Paper: Troubleshooting Oil-Water Separators By Measuring Their Residence-Time Distribution

Separation of oil and water (particularly the separation of small volumes of oil from large volumes of water) is a problem of increasing importance to the oil industry. The economics of handling large volumes of fluids often dictates the use of simple gravity-type separators for most of these separations. So long as the separators fulfill their function, there is little concern about flow characteristics, but when they fail in this function, normal oilfield practice is either to increase chemical, to add additional capacity, or to use some combination of both. It is often true, however, that the fault is due not to inadequate capacity or improper treatment, but is due rather to poor hydraulic characteristic of the separator. The only feasible method for determining the flowcharacteristics of a continuous separator is by the use of a tracer technique. The presence of such phenomena as short circuiting, the existence of stagnant flow regions, the presence of high rates of mixing and dispersion in the vessel, and other such hydrodynamic ills are easily identified by use of the tracer-response technique. Measurements can be made during the operation of the separator without interfering with it in any way, and thus measurements can be used to check the effect of variations in any of the operating parameters of the system. The causes of such hydrodynamic problems are numerous. They are rarely obvious from superficial examination of the system and, in our experience, are the rule rather than the exception. Plug flow is a very rare phenomenon in oil-field separators. Poor hydraulic characteristics of a separator can result in inadequate capacity, if a substantial fraction of the fluids spend insufficient time in the separator due to poor flow paths. Poor hydraulic characteristics may also contribute to poor treatment efficiency by introducing shear forces which decrease the droplet sizes and make separation more difficult for the same time interval. In this paper, the procedures required to make such measurements in the field are discussed, and the details of a successful method are described. Some of the results obtained in field measurements will be shown and the results discussed in terms of the hydraulic characteristics of the separators.