American Oil and Gas Reporter - February 2017 - 69

SpecialReport: Enhanced Oil Recovery
For the ethane evaluation, the model was placed on waterflood
for 4,000 days, and then 12 WAG cycles of ethane and water were
injected at 700 psi. A similar procedure was followed to inject
12 WAG cycles of CO2 and then 12 cycles of methane. In these
models, CO2 and ethane had the same gas-like relative permeability.
The ethane WAG ultimately recovered an additional 11.8 percent of the OOIP at a gross efficiency of 1.8 Mcf of ethane per
barrel of oil recovered. The net efficiency (including gas recycle) was a very attractive 0.9 Mcf/bbl. Figure 2A shows the results of the pattern model waterflood and ethane WAG simulations, while Figure 2B shows a comparison of the ethane WAG
and the CO2 WAG. The CO2 WAG recovered an additional 3.2
percent OOIP at a gross efficiency of 7.3 Mcf of CO2 per barrel. Net efficiency was 3.7 Mcf/bbl. As expected, the methane
WAG recovered no additional oil.
Oil saturation and viscosity maps from the ethane and CO2
WAG runs are in Figure 3. The ethane WAG was miscible and
reduced the oil viscosity by a factor of 10. The CO2 WAG was
not miscible, and reduced the oil viscosity by about half.
Cushing Field
The Mid-Continent also is a very promising target for ethane
EOR. Multiple CO2 floods are in operation in Oklahoma, but there
are several other mature fields with more than 1 billion barrels
of remaining oil in place and excellent reservoir properties for
CO2 WAG.
The Cushing Field in Oklahoma was chosen to evaluate ethane
EOR because it is centrally located in the "pipeline capital of the
world," with very large volumes of ethane flowing into and out
of the area. Slim tube simulations using 100 cells in a finite difference model were run to determine the MMP of both ethane and
CO2 at an oil composition of 38 degrees API gravity and a reservoir temperature of 108 degrees. The MMP for ethane in the Cushing Field was 875 psi, while the MMP for CO2 was 1,250 psi.
A quarter-five-spot model was built using the SPE3 solution
and reasonable estimates of depth, initial pressure, reservoir thickness, well spacing, fluid composition, average porosity, and average permeability. A 20-acre spacing base model was waterflooded for 20 years. For the ethane evaluation, the model was placed
on waterflood for 4,000 days, and then 16 WAG cycles of ethane
and water were injected at 1,600 psi (Figure 4A). A similar procedure was followed to inject 16 WAG cycles of CO2, and then
10 cycles of methane.
The ethane WAG recovered an additional 9.1 percent of OOIP
at a gross efficiency of 3.8 Mcf per barrel of oil. The net efficiency was a very attractive 1.6 Mcf/bbl. The CO2 WAG recovered
an additional 6.5 percent of OOIP at a gross efficiency of 4.3 Mcf
of CO2 per barrel. Net efficiency was 2.8 Mcf/bbl. The methane
WAG did not recover any additional oil.
Figure 4B compares the Cushing ethane WAG and the CO2
WAG results. Oil saturation and oil viscosity maps from the ethane
and CO2 WAG runs are shown in Figure 5. The ethane WAG was
miscible and reduced oil viscosity by almost an order of magnitude. The CO2 WAG was not miscible away from the injection
well, but significantly swelled the oil and reduced the oil viscosity by a factor of four. Both ethane and CO2 had good gross and

net gas efficiencies.
East Texas Field
The supergiant East Texas Field has an OOIP of 7 billion barrels. The field was chosen for ethane EOR evaluation because it
is a very large target and is connected to the ATEX ethane pipeline,
with significant volumes of ethane flowing through the area.
One major issue is the large number of old wells in the field.
With most wells more than 80 years old, a 2014 report concluded that the possibility of casing leaks in the aging wellbores could
FIGURE 4A
Cushing Waterflood and Ethane WAG Results
Ethane WAG
EOR, Mstb
GI, MMscf
GP, MMscf
Mscf/stb

2,000
1,800
1,600
1,400

224.5
841.7
472.4
3.75 Gross
1.64 Net

1,200
1,000

CUM PRD OIL

(MSTB)

800

OIL PRD RATE

(STB/DAY)

600

OIL PRD RATE

(STB/DAY)

CUM INJ GAS

(MMSCF)

CUM PRD OIL
CUM PRD OIL

400
200
0

1,000

2,000

(MSTB)
(MSTB)

3,000

4,000

5,000

6,000

7,000

FIGURE 4B
Cushing CO2 and Ethane WAG Comparison
2,000
1,800
1,600
1,400
1,200
1,000
800
600
400

2,000

OIL PRD RATE
(STB/DAY)

1,900

OIL PRD RATE
(STB/DAY)

1,800

OIL PRD RATE
(STB/DAY)

1,700

CUM INJ GAS
(MMSCF)

1,600

CUM INJ GAS
(MMSCF)

1,500

CUM PRD OIL
(MSTB)

Ethane WAG

CUM PRD OIL
(MSTB)

EOR, Mstb

CUM PRD OIL
(MSTB)

4,000

4,500

472.4

GP, MMscf

841.7

Mscf/stb

3.75 Gross

200
0
3,500

EOR, Mstb

GI, MMscf

GP, MMscf

5,000

CO2 WAG

224.6

GI, MMscf

691.9

Mscf/stb

4.27 Gross

1.64 Net

5,500

6,000

1,400

162.1

1,300

237.3

1,200

2.80 Net

6,500

1,100
7,000

1,000

FIGURE 5
Cushing Ethane and CO2 WAG Oil Saturation and
Oil Viscosity Simulations
SO

VISO

FEBRUARY 2017 69

American Oil and Gas Reporter - February 2017

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