Subsurface water purification method

A subsurface water purification method for producing purified water, comprising:

• introducing ambient subsurface source water into and through a first ultrafiltration membrane unit and a second ultrafiltration membrane unit during a forward operating cycle of a subsurface water treatment system using a system pump, thereby producing an ultrafiltrate substantially free of solid particulates having a largest dimension greater than 0.1 microns;
• preparing an aqueous solution comprising one or more hypohalous acid species in an electrochemical unit in fluid communication with the first ultrafiltration membrane unit;
• performing one of:
• (1) using the system pump to deliver at least a portion of the ultrafiltrate from one of the first and second ultrafiltration membrane units to the other one of the ultrafiltration membrane units in an opposite direction of flow as the source water during a backwash cycle;
• and
• (2) using the system pump to deliver at least a portion of the ultrafiltrate from one of the first and second ultrafiltration units and at least a portion of the aqueous solution comprising one or more hypohalous acid species to the other one of the first and second ultrafiltration membrane units during a chemical backwash cycle of the subsurface water treatment system;
• subjecting a first portion of the ultrafiltrate to a nanofiltration separation step in a nanofiltration unit of the subsurface water treatment system to produce a nanofiltrate and a retentate rejected by the nanofiltration unit;
• and
• subjecting a second portion of the ultrafiltrate to a reverse osmosis separation step in a reverse osmosis membrane unit of the subsurface water treatment system to produce a permeate and a retentate rejected by the reverse osmosis membrane unit;
• wherein a flux of the source water through each of the first and second ultrafiltration membrane units is less than thirty gallons per square foot per day.

A method of producing a hydrocarbon comprising:

• injecting purified water derived from an ambient subsurface source fluid into a hydrocarbon reservoir to stimulate flow of a hydrocarbon fluid from the reservoir;
• receiving the hydrocarbon fluid in a hydrocarbon production well;
• and
• transporting the hydrocarbon fluid from the production well to a storage facility;
• wherein the purified water is produced in a subsurface water treatment system comprising;
• a first ultrafiltration membrane unit and a second ultrafiltration membrane unit configured to produce from the source fluid an ultrafiltrate substantially free of solid particulates having a largest dimension greater than 0.1 microns during a forward operating cycle using a system pump of the subsurface water treatment system;
• a backwash unit comprising an electrochemical unit and the system pump, the electrochemical unit in fluid communication with at least one of the first and second ultrafiltration membrane units and capable of producing an aqueous solution comprising one or more hypohalous acid species, wherein the system pump is capable of delivering the ultrafiltrate from one of the first and second ultrafiltration membrane units to the other one of the ultrafiltration membrane units in an opposite direction of flow as the source fluid during a backwash cycle, and wherein the system pump is capable of delivering the ultrafiltrate from one of the first and second ultrafiltration membrane units and at least a portion of the aqueous solution from the electrochemical unit to the other one of the first and second ultrafiltration membrane units in an opposite direction of flow as the source fluid during a chemical backwash cycle;
• a nanofiltration membrane unit configured to receive the ultrafiltrate and produce therefrom a nanofiltrate containing less than 100 parts per million sulfate species;
• and
• a reverse osmosis membrane unit configured to receive the ultrafiltrate and produce therefrom a reverse osmosis membrane permeate and a reverse osmosis membrane retentate;
• wherein a flux of the source fluid through each of the first and second ultrafiltration membrane units is less than 30 gallons per square foot per day, and wherein the reverse osmosis membrane retentate is used as a source fluid for the electrochemical unit.