Process for pumping a gas/liquid mixture in an oil extraction well and device for implementing the process

The present invention relates to a process for pumping a two-phase gas/liquid mixture in an extraction well (11) whose initial gas percentage is greater than approximately 40% by volume, characterized in that it consists: PA1 in reducing the percentage of free gas to below 40% by the use of at least a first centrifugal separating module (3, 4); PA1 in cooling the drive motor by means of an annular flow of the mixture emerging from the first separating module (3, 4) around the motor (9), the proportion of gas in the said mixture having been brought down to below 40% in order to increase its heat capacity and its speed of passage around the motor; PA1 in reducing to below approximately 10% the percentage of gas by volume by the use of at least a second centrifugal separating module (14); PA1 in pumping the fluid thus obtained by means of a centrifugal pump (16) driven by the motor (9).

The present invention relates to a process for pumping a gas/liquid mixture 
in an oil extraction well and to the device for implementing the process. 
The production of hydrocarbons in an oil well is carried out, either within 
the framework of a naturally eruptive well, in a natural manner, or in an 
artificial manner, and in this case the well must be activated. In the 
first case, the base pressure is sufficient to enable the fluid to rise to 
the surface. In the second case, the pressure is insufficient to permit 
extraction, which requires a means of assistance to ensure that the fluid 
rises to the surface. The well is then activated. 
Moreover, after a certain exploitation time, eruptive wells are no longer 
eruptive and they must also be activated. 
Consequently, in order to effect the exploitation of the fluids formed by 
the hydrocarbons, various activation techniques are used, such as: 
gas lift (injection of gas at the bottom); 
alternate pumping; 
centrifugal pumping; 
jet-effect pumping, etc. 
Each of these various activation means will be used as a function of the 
characteristics of the well and of the range of application of the means. 
Thus, gas lift will be used when the fluid is already gassed, or, 
conversely, pumping will not be used if the gas quantities are large. 
Immersed electrical centrifugal pumping is one of the conventional means 
and is widespread. The conventional assembly is composed of a 
multicellular centrifugal pump, an electric motor and a protector located 
between the motor and the pump and whose role is to ensure a seal around 
the drive shaft in such a manner that the external fluids do not penetrate 
into the motor. 
However, this type of material has limitations due in particular to the 
proportion of gas contained in the mixture to be extracted from the well. 
Thus, when the proportion in the mixture produced reaches values of 10% by 
volume relative to the total effluent, the centrifugal pump is no longer 
able to function. The percentages mentioned in the text are the 
percentages by volume at the pressure and temperature base conditions. 
This disadvantage considerably limits the uses of centrifugal pumping 
according to the characteristics of the well, and, in order to tackle this 
problem, a centrifugal separator has been designed and used which, placed 
upstream of the pump between the latter and the motor, makes it possible 
for a partial separation of the gas to be achieved. 
When the flow rates are relatively low, less than 300 to 400 m.sup.3 per 
day, such a system makes it possible to obtain a partial elimination of 
the gas, which will permit normal operation of the pump. This is possible 
provided that the percentage of free gas in the initial mixture is less 
than approximately 40% by volume. In this case, the separator brings the 
percentage of free gas down to approximately 10%. 
In the case of mixtures which are greater than approximately 40%, besides 
the fact that the above device is no longer operating, the heat capacity 
of the mixtures is insufficient, and the cooling of the electric motor can 
no longer be ensured satisfactorily. 
A first object of the invention is therefore to propose a process for 
pumping a two-phase gas/liquid mixture which makes it possible to effect 
the exploitation of the well whose percentage of free gas in the initial 
mixture is greater than approximately 40% by volume. 
This object is achieved by the fact that the process for pumping a 
two-phase gas/liquid mixture in an extraction well whose initial gas 
percentage is greater than approximately 40% by volume, is characterized 
in that it consists: 
in reducing the percentage of free gas to below 40% by the use of at least 
a first centrifugal separating module; 
in cooling the drive motor by means of an annular flow of the mixture 
emerging from the first separating module around the motor, the proportion 
of gas in the said mixture having been decreased in order to increase its 
heat capacity and its speed of passage around the motor; 
in reducing to below approximately 10% the percentage of gas by volume by 
the use of at least a second centrifugal separating module; 
in pumping the fluid thus obtained by means of a centrifugal pump driven by 
the motor. 
A second object of the invention is to propose a device enabling the 
process to be implemented and which is capable of solving both the problem 
of pumping mixtures whose quantities of gas may be as much as 99% by 
volume and the problem of cooling the drive motor of the device. 
This object is achieved by the fact that the device comprises: 
a first module, which is a separator of gases in a mixture, is centrifugal, 
has an axial flow, is arranged in a cylindrical casing, and whose 
separated gases are discharged outside the casing; 
a cooling module comprising an outer casing connected in a sealing manner 
to the casing of the first separating module and comprising on the inside 
a second coaxial cylindrical casing containing an electric motor which is 
surrounded on either side, in the longitudinal direction, by protectors 
ensuring a seal towards the upstream and downstream sides at the level of 
the drive spindles of the motor; 
means at the entry of the cooling module, for diverting the axial flow of 
the first separating module outside the second casing, and 
means at the exit of the cooling module for bringing the flow along the 
axis of a centrifugal pump module connected to a pipe string for 
discharging the fluid leaving the cooling module, the said centrifugal 
pump module and the separating module being driven by the spindle of the 
motor. 
Another object is to adapt the system as a function of the characteristics 
of the well by the use of either a modular system or of a fixed system 
having a variable drive control. 
This object is achieved by the fact that the device comprises at least a 
second centrifugal separating module between the cooling module and the 
centrifugal pump module. 
According to another feature, this object is achieved by the fact that the 
first module comprises at least two centrifugal separators mounted in 
series, so that the axial exit flow of the one constitutes the entry flow 
of the second, and means for coupling the first and second separator in 
rotation. 
According to another feature, the coupling means consist of an 
electromagnetic clutch. 
According to another feature, the device comprises at each end a device for 
centering relative to the extraction well.

