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System Design
Pump sizing is governed by available API bores.
These range from 1-1/16" to 3-3/4" diameter. Systems
can be designed for 4-1/2", 5-1/2", and 7" casing
applications. A typical installation is for 5-1/2" casing
with 2-7/8" tubing and a 1-1/4" or 1-1/2" bore
insert pump on top and a 2-3/4" bore tubing pump on the bottom.
This gives good performance up to 200 m3/day total fluid in the
90%-95% water cut range.
To deal with lower water cuts down to 85% while still maintaining
the desired 50% water cut to surface it is merely a matter of
upsizing the top pump to an 1-3/4" or 2". Water cuts
above 95% simply mean a slightly higher than 50% cut to surface.
This would still be a vast improvement over any existing situation.
Take for example a well operating at a 98% water cut with 2 m3/day
of oil production and 98 m3/day of water production. A 1-1/4"
x 2-3/4" DPGS system could take this down to an 80% water
cut. We would then have the same 2 m3/day of oil but now only
8 m3/day of water, a 92% reduction in produced water to surface.
With the current design of tubulars for 5-1/2" casing applications,
it would be theoretically possible to run an 1-1/16" bore
pump on top and an 3-1/4" on bottom, giving better performance
at higher water cuts, but strength of top pump connections and
high velocities within the bottom pump system would need to be
given strong consideration.
The focus has certainly been on large bore bottom tubing pumps
and small bore top insert pumps; however, other non-typical applications
could be considered. As mentioned previously, a relatively low
existing water cut of 50% could still be addressed. Obviously,
the final water cut to surface would be below the 50% recommended
but the velocities and residence times involved in a system like
this would still allow for a high level of confidence that no
oil carry over was taking place. A system like this would require
downsizing the bottom pump to something equal to or smaller than
the top pump. Another application would be where the objective
is simply to maximize draw down (and hence oil production) with
no real concern about water cuts to surface. Here, two large bore
tubing pumps could be run. With of one of them injecting to a
low pressure disposal zone, more total fluid could be produced
into the wellbore without upgrading jack and rods. As an added
benefit, there would still end up being a decreased water cut
to surface.
Figure 2 shows the bottom
tubing pump situated below the production zone. This obviously
requires a certain amount of distance between the production and
disposal perforations to create a separation chamber. One possible
solution is depicted in Figure
5 where the entire system is situated above the production
perforations and "stingers" reach down to the isolation
packer. As mentioned previously, below zone is preferable, but
disposal to a zone above the production perforations would not
be impossible. Figure 6
depicts this concept.
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