Abstract

With the advent of stringent environmental regulations and more attractive secondary recovery propositions (due to increased oil prices), there has propositions (due to increased oil prices), there has been, over recent years, an industry effort to more effectively treat produced water. The upward flowing coalescer is a product of this emphasis. This process is being used to remove oil and particulate from produced water in a number of areas in the United produced water in a number of areas in the United States, as well as abroad.

Introduction

There are two primary reasons for more effective treatment of produced water:

  • efficient reservoir depletion programs through waterflood, and

  • concurrence with EPA regulations through disposal into a receiving strata.

In general, securing funds for water treatment for secondary recovery purposes is more successful than the same activity for disposal systems, because secondary recovery has a calculable return on investment. However, proper preparation of produced water has proven to be economical to many produced water has proven to be economical to many operators through reduced or eliminated acidizing, workovers and/or new wells. In many cases, the preparation of produced water for disposal should be preparation of produced water for disposal should be viewed with the same respect as the preparation of produced water for secondary recovery purposes. produced water for secondary recovery purposes. Most produced water contains some or all of the following: free floatable oil, entrained oil, water-in-oil and oil-in-water emulsions, oxides of iron, iron sulphide, clays, and formation sand. Any of these solid materials, in conjunction with the free floatable oil or emulsions, could act as an excellent plugging material at the well bore face. Over the plugging material at the well bore face. Over the recent years, a number of devices and/or processes have been developed to remove the undissolved oil and the associated particulate matter. The better known schemes employ the use of flotation cells, downward flowing coalescers, and storage vs time. Each of these methods commonly express their efficiency in terms of percent of oil removed. With these devices, sudden increases in oil content in the incoming stream results in increased oil content in the effluent stream.

The upward flowing coalescer on the other hand, was developed and installed for the primary purpose of treating produced water to a constant low level of oil in the effluent. This system delivers effluent with an oil content that is totally independent of the influent oil content. Thus if the influent oil content varies from 100 ppm to 14,000 ppm (Table 1), the oil content in the effluent will be held at a respectable range below 15 ppm. This can be seen graphically by Figure 1.

It should also be noted that acid insoluble particulate matter is reduced considerably (Table 2). particulate matter is reduced considerably (Table 2). The acid soluble particulate reduction is comparable.

PRINCIPLE PRINCIPLE The consistently low oil content of the effluent of this system is quite logical and predictable when considering the oil droplet formed by the coalescing agent (granular media) exerts a buoyancy force (Fb) in the same direction as the flow force (Ff) and not in opposition as is the case in downward flowing coalescers (Figure 2).

In operation, a downward flowing coalescer can develop high velocity areas due to channeling, which could result in the (Ff) overcoming the (Fb) with the outcome being oil droplet capture.

In other words:

Fb greater than Ff efficient coalescing/separation(Fb) less than Ff inefficient coalescing/separation

This same "capture" condition can be caused by surging flow rates.

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