DOWNHOLE CYCLONE SEPARATION An increasingly important technique in producing oil from high cut water wells involves carrying out a pre-separation of the oil and water down hole, pumping the oil component to the surface and injecting the water component back into the well. However, with existing techniques, the oil component which is pumped to the surface is still very wet and there is a need to reduce the water content of the oil component further. It is possible to separate the fluid to produce a much drier oil component for pumping to the surface, by utilizing as the pre-separator a hydrocyclone which is appropriately set up by the choice of overflow and underflow orifices sizes and working pressures, but only if a significant proportion of oil is accepted in the water component leaving the underflow of the hydrocyclone. This is not however generally acceptable as the water to be reinjected into the production zone should be as clean as possible.

We have appreciated that this problem can be overcome by pumping production fluid to a first hydrocyclone which is set up to provide a substantially dry oil component at its overflow, from which the oil component is pumped to the surface, a dirty water component from the underflow of the first hydrocyclone being fed to the inlet of a second hydrocyclone which is set up to produce a substantially clean water component at its underflow, form which the water component is reinjected into the well, a dirty water component provided at the overflow of the second hydrocyclone being either united with the production fluid for recycling through the first hydrocyclone, or pumped to the surface separately from or united with the substantially dry oil stream from the first hydrocyclone.

This method enables the amount of water which is pumped to the surface to be reduced whilst maintaining substantially clean water for reinjection into the production zone.

Two pumps are needed, one to pump the production fluid into the inlet of the first hydrocyclone, this pumping being necessary to ensure that the

substantially clean water component leaving the underflow of the second hydrocylone is at a higher pressure than that of the production fluid. The other pump is necessary for pumping the substantially dry oil component to the surface.

The invention also includes an apparatus for use downhole in an oil well, the apparatus comprising first and second hydrocyclone, the underflow of the first hydrocyclone being connected to the inlet of the second hydrocyclone ; a first pump for pumping production fluid into the inlet of the first hydrocyclone ; and a second pump for pumping liquid form the overflow of the first hydrocyclone, in use to the surface; the overflow of the second hydrocyclone being connected to the inlet of the first pump, or to the inlet of the second party, and the underflow outlet of the second hydrocylone being arranged to discharge, in use, into the well.

The invention is illustrated by way of example in the accompanying drawings, in which: Fig. 1 is a schematic side elevation of apparatus in accordance with the invention mounted within a well bore; and, Fig. 2 is a diagram showing the interconnections between the hydrocyclones and pumps.

As shown in Fig. 1 the apparatus A is mounted within a conventional well bore 3, having production perforations 4 and injection perforations 5 separated by a packer 6. In use production fluid flows through the perforations 4 into the well bore and is them pumped by an injection pump 7 into the inlet of a first hydrocyclone 8. The substantially dry oil component leaves the overflow of the hydrocyclone 8 through a line 9 which leads to the inlet of a production pump 10 which pumps the oil component through a line 11 to the surface.

The underflow of the hydrocyclone 8 is connected to the inlet of a second hydrocyclone 12, the overflow of which is either discharged into the well bore to be mixed with the production fluid as indicated by an arrow 13, or is fed through a conduit indicated by the chain dotted line 14 to the inlet of

the injection pump 7, and hence is recycled through the first hydrocyclone 8.

The substantially clean water leaving the underflow of the hydrocyclone 12 is discharged below the packer 6, as indicated by the arrows 15, and hence through the injection perforations 5 back into the production zone.

Both pumps 7 and 11 are driven by a common motor 16 separated from the motors by conventional protectors 17 and 18.

The connections between the hydrocyclones 8 and 12 and the pumps 7 and 10 are shown more clearly in Fig. 2.

In order to give an indication of the sort of ratios which are intended when referring to a substantially dry oil component and a substantially clean water component, the following oil/water ratios may be helpful. Thus if in unit time the incoming production fluid contains 20 units by volume of oil and 80 units of volume of water, and the dirty water leaving the overflow of the second hydrocyclone 12 for recirculation with the production fluid amounts to one part oil and four parts water, the pump 7 will pump 21 parts oil and 84 parts water into the inlet of the first hydrocyclone 8. As acceptably dry oil component leaving the overflow of the first hydrocyclone 8 might then contain 20 parts oil and four parts water, the remaining one part oil and 80 parts water leaving the underflow of the hydrocylone 8 and entering the inlet of the second hydrocyclone 12. Allowing for the continued recycling of one part oil and four parts water from the overflow of the second hydrocyclone 12, this leaves 76 parts water to leave the underflow of the second hydrocyclone 12 for reinjection into the well.