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Introduction
In the last ten years in Canada, the United States,
and overseas, there has been a significant amount of development
in the area of downhole oil/water separation with same wellbore
disposal. This technology has gained momentum due to increasing
water production(1) and producers aggressively pursuing lower
operating cost
alternatives along with aggressive environmental objectives. The
two key economic benefits of downhole oil/water separation are
decreased water handling costs and increased oil production.
There are and have been many different methods to facilitate the
separation of oil from water downhole. For flowing wells, a myriad
of unique packer and wellbore configurations have been developed
with and without the utilization of downhole pumps. In non-flowing
wells, theoretically, almost any form of artificial lift could
be used. Also, strictly speaking, only one energy source is needed
depending on whether the disposal zone is above or below the production
zone. However, most downhole oil water separation systems use
two pumps; a large volume and/or low horsepower/head pump to inject
water to a suitable (i.e. relatively low pressure and high permeability)
disposal zone, and a small volume and/or high horsepower/head
pump to lift an oil rich stream to surface. Obviously, the most
common in Canada would be reciprocating rod pumps, progressive
cavity (PC) pumps, and electric submersible pumps (ESP). Although
to the best of the authors' knowledge, no other form of artificial
lift has been commercially developed(2-4), the basic principles
of downhole oil/water separation do not preclude gas lift, jet
pumps, hydraulic pumps, etc.
In the early 1990's the Centre for Engineering Research (C-FER)
began developing the concept of separating oil and water downhole
using a hydrocyclone(5). The basic premise was to mate the two
mature technologies of bottom hole pumps and surface hydrocyclones
into a system that could separate oil and water downhole and inject
the majority of the water to a disposal zone in the same wellbore
without bringing it to surface.
The principles of operation are quite simple. One or two pumps
are used to pump normal production fluid through a cylindrical
cone (the hydrocyclone, Figure
1). Centrifugal forces are set up in the fluid as the oil
and water mixture move to the narrow end of the cone and velocities
increase. These centrifugal forces "spin" the generally
"heavier" (i.e. higher specific gravity) water to the
outside, leaving a core of oil rich fluid. These two phases are
then able to be produced as separate streams.
C-FER's downhole oil/water separation hydrocyclone systems have
matured fairly well for ESP applications with two vendors licensed
and distributing systems worldwide. Although technically successful,
and still progressing, this technology is somewhat less mature
for PC(6) and reciprocating rod pumps(7).
Although hydrocyclone downhole oil/water separation technology
is viable for many types of artificial lift, the mechanics are
fairly complex and relatively expensive as compared to conventional
pumping systems. Hydrocyclone systems therefore lend themselves
more to the high volume, high cost, ESPs than to the simpler,
lower cost, forms of artificial lift.
One novel concept, which has actually been around for decades,
is to simply use the wellbore, and configure it, and/or a pumping
system, such that the oil and water is allowed to segregate in
the wellbore just due to the force of gravity. In this manner,
it is possible to still pump an oil rich stream to surface and
dispose of the majority of the water to a zone in the same wellbore,
but without having to use a hydrocyclone or any other mechanical
means of separation.
There have actually been many systems developed (or in the process
of being developed) that utilize this concept. This paper will
look at one such system of dual reciprocating rod pumps.
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