American Oil and Gas Reporter - February 2017 - 68

SpecialReport: Enhanced Oil Recovery
FIGURE 2A
Bridgeport Waterflood and Ethane WAG Results
450

CUM PRD OIL

400

OIL PRD RATE

(STB/DAY)

OIL PRD RATE

(STB/DAY)

CUM INJ GAS

(MMSCF)

CUM PRD OIL

350

CUM PRD OIL

300
250

(MSTB)
(MSTB)
(MSTB)

350

Ethane WAG
EOR, Mstb 103.0
GI, MMscf
180.0
GP, MMscf
84.2
Mscf/stb 1.75 Gross
0.93 Net

300
250
200

200

150

450

OIL PRD RATE
(STB/DAY)
OIL PRD RATE
(STB/DAY)

400

OIL PRD RATE
(STB/DAY)
CUM INJ GAS
(MMSCF)

350

CUM INJ GAS
(MMSCF)

CUM PRD OIL
(MSTB)

300

CUM PRD OIL
(MSTB)

Ethane WAG
EOR, Mstb
103.0
GI, MMscf
180.0
GP, MMscf
84.2
Mscf/stb
1.75 Gross
0.93 Net

CUM PRD OIL
(MSTB)

100

50
0

FIGURE 2B
Bridgeport CO2 and Ethane WAG Comparison

1,000

2,000

3,000

4,000

5,000

6,000

7,000

is an excellent solvent, it will affect lube oils, greases, thread
sealants, and O-ring and valve-packing elastomers in a similar
manner to CO2.
As with any gas injection project, tubing collar leaks could be
a problem with ethane injection. New or reconditioned tubing with
premium threads and an ethane-resistant thread compound may
be warranted. The advantages of ethane WAG over CO2 WAG will
be particularly important in areas with limited corrosion problems where the produced oil is sweet.
An additional advantage of ethane is water solubility. CO2 is
about 20 times as soluble in water as ethane. In addition to increased corrosivity, the "parasitic losses" of CO2 to the large volumes of water contacted in a mature waterflood will increase significantly the volume of CO2 required for a WAG flood.
One study of a representative reservoir showed that more than
2 Bcf of CO2 per square mile was trapped by being dissolved in
the reservoir brine at a solubility of 140 cubic feet of CO2 per barrel. Ethane will not experience the solution trapping associated
with CO2. As a result, there is a large economic benefit for comparable ethane EOR projects.
A final advantage of ethane over CO2 is that the produced gas
from an ethane WAG flood has intrinsic value as sales gas or fuel,
and no acid-gas removal plant is required either for gas sales or
reinjection. High gas-to-oil ratio fields before significant ethane
breakthrough could produce lean gas that was too diluted with
methane to make recycling desirable. The gas could continue to
be sold or burned for fuel until the gas C2+ mole fraction becomes
high enough to be a good injectant. Eventually, returned ethane
will overwhelm the produced solution gas.
FIGURE 3
Bridgeport Ethane and CO2 WAG Oil Saturation and
Oil Viscosity Simulations
SO

68 THE AMERICAN OIL & GAS REPORTER

VISO

0
3,500

4,000

4,500

5,000

5,500

6,000

CO2 WAG
EOR, Mstb
GI, MMscf
GP, MMscf
Mscf/stb

6,500

26.1
190.8
94.7
7.32 Gross
3.69 Net

7,000

250
200
150
100

Example Target Fields
Extensive ethane infrastructure, including ethane and natural
gas liquids pipelines from the Bakken, Eagle Ford, Marcellus/Utica and other shale plays has opened ethane EOR opportunities
in areas that either lack sufficient CO2 supplies, or where fields
are not appropriate for CO2 WAG.
There are several types of ethane EOR targets. Evaluations of
three examples-the Lawrence Field in the Illinois Basin, Cushing Field in Oklahoma, and East Texas Field-provide insights on
the various types of fields that could be good candidates for ethane
EOR. These evaluations focus on WAG displacement efficiency and miscibility in the reservoir, as analyzed using a commercial reservoir simulator, and are not intended to accurately represent reservoir geology or sweep efficiency.
The Illinois Basin is a very promising target for gas EOR, but
no viable source of CO2 exists to develop large remaining reserves
in legacy assets such as the Lawrence Field. The largest field in
the basin, it was discovered in 1906 with an estimated 1.05 billion barrels of original oil in place. Cumulative oil recovery is
less than 40 percent of OOIP.
One obvious target for a WAG project in the Lawrence Field
is the shallow Bridgeport interval at a depth of 950 feet. Given
its depth, a safe injection pressure would be perhaps no more than
700 psi.
The field is located approximately 20 miles north of Enterprise Products Partners' ATEX ethane pipeline terminal at
Princeton, In. Slim tube simulations using 100 cells in a finite difference model were run to determine the MMP of both ethane
and CO2 in the Lawrence Field. The analysis used an oil composition of 34 degrees API gravity and reservoir temperatures of
71 and 80 degrees, respectively, for the shallow Bridgeport and
deeper Cypress intervals. The MMP for ethane in the Bridgeport
was slightly less than 600 psi, while the MMP for CO2 was 950
psi.
A quarter-five-spot model of the Bridgeport was built using
the SPE3 comparative solution project problem (SPE3 solution)
with 9x9x4-layer gas cycling and blow-down). Reasonable estimates of the depth, initial pressure, reservoir thickness, well spacing, fluid composition, average porosity, and average permeability from the Bridgeport were used in the model.
A 10-acre spacing base model was waterflooded for 20 years.

American Oil and Gas Reporter - February 2017

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