FIG. 1A shows the upstream part of the pumping device which enables the 
process according to the invention to be implemented. This device 
comprises a centering element (1), which may or may not be sealed, the 
central pipe (100) of which leads the gas and fluid mixture to a first 
centrifugal separator (3), whose gas discharge exit (30) discharges the 
gases into the annular space between the outer cylindrical casing (32) of 
the separator and the pipe (11) constituting the wall of the extraction 
well. The gas/fluid mixture whose percentage has been reduced by the first 
separator is discharged through an axial orifice (31), in the direction of 
a second separator (4), with a view to a further reduction in the 
percentage. 
This separator (4) discharges the gas through an orifice (40) into the 
annular space, and the gas/fluid mixture through an axial orifice (41), in 
the direction of a cooling module (10) constituted by an element (6) which 
diverts the axial flow of the separator (4) through lateral orifices (60) 
towards an annular space formed between an outer pipe (10 A) and the 
successive outer pipes (80, 90, 120) respectively of the protecting 
modules (8), motor (9) and protector (12) which are mounted inside and 
coaxially with the pipe (10 A), thus forming the cooling module (10). 
The protecting modules (8, 12) make it possible to ensure a seal at the 
level of the output shafts of the motor (9) towards the upstream side and 
towards the downstream side. Hereby, the motor element (9) is protected 
from contact with the fluids which circulate in the device. Moreover, the 
fluid flowing in the annular space formed between the pipe (10) and the 
outer casings (80, 90, 120) which form respectively the first protector, 
the motor and the second protector make it possible to ensure a cooling of 
the motor which is all the more efficient since the percentage of gas in 
the mixture has been brought down to as low a level as possible below 40%. 
At the downstream end of the cooling module, a diverting module (13) 
enables, by virtue of the orifices (130), the flow to be brought axially 
into the separating element (14) which follows the cooling module. 
This separator (14), which is similar in composition to the other 
separators, discharges the gas through the orifice (140) towards the 
annular space between the outside of the casing (142) of the separating 
device and the pipes (11) constituting the wall of the extraction well. 
This separator (14) discharges the axial flow of the mixture towards a 
centrifugal pump (16) via the axial orifice (141). The exit of the 
centrifugal pump (16) is connected to an assembly of pipes (18), which 
makes it possible for the liquid, which is virtually separated from its 
gas, to rise to the surface. 
A centering device (17) can also be used at the exit of the device. 
The motor (9) drives by means of drive shafts which extend inside the 
device, both towards the separators located upstream and downstream and 
towards the centrifugal pump. 
During operation, the two-phase mixture (2) penetrates into the system, and 
a part of the gas is separated and discharged via the annular space at the 
level of the first centrifugal separator (3) having axial flow. 
The remaining mixture penetrates into the second separator (4), where the 
same operation is carried out. For a flow rate of the order of 200 m.sup.3 
per day, using a separator having a diameter of 125 mm driven at 3,000 
revolutions/minute, and the percentage of free gas as the drawing means 
(2) being 99%, it will be possible for the percentage at the exit of the 
first separator (3) to be brought down to approximately 60%. The second 
separator will bring the percentage of gas to approximately 30%. 
The fluid is thus sufficiently degassed so as to have a heat capacity which 
is sufficient for ensuring an efficient cooling of the motor. The fluid 
leaving the second separator (4) passes into the cooling module of the 
motor and subsequently penetrates into the third separator (14) in order 
to terminate its journey in the centrifugal pump and to then be discharged 
up to the surface, inside the pipe string (18). The gas in turn reaches 
the surface via the annular space formed between the pipe string (18) and 
the pipes (11) constituting the wall of the extraction well. The third 
separator will bring the percentage of gas, which is 30% at the entry, to 
a percentage which is compatible with the smooth running of the pump (16), 
generally less than 8%. 
It is clearly evident that the third separator (14) is optional and depends 
on the percentage of gas contained in the initial two-phase liquid. Thus, 
in the case of an initial two-phase mixture whose gas percentage is 
slightly greater than 70%, the two separators (3, 4) will be used, but it 
may be possible to dispense with the last separator (14). On the other 
hand, in the case of a two-phase mixture between 70 and 40%, only a single 
separator (3) upstream of the motor and a second separator (14) downstream 
of the motor will be used. 
In the variant shown, the drive shafts of the first and second separators 
(3, 4) are connected mechanically in rotation by means of a muff (33). 
In an alternative embodiment of the invention, it will be possible for 
these shafts to be connected mechanically by means of an electromagnetic 
clutch controlled from the surface so as to implement, as required, one or 
two separating modules upstream of the motor. 
FIG. 2 shows a sealed centering element (1) used upstream of the device. 
This centering element consists of anchoring chocks (101) connected, on 
the one hand, to the outer pipe (11) of the extraction well by means of 
seals (103), and, on the other hand, to the inner pipe (100) for drawing 
the two-phase liquid (2), by means of gaskets (102), so as to channel the 
two-phase mixture towards the inside of the pipe (100). 
The unsealed centering device (17) located downstream of the pumping device 
will solely comprise spacers for supporting the pipes (18) to enable the 
gas to flow outside the pipe (18). 
It is clearly evident that, depending on the case, an unsealed centering 
piece (1) may be used. Similarly, it will be possible, if appropriate, to 
use a sealed centering piece for the exit centering piece (17).