American Oil and Gas Reporter - PBIOS 2014 Official Program Guide - 163

Permian Basin Reports
Upstream

in one of the SACROC project expansion
areas (Southwest Centerline), taking into
account the distance over which pressures
are measured. For each injector/producer
pair, the width of each arrow is proportional to the gradient. Some patterns exhibit
rather uniform pressure gradients in all directions, while others show wide variations.
Note that the high-permeability pattern
263-1A has rather low gradients.
Irregular Pattern Geometries
In an ideal arrangement of five-spot patterns, the producing wells on the edges
would all be in a line with one another,
with sufficient injection into the water curtain wells to exactly balance the withdrawal rates from the producers. To minimize
the capital cost of implementing each successive expansion area at SACROC, it is
desirable to minimize the number of new
wells to be drilled. Therefore, every attempt
is made to use existing wellbores whenever possible.
One consequence of this is that most of
the project areas contain asymmetrical
and/or irregular pattern geometries. When
this happens on a pattern on the boundary,
there will be an external producing well
that receives CO2 support from only one
direction, and the injection will be opposed
by water curtain injection (at a higher pressure) from the other three directions (Figure 3 at left).
Producing wells can be overrun by injection from other patterns within a project area also. While numerous studies have
failed to identify any systematic or directional trends in channeling between injectors and producers, these high-permeability channels and "void space conduits" are common enough that a significant number of patterns have one or
more producers connected directly to the
central injector.
These channels are swept very quickly and a rapid (but short-lived) initial oil
response is followed by gas breakthrough.
In such cases, the standard approach is to
attempt to plug the channel with polymer
or isolate the offending zone mechanically in the injection well. When this cannot
be done or is not successful, the producing well typically is shut in and the CO2
that it was formerly producing from the
thief zone is redirected elsewhere in the
pattern.

As an alternative, the injector may be
shut in, when for example, the entire pattern is about to be put on chase water. It
is sometimes found that the producing well
suddenly begins responding to CO2 injection from adjacent patterns that the direct
connection had prevented previously.

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Midstream

Downstream

Changing Streamlines
The Southwest Centerline area referenced in Figure 2 had been part of the water curtain for a prior phase and was
overpressured with respect to the average
reservoir pressure in the adjacent project
area. Static reservoir pressure was meas-

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OCTOBER PBIOS 2014 163

American Oil and Gas Reporter - PBIOS 2014 Official Program Guide

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