Introduction Poor selection of a water source and a miscalculation of the water requirements can affect adversely the economics of a potentially successful flood. A methodical examination of the sources, selection of the best one available, and application of engineering principles to the development of that source mean cheaper production of each barrel of waterflood oil. Following is the preferred order for approaching these basic problems which must be solved early in the launching of a waterflood program:determine the requirements as accurately as data permit;survey all possible water sources with special attention to the quantitative satisfaction of the requirements; anddevelop the selected source in the most economical manner permitted by good engineering practice. WATERFLOOD REQUIREMENTS Daily Water Injection Rate The heaviest drain on the make-up water source is during the fill-up period when there is no return water and the desired unit daily injection rate is as much as twice that needed after fill-up. The maximum daily total injection rate usually is tempered somewhat by progressive development of the flood acreage. Empirical relationships established during 18 years experience with injection projects throughout the country are the bases for the following general requirements. During the initial or fill-up period of injection into a depleted reservoir, a unit rate of between 1 and 2 B/D/acre-ft is desirable. This normally will result in increased oil production within six to nine months without adversely affecting the ultimate oil recovery. After fill-up has been achieved, the injection rate should be maintained at about 1 B/D/acre-ft and at not less than 1/2 B/D/acre-ft. Flood pattern, well spacing, and injection pressures should be designed to meet these requirements. For depleted reservoirs in which there is gas sand in the top of the sand section, together with basal water sand, the water requirements normally will be several times that for an oil sand section alone. The gas sand must be filled with water in order to prevent, as much as possible, migration and loss of oil into such sand.