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.