Secondary oil recovery

1. A process for injecting water into a subterranean petroleum-bearing formation for petroleum recovery, said method comprising:

a) positioning a selective membrane between an aqueous solution and formation water having a higher solute concentration than the aqueous solution, such that water-passes across the membrane by osmosis to dilute the formation water,

b) injecting the diluted formation water into the petroleum-bearing formation,

c) recovering formation water from the petroleum-bearing formation, and

d) using at least a portion of the recovered formation water in step a).

2. The process as claimed in claim 1 , wherein the aqueous solution is seawater, fresh water or a waste stream from an agricultural or industrial process.

3. The process as claimed in claim 1 , wherein the formation water is treated by filtration prior to injection into the petroleum-bearing formation.

4. The process as claimed in claim 1 , which further comprises the step of removing water from at least a portion of the recovered formation water prior to recycling said portion of recovered formation water to step a).

5. The process as claimed in claim 1 , which further comprises introducing at least one additive selected from antimicrobial agents, ant-corrosion agents, surfactants, emulsifiers, scale inhibitors and pH stabilisers to the petroleum-bearing formation.

6. The process as claimed in claim 5 , wherein the at least one additive is added to the formation water before it is introduced into the petroleum-bearing formation.

7. The process as claimed in claim 1 , wherein, prior to step a), at least one solute is added to the formation water to increase its solute concentration further relative to liquid.

8. The process as claimed in claim 7 , wherein the solute is selected from barium chloride and/or calcium chloride.

9. The process as claimed in claim 1 , which comprises:

dissolving at least one solute in water to provide a drive solution having a higher solute concentration than the liquid,

positioning a selective membrane between the aqueous solution and the drive solution, such that water passes across the membrane by osmosis to dilute the drive solution,

injecting the diluted drive solution into the petroleum-bearing formation, causing the solution to mix with formation water in the formation,

recovering formation water from the petroleum-bearing formation, and

using the recovered formation water in step a) of the process.

10. The process as claimed in claim 9 , wherein the solute is selected from barium chloride and/or calcium chloride.

11. The process as claimed in any claim 1 , wherein a waste stream from an industrial process is mixed with the formation water before or after step a).

12. An apparatus for injecting water into a subterranean petroleum-bearing formation for petroleum recovery, said apparatus comprising:

a housing comprising a selective membrane for separating an aqueous solution from formation water having a higher solute concentration than the aqueous solution and configured to allow water from the aqueous solution to pass across the membrane by osmosis to dilute the formation water,

means for injecting the diluted formation water into the petroleum-bearing formation,

means for recovering formation water from the petroleum-bearing formation, and

means for introducing the recovered formation water into the housing.