American Oil and Gas Reporter - February 2016 - 26

TECHCONNECTIONS
Study May Help
Increase Applicability
Of CO2 Flooding
Carbon dioxide injection for enhanced oil recovery has been
a common practice in various parts of the country for many years.
It is effective primarily because of the interaction of CO2 with crude
oil trapped in the pore spaces of reservoirs. In essence, when CO2
is pumped into a reservoir, it acts as a surfactant, bonding with
crude to increase its mobility and assist with recovery.
In addition, large underground deposits of natural CO2 are available and are relatively inexpensive, making CO2 injection more
cost effective than alternative chemical options. There also has
been a lot of talk regarding injecting CO2 from man-made sources,
however, this process is still developing and is not as cost effective as using CO2 from natural sources.
According to the National Energy Technology Laboratory, CO2
is miscible with crude oil, meaning that it becomes mutually soluble with the residual oil (hydrocarbons from the oil dissolve in
CO2 and CO2 dissolves in the oil). When CO2 is compressed and
injected into a reservoir, the miscibility reduces interfacial tension and viscosity, enabling CO2 to displace oil from rock pores
form an oil bank that is swept toward producing wells.
For a traditional CO2 flood in a mature field, operators must
consider reservoir pressure before they inject, and in some cases may repressure the field through waterflooding. Once the CO2
is injected and it becomes miscible with the oil, the interfacial tension holding the crude in place is reduced dramatically, allowing
the CO2 to displace the oil from the rock and send it toward a producing well.
Until recently, this process was thought to work only in reservoirs pressurized above miscibility pressure. However, injecting CO2
into reservoirs that are near minimum miscibility creates the possibility of utilizing the process in a significant number of reservoirs.
In a study funded by the Research Partnership to Secure Energy for America at the University of Kansas, researchers
showed that injecting CO2 can provide enhanced oil recovery in
mature reservoirs previously not thought to benefit from CO2 injection. The study, Characterization of Potential Sites for NearMiscible CO2 Applications to Improve Oil Recovery in Arbuckle Reservoirs" (RPSEA 09123-18), led by Jyun-Syung Tsau from
KU, describes how the origins of the work are rooted in the miscibility of CO2 with oil (a function of pressure and temperature)
within a reservoir.
Basically, greater pressure improves the miscibility of CO2 with
oil in the reservoir and its chances of providing improved oil recovery. Injecting CO2 in miscible conditions to enhance oil recovery has been a common practice for many years. Over time,
it was observed that there were several oil fields at shallower depths
with lower pressures-or near miscible conditions-that potentially
could benefit from CO2 injection, but often were overlooked in
spite of their large resource potential.
An initial laboratory study funded by RPSEA indicated favorable conditions for improved oil recovery in conditions
where pressures were 200 psi below minimum miscibility pressure (MMP) at reservoir temperature in the Ogallah Field in the
Arbuckle reservoir. Tsau found that among the more favorable
26 THE AMERICAN OIL & GAS REPORTER

"An initial laboratory study indicated favorable conditions for improved oil recovery

where pressures were 200 psi below MMP.

mechanisms for oil recovery using near-miscible CO2 injection
included oil swelling, reduction of oil viscosity, and extraction
of light components.
This study, in which researchers describe a single-well pilot
test program to demonstrate the applicability of CO2 injection at
near-miscible conditions for improved recovery, is aimed toward
small producers. The pilot test consisted of a single-well tracer
test before and after CO2 injection.
The first test was conducted to determine the oil saturation of
the formation prior to injection. This was followed by the actual CO2 injection at pressures below MMP with a follow-up water displacement, followed by a second tracer test to determine
whether there was a reduction in oil saturation in the formation.
The effectiveness of the process was determined by the differences
in saturation before and after the process.
After evaluating several wells producing from the Arbuckle
formation in the area, researchers selected well No. 10 in the Dreiling Field in Ellis County, Ks. The reservoir temperature was 106
degrees Fahrenheit with shut-in pressure of 1,150 psi. The MMP
was determined to be 1,500 psi at reservoir temperature. The initial tracer test measured oil saturation at 0.23, and after CO2 injection, it was reduced to 0.20, representing a 13 percent improvement in oil displacement in near-miscible conditions.
The study describes its results computationally with a simplified
well model to simulate the process of CO2 injection at near-miscible conditions to improve CO2 displacement efficiency. From
the single-well pilot test and simulation results, it was concluded that there was potential for using CO2 injection at near-miscible conditions to improve oil recovery in Kansas Arbuckle reservoirs and others where MMP couldn't be achieved.
Injecting CO2 to enhance oil recovery in mature fields is a proven
technique in many areas. The work conducted in Kansas shows
that there may be significant opportunities for operators working
in fields previously thought not to be candidates for injection. ❒

JEREMY VISCOMI is the di-

rector of technology transfer
for the University of Kansas
Tertiary Oil Recovery Program and the PTTC Midcontinent Region office. He has
more than a decade of experience in developing and organizing technical conferences
and special events.

American Oil and Gas Reporter - February 2016

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