EXPLOTATION OF FLOWING WELLS

By flowing or gushing of an oil well is meant the process of oil motion from the bottom hole to the head of the well under the action of the formational energy.

Natural flowing of an oil well is possible only if its bottom hole pressure is greater than the hydrostatic pressure exerted on the bottom hole by the weight of the column of gas-oil mixture rising to the well head. The gas which evolves from the oil when it approaches the heard of the well is exceedingly important for flowing. The numerous bubbles of gas dispersed in the oil not only reduce its specific gravity, but also actively participate in lifting the oil, since they entrain particles of the surrounding liquid when they float to the surface.

The energy of the expanding gas is best used by fitting flowing wells with small diameter tubings which are usually lowered almost to the bottom of the well, to the perforated interval. The gas disperses uniformly over the entire cross section of the tubing as it rises and also carries along the oil near the walls of the tubes. In tubings with a great diameter partial separation of the oil and gas occurs: the oil is more saturated with gas bubbles at the central section of the tube and it moves with a greater velocity, while the oil is much less mobile at the walls of the tube, it becomes gradually degassed, and, mixing with fresh oil, increases its specific gravity, thus creating additional back pressure on the stratum. This is why flowing wells are no longer operated by withdrawing oil directly through the casing string.

When a flowing well is in operation, an excess pressure, called the buffer pressure, appears at the well head. It is approximately equal to the difference between the bottom hole pressure and the hydrostatic pressure created by the weight of the column of the oil and gas-oil mixture filling the tubing. The buffer pressure characterizes to a certain extent the productive capacity of wells at a given oil field.

The oil and gas are arranged separately in the annular space, i.e., in the annular clearance between the production casing and the tubing. There is always a gas «cushion» – free compressed gas – in the upper part of the annular space, while its remaining part is filled with oil. Almost the entire volume of the annular space becomes filled with compressed gas, if oil and gas flow from the stratum into the well in a free state.

The pressure of the gas in the annular space is determined by the difference between the bottom hole pressure in the operating well and the hydrostatic pressure created by the column of fluid in if. This quantity does not characterize operation of the well, but does give an idea of the distribution of oil and gas in the annular space.

As oil rises to the well head along the tubing, the evolution of gas from it is accompanied by a number of secondary phenomena. In particular the composition of the oil changes and it cools down. When paraffin base oil is involved cooling of the oil results in paraffin deposition on the walls of the tubes.

An increase in the volume of the free gas results in an increase in the velocity of the gas-oil mixture, and it is much higher at the well head than lower in the well.

Список литературы

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