Method for secondary oil recovery
A method and a system for treating seawater to facilitate secondary recovery of petroleum from a location below a body of saline water by flooding a permeable stratum with the treated water. The system utilizes a spirally-wound cross flow semipermeable membrane cartridge incorporating a three layer membrane arrangement preferably having nanofiltration characteristics which facilitates treatment of open ocean water by a single pass without any chemical or mechanical pretreatment to result in a liquid stream of desired character that can have greater ionic strength than the input seawater, e.g. at least about 18,000 ppm of chloride ion and less than 50 ppm of sulfate ion. This surprising objective is obtained even at high operating temperatures, e.g. as high as about 70° C., through the combination of the construction of the feed spacer and the nanofiltration character of the three layer membrane arrangement.
1. A method for the secondary recovery of petroleum from a location below a body of seawater by flooding a permeable petroleum-bearing stratum, which method comprises
treating seawater from said body at a temperature between about 22° C. and about 75° C. to remove at least about 99% of all divalent anions and to simultaneously increase the chloride ion content of said seawater by at least 10% by pumping said seawater on a single pass through a nanofiltration membrane formed of a polyamide film created by the condensation reaction of a piperazine and an acyl chloride which film is supported upon a porous film of a different polymeric material, and
then pumping said treated seawater under pressure into said permeable stratum.
2. The method according to claim 1 wherein said porous supporting film is a polyurethane film about 1500-2000 angstroms thick.
3. The method in accordance with claim 1 wherein seawater at a temperature of about 30° C. or above is treated in a single pass through said membrane to provide a permeate containing at least about 18,000 ppm of chloride ion and less than 50 ppm of sulfate ion.
4. The method according to claim 3 wherein seawater is pumped to said membrane at a flow rate such that the rate of flow of concentrate is no more than about 20% of the rate of flow of permeate.
5. The method according to claim 1 wherein said seawater is pumped to said membrane at a pressure between about 150 and about 200 psig and provides a permeate flux of at least about 20 gallons/sq. foot/day.
6. The method according to claim 1 wherein said membrane is present in sheet form as a part of a spirally-wound cartridge.
7. The method according to claim 6 wherein said seawater being treated is pumped through spacers in the form of parallel ribs that provide about 3 to 10 axially extending channels per inch, which ribs are interconnected by transverse filaments of lesser size than said ribs, said spacers creating a concentration-polarization-enhancing effect at the feed surface of said membrane, which effect also keeps Ca sulfates from precipitating onto said membrane, negating the need for any ultrafiltration prefiltering or anti-sealant chemicals.