlorskoye deposit is known to be one of the top ten largest deposits of the world concerning oil reserves.

4. We know the existence of oil and gas resources in Siberia to have been predicted by I.M. Gubkin in 1934.

5. We know a laser to be a device that stimulates the electrons of a light-producing material to vibrate simultaneously.

6. Lasers are known to have found an application in the printing industry.

7. Light waves are now believed to be due to the motions of electrons themselves within the atoms.

8. Chemical investigations have shown each element to combine with another only in definite proportions.

 

3. Перепишите и переведите следующие предложения, содержащие придаточное условия. Определите тип условного предложения (см. образец выполнения № 3).

1. If this article were on radio-electronics we should read it without dictionary.

2. It would have been impossible to launch space rockets, if we had not had the necessary polymeric materials and synthetic fuels.

3. If oil is found in a region, the oil extractive and oil processing enterprises are created.

4. If liquids expand upon freezing, an increase of pressure lowers the freezing point.

5. If the metal had been heated slowly, the first changes in its appearance would have occurred at a temperature of 1000 К.

6. It would be impossible to determine the properties of these materials without intensive studies in our research laboratory.

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