"The objective of any water-injection operation is to inject water into the reservoir rock without plugging or permeability reduction from particulates, dispersed oil, scale formation, bacterial growth, or clay swelling." Introduction Ideally, injection water should enter the reservoir free of suspended solids or oil. It should also be compatible with the reservoir rock and fluids and would be sterile and non-scaling. The objective of any water-injection operation is to inject water into the reservoir rock without plugging or permeability reduction from particulates, dispersed oil, scale formation, bacterial growth, or clay swelling. In addition, souring of sweet reservoirs by sulfate-reducing bacteria should be prevented if possible. How Good Must Water Quality Be? Actually, the question posed by most producers is, how bad can water quality be without causing any significant problems? The methods by which people arrive at an answer to this question are both curious and diverse. In many injection projects, experience is the sole guide, and little, if any, technical investigation precedes design of the injection system. Those who wish to apply the best available technology should precede system design with a careful study of the water to be injected and the characteristics of the formation rock and fluids. Characterization of the Water and the Formation Injection Water. The water must be carefully sampled and characterized to assess the likely problem areas. The following measurements are considered essential. Chemical Composition. Knowledge of the chemical composition of the water is required to calculate scaling tendencies and to determine the likelihood of clay swelling. Certain parameters, such as pH, must be measured on site immediately after sampling. Dissolved Gases. The amounts of dissolved oxygen, carbon dioxide, and hydrogen sulfide should be known to anticipate the types of corrosion. Corrosivity. On-site corrosion-rate measurements are necessary to quantity the water corrosivity. Bacteria. Any bacteria in the water should be identified and their population estimated. Sulfate-reducing bacteria are of particular interest. Suspended Solids. The amount and composition of the suspended solids are primary indicators of water quality. The particle-size distribution can also be determined for surface waters and waters from water-supply wells. Particle-size measurements in produced waters have little meaning because particle-size-measurement instruments cannot distinguish between solid particles and oil droplets. Oil Content. Dispersed oil can decrease injectivity, especially when combined with suspended solids, such as iron sulfide. Also, emulsion blocks can form in injection wells. Injection Formation. The injection formation is often characterized simply by its permeability. Pore-size distributions, however, may also be useful in the determination of injection-water standards. Knowledge of the composition and distribution of clays in sandstone reservoirs is desirable to assess potential clay swelling and migration problems. Water-Treatment Objectives Once the water and the rock have been characterized, the difficult task of deciding how much water processing is required to create a nonplugging water begins. Suspended Solids and Oil. Methods to estimate the quantity and size distribution of suspended solids that can be tolerated in an oil-free water abound. Most methods attempt to correlate the suspended-solids concentration, particle-size distribution, or water-quality measurements with the formation permeability or pore-size distribution. Unfortunately, there is no consensus as to which method provides the most dependable estimate, and petroleum engineers are left with a bewildering array of choices. When produced water is injected, the water contains both solids and oil. Attempts to estimate acceptable levels of suspended solids and oil with theoretical models have not been successful to date. Empirical correlations currently offer the best hope.