Size of Oil Droplets Under High-Water-Cut Conditions

Abstract

Abstract In this paper the oil droplet size distribution as present during conventional high-water-cut oil production is discussed. The analysis is mainly based on the results of laboratory work on, firstly, core-flow experiments and, secondly, break-up in model chokes. Downhole, the size distribution is governed by the flow of oil and water out of the reservoir sections near the wellbore. It is expected that the smallest oil droplets will be in the 10–15 µm range, while the largest droplets are, presumably, of the order of centimeters. In the tubing, break-up of the largest droplets may occur, so that, eventually, the width of the size distribution will decrease somewhat in the sense that very large droplets will no longer be present. Subsequently, due to more intense break-up phenomena in the choke, the size distribution will shift to smaller droplet sizes. Downstream of the choke, the largest droplets will be in the 100–200 µm range. More information is needed for reliable predictions on the occurrence of very small droplets downstream of the choke. It is expected, however, that many droplets of sizes smaller than 10 µm will be present downstream of the choke. Introduction The simultaneous production of oil and much water (high-water-cut conditions) is encountered quite often. Because of the high water content, water is the continuous medium while the oil is present in the form of oil drops or droplets. The conventional productionmethod leads to the treatment, at surface, of large amounts of produced water. An alternative technique is downhole dehydration. Here oil and water are separated in a hydrocyclone positioned in the wellbore near the producing interval, an oil-rich stream is pumped to surface and, simultaneously, the produced water is injected into a suitable subsurface formation. For the success of this technique, the downhole oil droplet size distribution is crucial. The presence of small droplets (diameters of the order of, say, 15 µm or less) will lead to separation problems, because the hydrocyclone will be unable to remove these droplets from the produced water. In a recent paper1 we gave attention to some aspects of downhole dehydration. In addition, the possibilities - for conventional high-water-cut oil production - of reduction of oil droplet break-up were addressed. In this paper we focus specifically on the size of the oil droplets in a conventional production system, without giving attention to the technical implications. For convenience we divide the production system into three parts: the near-wellbore region, the tubing, and the section downstream of the choke. Successively the oil droplet sizes in each of the three parts will be treated. Oil Droplet Sizes in the Near-Wellbore Region For our analysis of oil droplet sizes in the near-wellbore region we base ourselves on the work of Janssen2. He carried out core-flow experiments (core dimensions: length 5.0 cm, diameter 2.5 cm) as well as micromodel-flow experiments, mimicking the flow of oil droplets, dispersed in water, in the reservoir near the wellbore. In the core-flow experiments, oil and water were simultaneously injected into the core, and the sizes of the oil droplets leaving the core were measured. We include and discuss two figures from his work.