A method for decreasing the dissolved oxygen content in water used for extracting oil from rocks includes directing water and gas into a housing containing at least one hydrophobic membrane such that the water contacts only a first surface of the membrane and the gas contacts only a second surface of the membrane. The pressure of the gas is decreased thereby causing the oxygen in the water to pass through the first side of the membrane to the second side of the membrane and mix with the gas. A system for decreasing the dissolved oxygen content in water used for extracting oil from rocks includes a plurality of membrane modules. Each module contains at least one hydrophobic membrane. A water supply inlet and a gas supply inlet direct water over the first and second surface respectively of each of the hydrophobic membranes. A water outlet and a gas outlet direct water and gas respectively out of the plurality of membrane modules.