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Candidate Selection
Following are basic guidelines which do not preclude
other possible candidates but are only intended to maximize potential
of system.
1. First and foremost are economic considerations. Most operators
that are considering downhole oil/water separation are going after
more oil because current production equipment is maxed out or
reduced water to surface because trucking/handling costs are high
or surface separation/handling facilities are constrained. Figure
4 shows a basic candidate selection and economic evaluation
spreadsheet. It is based on simple pump sizing and oil price/water
cost economics.
2. Disposal zone below production zone. Uphole disposal is possible
but significantly more complicated and expensive with many more
operational considerations and compromises.
3. Zonal isolation. Two separate zones or an impermeable barrier
between the production and injection perforations is required.
One operator has however reported success using a 40 m thick zone
with no impermeable barrier(14). It is simply a matter of confidence
that there is no near wellbore communication. Good cement is a
must.
4. 20 meters between production and disposal perforations. More
is always better for separation, limited only by rod loading considerations.
Gravity separation can be accomplished in as little as 10 m between
zones but an alternate pump configuration would be required.
5. Desk top separation time < 15 seconds. Remember, this is
just a conservative rule of thumb that will give about 5x residence
time with the above zonal separation. Further distances allow
for longer times.
6. Water cut > 85%. This is a good number for conventional
pump sizing to get about 50% water cut to surface and maximize
impact of system, but again, unusual applications have been looked
at where the existing water cut is only 50%.
7. 100 m3/m3 gas oil ratio (GOR). Note that the top pump intake
is above the production perforations so a high GOR will definitely
impact volumetric efficiency.
8. Sand cut < 1%. The pump can always be designed to handle
more sand. The problem is whatever sand gets carried with the
water can end up plugging the injection perforations. Also, a
large sump below the disposal zone would be required along with
regular sand clean out trips made all the more expensive because
of the packer in the hole.
9. 20 °API gravity oil. There have been installations down
to 17 API degrees(13) but at some point gravity separation is
no longer feasible. Also, the oil concentrate pump is typically
small bore and has more difficulty producing heavy oil.
10. Injectivity index > 0.01 m3/day/kPa. This is truly a moving
target because the limit depends so much on reservoir pressure,
pump size, and fluid levels. This is the load that the bottom
pump sees and that the small bore top pump has to move so more
is definitely better. Static reservoir pressure - less is more.
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