Downhole Dehydration vs. Reduction of Oil Droplet Break-Up

Abstract

In downhole dehydration the produced fluids - oil and water, with a low oil/water ratio - are separated at the bottom of the well by means of a hydrocyclone. In many cases this separation can be carried out successfully, because the size of the oil droplets is generally in such a range, that the hydrocyclone can remove the oil from the produced water. Potential problems are connected with the possible occurrence of very small oil droplets, because this may lead to the presence of some oil remaining in the produced water, which is unfavorable from an economical point of view and, moreover, which can lead to difficulties with subsurface injection of this water. Notably production from reservoirs with a relatively low permeability may lead to the presence of small oil droplets. For conventional high water-cut oil production, the presence of very small oil droplets is predominantly caused by droplet break-up in the choke valve. Further downstream these small droplets determine the complexity of the separation units. It is shown that the geometry of the choke valve influences the break-up process. Consequently, by modification of the geometry of this valve it must be possible to reduce the break-up. It is expected that in this way a substantial droplet size increase can be realized. Introduction As an oil field matures the water-cut increases, which gives risetoincreasedoperationalcostsand makes increasedseparation efforts necessary. To deal with these problems it is important to generate and develop new technical solutions. In this paper technical aspects of two alternative ways to deal with a high water-cut are treated, viz. downhole dehydration and reduction of oil droplet break-up. The idea of downhole dehydration is not new, but we look again at this technique, mainly with regard to separation aspects, and taking into account data from recent work on oil/water morphology by Janssen1. Break-up reduction concerns means to influence the average oil droplet size of the produced water in the production system (more specifically: in the choke valve), so that the oil-water separation equipment at surface can become somewhat less complicated. To the best of our knowledge this second option has not been considered up till now. We will treat the feasibility of this idea on the basis of work by Van der Zande2. Downhole Dehydration Production of oil making use of downhole dehydration has been treated by a number of authors, of which we mention Kjos et al.3, Verbeek et al.4, and Jacobs and Schmidt5. The idea is that, with respect to separation, the produced fluids are treated downhole and not at the surface. To this end a separation unit is positioned at the bottom of the well. The produced fluids are forced to enter this unit (emulsion pump), subsequently pass a separation device (hydrocyclone) and leave the unit as two streams, an oil-rich stream and a water-rich stream. The oil-rich stream is pumped to the surface (concentrate pump), while the water-rich stream is injected into a suitable formation. The two main objectives of downhole dehydration are to bring an oil-water mixture to the surface with a considerably higher oil concentration than in the absence of a downhole dehydration unit and, simultaneously, to prevent that the bulk of the produced water arrives at the surface. The higher oil concentration of the produced oil-water mixture will be advantageous, because oil droplet coalescence will be promoted. More important, however, is that a much lower capacity of the separation system at the surface is needed, as a result of the much lower amount of produced fluids.

Experimental study of a vane-type pipe separator for oil–water separation

“…If the oil phase is reemulsified, it would be quite difficult to separate oil droplets from water and even lead to the presence of some oil droplets in water injected to a disposal zone. The potential problems of this lasting oily-water stream reinjection will add to the difficulties in subsurface injection of the produced liquid (van den Broek et al, 2001) and reduce the field oil production. To solve the above problems, Sooran et al tried to improve the separation efficiency through the redesign of inlet structures.…”

Size of Oil Droplets Under High-Water-Cut Conditions

“…In a recent paper 1 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.…”