American Oil and Gas Reporter - PBIOS 2014 Official Program Guide - 159

Permian Basin Reports
Upstream

ducers in a pattern may not receive any
support from the central injector.
The differences in the pressure gradients that occur at SACROC are significant
because of the very high average permeability. This leads to a processing rate that
averages 30 percent of a pore volume per
year. To maintain the fluid throughput required to keep the injection/withdrawal ratio close to 1.0, nearly all of the producing wells produce with electric submersible pumps. While a much more expensive lift method than conventional
beam pumping, it does provide pump intake pressures (PIPs) on a continuous basis. PIPs are a key element in reservoir surveillance, and pressure and rate balancing.
The high processing rate at the
SACROC Unit makes managing the areal sweep efficiency by adjusting the withdrawal rates and pump intake pressures
much easier than it otherwise would be.
Changes in flow behavior can be observed
quickly and appropriate adjustments can
be made while the pattern is still responding to CO2 injection. If the desired
effects are not seen, corrective action
can be implemented in a timely fashion.
Generally, if the CO2 injection rate is
less than 20 percent pore volume/year for
a given pattern, it becomes difficult to assess the effects of pressure and rate rebalancing because of the time required for the
response to be observed from any given
pressure or rate adjustment. Slower processing rates increase the time between
cause and effect. In this case, other changing conditions can make the "signal"
ambiguous, and an observed response
may be related to adjustments in one pattern and incorrectly ascribed to changes
implemented in another pattern.

utilization of CO2, each project area had external "water curtain" injection wells surrounding the boundary wells of each expansion phase. By design, this caused the
reservoir pressure outside the project area
to be higher than that within the patterns

Midstream

Downstream

being flooded. Figure 1 displays a contour
map of the pump intake pressures (injector bottom-hole pressure is not shown), illustrating the higher pressures around the
margins maintained by the water curtain to
prevent CO2 migration out of the area.

Pressure Gradient Variability
At SACROC, the need to repressure the
reservoir coupled with the need to reinject
produced water has combined with these
other issues to produce wildly varying
pressure gradients within the field. The fully miscible CO2 flood in the SACROC
Unit began with injection into a limited
area and expanded outward, phase by
phase. These phases are referred to as "expansion areas," and generally proceeded
from the central area outward.
To help mitigate the migration of CO2
out of the project area and improve the net
OCTOBER PBIOS 2014 159

American Oil and Gas Reporter - PBIOS 2014 Official Program Guide

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